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Junio de 2014
3D inversion of gravity data in spherical coordinates with
application to the GRAIL data
Authors: Qing Liang, Chao Chen et al
Link: Click here
Three-dimensional (3D) inversion of gravity
data has been widely used to reconstruct the density distributions
of ore bodies, basins, crust, lithosphere and upper mantle.
At present, gravity inversions are based on density model representation
in the Cartesian coordinates, which is applicable for local
to regional scales. For global model of 3D density structures
of planetary interior, such as the Earth, the Moon or Mars,
it is necessary to use an inversion algorithm that operates
in the spherical coordinates. We develop a 3D inversion algorithm
formulated with
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specially
designed model objective function and radial weighting function
in the spherical coordinates. We present a regional and a global
synthetic example to illustrate the capability of the algorithm.
The inverted results show density distribution features consistent
with the true models. We also apply the algorithm to a set of
lunar Bouguer gravity anomaly, and obtain a lunar 3D density distribution.
High density anomalies are clearly identified underlying lunar
basins, and a wide region of the lateral density heterogeneities
are found that exist beneath the South Pole-Aitken (SPA) basin;
and low density anomalies are distributed beneath the Feldspathic
Highlands Terrane (FHT) on the lunar far-side. The consistency
of these results with those obtained independently from other
existing methods verifies the newly developed algorithm |
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Junio de 2014
Varying Seismic-Acoustic Properties of the Fluctuating Lava
Lake at Villarrica Volcano, Chile
Authors: Joshua P. Richardson, Gregory P. Waite et al
Link: Click here
Villarrica volcano outgasses through an open
lava lake, with bubbles ranging in size from sub-millimeter
to several meters, the largest of which produce strombolian
bursting events that are visible from the crater rim. Thousands
of shallow strombolian events identified through seismic waveform
cross-correlation were found to produce discrete and repetitive
long-period seismic and infrasonic signals. We identified variations
of up to 0.7 s in seismic-acoustic arrival delay times between
April and July 2010 at a station ~750 m from the vent, which
we interpret as due to fluctuations of hundreds of meters in
the level of
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lava lake below the conduit rim.
During time periods interpreted as having high lava lake levels,
based on reduced time delays, interevent times were also reduced,
and average seismic amplitude measurements, seismic and acoustic
event energies, and volcano acoustic-seismic ratios were all high
as compared to times when the lava lake was lower. The crater
is also a source of nearly continuous, monotonic infrasonic tremor.
We found that the peak frequency of this infrasonic tremor, typically
around 0.5 - 1.0?Hz, was inversely correlated with seismic-acoustic
delay times and therefore an indicator of lava lake level. We
use this correlation to propose a new model for infrasonic tremor
generation, namely using crater geometry to approximate a Bessel
horn. We interpret the two clearest cycles of elevated seismicity
and lava lake level as due to an increase in exsolved gas resulting
from an injection of volatile-rich magma or an overturn in a deeper
magma reservoir. |
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Junio de 2014
The 2010-2014.3 global earthquake rate increase
Authors: Tom Parsons and Eric L. Geist
Link: Click here
In light of a heightened global earthquake rate
during the first quarter of 2014 and recent studies concluding
that large earthquakes affect global seismicity for extended
periods, we revisit the question whether the temporal distribution
of global earthquakes shows clustering beyond that expected
from a time independent Poisson
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process. We examine a broad window
from 1979-2014.3 for M 7.0 shocks, and a narrow window for M 5.0
seismicity since 2010 that has higher than average rates. We test
whether a Poisson process can be falsified at 95% confidence to
assess the degree of dependent clustering in the catalogs. If
aftershocks within at least one rupture length from mainshocks/foreshocks
are filtered, then we find no evidence of global scale M 5.2-5.6
(depending on parameters) clustering since 2010 that demands a
physical explanation. There is evidence for interdependence below
this threshold that could be a consequence of catalog completeness
or a physical process. |
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Junio de 2014
Pyroclast Tracking Velocimetry illuminates bomb ejection
and explosion dynamics at Stromboli (Italy) and Yasur (Vanuatu)
volcanoes
Authors: Damien Gaudin , Jacopo Taddeucci et al
Link: Click here
A new image processing technique Pyroclast Tracking
Velocimetry was used to analyze a set of 30 high-speed videos
of Strombolian explosions from different vents at Stromboli
(Italy) and Yasur (Vanuatu) volcanoes. The studied explosions
invariably appear to result from the concatenation of up to
a hundred individual pyroclast ejection pulses. All these pulses
share a common evolution over time, including: 1) a non-linear
decrease of the pyroclast ejection velocity; 2) an increasing
spread of ejection angle; and 3) an increasing size of the ejected
pyroclast. These features reflects the dynamic burst of short-lived
gas pockets, in which the rupture area
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enlarges
while pressure differential decreases. We estimated depth of pyroclast
release to be approximately 1 and 8 m below the surface at Stromboli
and Yasur, respectively. In addition, explosions featuring more
frequent pulses also have higher average ejection velocities and
larger total masses of pyroclasts. These explosions release a
larger overall amount of energy stored in the pressurized gas
by a combination of more frequent and stronger ejection pulses.
In this context, the associated kinetic energy per explosion,
ranging 103 - 109 J appears to be a good proxy for the explosion
magnitude. Differences in the pulse-defining parameters among
the different vents suggest that this general process is modulated
by geometrical factors in the shallow conduit, as well as magma-specific
rheology. Indeed, the more viscous magma of Yasur, compared to
Stromboli, is associated with larger vents producing fewer pulses
but larger pyroclasts. |
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Junio de 2014
Experimental evidence for the formation of liquid saline
water on Mars
Authors: Erik Fischer, Germán M. Martínez
et al
Link: Click here
Evidence for deliquescence of perchlorate salts
has been discovered in the Martian polar region while possible
brine flows have been observed in the equatorial region. This
appears to contradict the idea that bulk deliquescence is too
slow to occur during the short periods of the Martian diurnal
cycle during which conditions are
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favorable for it. We conduct laboratory
experiments to study the formation of liquid brines at Mars environmental
conditions. We find that when water vapor is the only source of
water, bulk deliquescence of perchlorates is not rapid enough
to occur during the short periods of the day during which the
temperature is above the salts' eutectic value, and the humidity
is above the salts' deliquescence value. However, when the salts
are in contact with water ice, liquid brine forms in minutes,
indicating that aqueous solutions could form temporarily where
salts and ice co-exist on the Martian surface and in the shallow
subsurface. |
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Junio de 2014
Planetary ageostrophic instability leads to superrotation
Authors: Peng Wang, Jonathan L. Mitchell
Link: Click here
We demonstrate the existence of a global-scale,
linear instability in the atmospheres of slowly rotating and/or
small planets that spontaneously emerges and produces momentum
convergence at the equator, thus supporting the development
of planetary superrotation. We identify the instability as being
barotropic, ageostrophic in nature, coupling an equatorial Kelvin
wave with midlatitude or high-latitude Rossby waves. This
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coupling
requires a frequency matching of the Doppler-shifted wave components
and moderate spatial overlap between them, which are determined
by two nondimensional parameters, the Rossby and Froude numbers.
By diagnosing these parameters, we find that this instability
is an essential and necessary process to obtain superrotation
in dry atmospheric, general circulation models with axisymmetric
forcing. The Rossby and Froude numbers for Solar System bodies
are consistent with the presence or absence of superrotation,
which suggests that they provide useful diagnostics for predicting
the emergence of superrotation in the atmospheres of terrestrial
planets. |
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Junio de 2014
DYNAMICS OF THE YELLOWSTONE HYDROTHERMAL SYSTEM
Authors: Shaul Hurwitz and Jacob B. Lowenstern
Link: Click here
The Yellowstone Plateau Volcanic Field (YPVF)
is characterized by extensive seismicity, episodes of uplift
and subsidence, and a hydrothermal system that comprises more
than 10,000 thermal features, including geysers, fumaroles,
mud pots, thermal springs, and hydrothermal explosion craters.
The diverse chemical and isotopic compositions of waters and
gases derive from mantle, crustal, and meteoric sources and
extensive water-gas-rock interaction at variable pressures and
temperatures. The thermal features are host to all domains of
life that utilize diverse inorganic sources of energy for metabolism.
The unique and exceptional features of the hydrothermal system
have attracted numerous researchers to Yellowstone beginning
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with the Washburn and Hayden expeditions
in the 1870s. Since a seminal review published a quarter of a
century ago [Fournier, 1989], research in many fields has greatly
advanced our understanding of the many coupled processes operating
in and on the hydrothermal system. Specific advances include:
more refined geophysical images of the magmatic system, better
constraints on the time scale of magmatic processes, characterization
of fluid sources and water-rock interactions, quantitative estimates
of heat and magmatic volatile fluxes, discovering and quantifying
the role of thermophile microorganisms in the geochemical cycle,
defining the chronology of hydrothermal explosions and their relation
to glacial cycles, defining possible links between hydrothermal
activity, deformation, and seismicity; quantifying geyser dynamics,
and the discovery of extensive hydrothermal activity in Yellowstone
Lake. Discussion of these many advances forms the basis of this
review. |
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Junio de 2014
Long- and Short-term Postseismic Gravity Changes of Megathrust
Earthquakes from Satellite Gravimetry
Authors: Yusaku Tanaka and Kosuke Heki
Link: Click here
Using monthly satellite gravimetry data, we
studied time-variable gravity field after three M9 class earthquakes,
the 2004 Sumatra-Andaman, 2010 Chile (Maule), and 2011 Tohoku-oki
earthquakes. The observations showed that the gravity typically
(1) decreases coseismically, (2) continues to decrease for a
few months, and (3)
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increases
over a longer period. Therefore, postseismic gravity changes have
two components with different time constants and polarities. The
mechanisms of short- and long-term postseismic gravity changes
are not as clear as coseismic changes at the moment, but might
be explained to some extent with afterslip and the Maxwell viscoelasticity,
respectively. These two components are difficult to discriminate
with surface velocity measurements because the forearc area moves
trenchward at both stages. They appear in different polarities
in gravity, making satellite gravimetry a unique tool to separate
them. |
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Junio de 2014
Remotely triggered non-volcanic tremor in Sumbawa, Indonesia
Authors: F. Fuchs, M. Lupi et al
Link: Click here
We present, for the first time, evidence for
triggered tremor beneath the island of Sumbawa, Indonesia. We
show triggered tremor in response to three teleseismic earthquakes;
the Mw 9.0 2011 Tohoku earthquake, and two oceanic strike slip
earthquakes (Mw 8.6 and Mw 8.2) offshore of Sumatra in 2012.
We constrain an apparent
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triggering threshold of 1 mm/s ground
velocity that corresponds to about 8 kPa dynamic stress. Peak
tremor amplitudes of about 180?nm/s are observed, and scale with
the ground velocity induced by the remote earthquakes. Triggered
tremor responds to 45-65?s period surface waves and predominantly
correlates with Rayleigh waves, even though the 2012 oceanic events
have stronger Love wave amplitudes. We could not locate the tremor
because of minimal station coverage, but data indicates several
potential source volumes including the Flores Thrust, the Java
subduction zone or Tambora volcano. |
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Junio de 2014
Tsunami source and its validation of the 2014 Iquique, Chile,
earthquake
Authors: Chao An, Ignacio Sepúlveda et al
Link: Click here
The slip distribution of the 1 April 2014 Iquique
earthquake is obtained by using the least squares inversion
of tsunami data at three Deep-Ocean Assessment and Reporting
of Tsunamis stations. Most of the slip is concentrated along
a 60 km by 40 km slip patch near the hypocenter, with magnitude
ranging from 5 to 7 m and a depth of
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23
km. The earthquake magnitude from the inversion is estimated as
Mw 8.0. The slip distribution is converted into seafloor displacement
based on Okada's formula. A nonlinear shallow water equation model
is used to simulate tsunami wave propagation, and the simulated
water surface elevations are compared with the measured data at
10 tide gauges along the Chilean coast. The agreement is excellent
at gauges where the local bathymetry data are complete and the
gauges are open to the ocean; otherwise, mismatches of up to 10
min in arrival time and 1.0 m in amplitude are seen. |
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Junio de 2014
A spherical harmonic model of the lithospheric magnetic field
of Mars
Authors: A. Morschhauser, V. Lesur et al
Link: Click here
We present a model of the lithospheric magnetic
field of Mars which is based on Mars Global Surveyor orbiting
satellite data and represented by an expansion of spherical
harmonic (SH) functions up to degree and order 110. Several
techniques were applied in order to obtain a reliable and well-resolved
model of the Martian lithospheric magnetic field: A modified
Huber-Norm was used to properly treat data outliers, the mapping
phase orbit data was weighted based on an a priori analysis
of the data, and static external fields were treated by a joint
inversion of external and internal fields. Further, temporal
variabilities in the data which lead to unrealistically strong
anomalies were considered
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as noise and handled by additionally
minimizing a measure of the horizontal gradient of the vertically
down internal field component at surface altitude. Here we use
an iteratively reweighted least squares algorithm to approach
an absolute measure (L1 norm), allowing for a better representation
of strong localized magnetic anomalies as compared to the conventional
least squares measure (L2?norm). The resulting model reproduces
all known characteristics of the Martian lithospheric field and
shows a rich level of detail. It is characterized by a low level
of noise and robust when downward continued to the surface. We
show how these properties can help to improve the knowledge of
the Martian past and present magnetic field by investigating magnetic
signatures associated with impacts and volcanoes. Additionally,
we present some previously undescribed isolated anomalies, which
can be used to determine paleopole positions and magnetization
strengths. |
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Junio de 2014
Comprehensive observation and modeling of earthquake and
temperature-related seismic velocity changes in northern Chile
with passive image interferometry
Authors: Tom Richter, Christoph Sens-Schönfelder
et al
Link: Click here
We report on earthquake and temperature-related
velocity changes in high-frequency autocorrelations of ambient
noise data from seismic stations of the Integrated Plate Boundary
Observatory Chile project in northern Chile. Daily autocorrelation
functions are analyzed over a period of 5 years with passive
image interferometry. A short-term velocity drop recovering
after several days to weeks is observed for theMw 7.7 Tocopilla
earthquake at most stations. At the two stations PB05 and PATCX,
we observe a long-term velocity decrease recovering over the
course of around 2 years. While station PB05 is located in the
rupture area of the Tocopilla earthquake, this is not the case
for station PATCX. Station PATCX is situated in an
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area influenced by salt sediment
in the vicinity of Salar Grande and presents a superior sensitivity
to ground acceleration and periodic surface-induced changes. Due
to this high sensitivity, we observe a velocity response of several
regional earthquakes at PATCX, and we can show for the first time
a linear relationship between the amplitude of velocity drops
and peak ground acceleration for data from a single station. This
relationship does not hold true when comparing different stations
due to the different sensitivity of the station environments.
Furthermore, we observe periodic annual velocity changes at PATCX.
Analyzing data at a temporal resolution below 1 day, we are able
to identify changes with a period of 24 h, too. The characteristics
of the seismic velocity with annual and daily periods indicate
an atmospheric origin of the velocity changes that we confirm
with a model based on thermally induced stress. This comprehensive
model explains the lag time dependence of the temperature-related
seismic velocity changes involving the distribution of temperature
fluctuations, the relationship between temperature, stress and
velocity change, plus autocorrelation sensitivity kernels. |
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Junio de 2014
The stress shadow problem in physics-based aftershock forecasting:
Does incorporation of secondary stress changes help?
