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Julio de 2013
Determining the orientations of ocean bottom seismometers
using ambient noise correlation
Authors: Yang Zha, Spahr C. Webb et al
Link: Click here
Abstract
The cross-correlation of multi-component ambient seismic noise
can reveal both the velocity and polarization of surface waves
propagating between pairs of stations. We explore this property
to develop a novel method for determining the horizontal orientation
of ocean bottom seismometers (OBS) by analyzing the polarization
of Rayleigh waves retrieved from ambient noise cross-correlation.
We demonstrate
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that
the sensor orientations can be estimated through maximizing the
correlation between the radial-vertical component and the phase-shifted
vertical-vertical component of the empirical Green's tensor. We
apply this new method to the ELSC (Eastern Lau Spreading Center)
OBS experiment dataset, and illustrate its robustness by comparing
the obtained orientations with results from a conventional method
utilizing teleseismic P- and Rayleigh- wave polarizations. When
applied to a large OBS array, the ambient noise method provides
a larger number of orientation estimates and better azimuthal
coverage than typically is possible with traditional methods. |
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Julio de 2013
Efficient posterior exploration of a high-dimensional groundwater
model from two-stage Markov chain Monte Carlo simulation and
polynomial chaos expansion
Authors: Eric Laloy, Bart Rogiers et al
Link: Click here
Abstract
This study reports on two strategies for accelerating posterior
inference of a highly parameterized and CPU-demanding groundwater
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flow model. Our method builds on
previous stochastic collocation approaches, e.g., Marzouk and
Xiu (2009) and Marzouk and Najm (2009), and uses generalized polynomial
chaos (gPC) theory and dimensionality reduction to emulate the
output of a large-scale groundwater flow model. The resulting
surrogate model is CPU efficient and serves to explore the posterior
distribution at a much lower computational cost using two-stage
MCMC simulation. The case study reported in this paper demonstrates
a two to five times speed-up in sampling efficiency. |
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Julio de 2013
Stress tensor changes related to fluid injection at The Geysers
geothermal field, California
Authors: Patricia Martínez-Garzón, Marco Bohnhoff
et al
Link: Click here
Abstract
Studying variations of the stress field in reservoirs caused
by massive fluid injection is important toward an improved understanding
of geomechanical processes involved. We report on spatio-temporal
variations of the local stress tensor orientation at The Geysers
geothermal field, California. We apply two stress inversion
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methods with detailed uncertainty
assessments using a selection of events recorded between 2007
and 2012. Our results clearly indicate variations in the orientation
of the principal stress axes for the reservoir as a whole showing
a normal faulting regime at the reservoir depth between 2 and
3.7 km bounded by a strike-slip regime above and below. Analyzing
the temporal evolution of the stress tensor orientation for a
prominent seismicity cluster we observe a clear correlation of
changes in orientation for ?1–3 with the highest injection
rates. These results suggest that temporal changes in the stress
tensor orientation could contribute to characterize reservoirs
during stimulation. |
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Julio de 2013
Using hydroacoustic stations as water column seismometers
Authors: Selda Yildiz, Karim Sabra et al
Link: Click here
Abstract
Getting seismic data from the deep oceans usually involves ocean-bottom
seismometers, but hydrophone arrays may provide a practical
alternative means of obtaining vector data. We here explore
this possibility using hydrophone stations of the International
Monitoring System, which have been used to study icebergs and
T-wave propagation among others. These stations
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consist of three hydrophones at about the depth of the deep sound
channel in a horizontal triangle array with 2 km sides. We use
data from these stations in the very low-frequency regime (0.01–0.05
Hz band) to demonstrate that these stations can also be used as
water column seismometers. By differencing the acoustic pressure,
we obtain vector quantities analogous to what a seismometer would
record. Comparing processed hydrophone station records of the
2004 Great Sumatra-Andaman Earthquake with broadband seismograms
from a nearby island station, we find that the differenced hydrophones
are indeed a practical surrogate for seismometers. |
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Julio de 2013
Monitoring deformation at the Geysers Geothermal Field, California
using C-band and X-band interferometric synthetic aperture radar
Authors:D. W. Vasco, Jonny Rutqvist et al
Link: Click here
Abstract
We resolve deformation at The Geysers Geothermal Field using
two distinct sets of interferometric synthetic aperture radar
(InSAR) data. The first set of observations utilize archived
European Space Agency C-band synthetic aperture radar data from
1992 through 1999 to image the long-term and large-scale subsidence
at The Geysers. The peak range velocity of approximately 50
mm/year agrees with previous
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estimates from leveling and global
positioning system observations. Data from a second set of measurements,
acquired by TerraSAR-X satellites, extend from May 2011 until
April 2012 and overlap the C-band data spatially but not temporally.