Authors: M. Segou and T. Parsons et al
Link: Click here
Main shocks are calculated to cast stress shadows
across broad areas where aftershocks occur. Thus, a key problem
with stress-based operational forecasts is that they can badly
underestimate aftershock occurrence in the shadows. We examine
the performance of two physics-based earthquake forecast models
(Coulomb rate/state (CRS)) based on Coulomb stress changes and
a rate-and-state friction law
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for
their predictive power on the 1989 Mw>=6.9 Loma Prieta aftershock
sequence. The CRS-1 model considers the stress perturbations associated
with the main shock rupture only, whereas CRS-2 uses an updated
stress field with stresses imparted by M>=3.5 aftershocks.
Including secondary triggering effects slightly improves predictability,
but physics-based models still underestimate aftershock rates
in locations of initial negative stress changes. Furthermore,
CRS-2 does not explain aftershock occurrence where secondary stress
changes enhance the initial stress shadow. Predicting earthquake
occurrence in calculated stress shadow zones remains a challenge
for stress-based forecasts, and additional triggering mechanisms
must be invoked. |
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Junio de 2014
The 1 April 2014 Iquique, Chile, Mw 8.1 earthquake rupture
sequence
Authors: Thorne Lay, Han Yue et al
Link: Click here
On 1 April 2014, a great (Mw 8.1) interplate
thrust earthquake ruptured in the northern portion of the 1877
earthquake seismic gap in northern Chile. The sequence commenced
on 16 March 2014 with a magnitude 6.7 thrust event, followed
by thrust-faulting aftershocks that migrated northward ~40?km
over 2?weeks to near the main shock hypocenter. Guided by short-period
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teleseismic P wave backprojections
and inversion of deepwater tsunami wave recordings, a finite-fault
inversion of teleseismic P and SH waves using a geometry consistent
with long-period seismic waves resolves a spatially compact large-slip
(~2-6.7?m) zone located ~30?km downdip and ~30?km along-strike
south of the hypocenter, downdip of the foreshock sequence. The
main shock seismic moment is 1.7?×?1021?N m with a fault
dip of 18°, radiated seismic energy of 4.5-8.4?×?1016?J,
and static stress drop of ~2.5?MPa. Most of the 1877 gap remains
unbroken and hazardous. |
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Junio de 2014
Volcanic signatures in time gravity variations during the
volcanic unrest on El Hierro (Canary Islands)
Authors: S. Sainz-Maza Aparicio, J. Arnoso Sampedro
et al
Link: Click here
Gravity changes occurring during the initial
stage of the 2011-2012 El Hierro submarine eruption are interpreted
in terms of the preeruptive signatures during the episode of
unrest. Continuous gravity measurements were made at two sites
on the island using the relative spring gravimeter LaCoste and
Romberg gPhone-054. On 15 September 2011, an observed gravity
decrease of 45??Gal, associated with the
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southward
migration of seismic epicenters, is consistent with a lateral
magma migration that occurred beneath the volcanic edifice, an
apparently clear precursor of the eruption that took place 25
days later on 10 October 2011. High-frequency gravity signals
also appeared on 6-11 October 2011, pointing to an occurring interaction
between a magmatic intrusion and the ocean floor. These important
gravity changes, with amplitudes varying from 10 to ?90??Gal,
during the first 3?days following the onset of the eruption are
consistent with the northward migration of the eruptive focus
along an active eruptive fissure. An apparent correlation of gravity
variations with body tide vertical strain was also noted, which
could indicate that concurrent tidal triggering occurred during
the initial stage of the eruption. |
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Junio de 2014
Multisatellite observations of an intensified equatorial
ionization anomaly in relation to the northern Sumatra earthquake
of March 2005
Authors: K. Ryu, E. Lee et al
Link: Click here
Here we report multisatellite observations of
ionospheric disturbances in relation to the occurrence of the
M8.7 northern Sumatra earthquake of 28 March 2005. The DEMETER
(Detection of Electro-Magnetic Emissions Transmitted from Earthquake
Regions) and CHAMP (Challenging Minisatellite Payload) satellite
data were investigated to find possible precursory and postevent
phenomena. It was found that EIA (equatorial ionization anomaly)
strength expressed in the apex height, derived from the CHAMP
plasma density profile, was intensified along the orbits whose
longitudes were close to the epicenter within about a week before
and after occurrence of the earthquake.
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Increases in electron and O+ density
along the orbits close to the epicenter were also observed in
the DEMETER measurements. The normalized equatorial plasma density
derived from the DEMETER measurements showed intensification about
a week before and after the earthquake reaching maximum the day
after the shock and afterward disappearing. In addition, similar
behavior of the EIA enhancements related to the M8.0 Pisco earthquake
of 15 August 2007 was observed. Surveys of space weather and geomagnetic
activities excluded the possibility that these fluctuations were
caused by changes in space weather or by a geomagnetic storm.
Statistical analyses of the longitudinal variation revealed that
the EIA was enhanced in the west of the epicenter and reduced
in the east of the epicenter, and this fits the "increased
conductivity" model. Based on these observations, we proposed
a revised view of seismo-ionospheric coupling in the region of
the geomagnetic equator, to explain the EIA features observed
in this study. |
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Junio de 2014
East Texas's biggest earthquake may have been induced
Author: Colin Schultz
Link: Click here
Aside from a few small events, east Texas has
been largely devoid of earthquakes. However, operations began
in 2006 to pump waste water
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from oil and gas production into
wells around the region, with some sites being injected with just
shy of 43,000 cubic meters of water per month. Within a few years,
this seismic quiet zone began to feel some temblors: In 2008 the
preshocks started, small events with magnitudes from 0.5 to 2.2.
Then, on 10 May 2012, a magnitude 3.9 earthquake hit, chased a
week later by a magnitude 4.8 earthquake. |
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Junio de 2014
The two types of El-Niño and their impacts on the
length of day
Authors: O. de Viron and J. O. Dickey
Link: Click here
At the interannual to decadal time scale, the
changes in the Earth rotation rate are linked with the El-Niño-Southern
Oscillation phenomena through changes in the atmospheric angular
momentum. As climatic studies demonstrate that
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there
were two types of El-Niño events, namely, eastern Pacific
(EP) and central Pacific (CP) events, we investigate how each
of them affects the atmospheric angular momentum. We show in particular
that EP events are associated with stronger variations of the
atmospheric angular momentum and length of day. We explain this
difference by the stronger pressure gradient over the major mountain
ranges, due to a stronger and more efficiently localized pressure
dipole over the Pacific Ocean in the case of EP events. |
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Mayo de 2014
Decrypting geophysical signals at Stromboli Volcano (Italy):
Integration of seismic and Ground-Based InSAR displacement data
Authors: F. Di Traglia, L. Cauchie
Link: Click here
We present the integration of seismic and Ground-Based
Interferometric Synthetic Aperture Radar system (GBInSAR) displacement
data at Stromboli Volcano. Ground deformation in the area of
summit vents is positively correlated with both seismic tremor
amplitude and cumulative amplitudes of very long period (VLP)
signals associated with Strombolian explosions. Changes in VLP
amplitudes precede by a few
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days the variations in ground deformation
and seismic tremor. We propose a model where the arrival of fresh,
gas-rich magma from depth enhances gas slug formation, promoting
convection and gas transfer throughout the conduit system. At
the shallowest portion of the conduit, an increase in volatile
content causes a density decrease, expansion of the magmatic column
and augmented degassing activity, which respectively induce inflation
of the conduit, and increased tremor amplitudes. The temporal
delay between increase of VLP and tremor amplitudes/conduit inflation
can be interpreted in terms of the different timescales characterizing
bulk gas transfer versus slug formation and ascent. |
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Mayo de 2014
Expected seismic shaking in Los Angeles reduced by San Andreas
fault zone plasticity
Authors: D. Roten, K. B. Olsen
Link: Click here
Computer simulations of large (M?7.8) earthquakes
rupturing the southern San Andreas Fault from SE to NW (e.g.,
ShakeOut, widely used for earthquake drills) have predicted
strong long-period ground motions in the densely populated Los
Angeles Basin due to channeling of waves through a series of
interconnected sedimentary basins. Recently, the importance
of this waveguide amplification effect for seismic shaking in
the Los Angeles Basin has also been
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confirmed
from observations of the ambient seismic field. By simulating
the ShakeOut earthquake scenario (based on a kinematic source
description) for a medium governed by Drucker-Prager plasticity,
we show that nonlinear material behavior could reduce the earlier
predictions of large long-period ground motions in the Los Angeles
Basin by up to 70% as compared to viscoelastic solutions. These
reductions are primarily due to yielding near the fault, although
yielding may also occur in the shallow low-velocity deposits of
the Los Angeles Basin if cohesions are close to zero. Fault zone
plasticity remains important even for conservative values of cohesions,
suggesting that current simulations assuming a linear response
of rocks are overpredicting ground motions during future large
earthquakes on the southern San Andreas Fault. |
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Mayo de 2014
Recognizing Foreshocks from the 1 April 2014 Chile Earthquake
Authors: Emily E. Brodsky and Thorne Lay
Link: Click here
Are there measurable, distinctive precursors
that can warn us in advance of the planet's largest earthquakes?
Foreshocks have long been
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considered the most promising candidates
for predicting earthquakes. At least half of large earthquakes
have foreshocks, but these foreshocks are difficult or even impossible
to distinguish from non-precursory seismic activity. The foreshocks
for the 1 April 2014 Chile event and other recent large earthquakes
suggest that observable precursors may exist before large earthquakes.
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Mayo
de 2014
Observed change in plate coupling close to the rupture initiation
area before the occurrence of the 2011 Tohoku earthquake: Implications
from an earthquake cycle model
Authors: Makiko Ohtani, Kazuro Hirahara et al
Link: Click here
Changes in plate coupling off the coast of Fukushima
have been detected by GPS since 2000. These changes occurred
close to the rupture initiation area of the Mw9.0 2011 Tohoku
earthquake and possibly initiated the earthquake. We investigated
these changes with quasi-dynamic earthquake cycle simulations
using a hierarchical asperity model. We modeled the
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entire
rupture region as rate weakening but conditionally stable, while
areas with huge slips and Mw7 asperities were modeled as strong
and ordinary rate-weakening unstable friction, respectively. The
following observed characteristics were reproduced: long recurrence
time, large rupture region (including a localized huge coseismic
slip area and source area of recurring Mw7 earthquakes), and a
Mw7 foreshock triggering the Tohoku earthquake. Off Fukushima,
repeated aseismic slips propagating northward appeared in the
last half of the cycle and possibly caused the decrease in plate
coupling. However, it is not necessarily related to the immediate
occurrence of the giant earthquake. |
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Mayo de 2014
On the origin and evolution of electrical signals during
frictional stick slip in sheared granular material
Authors: J.R. Leeman, M.M. Scuderi et al
Link: Click here
Electromagnetic signals have been reported in
association with geophysical phenomena including earthquakes,
landslides, and volcanic events. Mechanisms that suggested to
explain seismoelectrical signals include triboelectricity, piezoelectricity,
streaming potentials, and the migration of electron holes, yet
the origin of such phenomena remains poorly understood. We present
results from laboratory experiments regarding the relationship
between electrical and mechanical signals for frictional stick-slip
events in sheared soda-lime glass bead layers. The results are
interpreted in the context of lattice defect migration and granular
force chain mechanics. During stick-slip events, we observe
two distinct behaviors delineated by the
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attainment of a frictional stick-slip
steady state. During initial shear loading, layers charge during
stick-slip events and the potential of the system rises. After
steady state stick-slip behavior is attained, the system begins
to discharge. Coseismic signals are characterized by potential
drops superimposed on a longer-term trend. We suggest that the
observed signal is a convolution of two effects: charging of the
forcing blocks and signals associated with the stress state of
the material. The long-term charging of the blocks is accomplished
by grain boundary movement during the initial establishment of
force chain networks. Short-term signals associated with stick-slip
events may originate from produced electron holes. Applied to
tectonic faults, our results suggest that electrical signals generated
during frictional failure may provide a way to monitor stress
and the onset of earthquake rupture. Potential changes could produce
detectable signals that may forecast the early stages of failure,
providing a modest warning of the event. |
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Mayo de 2014
Data from Tohoku earthquake offer new insights
Author: JoAnna Wendel
Link: Click here
The subduction zone megathrust earthquake that
struck offshore of Tohoku, Japan, in 2011 was so strong that
it shifted the tilt of the Earth's axis and
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moved
the island of Japan itself four meters. This earthquake puzzles
scientists in part because the hypocenter-where the earthquake
originated, deep below the epicenter-occurred on a fault previously
thought to be inactive. The lack of data about the rock surrounding
the hypocenter makes it hard for scientists to pinpoint specific
details about how the earthquake was triggered and propagated. |
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Mayo de 2014
Effects of various lithospheric yield stresses and different
mantle-heating modes on the breakup of the Pangea supercontinent
Author: Masaki Yoshida
Link: Click here
Numerical simulations of three-dimensional spherical
mantle convection were performed to investigate the effects
of various lithospheric yield stresses and two different mantle-heating
modes (i.e., mixed heating from the bottom and interior and
purely internal heating) on the breakup of the Pangea supercontinent
and the subsequent
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continental drift for the past 200
Myr. Results show that the continental breakup and subsequent
continental drift are accomplished in mantle convection models
with two different heating modes. This implies that active upwelling
plumes from the core-mantle boundary are not necessarily required
for the breakup of Pangea. In addition, I found that the continental
breakup is only realized when choosing a moderate value of the
yield stress (~120 MPa). The bound on the yield stress derived
in the present study will potentially enable the self-consistent
reconstruction of continental breakup and drift as well as the
accompanying pattern on mantle convection since 200 Ma. |
|
Mayo de 2014
Tomography from 26 years of seismicity revealing that the
spatial extent of the Yellowstone crustal magma reservoir extends
well beyond the Yellowstone caldera
Authors: Jamie Farrell, Robert B. Smith et al
Link: Click here
The Yellowstone volcanic field has experienced
three of Earth's most explosive volcanic eruptions in the last
2.1 Ma. The most recent eruption occurred 0.64 Ma forming the
60 km long Yellowstone caldera. We have compiled earthquake
data from the Yellowstone Seismic Network from 1984 to 2011
and tomographically
|
imaged
the three-dimensional P wave velocity (Vp) structure of the Yellowstone
volcanic system. The resulting model reveals a large, low Vp body,
interpreted to be the crustal magma reservoir that has fueled
Yellowstone's youthful volcanism. Our imaged magma body is 90
km long, 5-17 km deep, and 2.5 times larger than previously imaged.
The magma body extends ~15 km NE of the caldera and correlates
with the location of the largest negative gravity anomaly, a 80
mGal gravity low. This new seismic image provides important constraints
on the dynamics of the Yellowstone magma system and its potential
for future volcanic eruptions and earthquakes. |
|
Mayo de 2014
Bouguer gravity anomaly and isostasy at western Sichuan Basin
revealed by new gravity surveys
Authors: Guangyu Fu, Shanghua Gao et al
Link: Click here
A dense relative gravity and Global Positioning
System observation network with 302 stations was measured in
the western Sichuan Basin (SB) to study gravity and isostasy.
Bouguer Gravity Anomalies are negative in the study area, and
change gradually from about ?110 mGal (10-?5m?s-?2) in the southeast
to ?220 mGal in the northwest. The new data reveal that the
Moho beneath the western SB changes gradually from 39.5?km in
the southeast to 43.7?km in the northwest and is inclined slightly
to the northwest beneath the Chengdu Plain. The isostatic crustal
|
thickness calculated by Airy isostatic
theory varies between 39.5 and 42.0?km. The Longquan Shan (LQS)
marks a clear boundary in the state of isostastic compensation
in the study area. The Moho depth is almost identical to the Airy
isostatic crustal thickness east of the LQS, where the crust is
in isostatic balance. However, on the Chengdu Plain west of the
LQS, differences between the Moho depth and Airy isostatic crustal
thickness became larger from east to west, where the crust is
out of isostatic balance. This indicates that the load of the
Longmen Shan (LMS) on the west part of the crust of SB is mainly
borne by the crust beneath the Chengdu Plain, and does little
influence the crust east of the LQS. As a result we deduce that
the LQS fault zone is a deeply rooted high angle fault zone that
was broken by the load of the LMS. |
|
Mayo de 2014
Geophysical Advances Triggered by 1964 Great Alaska Earthquake
Authors: Peter Haeussler, William Leith et al
Link: Click here
A little more than 50?years ago, on 27 March
1964, the Great Alaska earthquake and tsunami struck. At moment
magnitude 9.2, this earthquake
|
is notable as the largest in U.S.
written history and as the second-largest ever recorded by instruments
worldwide. But what resonates today are its impacts on the understanding
of plate tectonics, tsunami generation, and earthquake history
as well as on the development of national programs to reduce risk
from earthquakes and tsunamis. |
|
Mayo de 2014
Direct measurement of subsurface mass change using the variable
baseline gravity gradient method
Authors: Jeffrey Kennedy, Ty P. A. Ferré et al
Link: Click here
Time-lapse gravity data provide a direct, nondestructive
method to monitor mass changes at scales from centimeter to
kilometer. But, the effectively infinite spatial sensitivity
of gravity measurements can make it difficult to isolate the
signal of interest. The variable baseline gravity gradient method,
based on the difference of measurements between two gravimeters,
is an
|
alternative
to the conventional approach of individually modeling all sources
of mass and elevation changes. This approach can improve the signal-to-noise
ratio for many applications by removing the contributions of Earth
tides, loading, and other signals that have the same effect on
both gravimeters. At the same time, this approach can focus the
support volume within a relatively small user-defined region of
the subsurface. The method is demonstrated using paired superconducting
gravimeters to make for the first time a large-scale, noninvasive
measurement of infiltration wetting front velocity and change
in water content above the wetting front. |
|
Abril de 2014
Geophysical Advances Triggered by 1964 Great Alaska Earthquake
Authors: Peter Haeussler, William Leith et al
Link: Click here
A little more than 50 years ago, on 27 March
1964, the Great Alaska earthquake and tsunami struck. At moment
magnitude 9.2, this earthquake is
|
notable as the largest in U.S. written
history and as the second-largest ever recorded by instruments
worldwide. But what resonates today are its impacts on the understanding
of plate tectonics, tsunami generation, and earthquake history
as well as on the development of national programs to reduce risk
from earthquakes and tsunamis. |
|
Abril de 2014
New algorithm to improve earthquake early warning systems
Author: Colin Schultz
Link: Click here
When a fault line ruptures, seismic waves race
out from the earthquake epicenter.
|
Compressional
seismic waves, known as primary (P) waves, travel fastest. Shear
waves-secondary (S) waves-travel more slowly but are the source
of the bulk of earthquake-induced damage. Using the opportunity
afforded by the difference in travel times between these two types
of waves, researchers have begun to design and implement earthquake
early warning systems. |
|
Abril de 2014
A spherical harmonic model of the lithospheric magneticfield
of Mars
Authors: A. Morschhauser, V. Lesur et al
Link: Click here
We present a model of the lithospheric magnetic
field of Mars which is based on Mars Global Surveyor orbiting
satellite data and represented by an expansion of spherical
harmonic functions up to degree and order 110. Several techniques
were applied in order to obtain a reliable and well-resolved
model of the Martian lithospheric magnetic field. A modified
Huber-Norm was used to properly treat data outliers and the
MPO data was weighted based on an a priori analysis of the data.
Static external fields were treated by a joint inversion of
external and internal fields. Further, temporal variabilities
in the data which lead to unrealistically strong anomalies were
considered as noise and handled by additionally minimizing a
|
measure of the horizontal gradient
of the vertically down internal field component at surface altitude.
Here, we use an iteratively reweighted least-squares algorithm
to approach an absolute measure (L1-norm), allowing for a better
representation of strong localized magnetic anomalies as compared
to the conventional least-squares measure (L2-norm). The resulting
model reproduces all known characteristics of the Martian lithospheric
field and shows a rich level of detail. It is characterized by
a low level of noise and robust when downward-continued to the
surface. We show how these properties can help to improve the
knowledge of the Martian past and present magnetic field by investigating
magnetic signatures associated with impacts and volcanoes. Additionally,
we present some previously undescribed isolated anomalies, which
can be used to determine paleopole positions and magnetization
strengths. |
|
Abril de 2014
Advantage of wavelet technique to highlight the observed
geomagnetic perturbations linked to the Chilean tsunami (2010)
Authors: V. Klausner, Odim Mendes et al
Link: Click here
The vertical component (Z) of the geomagnetic
field observed by ground-based observatories of the International
Real-Time Magnetic Observatory Network has been used to analyze
the induced magnetic fields produced by the movement of a tsunami,
electrically conducting sea water through the geomagnetic field.
We focus on the survey of minutely sampled geomagnetic variations
induced by the tsunami of 27 February 2010 at Easter Island
(IPM) and Papeete (PPT) observatories. In order to detect the
tsunami
|
disturbances in the geomagnetic data,
we used wavelet techniques. We have observed an 85% correlation
between the Z component variation and the tide gauge measurements
in period range of 10 to 30 min which may be due to two physical
mechanisms: gravity waves and the electric currents in the sea.
As an auxiliary tool to verify the disturbed magnetic fields,
we used the maximum variance analysis (MVA). At PPT, the analyses
show local magnetic variations associated with the tsunami arriving
in advance of sea surface fluctuations by about 2 h. The first
interpretation of the results suggests that wavelet techniques
and MVA can be effectively used to characterize the tsunami contributions
to the geomagnetic field and further used to calibrate tsunami
models and implemented to real-time analysis for forecast tsunami
scenarios. |
|
Abril de 2014
Remarkably consistent thermal state of the south central
Chile subduction zone from 36°S to 45°S
Authors: Holly M. M. Rotman and Glenn A. Spinelli
Link: Click here
Delineating rupture areas of subduction zone
earthquakes is necessary for understanding the controls on seismic
and aseismic slip. For the largest recorded earthquake, the
1960 Chile event with moment magnitude 9.5, the rupture area
is only loosely defined due to limitations in the global seismic
network at the time. The rupture extends ~900?km along strike.
Coastal deformation is consistent with either a constant rupture
width of ~180-200?km along the entire length or a narrower (~115?km)
rupture in the southern half. A southward narrowing of the seismogenic
zone has been hypothesized to result from warming of the subduction
zone to the
|
south, where the subducting plate is younger. We present results
of thermal models at 36°S, 38°S, 43°S, and 45°S
to examine potential along-strike changes in thermal state. Models
most consistent with observed surface heat flux include fluid
circulation in the oceanic crust that advects heat to the ocean.
This ventilated hydrothermal circulation preferentially cools
transects with young subducting lithosphere; frictional heating
preferentially warms transects with older subducting lithosphere.
The combined effects of frictional heating and hydrothermal circulation
increase décollement temperatures in the 36°S and 38°S
transects by up to ~155°C and decrease temperatures in the
45°S transect by up to ~150°C. In our preferred models,
décollement temperatures 200?km landward of the trench
in all four transects are ~350-400°C. This is consistent with
a constant ~200?km wide seismogenic zone for the 1960 Mw 9.5 rupture,
with decreasing slip magnitude in the southern half of the rupture. |
|
Abril de 2014
Human Activity May Have Triggered Fatal Italian Earthquakes,
Panel Says
Author: Edwin Cartlidge
Link: Click here
In an as-yet-unpublished report, an international
panel of geoscientists has concluded that a pair of deadly earthquakes
that struck the Italian region
|
of Emilia-Romagna in 2012 could have
been triggered by the extraction of petroleum at a local oil field.
Fear of humanmade seismicity has already sparked fierce opposition
against new oil and gas drilling efforts in Italy, and some say
the report could lead the country's regional presidents to turn
down new requests for fossil-fuel exploration. |
|
Abril de 2014
Human Activity May Have Triggered Fatal Italian Earthquakes,
Panel Says
Authors: Julien Monteux and Jafar Arkani-Hamed
Link: Click here
A giant impact is an increasingly popular explanation
for the formation of the northern lowland on Mars. It is plausible
that at the impact time both Mars and the impactor were differentiated
with solid silicate mantles and liquid iron cores. Such a large
impact likely resulted in merging of the cores of both bodies,
a process which will have implications on the thermal state
of the planet. We model the evolution of the Martian mantle
following a giant impact and characterize the thermochemical
consequences of the sinking of an impactor's core as a single
diapir. The impact heating and the viscous
|
heating
induced during the core merging may affect the early thermal state
of Mars during several tens of million years. Our results show
that large viscosity contrasts between the impactor's core and
the surrounding mantle silicates can reduce the duration of the
merging down to 1 kyr but do not modify the merging temperature.
When the viscosity contrast between the diapir and the surrounding
silicates is larger than a factor of 1000, the descent of the
diapir can lead to some entrainment of the relatively shallow
silicates to deepest regions close to the core-mantle boundary.
Finally, the direct impact heating of Martian core leads to thermal
stratification of the core and kills the core dynamo. It takes
on the order of 150-200 Myr to reinitiate a strong dynamo anew.
The merging of the impactor's core with the Martian core only
delays the reinitiation of the dynamo for a very short time. |
|
Abril de 2014
Assessment of the tsunami-induced current hazard
Authors: Patrick J. Lynett, Jose Borrero et al
Link: Click here
The occurrence of tsunami damage is not limited
to events causing coastal inundation. Even without flooding,
maritime assets are vulnerable to significant damage from strong
currents and associated drag forces. While such impacts have
been observed in the past, they have not been well studied in
any context. Nearshore tsunami
|
currents are governed by nonlinear
and turbulent physics and often have large spatial and temporal
variability making high-fidelity modeling particularly challenging.
Furthermore, measured data for the validation of numerical simulations
is limited, with few quality data sets appearing after recent
tsunami events. In this paper, we present a systematic approach
for the interpretation of measured tsunami-induced current impacts
as well as a validation approach for simulation tools. The methods
and results provided here lay the foundation for much needed efforts
to assess tsunami hazards in ports and harbors. |
|
Abril de 2014
Bouguer Gravity Anomaly and Isostasy at Western Sichuan Basin
Revealed by New Gravity Surveys
Authors: Guangyu Fu, Shanghua Gao et al
Link: Click here
A dense relative gravity and Global Positioning
System (GPS) observation network with 302 stations was measured
in the western Sichuan Basin (SB) to study gravity and isostasy.
Bouguer Gravity Anomalies (BGAs) are negative in the study area,
and change gradually from about -110 mGal (10-5?ms-2) in the
southeast to -220 mGal in the northwest. The new data reveal
that the Moho beneath the western SB changes gradually from
39.5?km in the southeast to 43.7?km in the northwest, and is
inclined slightly to the northwest beneath the Chengdu Plain.
The isostatic crustal
|
thickness
calculated by Airy isostatic theory varies between 39.5 and 42.0?km.
The Longquan Shan (LQS) marks a clear boundary in the state of
isostastic compensation in the study area. The Moho depth is almost
identical to the Airy isostatic crustal thickness east of the
LQS, where the crust is in isostatic balance. However, on the
Chengdu Plain west of the LQS, differences between the Moho depth
and Airy isostatic crustal thickness became larger from east to
west, where the crust is out of isostatic balance. This indicates
that the load of the Longmen Shan (LMS) on the west part of the
crust of SB is mainly borne by the crust beneath the Chengdu Plain,
and does little influence the crust east of the LQS. As a result
we deduce that the LQS fault zone is a deeply rooted high angle
fault zone that was broken by the load of the LMS. |
|
Abril de 2014
Triggering and modulation of geyser eruptions in Yellowstone
National Park by earthquakes, earth tides, and weather
Authors: Shaul Hurwitz, Robert A. Sohn et al
Link: Click here
We analyze intervals between eruptions (IBEs)
data acquired between 2001 and 2011 at Daisy and Old Faithful
geysers in Yellowstone National Park. We focus our statistical
analysis on the response of these geysers to stress perturbations
from within the solid earth (earthquakes and earth tides) and
from weather (air pressure and temperature, precipitation, and
wind). We conclude that (1) the IBEs of these geysers are insensitive
to periodic stresses induced by solid earth tides and barometric
pressure variations; (2) Daisy (pool geyser) IBEs lengthen by
evaporation and heat loss in response to large wind stormsand
cold air; and (3) Old Faithful (cone
|
geyser) IBEs are not modulated by
air temperature and pressure variations, wind, and precipitation,
suggesting that the subsurface water column is decoupled from
the atmosphere. Dynamic stress changes of 0.1?0.2?MPa resulting
from the 2002 M-7.9 Denali, Alaska, earthquake surface waves caused
a statistically significant shortening of Daisy geyser's IBEs.
Stresses induced by other large global earthquakes during the
study period were at least an order of magnitude smaller. In contrast,
dynamic stresses of >0.5?MPa from three large regional earthquakes
in 1959, 1975, and 1983 caused lengthening of Old Faithful's IBEs.
We infer that most subannual geyser IBE variability is dominated
by internal processes and interaction with other geysers. The
results of this study provide quantitative bounds on the sensitivity
of hydrothermal systems to external stress perturbations and have
implications for studying the triggering and modulation of volcanic
eruptions by external forces. |
|
Marzo de 2014
Moment tensor inversions of M ~ 3 earthquakes in the Geysers
geothermal fields, California
Authors: A. Guilhem, L. Hutchings et al
Link: Click here
Microearthquakes have come into high public
awareness due to being induced by the development and exploitation
of enhanced and natural geothermal fields, hydrofracturing,
and CO2 sequestration sites. Characterizing and understanding
the faulting process of induced earthquakes, which is generally
achieved through moment tensor inversion, could both help in
risk prediction and in reservoir development monitoring. However,
this is a challenging task because of their lower signal-to-noise
ratio at frequencies typically used in earthquake source analyses.