These X-band data, analyzed using a combined permanent and distributed
scatterer algorithm, provide a higher density of scatterers (1122
per square kilometer) than do the C-band data (12 per square kilometer).
The TerraSAR-X observations resolve 1 to 2 cm of deformation due
to water injection into a Northwest Geysers enhanced geothermal
system well, initiated on October 2011. The temporal variation
of the deformation is compatible with estimates from coupled numerical
modeling. |
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Julio de 2013
The 2011 M = 9.0 Tohoku oki earthquake more than doubled
the probability of large shocks beneath Tokyo
Authors: Shinji Toda and Ross S. Stein
Link: Click here
Abstract
The Kanto seismic corridor surrounding Tokyo has hosted four
to five M ? 7 earthquakes in the past 400 years. Immediately
after the Tohoku earthquake, the seismicity rate in the corridor
jumped 10-fold, while the rate of normal focal mechanisms dropped
in half. The seismicity rate
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decayed
for 6–12 months, after which it steadied at three times the
pre-Tohoku rate. The seismicity rate jump and decay to a new rate,
as well as the focal mechanism change, can be explained by the
static stress imparted by the Tohoku rupture and postseismic creep
to Kanto faults. We therefore fit the seismicity observations
to a rate/state Coulomb model, which we use to forecast the time-dependent
probability of large earthquakes in the Kanto seismic corridor.
We estimate a 17% probability of a M ? 7.0 shock over the 5 year
prospective period 11 March 2013 to 10 March 2018, two-and-a-half
times the probability had the Tohoku earthquake not struck. |
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Julio de 2013
Electrical resistivity tomography revealing the internal
structure of monogenetic volcanoes
Authors: Stéphanie Barde-Cabusson, Xavier Bolós
et al
Link: Click here
Abstract
Eruptive activity of individual monogenetic volcanoes usually
lasts a few days or weeks. However, their short lifetime does
not always mean that their dynamics and structure are simple.
Monogenetic cones construction is rarely witnessed from the
beginning to the end, and conditions for observing their internal
structure are hardly reached. We provide high-resolution
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electrical resistivity sections
(10 m electrode spacing) of three monogenetic cones from northeastern
Spain, comparing our results to geological observations to interpret
their underground continuation. The 100 m maximum depth of exploration
provides information on almost the entire edifices, highlighting
the relationships between Strombolian and hydromagmatic deposits
in two multiphase edifices. A main observation is a column of
distinct resistivity centered on the Puig d'Adri volcano, which
we interpret as the eruptive conduit. This method can provide
valuable information on the past volcanic dynamics of monogenetic
volcanic fields, which has real implications for the forecast
of future activity. |
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Junio de 2013
A decade of horizontal deformation from great earthquakes
Authors: P. Tregoning, R. Burgette et al
Link: Click here
Abstract
The 21st Century has seen the occurrence of 17 great earthquakes
(Mw >8), including some of the largest earthquakes ever recorded.