Therefore, higher-resolution velocity models and modeling of
seismic waves at higher frequencies are required. In this study,
we
|
examine both the potentials to obtain
moment tensor solutions for small earthquakes and the uncertainty
of those solutions. We utilize a short-period seismic network
located in the Geysers geothermal field in northern California
and limit our study to that which would be achieved by industry
in a typical reservoir environment. We obtain full moment tensor
solutions of M ~ 3 earthquakes using waveform modeling and first-motion
inversions. We find that these two data sets give complimentary
but yet different solutions. Some earthquakes correspond possibly
to complex processes in which both shear and tensile failures
occur simultaneously or sequentially. This illuminates the presence
of fluids at depth and their role for the generation of these
small-magnitude earthquakes. Finally, since first motions are
routinely obtained for all magnitude earthquakes, our approach
could be extended to small earthquakes where noise level and complex
Green's functions prohibit using waveforms in moment tensor inversions. |
|
Marzo de 2014
Ensemble prediction and intercomparison analysis of GRACE
time-variable gravity field models
Authors: C. Sakumura, S. Bettadpur et al
Link: Click here
Precise measurements of the Earth's time-varying
gravitational field from the NASA/DLR GRACE mission allow unprecedented
tracking of the transport of mass across and underneath the
surface of the Earth and give insight into secular, seasonal,
and sub-seasonal variations in the global water supply. Several
groups produce
|
these
estimates and while the various gravity fields are similar, differences
in processing strategies and tuning parameters result in solutions
with regionally specific variations and error patterns. This study
examined the spatial, temporal, and spectral variations between
the different gravity field products and developed an ensemble
gravity field solution from the products of four such analysis
centers. The solutions were found to lie within a certain analysis
scatter regardless of the local relative water height variation,
and the ensemble model is clearly seen to reduce the noise in
the gravity field solutions within the available scatter of the
solutions. |
|
Marzo de 2014
Can we test for the maximum possible earthquake magnitude?
Authors: M. Holschneider, G. Zöller et al
Link: Click here
We explore the concept of maximum possible earthquake
magnitude, M, in a region represented by an earthquake catalog
from the viewpoint of statistical testing. For this aim, we
assume that earthquake magnitudes are independent events that
follow a doubly truncated Gutenberg-Richter distribution and
focus on the upper truncation M. In earlier work, it has been
shown that the value of M cannot be well constrained from earthquake
catalogs alone. However, for two hypothesized values M and M?,
alternative statistical tests may address the question: Which
value is more consistent with the data? In other words, is it
possible to reject a magnitude within reasonable
|
errors, i.e., the error of the first
and the error of the second kind? The results for realistic settings
indicate that either the error of the first kind or the error
of the second kind is intolerably large. We conclude that it is
essentially impossible to infer M in terms of alternative testing
with sufficient confidence from an earthquake catalog alone, even
in regions like Japan with excellent data availability. These
findings are also valid for frequency-magnitude distributions
with different tail behavior, e.g., exponential tapering. Finally,
we emphasize that different data may only be useful to provide
additional constraints for M, if they do not correlate with the
earthquake catalog, i.e., if they have not been recorded in the
same observational period. In particular, long-term geological
assessments might be suitable to reduce the errors, while GPS
measurements provide overall the same information as the catalogs. |
|
Marzo de 2014
Assessment of the tsunami-induced current hazard
Authors: Patrick J. Lynett, Jose Borrero et al
Link: Click here
The occurrence of tsunami damage is not limited
to events causing coastal inundation. Even without flooding,
maritime assets are vulnerable to significant damage from strong
currents and associated drag forces. While such impacts have
been observed in the past, they have not been well studied in
any context. Nearshore tsunami
|
currents are governed by nonlinear
and turbulent physics and often have large spatial and temporal
variability making high-fidelity modeling particularly challenging.
Furthermore, measured data for the validation of numerical simulations
is limited, with few quality data sets appearing after recent
tsunami events. In this paper, we present a systematic approach
for the interpretation of measured tsunami-induced current impacts
as well as a validation approach for simulation tools. The methods
and results provided here lay the foundation for much needed efforts
to assess tsunami hazards in ports and harbors. |
|
Marzo de 2014
Migrating quake swarm may indicate magma conduit clog
Author: Colin Schultz
Link: Click here
On 13 January 2006, Augustine Volcano, a towering
volcano offshore from the Alaska Peninsula, erupted explosively.
In the days leading up to the eruption, a series of explosions
|
and
earthquake swarms had warned of the impending activity. On 12
January, 36?hours before the first magmatic explosions, a swarm
of 54 earthquakes was detected across the 13-station seismic network
on Augustine Island. Analyzing the seismic waves produced by the
earthquakes, Buurman and West found that the earthquakes were
being triggered from point sources within the magma conduit itself. |
|
Marzo de 2014
Quasi-dynamic versus fully dynamic simulations of earthquakes
and aseismic slip with and without enhanced coseismic weakening
Authors: Marion Y. Thomas, Nadia Lapusta et al
Link: Click here
Physics-based numerical simulations of earthquakes
and slow slip, coupled with field observations and laboratory
experiments, can, in principle, be used to determine fault properties
and potential fault behaviors. Because of the computational
cost of simulating inertial wave-mediated effects, their representation
is often simplified. The quasi-dynamic (QD) approach approximately
accounts for inertial effects through a radiation damping term.
We compare QD and fully dynamic (FD) simulations by exploring
the long-term behavior of rate-and-state fault models with and
without additional weakening during seismic slip. The models
incorporate a velocity-strengthening (VS) patch in a velocity-weakening
(VW) zone, to consider rupture interaction with a
|
slip-inhibiting heterogeneity. Without
additional weakening, the QD and FD approaches generate qualitatively
similar slip patterns with quantitative differences, such as slower
slip velocities and rupture speeds during earthquakes and more
propensity for rupture arrest at the VS patch in the QD cases.
Simulations with additional coseismic weakening produce qualitatively
different patterns of earthquakes, with near-periodic pulse-like
events in the FD simulations and much larger crack-like events
accompanied by smaller events in the QD simulations. This is because
the FD simulations with additional weakening allow earthquake
rupture to propagate at a much lower level of prestress than the
QD simulations. The resulting much larger ruptures in the QD simulations
are more likely to propagate through the VS patch, unlike for
the cases with no additional weakening. Overall, the QD approach
should be used with caution, as the QD simulation results could
drastically differ from the true response of the physical model
considered. |
|
Marzo de 2014
Ensemble prediction and intercomparison analysis of GRACE
time-variable gravity field models
Authors: Chunli Dai, C. K. Shum et al
Link: Click here
A new approach of using only the north component
of gravity change from Gravity Recovery and Climate Experiment
(GRACE) data reveals that a substantially higher spatial resolution
of the observed seismic deformation following the 2011 Tohoku
earthquake is achievable at 333 km or longer. Here we show
|
that
GRACE-observed north component of gravity change, 17.6 ±
1.1 Gal, and the corresponding gravity gradient change, e.g.,
Txz at 1.25 ± 0.09 mEötvös, agree well with seismic/GPS
model predictions. Localized Slepian spectrum analysis further
confirms that the GRACE gravity and gravity gradient changes agree
well with seismic model spectra and have powers up to the limit
of the GRACE solution complete to spherical harmonic degree 60.
Using the gravity observations for the fault parameter inversion
via simulated annealing algorithm, we show that the estimated
slip orientation and centroid location are different from GPS/seismic
solutions and potentially due to the additional offshore constraint
from GRACE data. |
|
Marzo de 2014
Gutenberg-Richter relation originates from Coulomb stress
fluctuations caused by elastic rock heterogeneity
Authors: C. Langenbruch and S. A. Shapiro
Link: Click here
Based on measurements along boreholes, a characterization
of the Earth's crust elastic heterogeneity is presented. We
investigate its impact on Coulomb stress distribution and earthquake
magnitude scaling. The analysis of elastic modulus distribution
at various borehole locations in different regions reveals universal
fractal nature of elastic heterogeneity. By applying a homogeneous
far-field stress to a representative model of elastic rock heterogeneity,
we show that it causes strong Coulomb stress fluctuations. In
situ fluctuations of Coulomb stress
|
are mainly controlled by in situ
elastic moduli. Fluctuations caused by surrounding heterogeneities
are only of minor importance. Hence, the fractal nature of elastic
heterogeneity results in Coulomb stress fluctuations with power
law size distribution. As a consequence, fault sizes and magnitudes
of earthquakes scale according to the Gutenberg-Richter relation.
Due to the universal fractal nature of elastic heterogeneity,
the b value should be universal. Deviation from its universal
value of b?1 occurs due to characteristic scales of seismogenic
processes, which cause limitations or changes of fractal scaling.
Scale limitations are also the reason for observed stress dependency
of the b value. Our analysis suggests that the Gutenberg-Richter
relation originates from Coulomb stress fluctuations caused by
elastic rock heterogeneity. |
|
Marzo de 2014
A microphysical model for fault gouge friction applied to
subduction megathrusts
Authors: Sabine A. M. den Hartog and Christopher J. Spiers.
Link: Click here
A microphysical model is developed for the steady
state frictional behavior of illite-quartz fault gouge and applied
to subduction megathrust P-T conditions. The model assumes a
foliated, phyllosilicate-supported microstructure which shears
by rate-independent frictional slip on the aligned phyllosilicates
plus thermally activated deformation of the intervening quartz
clasts. At low slip rates or high temperatures, the deformation
of the clasts is easy, accommodating slip on the foliation without
dilatation. With increasing velocity or decreasing temperature,
the shear of the clasts becomes more difficult, increasing bulk
shear strength, until slip is activated on inclined portions
of the phyllosilicate foliation, where it anastomoses around
the clasts. Slip at these
|
sites
leads to dilation involving clast/matrix debonding, balanced,
at steady state, by compaction through thermally activated clast
deformation. Model predictions, taking pressure solution as the
thermally activated mechanism, show three regimes of velocity-dependent
frictional behavior at temperatures in the range of 200-500°C,
with velocity weakening occurring at 300-400°C, in broad agreement
with previous experiments on illite-quartz gouge. Effects of slip
rate, normal stress, and quartz fraction predicted by the model
also resemble those seen experimentally. Extrapolation of the
model to earthquake nucleation slip rates successfully predicts
the onset of velocity-weakening behavior at the updip seismogenic
limit on subduction megathrusts. The model further implies that
the onset of seismogenesis is controlled by the thermally activated
initiation of fault rock compaction through pressure solution
of quartz, which counteracts dilatation due to slip on the fault
rock foliation. |
|
Marzo de 2014
Maximum magnitude earthquakes induced by fluid injection
Authors: A. McGarr
Link: Click here
Analysis of numerous case histories of earthquake
sequences induced by fluid injection at depth reveals that the
maximum magnitude appears to be limited according to the total
volume of fluid injected. Similarly, the maximum seismic moment
seems to have an upper bound proportional to the total volume
of injected fluid. Activities involving fluid injection include
(1) hydraulic fracturing of shale formations or coal seams to
extract gas and oil, (2) disposal of wastewater from these gas
and oil activities by injection into deep aquifers, and (3)
the development of enhanced geothermal systems by injecting
water into hot, low-permeability rock. Of these three operations,
wastewater disposal is
|
observed to be associated
with the largest earthquakes, with maximum magnitudes sometimes
exceeding 5. To estimate the maximum earthquake that could be
induced by a given fluid injection project, the rock mass is assumed
to be fully saturated, brittle, to respond to injection with a
sequence of earthquakes localized to the region weakened by the
pore pressure increase of the injection operation and to have
a Gutenberg-Richter magnitude distribution with a b value of 1.
If these assumptions correctly describe the circumstances of the
largest earthquake, then the maximum seismic moment is limited
to the volume of injected liquid times the modulus of rigidity.
Observations from the available case histories of earthquakes
induced by fluid injection are consistent with this bound on seismic
moment. In view of the uncertainties in this analysis, however,
this should not be regarded as an absolute physical limit. |
|
Marzo de 2014
A wet, heterogeneous lunar interior: Lower mantle and core
dynamo evolution
Authors: A. J. Evans, M. T. Zuber et al
Link: Click here
While recent analyses of lunar samples indicate
the Moon had a core dynamo from at least 4.2-3.56?Ga, mantle
convection models of the Moon yield inadequate heat flux at
the core-mantle boundary to sustain thermal core convection
for such a long time. Past investigations of lunar dynamos have
focused on a generally homogeneous, relatively dry Moon, while
an initial compositionally stratified mantle is the expected
consequence of a post-accretionary lunar magma ocean. Furthermore,
recent re-examination of Apollo samples and geophysical data
suggests that the Moon contains at least some regions with high
water content. Using a finite-element model,
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we
investigate the possible consequences of a heterogeneously wet,
compositionally stratified interior for the evolution of the Moon.
We find that a post-overturn model of mantle cumulates could result
in a core heat flux sufficiently high to sustain a dynamo through
2.5 Ga and a maximum surface, dipolar magnetic field strength
of less than 1 ?T for a 350-km core and near ~2 ?T for a 450?km
core. We find that if water was transported or retained preferentially
in the deep interior, it would have played a significant role
in transporting heat out of the deep interior and reducing the
lower mantle temperature. Thus, water, if enriched in the lower
mantle, could have influenced core dynamo timing by over 1.0 Gyr
and enhanced the vigor of a lunar core dynamo. Our results demonstrate
the plausibility of a convective lunar core dynamo even beyond
the period currently indicated by the Apollo samples. |
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Marzo de 2014
Remarkably consistent thermal state of the south central
Chile subduction zone from 36°S to 45°S
Authors: Holly M. M. Rotman and Glenn A. Spinelli
Link: Click here
Delineating rupture areas of subduction zone
earthquakes is necessary for understanding the controls on seismic
and aseismic slip. For the largest recorded earthquake, the
1960 Chile event with moment magnitude 9.5, the rupture area
is only loosely defined due to limitations in the global seismic
network at the time. The rupture extends ~900 km along strike.
Coastal deformation is consistent with either a constant rupture
width of ~180-200?km along the entire length, or a narrower
(~115?km) rupture in the southern half. A southward narrowing
of the seismogenic zone has been hypothesized to result from
warming of the subduction zone to the
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south, where the subducting plate
is younger. We present results of thermal models at 36?S, 38?S,
43°S, and 45°S to examine potential along-strike changes
in thermal state. Models most consistent with observed surface
heat flux include fluid circulation in the oceanic crust that
advects heat to the ocean. This ventilated hydrothermal circulation
preferentially cools transects with young subducting lithosphere;
frictional heating preferentially warms transects with older subducting
lithosphere. The combined effects of frictional heating and hydrothermal
circulation increase décollement temperatures in the 36?S
and 38?S transects by up to ~155?C, and decrease temperatures
in the 45?S transect by up to ~150?C. In our preferred models,
décollement temperatures 200?km landward of the trench
in all four transects are ~350-400?C. This is consistent with
a constant ~200?km wide seismogenic zone for the 1960 Mw 9.5 rupture,
with decreasing slip magnitude in the southern half of the rupture. |
Las erupciones volcánicas de
principios del siglo XXI han frenado el calentamiento global.
Gracias a los volcanes, la temperatura global media de
la superficie y de la
troposfera terrestres han sufrido sólo un ligero aumento
desde 1998, revela un estudio.
Por Yaiza Martínez - Tendencias21.net
Las erupciones volcánicas
acaecidas en la primera parte del siglo XXI han enfriado el
planeta, según un estudio liderado por el Lawrence
Livermore National Laboratory de Estados Unidos.
Este enfriamiento habría contrarrestado en parte el
calentamiento producido por los gases de efecto invernadero,
publica dicho Laboratorio.
A pesar del continuo incremento de los niveles atmosféricos
de gases de efecto invernadero y de la entalpía o energía
absorbida por los océanos, las temperaturas medias
globales de la superficie del planeta y de la troposfera (capa
de la atmósfera terrestre que está en contacto
con la superficie de la Tierra) han mostrado un aumento relativamente
escaso desde 1998.
Esta ralentización del calentamiento ha recibido una
atención política, científica y popular
notable. La contribución de los volcanes a ella ha
sido objeto de esta investigación, que aparece detallada
en la revista Nature Geoscience.
La causa del enfriamiento
Las erupciones volcánicas inyectan un gas, el dióxido
de azufre, a la atmósfera. Si son lo suficientemente
potentes como para añadir dióxido de azufre
a la estratosfera (capa de la atmósfera situada sobre
la troposfera), el gas forma pequeñas gotas de ácido
sulfúrico, también conocidas como "aerosoles
volcánicos". Estas gotitas reflejan una parte
de la luz solar entrante de vuelta al espacio, enfriando la
superficie terrestre y la atmósfera inferior.
"En la última década, la cantidad de aerosol
volcánico en la estratosfera ha aumentado, por lo que
mayor cantidad de luz solar se ha reflejado hacia el espacio",
explica el científico del clima del Lawrence Livermore
Laboratory, Benjamin Santer, autor principal del estudio.
"Esto ha provocado un enfriamiento natural del planeta
y ha compensado en parte el aumento de la temperatura en la
superficie y en la atmósfera, derivado de la influencia
humana".
Efectos registrados
Entre los años 2000 y 2012, las emisiones de gases
de efecto invernadero en la atmósfera han aumentado,
al igual que viene sucediendo desde la Revolución Industrial.
Este cambio inducido por el hombre por lo general hace que
la troposfera se caliente y la estratosfera se enfríe.
Por el contrario, las grandes erupciones volcánicas
enfrían la troposfera y calientan la estratosfera.
Los investigadores informan de que las erupciones volcánicas
a principios del siglo XXI han contribuido a este reciente
"hiato en el calentamiento", y que la mayoría
de los modelos climáticos no han registrado con exactitud
este efecto.
Los científicos realizaron dos pruebas estadísticas
diferentes para determinar si las recientes erupciones volcánicas
habían tenido efectos distinguibles en la variabilidad
intrínseca del enfriamiento del clima.
Descubrieron evidencias de correlaciones significativas entre
las observaciones sobre aerosoles volcánicos y las
estimaciones basadas en datos de satélite de la temperatura
de la troposfera inferior; así como la luz solar reflejada
de vuelta al espacio por las partículas de esos aerosoles.
"Esta es la evaluación observacional más
completa del papel de la actividad volcánica en el
clima en la primera parte del siglo XXI", asegura otra
de las autoras del estudio, la especialista en química
atmosférica Susan Solomon.
Volcanes e invierno volcánico
En términos generales, los aerosoles son sustancias
sólidas o líquidas presentes en la atmósfera
que pueden afectar al cambio climático e incluso, según
los expertos, ocasionar un ‘invierno nuclear’, un
enfriamiento generalizado del planeta por el oscurecimiento
de la atmósfera.
Aunque el estudio sobre este fenómeno surgió
en los años 60 por el temor a un intercambio nuclear
entre las grandes potencias, no hace falta que haya una explosión
nuclear para que ocurra este fenómeno, pues una erupción
volcánica de alta intensidad también podría
enviar grandes cantidades de aerosoles a la estratosfera,
provocando el mismo efecto. En ese caso estaríamos
hablando de un “invierno volcánico”.
Según ha explicado recientemente a Tendencias21 el
catedrático de Física de la Universidad de Valladolid
y director del grupo de Óptica Atmosférica de
dicha Universidad, Ángel M. de Frutos Baraja, en la
historia ya ha habido casos de "inviernos” provocados
por explosiones volcánicas. Se han conocido por los
relatos de los presentes, que tienen más o menos fiabilidad
en función de la cercanía o lejanía del
evento.
En 2002, un especialista de la Universidad
de Cambridge sugirió que un invierno volcánico
se habría producido hace entre 71.000 y 73.000 años
tras una súpererupción en el lago Toba, en la
isla indonesia de Sumatra. Los seis años subsiguientes
se concentró la mayor cantidad de azufre volcánico
de los últimos 110.000 años, causando, probablemente,
una deforestación significativa en el sudeste asiático
y enfriando la temperatura global de la Tierra en 1 °C.
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Marzo de 2014
Drilling Into the Track of the Yellowstone Hot Spot
Authors: C. Sakumura, S. Bettadpur et al
Link: Click here
The Yellowstone supervolcano erupted roughly
640,000 years ago, covering much of North America in a thick
coat of ash. Material ejected
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from
the volcano devastated the surrounding area, and particles injected
into the atmosphere changed the Earth's climate. Over the past
18 million years the Yellowstone hot spot has powered a series
of similar eruptions. In southern Idaho, the 640-kilometer-long
Snake River Plain traces the path of the Yellowstone hot spot
over this period. |
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Marzo de 2014
Constraining particle size-dependent plume sedimentation
from the 17 June 1996 eruption of Ruapehu Volcano, New Zealand,
using geophysical inversions
Authors: John W. Shervais, James P. Evans et al
Link: Click here
Weak subplinian-plinian plumes pose frequent
hazards to populations and aviation, yet many key parameters
of these particle-laden plumes are, to date, poorly constrained.
This study recovers the particle size-dependent mass distribution
along the trajectory of a well-constrained weak plume by inverting
the dispersion process of tephra fallout. We use the example
of the 17 June 1996 Ruapehu eruption in New Zealand and base
our computations on mass per unit area tephra measurements and
grain size distributions at 118 sample locations. Comparisons
of particle fall times and time of sampling collection, as well
as observations during the eruption, reveal that particles smaller
than 250 m likely settled as
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aggregates. For simplicity we assume
that all of these fine particles fell as aggregates of constant
size and density, whereas we assume that large particles fell
as individual particles at their terminal velocity. Mass fallout
along the plume trajectory follows distinct trends between larger
particles (d 250 m) and the fine population (d<250 m) that
are likely due to the two different settling behaviors (aggregate
settling versus single-particle settling). In addition, we computed
the resulting particle size distribution within the weak plume
along its axis and find that the particle mode shifts from an
initial 1? mode to a 2.5? mode 10 km from the vent and is dominated
by a 2.5 to 3 mode 10-180?km from vent, where the plume reaches
the coastline and we do not have further field constraints. The
computed particle distributions inside the plume provide new constraints
on the mass transport processes within weak plumes and improve
previous models. The distinct decay trends between single-particle
settling and aggregate settling may serve as a new tool to identify
particle sizes that fell as aggregates for other eruptions. |
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Marzo de 2014
Seismic structure of the north-central Chilean convergent
margin: Subduction erosion of a paleomagmatic arc
Authors: Eduardo Contreras-Reyes, Juan Becerra et al
Link: Click here
We study the erosive convergent margin of north-central
Chile (at ~31°S) by using high-resolution bathymetric, wide-angle
refraction, and multichannel seismic reflection data to derive
a detailed tomographic 2-D velocity-depth model. In the overriding
plate, our velocity model shows that the lowermost crustal velocities
beneath the upper continental slope are 6.0-6.5 km/s, which
are interpreted as the continental basement composed by characteristic
metamorphic and igneous rocks of the Coastal Cordillera. Beneath
the lower and middle continental slope, however,
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the
presence of a zone of reduced velocities (3.5- 5.0 km/s) is interpreted
as the outermost fore arc composed of volcanic rocks hydrofractured
as a result of frontal and basal erosion. At the landward edge
of the outermost fore arc, the bathymetric and seismic data provide
evidence for the presence of a prominent trenchward dipping normal
scarp (~1 km offset), which overlies a strong lateral velocity
contrast from ~5.0 to ~6.0?km/s. This pronounced velocity contrast
propagates deep into the continental crust, and it resembles a
major normal listric fault. We interpret this seismic discontinuity
as the volcanic-continental basement contact of the submerged
Coastal Cordillera characterized by a gravitational collapse of
the outermost fore arc. Subduction erosion has, most likely, caused
large-scale crustal thinning and long-term subsidence of the outermost
fore arc. |
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Marzo de 2014
Triggering of repeating earthquakes in central California
Authors: Chunquan Wu, Joan Gomberg et al
Link: Click here
Dynamic stresses carried by transient seismic
waves have been found capable of triggering earthquakes instantly
in various tectonic settings. Delayed triggering may be even
more common, but the mechanisms are not well understood. Catalogs
of repeating earthquakes, earthquakes that recur repeatedly
at the same location, provide ideal data sets to test the effects
of transient dynamic perturbations on the timing of earthquake
occurrence. Here we employ a catalog of 165 families containing
~2500 total repeating earthquakes to test whether dynamic perturbations
from local, regional, and
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teleseismic earthquakes change recurrence
intervals. The distance to the earthquake generating the perturbing
waves is a proxy for the relative potential contributions of static
and dynamic deformations, because static deformations decay more
rapidly with distance. Clear changes followed the nearby 2004
Mw6 Parkfield earthquake, so we study only repeaters prior to
its origin time. We apply a Monte Carlo approach to compare the
observed number of shortened recurrence intervals following dynamic
perturbations with the distribution of this number estimated for
randomized perturbation times. We examine the comparison for a
series of dynamic stress peak amplitude and distance thresholds.
The results suggest a weak correlation between dynamic perturbations
in excess of ~20?kPa and shortened recurrence intervals, for both
nearby and remote perturbations. |
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Marzo de 2014
North Cascadia heat flux and fluid flow from gas hydrates:
Modeling 3-D topographic effects
Authors: Hong-lin Li, Tao He et al
Link: Click here
The bottom-simulating reflector (BSR) of gas
hydrate is well imaged from two perpendicular seismic grids
in the region of a large carbonate mound, informally called
Cucumber Ridge off Vancouver Island. We use a new method to
calculate 3-D heat flow map from the BSR depths, in which we
incorporate 3-D topographic corrections after calibrated by
the drilling results from nearby (Integrated) Ocean Drilling
Program Site 889 and Site U1327. We then estimate the associated
fluid flow by relating it to the topographically corrected heat
flux anomalies. In the midslope region, a heat flux anomaly
of 1 mW/m2 can be associated with an approximate
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focused fluid flow rate of 0.09?mm/yr.
Around Cucumber Ridge, high rates of focused fluid flow were observed
at steep slopes with values more than double the average regional
diffusive fluid discharge rate of 0.56?mm/yr. As well, in some
areas of relatively flat seafloor, the focused fluid flow rates
still exceeded 0.5?mm/yr. On the seismic lines the regions of
focused fluid flow were commonly associated with seismic blanking
zones above the BSR and sometimes with strong reflectors below
the BSR, indicating that the faults/fractures provide high-permeability
pathways for fluids to carry methane from BSR depths to the seafloor.
These high fluid flow regions cover mostly the western portion
of our area with gas hydrate concentration estimations of ~6%
based on empirical correlations from Hydrate Ridge in south off
Oregon, significantly higher than previously recognized values
of ~2.5% in the eastern portion determined from Site U1327. |
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Marzo de 2014
Seismic and gravity constraints on the nature of the basement
in the Africa-Eurasia plate boundary: New insights for the geodynamic
evolution of the SW Iberian margin
Authors: Sara Martínez-Loriente, Valentí Sallarès
et al
Link: Click here
We present a new classification of geological
domains at the Africa-Eurasia plate boundary off SW Iberia,
together with a regional geodynamic reconstruction spanning
from the Mesozoic extension to the Neogene-to-present-day convergence.
It is based on seismic velocity and density models along a new
transect running from the Horseshoe to the Seine abyssal plains,
which is combined with previously available geophysical models
from the region. The basement velocity structure at the Seine
Abyssal Plain indicates the presence of a highly heterogeneous,
thin oceanic crust with local high-velocity anomalies possibly
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representing zones related to the
presence of ultramafic rocks. The integration of this model with
previous ones reveals the presence of three oceanic domains offshore
SW Iberia: (1) the Seine Abyssal Plain domain, generated during
the first stages of slow seafloor spreading in the NE Central
Atlantic (Early Jurassic); (2) the Gulf of Cadiz domain, made
of oceanic crust generated in the Alpine-Tethys spreading system
between Iberia and Africa, which was coeval with the formation
of the Seine Abyssal Plain domain and lasted up to the North Atlantic
continental breakup (Late Jurassic); and (3) the Gorringe Bank
domain, made of exhumed mantle rocks, which formed during the
first stages of North Atlantic opening. Our models suggest that
the Seine Abyssal Plain and Gulf of Cadiz domains are separated
by the Lineament South strike-slip fault, whereas the Gulf of
Cadiz and Gorringe Bank domains appear to be limited by a deep
thrust fault located at the center of the Horseshoe Abyssal Plain. |
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Marzo de 2014
Evolution of gas saturation and relative permeability during
gas production from hydrate-bearing sediments: Gas invasion
vs. gas nucleation
Authors: Jaewon Jang and J. Carlos Santamarina
Link: Click here
Capillarity and both gas and water permeabilities
change as a function of gas saturation. Typical trends established
in the discipline of unsaturated soil behavior are used when
simulating gas production from hydrate-bearing sediments. However,
the evolution of gas saturation and water drainage in gas invasion
(i.e., classical soil behavior) and gas nucleation (i.e., gas
production) is inherently different: micromodel experimental
results show that gas invasion forms a continuous flow path
while gas nucleation forms isolated gas clusters. Complementary
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simulations
conducted using tube networks explore the implications of the
two different desaturation processes. In spite of their distinct
morphological differences in fluid displacement, numerical results
show that the computed capillarity-saturation curves are very
similar in gas invasion and nucleation (the gas-water interface
confronts similar pore throat size distribution in both cases);
the relative water permeability trends are similar (the mean free
path for water flow is not affected by the topology of the gas
phase); and the relative gas permeability is slightly lower in
nucleation (delayed percolation of initially isolated gas-filled
pores that do not contribute to gas conductivity). Models developed
for unsaturated sediments can be used for reservoir simulation
in the context of gas production from hydrate-bearing sediments,
with minor adjustments to accommodate a lower gas invasion pressure
Po and a higher gas percolation threshold. |
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Marzo de 2014
Three-dimensional simulation of the electromagnetic fields
induced by the 2011 Tohoku tsunami
Authors: Luolei Zhang, Hisashi Utada et al
Link: Click here
The motion of seawater induces electromotive
force of significant intensity due to Faraday's law, and the
resulting electromagnetic (EM) field can be recorded by instruments
installed on land or on the ocean bottom. However, few studies
have successfully made a quantitative interpretation to obtain
geophysical information from observations of tsunami-induced
EM signals by an exact and accurate application of Maxwell equations.