Numerical modeling of the earthquakes shows that nearly half
of the Earth's surface has undergone horizontal coseismic deformation
>1 mm, with the 2004 Sumatra-Andaman earthquake dominating
the global deformation field. This has important implications
for both the realization of a terrestrial reference frame and
in the interpretation of regional tectonic studies based on
GPS velocities. We show that far-field coseismic deformations
from great earthquakes will, if unaccounted for, introduce errors
in estimates of linear site velocities of at least 0.1-0.3 mm/yr
across most of
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the
surface of the Earth. The accumulated global deformation field
shows that two regions, Australia and the north Atlantic/Arctic
Ocean, have been largely undeformed by these great earthquakes,
with accumulated deformations generally <0.5 mm. Using GPS
estimates of surface deformation, we show that the majority of
the Australian continent is deforming at <0.2 mm/yr, the northern
part of New Zealand is rotating clockwise relative to the Australian
Plate with relative horizontal velocities of ?2 mm/yr, while the
southeastern coast of Australia is undergoing post-seismic relaxation
caused by the 2004 Mw = 8.1 Macquarie Ridge earthquake. The presence
of ongoing post-seismic relaxation thousands of kilometers from
plate margins violates the secular/linear assumption made in current
terrestrial reference frame definitions. These effects have significant
ramifications for regional tectonic interpretations and global
studies such as sea level rise that require reference frame accuracy
greater than this level. |
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Junio de 2013
The Development and Evaluation of the Earth Gravitational
Model 2008 (EGM2008)
Authors: Nikolaos K. Pavlis, Simon A. Holmes et al
Link: Click here
Abstract
EGM2008 is a spherical harmonic model of the Earth's gravitational
potential, developed by a least squares combination of the ITG
GRACE03S gravitational model and its associated error covariance
matrix, with the gravitational information obtained from a global
set of area mean free air gravity anomalies defined on a 5 arc
minute equiangular grid. This grid was formed by merging terrestrial,
altimetry derived, and airborne gravity data. Over areas where
only lower resolution gravity data were available, their spectral
content was supplemented with gravitational information implied
by the topography. EGM2008 is complete to degree and order 2159,
and contains additional coefficients
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up to degree 2190 and order 2159.
Over areas covered with high quality gravity data, the discrepancies
between EGM2008 geoid undulations and independent GPS/Leveling
values are on the order of {plus minus}5 to {plus minus}10 cm.
EGM2008 vertical deflections over USA and Australia are within
{plus minus}1.1 to {plus minus}1.3 arc seconds of independent
astrogeodetic values. These results indicate that EGM2008 performs
comparably with contemporary detailed regional geoid models. EGM2008
performs equally well with other GRACE based gravitational models
in orbit computations. Over EGM96, EGM2008 represents improvement
by a factor of six in resolution, and by factors of three to six
in accuracy, depending on gravitational quantity and geographic
area. EGM2008 represents a milestone and a new paradigm in global
gravity field modeling, by demonstrating for the first time ever,
that given accurate and detailed gravimetric data, a single global
model may satisfy the requirements of a very wide range of applications. |
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Junio de 2013
Interplate coupling off northeastern Japan before the 2011
Tohoku-oki earthquake, inferred from seafloor geodetic data
Authors: Mariko Sato, Masayuki Fujita et al
Link: Click here
Abstract
Seafloor crustal movements off Miyagi and Fukushima Prefectures,
northeastern Japan, observed for about nine years before the
2011 Tohoku-oki earthquake (M9.0) with the GPS/Acoustic geodetic
technique, revealed the spatio-temporal variation of interplate
coupling in this region. Off Miyagi Prefecture, temporal variations
of the crustal movements are characterized as a strain accumulation-release
process in relation to the 2005 Miyagi-oki earthquake (M7.2).
The intraplate velocity since
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December
2006, when the strain accumulation seemed to have restarted, is
4–5?cm/year relative to the North American plate, which is
significantly lower than that before the 2005 event, 6–7?cm/year.
This suggests that interplate coupling off Miyagi Prefecture since
around 2007 is weaker than that before the 2005 event. On the
other hand, off Fukushima Prefecture, a westward crustal movement
at a constant rate of about 2?cm/year relative to the North American
plate has been observed for the interseismic period from 2002
to 2008. The interplate coupling off Fukushima Prefecture is significantly
weak compared with that off Miyagi Prefecture. This is consistent
with the observed co-seismic displacements associated with the
2011 event, which are much smaller off Fukushima Prefecture than
off Miyagi Prefecture. |
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Junio de 2013
Preservation of contrasting geothermal gradients across the
Caribbean-North America plate boundary (Motagua Fault, Guatemala)
Authors: Thibaud Simon-Labric, Gilles Y. Brocard et al
Link: Click here
Abstract
Strike-slip plate boundaries juxtapose crustal blocks that may
have different geodynamic origins and therefore different thermal
structures. Thermo-kinematic modeling of this type of strike-slip
plate boundary predicts an asymmetric signature in the low-temperature
thermochronologic record across the fault. Age-elevation profiles
of zircon (U-Th)/He ages across the Motagua Fault, a 500?km-long
segment of the transform boundary between the North American
and Caribbean plates, document a sharp cooling
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age discontinuity across the fault.
This discontinuity could be interpreted as a difference in denudation
history on each side of the fault. However, a low-relief Miocene
erosional surface extends across the fault; this surface has been
uplifted and incised and provides a geomorphic argument against
differential denudation across the fault. By integrating magmatic,
volcanic, and heat-flow data, age-elevation profiles, and thermo-kinematic
modeling, we propose that large horizontal displacement along
the Motagua Fault has offset a persistent geothermal asymmetry
across the fault and explains both the age discontinuities and
the age-elevation patterns. This study illustrates how thermochronology
can be used to detect large strike-slip displacements and more
generally opens new perspectives in understanding the impact of
non-uniform thermal structures on thermochronologic results. |
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Junio de 2013
Response of seismicity to static and dynamic stress changes
induced by the 2011?M9.0 Tohoku-Oki earthquake
Authors: Aitaro Kato, Jun'ichi Fukuda et al
Link: Click here
Abstract
A growing body of evidence indicates that many earthquakes are
triggered by static and dynamic stress changes following large
earthquakes. However, it has been difficult to differentiate
the roles of static and dynamic stress transfer on earthquake
triggering. Here we focus on two adjacent seismic clusters aligned
E–W beneath
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the
Hida mountain range, central Japan, where spatially uniform static
and dynamic stress changes resulted from Tohoku-Oki earthquake.
The rate of seismicity in the western cluster, detected by the
matched filter technique, showed a slight increase after the mainshock,
which can be explained by static stress transfer. In contrast,
seismicity in the eastern cluster showed a marked increase due
to dynamic stress change. The difference in the number of triggered
events between these clusters indicates that dynamic stress change
is more important than static one in terms of remote triggering,
provided that large-sized potential earthquake-nucleation patches
are close to failure. |
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Junio de 2013
Verification of velocity-resistivity relationships derived
from structural joint inversion with borehole data
Authors: M. Moorkamp, A. W. Roberts et al
Link: Click here
Abstract
We present results of three-dimensional joint inversion of seismic,
magnetotelluric and gravity data over a marine salt dome. Such
structures are difficult to image with a single method and our
results demonstrate how combining different techniques can yield
improved results. More importantly, we examine the reliability
of velocity-
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conductivity relationships derived
from structure-coupled joint inversion approaches. Comparison
with a seismic reflection section shows that our models match
the upper limit of the salt. Furthermore, velocity and resistivity
logs from a borehole drilled into the salt dome's flank match,
within error, those recovered by the inversion. The good match
suggests that the difference in length scale does not have a significant
effect in this case. This provides a strong incentive to incorporate
borehole data into the joint inversion in the future and substantiates
approaches that use the relationships derived from joint inversion
models for lithological classification. |
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Junio de 2013
A double seismic zone in the subducting Juan Fernandez Ridge
of the Nazca Plate (32°S), central Chile
Authors: M. Marot, T. Monfret et al
Link: Click here
Abstract
The region of central Chile offers a unique opportunity to study
the links between the subducting Juan Fernandez Ridge, the flat
slab, the Double Seismic Zone (DSZ) and the absence of modern
volcanism. Here, we report the presence and characteristics
of the first observed DSZ within the intermediate-depth Nazca
slab using two temporary seismic catalogues (OVA99 and CHARSME).
The lower plane of seismicity (LP) is located 20-25?km below
the upper plane (UP), begins at 50?km depth and merges with
the lower plane at 120?km depth, where the slab
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becomes
horizontal. Focal mechanism analysis and stress tensor calculations
indicate that the slab's state of stress is dominantly controlled
by plate convergence and overriding crust thickness: Above 60-70?km
depth, the slab is in horizontal compression, and below, it is
in horizontal extension, parallel to plate convergence, which
can be accounted for by vertical loading of the overriding lithosphere.
Focal mechanisms below 60-70?km depth are strongly correlated
with offshore outer rise bend faults, suggesting the reactivation
of pre-existing faults below this depth. The large interplane
distances for all Nazca DSZs can be related to the slab's unusually
cold thermal structure with respect to its age. Since LPs globally
seem to mimic mantle mineral dehydration paths, we suggest that
fluid migration and dehydration embrittlement provide the mechanism
necessary to weaken the rock and that the stress field determines
the direction of rupture. |
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Junio de 2013
Teleseismic magnetic effects (TMDs) of 2011 Tohoku earthquake
Authors: Y. Q. Hao, Z. Xiao et al
Link: Click here
Abstract
Anomalous magnetic variations were observed by ground magnetometers
in East Asia area after the 2011 Tohoku earthquake. Some earlier
reports showed that the seismo-magnetic variations have obvious
amplitude around the epicenter, we emphasis here that the variations
can still be notable at stations 2000–4000 km away from
epicenter, and we define it as teleseismic magnetic disturbances
(TMDs). TMDs appear about 8 min later after the arrival of seismic
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Rayleigh waves at teleseismic distances
and propagate at a horizontal velocity of 3.9 ± 0.1 km/s.