We built a scheme for three-dimensional numerical simulation
to calculate EM fields due to ocean tidal flow and tested the
system's accuracy by using the source currents in the ocean
as expected from a Tohoku tsunami simulation. Here
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we show examples of a comparison
of data from one land observatory in the Tohoku district, one
island observatory in the Izu-Bonin arc, and one seafloor station
in the northwest Pacific Ocean. The water motion that generates
source current in the sea consists of both the primary poloidal
and toroidal magnetic modes. Our numerical simulation shows that
the field of the primary toroidal magnetic mode can be effective
for seafloor observations but only when the seafloor is highly
conductive. We examined how the conductivity of the shallower
part of the seabed, composed of sediments, can be constrained
by the tsunami-induced EM field observations, which is difficult
in case of using ordinary seafloor magnetotelluric signals. We
also defined the motional impedance just as the case of ordinary
magnetotellurics and showed that both modeled and observed impedances
approximately indicate the phase velocity of the long wave as
predicted by a simple theory. |
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Marzo de 2014
The impact of Rayleigh number on assessing the significance
of supercontinent insulation
Authors: Philip J. Heron and Julian P. Lowman
Link: Click here
Several processes unfold during the supercontinent
cycle, more than one of which might result in an elevation in
subcontinental mantle temperatures, thus multiple interpretations
of the concept of continental insulation exist. Although a consensus
seems to have formed that subcontinental mantle upwellings appear
below large continents extensively ringed by subduction zones,
there are differing views on what role continental insulation
plays in the production of elevated mantle temperatures. Here
we investigate how the heating mode of the mantle can change
the influence of the "thermal blanket" effect. We
present 2?D and 3?D Cartesian geometry mantle convection simulations
with thermally and mechanically distinct oceanic and
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continental
plates. The evolution of mantle thermal structure is examined
after continental accretion at subduction zones (e.g., the formation
of Pangea) for a variety of different mantle?heating modes. Our
results show that in low Rayleigh number models the impact of
the role of continental insulation on subcontinental temperatures
increases, when compared to models with higher convective vigor.
Broad, hot upper mantle features generated in low?Rayleigh number
models (due, in part, to the thermal blanket effect) are absent
at higher Rayleigh numbers. We find that subcontinental heating
in a high Rayleigh number flow occurs almost entirely as a consequence
of the influence of subduction initiation at the continental margin,
rather than the influence of continental insulation. In our models
featuring Earth?like convective vigor, we find that it is difficult
to obtain subcontinental temperatures in significant excess of
suboceanic temperatures over timescales relevant to supercontinent
aggregation. |
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Marzo de 2014
First satellite detection of volcanic OClO after the eruption
of Puyehue-Cordón Caulle
Authors: Nicolas Theys, Isabelle De Smedt et al
Link: Click here
Volcanoes release large amounts of halogen species
such as HCl and HBr, which can be converted into reactive halogens
by heterogeneous photochemical reactions that are currently
not fully characterized. Here we report on the first satellite
detection of volcanic chlorine dioxide (OClO). Measurements
were performed using the Scanning Imaging Absorption
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Spectrometer for Atmospheric Chartography
instrument for the ash-laden plume emitted after the 2011 eruption
of Puyehue-Cordón Caulle in Chile. We also identified volcanic
BrO using the Ozone Monitoring Instrument, as well as enhanced
HCl in data of the Microwave Limb Sounder instrument. These observations
suggest that OClO was formed in the plume by the ClO+BrO reaction
in presence of a large excess of ClO. The present satellite data
set could help better understand reactive halogen chemistry in
volcanic plumes and its impact on atmospheric composition. |
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Marzo de 2014
Ensemble prediction and intercomparison analysis of GRACE
time-variable gravity field models
Authors: John F. Hoffecker, Scott A. Elias et al
Link: Click here
Based on the distribution of tundra plants around
the Bering Strait region, Eric Hultén proposed in the
1930s that the now-submerged plain between Chukotka and Alaska-the
Bering land bridge-became a refugium for shrub tundra vegetation
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during
cold periods (1), which include the last glacial maximum (LGM)
between ?28,000 and 18,000 cal BP (calibrated radiocarbon years
before the present). Adjoining areas to the west and east supported
drier plant communities with a higher percentage of grasses during
glacial periods. According to Hultén, when warmer and wetter
conditions returned to these areas, the land bridge, which he
named Beringia, became a center of dispersal for tundra plants.
Now it appears that it also may have been a glacial refugium and
postglacial center of dispersal for the people who first settled
the Americas. |
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Marzo de 2014
Mean dynamic topography estimates purely based on GOCE gravity
field models and altimetry
Authors: S. Becker, J. M. Brockmann et al
Link: Click here
The quality of mean dynamic topography (MDT)
models derived from an altimetric mean sea surface and a gravity
field model mainly depends on the spatial resolution and accuracy
of the particular gravity field model. We use an integrated
approach which allows for estimating the MDT and its (inverse)
covariance matrix on a
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predefined grid which is one of the
requirements for ocean data assimilation. The quality and accuracy
of the MDT directly reflects the quality and accuracy of the used
gravity field model. For the first time, MDT estimates along with
its full error covariance matrix based on GOCE data can be provided.
We demonstrate the progress accomplished with GOCE processing
and the valuable contribution of the GOCE gravity field models
regarding the estimation of the MDT by showing results based on
altimetric observations of Jason-1 and Envisat in combination
with different GOCE gravity field models for the North Atlantic. |
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Marzo de 2014
Significant groundwater contribution to Antarctic ice streams
hydrologic budget
Authors: Poul Christoffersen, Marion Bougamont et al
Link: Click here
Satellite observations have revealed active
hydrologic systems beneath Antarctic ice streams, but sources
and sinks of water within these systems are uncertain. Here
we use numerical simulations of ice streams to estimate the
generation, flux and budget of water beneath five ice streams
on the Siple Coast. We estimate that
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47%
of the total hydrologic input (0.98?km3?yr-1) to Whillans (WIS),
Mercer (MIS) and Kamb (KIS) ice streams comes from the ice sheet
interior, and that only 8% forms by local basal melting. The remaining
45% comes from a groundwater reservoir, an overlooked source in
which depletion significantly exceeds recharge. Of the total input
to Bindschadler (BIS) and MacAyeal (MacIS) ice streams (0.56?km3?yr-1),
72% comes from the interior, 19% from groundwater and 9% from
local melting. This contrasting hydrologic setting modulates the
ice streams flow and has important implications for the search
for life in subglacial lakes. |
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Marzo de 2014
In Situ Stress Analysis in the Northern Newark Basin: Implications
for Induced Seismicity from CO2 Injection
Authors: Natalia V. Zakharova and David S. Goldberg.
Link: Click here
We present detailed stress analysis in a deep
well drilled in the northern Newark Rift basin, and evaluate
the risk of induced seismicity for underground fluid injection
at the locality. In situ stress orientation and magnitudes were
estimated using quantitative analysis and modeling of borehole
breakouts identified in high-resolution wellbore images. The
distribution of breakouts and natural fractures in the well
suggests significant variability in orientation of the principal
horizontal stresses in the depth range of 450-1450?m. Evidence
from surface seismic
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reflection imaging indicates potential
presence of faults at about 800?m and 1200?m that bound zones
of distinctly different stress orientation at this locality. Stability
of natural fractures and faults under injection conditions was
evaluated for a range of potential stress magnitudes and the observed
stress variability with depth. Shallow crust above ~800?m appears
to be critically stressed under ambient conditions, and further
pore pressure increase would put it at risk of frictional failure
on favorably oriented fractures and faults. Deeper reservoirs,
however, may allow over 10 MPa increase in pore pressure without
fault reactivation due to a more relaxed state of stress. Additional
in situ test data are needed to more accurately constrain the
magnitude of the minimum horizontal stress in the basin and to
enable a more complete assessment of the induced seismic risk
from potential CO2 injection in the region. |
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Febrero de 2014
What is responsible for the strong observed asymmetry in
teleconnections between El Niño and La Niña?
Authors: Tao Zhang, Judith Perlwitz et al
Link: Click here
A large asymmetric component (El Niño?+?La
Niña) of El Niño-Southern Oscillation (ENSO)-related
teleconnections over North America is found during 1984-2009
that is comparable in strength to the commonly studied symmetric
component (El Niño-La Niña). Climate reforecasts
spanning this period are diagnosed in order to understand the
processes responsible for the observed asymmetry. It is confirmed
that an
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asymmetric
component is indeed a fundamental property of atmospheric responses
to recent ENSO forcing. Each and every composite of a 16-member
reforecast ensemble has appreciable asymmetry in tropical Pacific
rainfall, upper tropospheric Pacific-North American circulation
patterns, and contiguous U.S. surface temperatures. There is considerable
sampling variability in the magnitude of this asymmetric component
among individual reforecast composites. We argue therefore that
the true sea surface temperature boundary-forced signal of ENSO
teleconnections is likely composed of a symmetric component having
greater magnitude than its asymmetric component, though the latter
is an important property of how ENSO affects North American climate. |
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Febrero de 2014
Maximum magnitude earthquakes induced
by fluid injection
Author: A. McGarr
Link: Click here
Analysis of numerous case histories of earthquake
sequences induced by fluid injection at depth reveals that the
maximum magnitude appears to be limited according to the total
volume of fluid injected. Similarly, the maximum seismic moment
seems to have an upper bound proportional to the total volume
of injected fluid. Activities involving fluid injection include
(1) hydraulic fracturing of shale formations or coal seams to
extract gas and oil, (2) disposal of wastewater from these gas
and oil activities by injection into deep aquifers, and (3)
the development of enhanced geothermal systems by injecting
water into hot, low-permeability rock. Of these three operations,
wastewater disposal is
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observed to be associated with the
largest earthquakes, with maximum magnitudes sometimes exceeding
5. To estimate the maximum earthquake that could be induced by
a given fluid injection project, the rock mass is assumed to be
fully saturated, brittle, to respond to injection with a sequence
of earthquakes localized to the region weakened by the pore pressure
increase of the injection operation and to have a Gutenberg-Richter
magnitude distribution with a b value of 1. If these assumptions
correctly describe the circumstances of the largest earthquake,
then the maximum seismic moment is limited to the volume of injected
liquid times the modulus of rigidity. Observations from the available
case histories of earthquakes induced by fluid injection are consistent
with this bound on seismic moment. In view of the uncertainties
in this analysis, however, this should not be regarded as an absolute
physical limit. |
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Febrero de 2014
Crustal earthquake triggering by modern great earthquakes
on subduction zone thrusts
Authors: Joan Gomberg and Brian Sherrod
Link: Click here
Among the many questions raised by the recent
abundance of great (M?>?8.0) subduction thrust earthquakes
is their potential to trigger damaging earthquakes on crustal
faults within the overriding plate and beneath many of the world's
densely populated urban centers. We take advantage of the coincident
abundance of great earthquakes globally and instrumental observations
since 1960 to assess this triggering potential by analyzing
centroids and focal mechanisms from the centroid moment tensor
catalog for events starting in 1976 and published reports about
the
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M9.5
1960 Chile and M9.2 1964 Alaska earthquake sequences. We find
clear increases in the rates of crustal earthquakes in the overriding
plate within days following all subduction thrust earthquakes
of M?>?8.6, within about ±10° of the triggering
event centroid latitude and longitude. This result is consistent
with dynamic triggering of more distant increases of shallow seismicity
rates at distances beyond ±10°, suggesting that dynamic
triggering may be important within the near field too. Crustal
earthquake rate increases may also follow smaller M?>?7.5 subduction
thrust events, but because activity typically occurs offshore
in the immediately vicinity of the triggering rupture plane, it
cannot be unambiguously attributed to sources within the overriding
plate. These observations are easily explained in the context
of existing earthquake scaling laws. |
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Febrero de 2014
Experimental investigation of strong ground motion due to
thrust fault earthquakes
Authors: Vahe Gabuchian, Ares J. Rosakis et al
Link: Click here
Thrust fault earthquakes are studied in a laboratory
earthquake setup previously used to investigate analog strike-slip
seismic events. Dynamic mode II ruptures are generated along
preexisting faults in an analog material, Homalite H-100, and
their interaction with the free surface is studied for both
sub-Rayleigh and supershear rupture speeds. High-speed digital
photography and laser velocimeter diagnostics are used synergistically
to identify and study the ground velocity signatures caused
by the various features of the generated ruptures. The obtained
surface-normal motions of both sub-Rayleigh and supershear ruptures
show substantial asymmetry between the hanging and footwall,
with the
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hanging wall experiencing much larger
velocity amplitudes. The main features of the surface velocity
traces at various stations can be explained by the calculated
arrivals of various waves and fronts-Mach cones, Pand S waves,
and sub-Rayleigh features. In both the sub-Rayleigh and supershear
cases, the arrival of the rupture tip generates a prominent Rayleigh
wave traveling along the simulated Earth's surface. Supershear
events feature larger amplitudes of ground shaking profiles. All
signatures in the surface motion records attenuate and broaden
with increasing distance from the fault trace. The signatures
corresponding to the arrival of the Mach fronts attenuate with
distance at a slower rate than those from sub-Rayleigh ruptures.
The arrival of the updip supershear rupture at the free surface
creates a downdip propagating slip feature with its own Mach cone.
These additional Mach fronts further amplify ground shaking on
the hanging and footwalls. |
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Febrero de 2014
Forearc structure and morphology along the Sumatra-Andaman
subduction zone
Authors: Lisa C. McNeill and Timothy J. Henstock
Link: Click here
The Sunda subduction margin, the location of
recent magnitude 8-9 megathrust earthquakes, is characterized
by major changes in prism and forearc morphology and structure
along its 5000?km length. On the Sumatra-Andaman section, measurements
of prism width and surface slope (?) indicate along-strike segments,
inclu1ding abrupt changes, of prism morphology: (1) a narrow
and steep prism between Burma and the Andamans; (2) broad with
an averaged gentle slope in the Andamans, Nicobars, and North
Sumatra; (3) steep and narrow in Central Sumatra; and (4) wider
and less steep offshore South Sumatra, decreasing in width to
West Java. Prism width varies from ~90 to 180?km and
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average
surface slope from ~1 to 3° with a strong inverse correlation
between width and slope, also observed globally. The prism deviates
from typical taper geometry in parts of the margin, notably offshore
North Sumatra where it is characterized by a steep toe and broad
plateau. Along-strike changes in morphology are strongly linked
to input sediment thickness. Sections of the prism toe represent
key global examples of unusual landward vergent thrusting. These
sections correspond to a thick sediment input and to a wide prism
with shallow surface slope. A low basal shear stress or backstop
mechanism may drive this style of faulting. Prism morphology and
structure appear to be driven predominantly by input sediment
thickness linked to oceanic basement topography, with sediment
properties, plate smoothness, and orthogonal subduction rate and
obliquity also contributing, and no clear role of plate age or
dip. |
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Febrero de 2014
Crustal earthquake triggering by pre-historic great earthquakes
on subduction zone thrusts
Authors: Brian Sherrod and Joan Gomberg
Link: Click here
Triggering of earthquakes on upper plate faults
during and shortly after recent great (M?>?8.0) subduction
thrust earthquakes raises concerns about earthquake triggering
following Cascadia subduction zone earthquakes. Of particular
regard to Cascadia was the previously noted, but only qualitatively
identified, clustering of M?>?~6.5 crustal earthquakes in
the Puget Sound region between about 1200-900 cal years B.P.
and the possibility that this was triggered by a great
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Cascadia thrust subduction thrust
earthquake, and therefore portends future such clusters. We confirm
quantitatively the extraordinary nature of the Puget Sound region
crustal earthquake clustering between 1200-900 cal years B.P.,
at least over the last 16,000. We conclude that this cluster was
not triggered by the penultimate, and possibly full-margin, great
Cascadia subduction thrust earthquake. However, we also show that
the paleoseismic record for Cascadia is consistent with conclusions
of our companion study of the global modern record outside Cascadia,
that M?>?8.6 subduction thrust events have a high probability
of triggering at least one or more M?>?~6.5 crustal earthquakes. |
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Febrero de 2014
Modeling the seismic cycle in subduction zones: The role
and spatiotemporal occurrence of off-megathrust earthquakes
Authors: Y. van Dinther, P. M. Mai et al
Link: Click here
Shallow off-megathrust subduction events are
important in terms of hazard assessment and coseismic energy
budget. Their role and spatiotemporal occurrence, however, remain
poorly understood. We simulate their spontaneous activation
and propagation using a newly developed 2-D, physically consistent,
continuum, viscoelastoplastic seismo-thermo-mechanical modeling
approach. The
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characteristics
of simulated normal events within the outer rise and splay and
normal antithetic events within the wedge resemble seismic and
seismological observations in terms of location, geometry, and
timing. Their occurrence agrees reasonably well with both long-term
analytical predictions based on dynamic Coulomb wedge theory and
short-term quasi-static stress changes resulting from the typically
triggering megathrust event. The impact of off-megathrust faulting
on the megathrust cycle is distinct, as more both shallower and
slower megathrust events arise due to occasional off-megathrust
triggering and increased updip locking. This also enhances tsunami
hazards, which are amplified due to the steeply dipping fault
planes of especially outer rise events. |
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Febrero de 2014
El Mayor-Cucapah (Mw 7.2) earthquake: Early near-field postseismic
deformation from InSAR and GPS observations
Authors: Alejandro Gonzalez-Ortega, Yuri Fialko et al
Link: Click here
El Mayor-Cucapah earthquake occurred on 4 April
2010 in northeastern Baja California just south of the U.S.-Mexico
border. The earthquake ruptured several previously mapped faults,
as well as some unidentified ones, including the Pescadores,
Borrego, Paso Inferior and Paso Superior faults in the Sierra
Cucapah, and the Indiviso fault in the Mexicali Valley and Colorado
River Delta. We conducted several Global Positioning System
(GPS) campaign surveys of preexisting and newly established
benchmarks within 30?km of the earthquake rupture. Most of the
benchmarks were occupied within days after the earthquake, allowing
us to capture the very early postseismic transient motions.