The wave-like TMDs last for no longer than 10 min and have a main
period of 2.1–3.3 min. TMDs are not generated by direct effects
of processes in focal area crust or tsunami waves, instead, their
properties consist with the Rayleigh wave model of seismo-ionospheric
disturbances. Hence, we conclude that the TMDs are the magnetic
manifestation of seismotraveling ionospheric disturbances (STIDs)
generated by the interaction between the ionosphere and atmosphere
through acoustic waves launched by traveling Rayleigh waves. Our
findings contribute to the knowledge of seismo-electromagnetic
effects in the atmosphere-ionosphere system and further our understanding
of couplings between various spheres of the Earth. |
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Junio de 2013
Extreme value analysis unveils dramatic Antarctic warming
Author: Colin Schultz
Link: Click here
Abstract
Since the late 1940s the average temperature of
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the
coldest days on the Antarctic Peninsula has soared by 14°C.
Through a novel analysis of daily temperature records collected
at the peninsula's Vernadsky Research Base, Frankze identified
that the strong warming trend has affected the coldest 5% of days
on the Antarctic Peninsula over the course of the observational
period, from 1947 to 2011. |
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Junio de 2013
Is thermospheric global cooling caused by gravity waves?
Authors: W. L. Oliver, S.-R. Zhang et al
Link: Click here
Abstract
We analyze ion temperature data near 350 km altitude over the
years 1966–2012 to seek explanations for three outstanding
questions concerning the long-term cooling observed in the upper
thermosphere: (1) Why is the cooling so much larger than expected,
(2) why has the cooling lasted so long, and (3) why is the thermospheric
density response to the cooling so small? We speculate that
gravity waves may cause this cooling and provide answers to
these questions. Recent simulations have shown that gravity
waves are expected to cool the upper thermosphere by an amount
comparable to that observed over our data timeline. A gravity
wave proxy formed from the nontidal fluctuations in
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temperature shows a positive long-term
trend throughout its timeline, consistent with the increasing
cooling observed. The time scales of the long-term trend and its
decadal fluctuations are characteristic of the ocean, not the
atmosphere. We suggest that the following scenario may explain
these behaviors: (a) the climate regime shift of 1976–1977
launched slow Rossby waves across the oceans which continue to
propagate to this day, (b) winds over this increasingly corrugated
ocean have launched increasing fluxes of gravity waves into the
atmosphere, and (c) these increasing fluxes of gravity waves have
propagated to the thermosphere to produce increasing amounts of
cooling. The strong thermospheric cooling seen would be expected
to produce thermospheric density declines much larger than those
observed via satellite drag. These temperature and density results
would be compatible if the turbopause were lowered 4 km over the
time span of observations. |
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Junio de 2013
Band-limited Bouguer gravity identifies new basins on the
Moon
Authors: W. E. Featherstone, C. Hirt et al
Link: Click here
Abstract
Spectral domain forward modeling is used to generate topography-implied
gravity for the Moon using data from the Lunar Orbiter Laser
Altimeter instrument operated on board the Lunar Reconnaissance
Orbiter mission. This is subtracted from Selenological and Engineering
Explorer (SELENE)-derived gravity to generate band-limited Bouguer
gravity maps of the Moon so
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as
to enhance the gravitational signatures of anomalous mass densities
nearer the surface. This procedure adds evidence that two previously
postulated basins on the lunar farside, Fitzgerald-Jackson (25°N,
191°E) and to the east of Debye (50°N, 180°E), are
indeed real. When applied over the entire lunar surface, band-limited
Bouguer gravity reveals the locations of 280 candidate basins
that have not been identified when using full-spectrum gravity
or topography alone, showing the approach to be of utility. Of
the 280 basins, 66 are classified as distinct from their band-limited
Bouguer gravity and topographic signatures, making them worthy
of further investigation. |
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Junio de 2013
Seismo-acoustic coupling induced by the breakup of the 15
Feb 2013 Chelyabinsk Meteor
Authors: Benoit Tauzin, Eric Debayle et al
Link: Click here
Abstract
On 15th Feb 2013 around 03:20:00 UTC, the largest meteor reported
since the 1908 Tunguska event was observed as a fireball traveling
through the Earth's atmosphere, exploding in an air burst, near
the city of Chelyabinsk, Russia. The rarity of such an event
provides a unique window on the
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physics of meteoroid collision. We
report the fine seismic detection of Rayleigh waves produce by
the coupling of ground motion with the incident shock wave at
distances up to 4000?km from the event. Combining informations
from seismic beam-forming analysis, reconstructed trajectory from
casual video records, and remote sensing, we identify the Rayleigh
waves as being initiated by the shock wave produced by the main
blast that occasioned damages and injuries in Chelyabinsk. From
the Rayleigh wave observations, we report a magnitude Ms ~ 3.7
seismic source. |
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Junio de 2013
Induced seismicity associated with fluid injection into a
deep well in Youngstown, Ohio
Author: Won-Young Kim
Link: Click here
Abstract
Over 109 small earthquakes (MW 0.4–3.9) were detected during
January 2011– February 2012 in the Youngstown, Ohio area,
where there were no known earthquakes in the past. These shocks
were close to a deep fluid injection well. The 14-months seismicity
included six felt earthquakes and culminated with a MW 3.9 shock
on 31 December 2011. Among the 109 shocks, 12 events greater
than MW 1.8 were detected by regional network and accurately
relocated, whereas 97 small earthquakes (0.4 < MW < 1.8)
were detected by the waveform correlation detector. Accurately
located earthquakes were
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along
a subsurface fault trending ENE–WSW – consistent with
the focal mechanism of the mainshock and occurred at depths 3.5–4.0
km in the Precambrian basement. We conclude that the recent earthquakes
in Youngstown, Ohio were induced by the fluid injection at a deep
injection well due to increased pore pressure along the preexisting
subsurface faults located close to the wellbore. We found that
the seismicity initiated at the eastern end of the subsurface
fault – close to the injection point, and migrated toward
the west – away from the wellbore, indicating that the expanding
high fluid pressure front increased the pore pressure along its
path and progressively triggered the earthquakes. We observe that
several periods of quiescence of seismicity follow the minima
in injection volumes and pressure, which may indicate that the
earthquakes were directly caused by the pressure buildup and stopped
when pressure dropped. |
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Junio de 2013
A double seismic zone in the subducting Juan Fernandez Ridge
of the Nazca Plate (32°S), central Chile
Author: S. R. Proud
Link: Click here
Abstract
The large number of objects in a range of orbits around the
Sun means that some will inevitably intersect the Earth, becoming
a meteor. These objects are commonly comet fragments or asteroids.
To determine the type of a particular meteor requires knowledge
of its trajectory and
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orbital path that is typically estimated
by using ground-based observations such as images or radar measurements.
A lack of data can, however, make this difficult and create large
uncertainties in the reconstructed orbit. Here I show a new method
for estimating a meteor's trajectory, and hence allowing computation
of the orbit, based upon measurements from satellite sensors.
The meteor that fell on 15th Feb 2013 is used as an example and
the resulting orbit is in broad agreement with estimates from
other observations. This new technique represents an alternative
method for trajectory determination that may be particularly useful
in areas where ground-based observations are sparse. |
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Junio de 2013
Simple and reliable finite fault solutions for large earthquakes
using the W-phase: The Maule (Mw = 8.8) and Tohoku (Mw = 9.0)
earthquakes
Authors: Roberto Benavente and Phil Cummins
Link: Click here
Abstract
We explore the ability of W-phase waveform inversions to recover
a first-order coseismic slip distribution for large earthquakes.
To date W-phase inversions for point sources provide fast and
accurate moment tensor solutions for moderate to large events.
We have applied W-
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phase
finite fault inversion to seismic waveforms recorded following
the 2010 Maule earthquake (Mw = 8.8) and 2011 Tohoku earhquake
(Mw = 9.0). Firstly, a W-phase point source inversion was performed
to assist us in selecting the data for the finite fault solution.