The GPS
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data show postseismic displacements
in the same direction as the coseismic displacements; time series
indicate a gradual decay in postseismic velocities with characteristic
time scales of 66?±?9?days and 20?±?3?days, assuming
exponential and logarithmic decay, respectively. We also analyzed
interferometric synthetic aperture radar (InSAR) data from the
Envisat and ALOS satellites. The main deformation features seen
in the line-of-sight displacement maps indicate subsidence concentrated
in the southern and northern parts of the main rupture, in particular
at the Indiviso fault, at the Laguna Salada basin, and at the
Paso Superior fault. We show that the near-field GPS and InSAR
observations over a time period of 5?months after the earthquake
can be explained by a combination of afterslip, fault zone contraction,
and a possible minor contribution of poroelastic rebound. Far-field
data require an additional mechanism, most likely viscoelastic
relaxation in the ductile substrate. |
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Febrero de 2014
Crustal and uppermost mantle structure beneath western Tibet
using seismic traveltime tomography
Authors: Ayda Shokoohi Razi,Vadim Levin et al
Link: Click here
We study the elastic wave speed structure of
the crust and the uppermost mantle in western Tibet using P
wave and S wave arrival times from regional earthquakes recorded
by a temporary seismic network. We relocate the earthquakes,
and subsequently invert travel time residuals for 3-D distributions
of wave speed. Resolution tests with a variety of input structures
are used to verify the reliability of our results. The crust
beneath western Tibet has low P wave speed (5.9-6.3 km/s) throughout
its nearly 80 km thickness, with lower values in this range
concentrated within the Lhasa block. Beneath the Himalaya wave
speeds
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are higher. Southern and western
limits of the slow material beneath the Tibetan Plateau correlate
with the Karakoram fault, and dip beneath the plateau at ?40°
angle. We find no evidence of a subhorizontal low velocity zone
in the crust. In the uppermost mantle, we find a long and narrow
region of fast (up to 8.4 km/s) P wave speed extending from the
Karakoram fault in NE direction, and crossing the Bangong-Nujiang
suture. In a north-south cross section, the distribution of relatively
fast P wave speed suggests a ramp-flat geometry consistent with
India underthrusting the Tibetan Plateau at least as far as 32.5°N.
A plausible interpretation of the upper mantle fast feature is
the formation of eclogite from the mafic lower-crustal material
of India after it is underthrust beneath Tibet. Notably, in western
Tibet this process only takes place in a narrow region. |
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Febrero de 2014
Source mechanism of Vulcanian eruption at Tungurahua Volcano,
Ecuador, derived from seismic moment tensor inversions
Authors: Keehoon Kim, Jonathan M. Lees et al
Link: Click here
Source mechanisms of explosive volcanic eruptions
are critical for understanding magmatic plumbing systems and
magma transport. Tungurahua is a large andesitic stratovolcano
where seismoacoustic data has been recorded over several years.
In May 2010, an energetic eruption cycle began with a midsize
Vulcanian explosion followed by swarms of explosive eruptions.
The five-station seismoacoustic network recorded significant
seismic and infrasonic signals from the explosions. Source mechanisms
of 50 explosion earthquakes associated with Vulcanian explosions
during this eruptive period are investigated here. The source
centroid locations and geometries of explosive
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signals
in the 10-2 s band were determined by full-waveform moment tensor
inversion. The observed waveforms are well explained by a combination
of volumetric moment tensor components and a single, vertical,
downward force component. The source centroids are positioned
about 1.5 km below and about 320 m north of the active crater.
Eigenvalue and eigenvector analysis indicates that the source
geometries can be described by a subhorizontal, thin ellipsoid
representing a sill-like magma accumulation. Resultant source
time histories show a repetitive sequence of inflation and deflation
from event to event, indicating identical source processes frequently
occurred over the period. The inflation/deflation in the deep
source region may be the result of crack opening. Volatile bubble
growth at depth opens a pathway for gases to escape and triggers
shallow explosions at the summit crater. The associated downward
single force is interpreted as an exchange of linear momentum
between the source and the surrounding region during the escaping
gas flow. |
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Febrero de 2014
Seismic attenuation tomography of the Northeast Japan arc:
Insight into the 2011 Tohoku earthquake (Mw 9.0) and subduction
dynamics
Authors: Xin Liu,Dapeng Zhao et al
Link: Click here
Detailed three-dimensional (3-D) P and S wave
attenuation (Qp and Qs) models of the crust and upper mantle
under the entire Northeast Japan (Tohoku) arc from the Japan
Trench to the Japan Sea coast are determined, for the first
time, using a large number of high-quality t* data measured
precisely from P and S wave spectra of local earthquakes. The
suboceanic earthquakes used in this work are relocated precisely
using sP depth phases. Our results reveal a prominent landward
dipping high-Q zone representing the subducting Pacific slab,
a landward dipping intermediate- to high-Q zone in the mantle
wedge
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between the Pacific coast and the
volcanic front, and significant low-Q anomalies in the crust and
mantle wedge between the volcanic front and the Japan Sea coast.
Prominent high-Q patches surrounded by low-Q anomalies are revealed
in the interplate megathrust zone under the Tohoku fore arc where
the great 2011 Tohoku-oki earthquake (Mw 9.0) occurred. The high-Q
patches in the megathrust zone generally exhibit large coseismic
slips of megathrust earthquakes and large slip deficit on the
plate interface. We think that these high-Q patches represent
asperities in the megathrust zone, whereas the low-Q anomalies
reflect weakly coupled areas. We also find that the hypocenters
of the 2011 Tohoku-oki interplate earthquakes (Mw > 7.0) are
located in areas where Qp, Qs, and Qp/Qs change abruptly. These
results suggest that structural heterogeneities in the megathrust
zone control the interplate seismic coupling and the nucleation
of megathrust earthquakes. |
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Febrero de 2014
Gutenberg-Richter relation originates from Coulomb stress
fluctuations caused by elastic rock heterogeneity
Authors: C. Langenbruch and S. A. Shapiro.
Link: Click here
Based on measurements along boreholes, a characterization
of the Earth's crust elastic heterogeneity is presented. We
investigate its impact on Coulomb stress distribution and earthquake
magnitude scaling. The analysis of elastic modulus distribution
at various borehole locations in different regions reveals universal
fractal nature of elastic heterogeneity. By applying a homogeneous
far-field stress to a representative model of elastic rock heterogeneity,
we show that it causes strong Coulomb stress fluctuations. In
situ fluctuations of Coulomb stress
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are mainly controlled by in situ
elastic moduli. Fluctuations caused by surrounding heterogeneities
are only of minor importance. Hence, the fractal nature of elastic
heterogeneity results in Coulomb stress fluctuations with power
law size distribution. As a consequence, fault sizes and magnitudes
of earthquakes scale according to the Gutenberg-Richter relation.
Due to the universal fractal nature of elastic heterogeneity,
the b value should be universal. Deviation from its universal
value of b?1 occurs due to characteristic scales of seismogenic
processes, which cause limitations or changes of fractal scaling.
Scale limitations are also the reason for observed stress dependency
of the b value. Our analysis suggests that the Gutenberg-Richter
relation originates from Coulomb stress fluctuations caused by
elastic rock heterogeneity. |
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Febrero de 2014
Estimating the ETAS model from an early
aftershock sequence
Authors: Takahiro Omi, Yosihiko Ogata et al
Link: Click here
Forecasting aftershock probabilities, as early
as possible after a main shock, is required to mitigate seismic
risks in the disaster area. In general, aftershock activity
can be complex, including secondary aftershocks or even triggering
larger earthquakes. However, this early forecasting implementation
has been difficult because numerous aftershocks are unobserved
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immediately after the main shock
due to dense overlapping of seismic waves. Here we propose a method
for estimating parameters of the epidemic type aftershock sequence
(ETAS) model from incompletely observed aftershocks shortly after
the main shock by modeling an empirical feature of data deficiency.
Such an ETAS model can effectively forecast the following aftershock
occurrences. For example, the ETAS model estimated from the first
24 h data after the main shock can well forecast secondary aftershocks
after strong aftershocks. This method can be useful in early and
unbiased assessment of the aftershock hazard. |
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Febrero de 2014
Visualization of space competition and plume formation with
complex potentials for multiple source flows: some examples
and novel application to Chao lava flow (Chile)
Author: R. Weijermars
Link: Click here
Fluid displacement in a continuum pressured
by a variable constellation of source flows can be visualized
as solutions of line integrals. The algorithms are based on
complex potentials that provide exact solutions of the Navier-Stokes
equation and allow users to specify both the location and flux
strength of multiple sources. If relative strength and positioning
of the sources are varied, a wide range of flow patterns and
particle paths can be traced. Time-dependent variations in the
strength of the sources can
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account
for transient-flow problems. A further expansion is superposition
of a background flow, which displaces the particle paths issued
from the sources into concentrated plumes. The outlined modeling
technique for visualization of multiple plume displacements is
potentially relevant for a wide spectrum of practical situations.
Detailed applications are possible, such as time-tracking of groundwater-plume
migration from a series of pollution sources, tracking of salt-feeder-stock
flow and suture-zone formation when salt issued from the stocks
coalesces into a salt canopy, and designing of optimal spacing
and relative pressure strength of multiple water-injection wells
in field-development plans for hydrocarbon production. Further
applications are highlighted in the main text, including a simulation
of geometrical features exposed in the Chao coulee (Chilean Andes). |
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Febrero de 2014
Distinguishing high surf from volcanic long-period earthquakes
Authors: John J. Lyons, Matthew M. Haney et al
Link: Click here
Repeating long-period (LP) earthquakes are observed
at active volcanoes worldwide and are typically attributed to
unsteady pressure fluctuations associated with fluid migration
through the volcanic plumbing system. Nonvolcanic sources of
LP signals include ice movement and glacial outburst floods,
and the
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waveform characteristics and frequency
content of these events often make them difficult to distinguish
from volcanic LP events. We analyze seismic and infrasound data
from an LP swarm recorded at Pagan volcano on 12-14 October 2013
and compare the results to ocean wave data from a nearby buoy.
We demonstrate that although the events show strong similarity
to volcanic LP signals, the events are not volcanic but due to
intense surf generated by a passing typhoon. Seismo-acoustic methods
allow for rapid distinction of volcanic LP signals from those
generated by large surf and other sources, a critical task for
volcano monitoring. |
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Febrero de 2014
Structural heterogeneities around the megathrust zone of
the 2011 Tohoku earthquake from tomographic inversion of onshore
and offshore seismic observations
Authors: Yojiro Yamamoto, Koichiro Obana et al
Link: Click here
We performed a three-dimensional seismic tomography
around the coseismic slip area of the 2011 Tohoku earthquake.
By combining data from the aftershock period collected by ocean
bottom seismographs (OBSs), OBS data from previous studies off
the Miyagi coast, and land seismic data, we modeled the detailed
seismic velocity structure along the plate boundary from the
Japan Trench to near the coastline. Our results indicate that
VP, VS, and VP/VS along the plate boundary change drastically
about 60 km landward from the trench axis. Trenchward of this
boundary, velocities are consistent with a fluid-rich environment
(low VP and VS and high VP/VS)
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associated with the occurrence of
sediment compaction and opal-to-quartz metamorphism. This area
also corresponds to the contact between the slab and upper crust
or between the slab and the sediment layer. A comparison of our
results with numerical simulations and geological studies suggests
that thermal pressurization might have occurred near the trench
axis during the 2011 Tohoku earthquake. To the west of this boundary,
where VP, VS, and VP/VS have moderate values, our model showed
small-scale heterogeneous structures in the subducted oceanic
crust near the hypocenter of the Tohoku earthquake with regions
of low and high VP/VS on the updip and downdip sides of the hypocenter,
respectively. We interpret the former as corresponding to the
strong coupling patch and the latter as the localized fluid-rich
zone. Small-scale heterogeneities thus may affect the nucleation
and rupture processes of large earthquakes. |
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Febrero de 2014
The stress field beneath a quiescent stratovolcano: The case
of Mount Vesuvius
Authors: Luca D'Auria, Bruno Massa et al
Link: Click here
We have analyzed a focal mechanism data set
for Mount Vesuvius, consisting of 197 focal mechanisms of events
recorded from 1999 to 2012. Using different approaches and a
comparison between observations and numerical models, we have
determined the spatial variations in the stress field beneath
the volcano. The main results highlight the presence of two
seismogenic volumes characterized by markedly different stress
patterns. The two volumes are separated by a layer where the
seismic strain release shows a significant decrease. Previous
studies postulated the existence, at about the same depth, of
a ductile layer allowing the
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spreading of the Mount Vesuvius edifice.
We interpreted the difference in the stress pattern within the
two volumes as the effect of a mechanical decoupling caused by
the aforementioned ductile layer. The stress pattern in the top
volume is dominated by a reverse faulting style, which agrees
with the hypothesis of a seismicity driven by the spreading process.