Then, we use a simple linear multiple-time-window method accounting
for changes in the rupture velocity with smoothing and moment
minimisation constrains to infer slip and rake variations over
the fault. Our resultsdescribe well the main features of the slip
pattern previously found for both events. This suggests that fast
slip inversions may be carried out relying purely on W-phase records. |
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Junio de 2013
Graphite lubricates fault zones
Author: Ernie Balcerak
Link: Click here
Abstract
Graphite is known to be a low-friction material, and rocks rich
in graphite are often found in fault zones. Oohashi et al. conducted
laboratory studies to determine how much graphite is
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needed to reduce the frictional strength
of a fault. Their experiments included samples with various mixtures
of graphite and quartz, as well as pure quartz and pure graphite,
and they covered large displacements (up to 100 meters), a range
of slip rates (from 200 micrometers per second to 1.3 meters per
second), and a range of shear strains (up to several tens of thousands). |
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Junio de 2013
From Sumatra 2004 to Tohoku-Oki 2011: The systematic GPS
detection of the ionospheric signature induced by tsunamigenic
earthquakes
Authors: Giovanni Occhipinti, Lucie Rolland et al
Link: Click here
Abstract
The recent tsunamigenic earthquake in Tohoku (11 March 2011)
strongly affirms, one more time after the Sumatra event (26
December 2004), the necessity to open new paradigms in oceanic
monitoring. Detection of ionospheric anomalies following the
Sumatra tsunami demonstrated that ionosphere is sensitive to
the tsunami propagation. Observations supported by modeling
proved that tsunamigenic ionospheric anomalies are deterministic
and reproducible by numerical modeling via the ocean/neutral-atmosphere/ionosphere
coupling mechanism. In essence, tsunami induces internal gravity
waves propagating within the neutral atmosphere and detectable
in the ionosphere. Most of the ionospheric anomalies produced
by tsunamis were observed in the far field where the tsunami
signature in the ionosphere is clearly identifiable.
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In
this work, we highlight the early signature in the ionosphere
produced by tsunamigenic earthquakes and observed by GPS, measuring
the total electron content, close to the epicenter. We focus on
the first hour after the seismic rupture. We demonstrate that
acoustic-gravity waves generated at the epicenter by the direct
vertical displacement of the source rupture and the gravity wave
coupled with the tsunami can be discriminated with theoretical
support. We illustrate the systematic nature of those perturbations
showing several observations: nominally the ionospheric perturbation
following the tsunamigenic earthquakes in Sumatra on 26 December
2004 and 12 September 2007; in Chile on 14 November 2007; in Samoa
on 29 September 2009; and the recent catastrophic Tohoku-Oki event
on 11 March 2011. Based on the analytical description, we provide
tracks for further modeling efforts and clues for the interpretation
of complex-and thus often misleading-observations. The routine
detection of the early ionospheric anomalies following the rupture
highlights the role of ionospheric sounding in the future ocean
monitoring and tsunami detection. |
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Junio de 2013
Rapid modeling of the 2011 Mw 9.0 Tohoku-oki earthquake with
seismogeodesy
Authors: Diego Melgar, Brendan W. Crowell et al
Link: Click here
Abstract
Rapid characterization of finite fault geometry and slip for
large earthquakes is important for mitigation of seismic and
tsunamigenic hazards. Saturation of near-source weak motion
and problematic integration of strong-motion data into displacements
make this difficult in real time. Combining GPS and accelerometer
data to estimate seismogeodetic displacement
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waveforms overcomes these limitations
by providing mm-level three-dimensional accuracy and improved
estimation of coseismic deformation compared to GPS-only methods.