This agrees also with the persistent character of the seismicity
located within this volume. Conversely, the stress field determined
for the deep volume is consistent with a background regional field
locally perturbed by the effects of the topography and of heterogeneities
in the volcanic structure. Since the seismicity of the deep volume
shows an intermittent behavior and has shown to be linked to geochemical
variations in the fumaroles of the volcano, we hypothesize that
it results from the effect of fluid injection episodes, possibly
of magmatic origin, perturbing the pore pressure within the hydrothermal
system. |
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Febrero de 2014
On the use of geomagnetic indices and ULF waves for earthquake
precursor signatures
Authors: J. L. Currie and C. L. Waters
Link: Click here
Ultralow frequency (ULF: 0.001-5 Hz) magnetic
records have recently been used in the search for short-term
earthquake prediction methods. The separation of local and global
effects in the magnetic records is the greatest challenge in
this research area. Geomagnetic indices are often used to predict
global ULF magnetic behavior where it is assumed that increases
in a geomagnetic index correspond with an increase
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in
ULF power. This paper examines the relationships between geomagnetic
indices and ULF power, spectral polarization ratio, and the relationship
between the spectral polarization ratio and solar wind parameters.
The power in the ULF, Pc3-5 bands (10-600 s), shows a linear correlation
coefficient of 0.2 with the Kp magnetic activity index. The correlation
varies with magnetic local time (MLT) and latitude. The correlation
coefficient is inversely related to the integrated power in the
ULF Pc3 band (10-45 s) over MLT and magnetic latitude. The ratio
of spectral powers Z(?)/G(?) is discussed and shown to be a promising
parameter in the search for earthquake precursor signals in ULF
records. |
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Febrero de 2014
Importance of temporal variability for hydrological predictions
based on the maximum entropy production principle
Authors: Martijn C. Westhoff, Erwin Zehe et al
Link: Click here
This work builds on earlier work by Kleidon
and Schymanski (2008) who explored the use of the maximum entropy
production (MEP) principle for modeling hydrological systems.
They illustrated that MEP can be used to determine the partitioning
of soil water into runoff and evaporation-which determines hydroclimatic
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conditions around the Globe-by optimizing
effective soil and canopy conductances in a way to maximize entropy
production by these fluxes. In the present study, we show analytically
that under their assumption of constant rainfall, the proposed
principle always yields an optimum where the two conductances
are equal, irrespective of rainfall rate, evaporative demand,
or gravitational potential. Subsequently, we show that under periodic
forcing or periodic variations in one resistance (e.g., vegetation
seasonality), the optimal conductance does depend on climatic
drivers such as the length of dry spells or the time of closure
of stomata. |
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Febrero de 2014
Asthenospheric Pacific-Atlantic flow barriers and the West
Scotia Ridge extinction
Authors: Yasmina M. Martos, Jesús Galindo-Zaldívar
et al
Link: Click here
The Drake Passage is considered a gateway for
oceanic and asthenospheric flows since its opening, entailing
widespread consequences for climate and plate tectonics, respectively.
Both the surface and the 50 km upward continued Bouguer anomaly
maps of the Scotia Sea and surrounding areas, based on Gravity
Recovery and Climate Experiment gravity satellite data, improve
our
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knowledge
of deep lithospheric structures and the asthenosphere. We show
that the West Scotia Sea is likely to be underlain by an anomalously
low-density upper mantle. Gravity data are compatible with variable
lithospheric thicknesses related to asthenospheric currents. The
new data suggest that the development of the Shackleton Fracture
Zone since the middle Miocene was probably a main factor that
determined the evolution of the eastward Pacific mantle flows
and the extinction of the West Scotia Sea oceanic spreading around
6 Ma ago. Deep lithospheric roots are likely to divert asthenospheric
currents around them, flowing eastward through Drake Passage. |
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Febrero de 2014
Mantle compression affects seismic wave propagation
Author: Colin Schultz
Link: Click here
To try to understand the direction of motion
of the Earth's mantle, which lies hidden beneath tens of kilometers
of crust, researchers have relied on the property of seismic
anisotropy. When seismic
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shear waves pass through some types
of materials, known as anisotropic materials, the speed of the
wave can vary depending on the direction in which it is moving.
Traditionally, scientists have assumed that the direction in which
waves move more quickly aligns with the direction of mantle motion.
For subduction zones, however, this general rule seemed to break
down-a discrepancy exists between numerical model simulations
and observed seismic data. |
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Enero de 2014
Can we test for the maximum possible
earthquake magnitude?
Authors: M. Holschneider, G. Zöller et al
Link: Click here
We explore the concept of maximum possible earthquake
magnitude, M, in a region represented by an earthquake catalog
from the viewpoint of statistical testing. For this aim, we
assume that earthquake magnitudes are independent events that
follow a doubly-truncated Gutenberg-Richter distribution and
focus on the upper truncation M. In earlier work, it has been
shown that the value of M cannot be well constrained from earthquake
catalogs alone. However, for two hypothesized values M and M?,
alternative statistical tests may address the question: Which
value is more consistent with the data? In other words: Is it
possible to reject a magnitude within reasonable
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errors,
i.e. the error of the first and the error of the second kind?
The results for realistic settings indicate that either the error
of the first kind or the error of the second kind is intolerably
large. We conclude that it is essentially impossible to infer
M in terms of alternative testing with sufficient confidence from
an earthquake catalog alone, even in regions like Japan with excellent
data availability. These findings are also valid for frequency-magnitude
distributions with different tail behavior, e.g. exponential tapering.
Finally, we emphasize that different data may only be useful to
provide additional constraints for M, if they do not correlate
with the earthquake catalog, i.e. if they have not been recorded
in the same observational period. In particular, longterm geological
assessments might be suitable to reduce the errors, while GPS
measurements provide overall the same information as the catalogs. |
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Enero de 2014
Microearthquake evidence for reaction-driven cracking within
the Trans-Atlantic Geotraverse (TAG) active hydrothermal deposit
Authors: Claire W. Pontbriand and Robert A. Sohn
Link: Click here
We detected 32,078 very small, local microearthquakes
(average ML?=?-1) during a 9-month deployment of five ocean
bottom seismometers (OBS) on the periphery of the Trans-Atlantic
Geotraverse (TAG) active mound. Seismicity rates were constant
without any mainshock-aftershock behavior at ~243 events per
day at the beginning of the experiment, 128 events per day after
an instrument failed, and 97 events per day at the end of the
experiment when
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whale calls increased background
noise levels. The microearthquake seismograms are characterized
by durations of <1?second and most have single-phase P-wave
arrivals (i.e., no S-arrivals). We accurately located 6,207 of
the earthquakes, with hypocenters clustered within a narrow depth
interval from ~50-125 mbsf on the south and west flanks of the
deposit. We model the microearthquakes as reaction-driven fracturing
events caused by anhydrite deposition in the secondary circulation
system of the hydrothermal mound, and show that under reasonable
modeling assumptions an average event represents a volume increase
of 31-58?cm3, yielding an annual (seismogenic) anhydrite deposition
rate of 27-51?m3. |
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Enero de 2014
The 17 May 2012 M4.8 earthquake near Timpson, East Texas:
An event possibly triggered by fluid injection
Authors: Cliff Frohlich, William Ellsworth et al
Link: Click here
This study summarizes our investigation of the
17 May 2012 MW-RMT4.8 earthquake near Timpson, Texas, the largest
earthquake recorded historically in eastern Texas. To identify
preshocks and aftershocks of the 17 May event we examined the
arrivals recorded at Nacogdoches (NATX) 30?km from the 17 May
epicenter, at nearby USArray Transportable Array stations, and
at eight temporary stations deployed between 26 May 2012 and
mid-2013. At NATX we identified seven preshocks, the earliest
occurring in April 2008. Reliably located aftershocks recorded
by the temporary stations lie along a 6?km long NW-SE linear
trend corresponding to a previously
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mapped basement
fault that extends across the highest-intensity (MMI VII) region
of the 17 May main shock. Earthquakes in this sequence are relatively
shallow-with focal depths ranging from 1.6 to 4.6?km. Evidence
supporting these depths include: hypocentral locations of exceptionally
well-recorded aftershocks, S-P intervals at the nearest stations,
and comparisons of synthetics and observed seismograms. Within
3?km of the linear trend of aftershock activity there are two
Class II injection disposal wells injecting at 1.9?km depth beginning
in August 2006 and February 2007, with injection rates averaging
42,750?m3/mo and 15,600?m3/mo, respectively. Several observations
support the hypothesis that fluid injection triggered the Timpson
sequence: well-located epicenters are situated near a mapped basement
fault and near high-volume injection wells, focal depths are at
or below the depths of injection, and the earliest preshock (April
2008) occurred after the onset of injection in 2006. |
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Enero de 2014
Detection of supershear rupture in 2013 Craig, Alaska, earthquake
Author: Colin Schultz
Link: Click here
Seismic ruptures are akin to opening a zipper-a
gap in the crust starts in one location and travels along the
fault in a particular direction. When a strained fault ruptures
in an earthquake, seismic waves also spread out from the epicenter.
In
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some
cases, the waves' passage can trigger the initiation of a new
rupture ahead of the initial expanding rupture in locked portions
of the fault. If the triggered rupture grows successfully, the
overall rupture front can then outpace the passage of the shear
waves, secondary seismic waves that travel slowly after the earthquake
begins and are responsible for the bulk of violent shaking. These
earthquakes display what is known as supershear rupture; only
seven such earthquakes have previously been recorded. |
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Enero de 2014
Three-dimensional micromagnetic modeling of randomly oriented
magnetite grains (0.03-0.3 µm)
Authors: Huseyin Serdar Kuyuk and Richard M. Allen
Link: Click here
We examine five different methods to estimate
an earthquake's magnitude using only P wave data for use in
earthquake early warning systems. We test two input parameters:
the maximum predominant period of the P wave (?pmax) and the
displacement amplitude of the Pwave's vertical component (Pd).
We apply our algorithms
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to 174 earthquakes
3.0?<?M?<?8.0 from California and Japan that have also been
used in previous calibration studies. We also apply them to 1992
0.2?<?M?<?5.7 earthquakes that were processed by the real-time
Earthquake Alarm Systems in California. We find that ?pmax does
not scale with magnitude for small earthquakes (M?<?3) and
is less accurate for large-magnitude earthquakes than using Pd
alone. We derive a global scaling relation between Pd and magnitude
and conclude that this global relationship provides the most accurate
and robust magnitude estimate. This relationship could be applied
in earthquake source zones around the world. |
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Enero de 2014
Distribution of gas hydrates on continental margins by means
of a mathematical envelope: A method applied to the interpretation
of 3D Seismic Data
Authors: Sean Bale, Tiago M. Alves et al
Link: Click here
A 3D seismic volume from the Nankai Trough
accretionary wedge (SE Japan) is used to evaluate the subsurface
distribution of gas hydrates as a function of structural and
stratigraphic complexity, variable heat flow patterns and the
presence of subsurface fluid conduits. Eleven equations were
modified for depth, pressure, and temperature, modeled in 3D,
and compared with the distribution of Bottom-Simulating Reflections
(BSRs) offshore Nankai. The results show that the equations
produce overlapping-and thus potentially consistent-predictions
for the distribution of BSRs, leading us to propose the concept
of a "BSR Stability
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Envelope"
as a method to quantify the subsurface distribution of gas hydrates
on continental margins. In addition, we show that the ratio (R)
between shallow and deep BSRs of seven subenvelopes, which are
defined by BSR stability equations, indicates local gas hydrate
equilibrium conditions. Values of R?<?1 relate to cooler regions,
whereas when R? >?1 the majority of BSRs are located in warmer
structural traps. The method in this paper can be used to recognize
any divergence between observed and theoretical depths of occurrence
of BSRs on 3D or 4D (time lapse) seismic volumes. In the Nankai
Trough, our results point out for equilibrium conditions in BSRs
located away from the Megasplay Fault Zone and major thrust faults.
This latter observation demonstrates the applicability of the
method to: (a) the recognition of subsurface fluid conduits and
(b) the prediction of maximum and minimum depths of occurrence
of gas hydrates on continental margins, under distinct thermal
and hydrologic conditions. |
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Enero de 2014
Probing the slip-weakening mechanism of earthquakes with
electrical conductivity: Rapid transition from asperity contact
to gouge comminution
Authors: Futoshi Yamashita, Eiichi Fukuyama et al
Link: Click here
Despite the great importance of slip weakening
during earthquakes, the fault-weakening mechanism inside the
fault remains to be clarified. Here we propose a micromechanism
of slip weakening at seismic and subseismic slip rates as demonstrated
by electrical conductivity
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observation across
the fault. At the seismic slip rate, the formation of melt patches
and the subsequent growth to molten layer during frictional melting
were confirmed by the rapid increases in conductivity in two stages.
At the subseismic slip rate, the conductivity data successfully
captured rapid asperity breakage phases and the subsequent gouge
evolution phases during the slip-weakening process. The present
results suggest that the gouge comminution process is one of the
key mechanisms controlling the slip stability at the subseismic
slip rate, which may cause a variety of natural earthquakes. |
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Enero de 2014
Kilauea magma chamber inflation triggered strong 2007 earthquakes
Author; Colin Schultz
Link: Click here
Hawaii's Kilauea volcano has been erupting since
1983. Starting in 2003, researchers noticed an inflation of
the magma chamber beneath the volcano; this inflation accelerated
in 2006. In June 2007, the volcano's eruption peaked with a
burst
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of
activity known as the Father's Day event. In the buildup to those
eruptions, according to new research by Wauthier et al., a pair
of large-magnitude earthquakes hit, with an epicenter beneath
the volcano. Earthquakes are common around the volcano, which
sits on Hawaii's Big Island, but with magnitudes of 4.7 and 4.1,
the two 2007 earthquakes were among the most powerful recorded
in the region since records began in 1959 |
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Enero de 2014
The Science Behind Laboratory-Scale Models of the Earth
Authors: Karen Leever, Olivier Galland et al
Link: Click here
Deformation processes involve systems of tens
to hundreds of kilometers moving at highly variable rates. Because
direct kinematic observations have proven difficult, geoscientists
are sometimes found in the lab creating analogs, or laboratory
models of the Earth. The main scope-and a key
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challenge-of laboratory
modeling of Earth systems is to provide a quantitative approach
for understanding geological processes. Since the last decade,
various image analysis techniques in laboratory modeling have
been designed and used to constrain the kinematics of the simulated
processes at an unprecedented resolution. These techniques prove
to be the key quantification tools in experimental modeling, not
only allowing mechanical analysis of the experiments but also
producing experimental data sets that are directly comparable
to geophysical and geodetic data in nature and numerical models. |
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Enero de 2014
An Extreme Coronal Mass Ejection and Consequences for the
Magnetosphere and Earth
Authors: Bruce T. Tsurutani and Gurbax S. Lakhina
Link: Click here
Abstract
A "perfect" ICME could create a magnetic storm with
intensity up to the saturation limit (Dst?~?-2500 nT), a value
greater than the Carrington storm. Many of the other space weather
effects will not be limited by saturation effects, however.
The interplanetary shock would arrive at Earth within ~12?hrs
with a magnetosonic Mach number ~45.
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The
shock impingement onto the magnetosphere will create a SI+ of
~234 nT, the magnetic pulse duration in the magnetosphere will
be ~22?s with a dB/dt of ~30 nT s-1, and the magnetospheric electric
field associated with the dB/dt ~1.9?V?m-1, creating a new relativistic
electron radiation belt. The magnetopause location of 4 Re from
the Earth's surface will allow expose of orbiting satellites to
extreme levels of flare and ICME shock-accelerated particle radiation.
The results of our calculations are compared with current observational
records. Comments are made concerning further data analysis and
numerical modeling needed for the field of space weather. |