We leverage collocated GPS and accelerometer data from the 2011
Mw 9.0 Tohoku-oki, Japan earthquake by replaying them in simulated
real-time mode. Using a novel approach to account for fault finiteness,
we generate an accurate centroid moment tensor solution independently
of any constraint on the slab geometry followed by a finite fault
slip model. The replay of GPS and seismic data demonstrates that
robust models could have been made available within 3 min of earthquake
initiation. |
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Junio de 2013
Coseismic and postseismic slip associated with the 2010 Maule
Earthquake, Chile: Characterizing the Arauco Peninsula barrier
effect
Authors: Yu-nung Nina Lin, Anthony Sladen, Francisco Ortega-Culaciati
et al
Link: Click here
Abstract
Observations of coseismic and postseismic deformation associated
with the 2010 Mw = 8.8 Maule earthquake in south-central Chile
provide constraints on the spatial heterogeneities of frictional
properties on a major subduction megathrust and how they have
influenced the seismic rupture and postseismic effects. We find
that the bulk of coseismic slip occurs within a single elongated
patch approximately 460 km long and 100 km wide between the
depths of 15 and 40 km. We infer three major patches of afterslip:
one extends northward along strike and downdip of the major
coseismic patch between
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40 and 60 km depth; the other two bound the northern and southern
ends of the coseismic patch. The southern patch offshore of the
Arauco Peninsula is the only place showing resolvable afterslip
shallower than 20 km depth. Estimated slip potency associated
with postseismic slip in the 1.3 years following the earthquake
amounts to 20–30% of that generated coseismically. Our estimates
of the megathrust frictional properties show that the Arauco Peninsula
area has positive but relatively low (a-b)?n values (0.01 ~ 0.22
MPa), that would have allowed dynamic rupture propagation into
this rate-strengthening area and afterslip. Given the only modestly
rate-strengthening megathrust friction in this region, the barrier
effect may be attributed to its relatively large size of the rate-strengthening
patch. Coseismic and postseismic uplift of the Arauco Peninsula
exceeds interseismic subsidence since the time of the last major
earthquake in 1835, suggesting that coseismic and postseismic
deformation has resulted in some permanent strain in the forearc. |
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Junio de 2013
Statistical analysis of an ionospheric parameter as a base
for earthquake prediction
Authors: Mei Li and Michel Parrot
Link: Click here
Abstract
This paper is related to the use of ionospheric density variations
to tentatively predict earthquakes. The results of this statistical
analysis are presented as a function of various parameters.
The ion density was recorded by the low-altitude satellite DEMETER
during more than 6 years, and a search for anomalies was automatically
conducted with the complete data set. In a second time, a software
checked if each anomaly could correspond to an earthquake. The
search was conducted at less than 1500 km from the anomaly positions,
and until 15 days after the anomaly time. The earthquakes have
been
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classified depending on their magnitude,
depth, and position (below the sea or inland). This attempt to
predict earthquakes of course generates a lot of false alarms
and wrong detections. Nevertheless, it is shown that the number
of good detections increases with the magnitude of the earthquakes.
In average the number of perturbations is higher the day of the
earthquake, and then smoothly decreases for the days before. Earthquakes
below the sea are better detected. There are seismic areas close
to the South Atlantic Magnetic Anomaly and at high latitudes where
the number of natural perturbations is too important to expect
a high number of good detections. Finally, when there are several
perturbations corresponding to a single earthquake, it is possible
to combine their positions to have a better estimation of the
location of the future epicenter. However, uncertainties about
the time and the magnitude are large. |
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Junio de 2013
Discovery and analysis of topographic features using learning
algorithms: A seamount case study
Authors: Andrew P. Valentine Lara M. Kalnins et al
Link: Click here
Abstract
Identifying and cataloging occurrences of particular topographic
features are important but time-consuming tasks. Typically,
automation is challenging, as simple models do not fully
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describe the complexities of natural
features. We propose a new approach, where a particular class
of neural network (the “autoencoder”) is used to assimilate
the characteristics of the feature to be cataloged, and then applied
to a systematic search for new examples. To demonstrate the feasibility
of this method, we construct a network that may be used to find
seamounts in global bathymetric data. We show results for two
test regions, which compare favorably with results from traditional
algorithms. |
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Junio de 2013
Recurrence statistics of great earthquakes
Authors: E. Ben-Naim, E. G. Daub A. Johnson et al
Link: Click here
Abstract
We investigate the sequence of great earthquakes over the past
century. To examine whether the earthquake record includes temporal
clustering, we identify aftershocks and remove those from the
record. We focus on the recurrence time, defined as the time
between two consecutive earthquakes. We study the variance in
the recurrence time and the maximal recurrence time. Using these
quantities, we compare the
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earthquake record with sequences
of random events, generated by numerical simulations, while systematically
varying the minimal earthquake magnitude Mmin. Our analysis shows
that the earthquake record is consistent with a random process
for magnitude thresholds 7.0?Mmin?8.3, where the number of events
is larger. Interestingly, the earthquake record deviates from
a random process at magnitude threshold 8.4?Mmin?8.5, where the
number of events is smaller; however, this deviation is not strong
enough to conclude that great earthquakes are clustered. Overall,
the findings are robust both qualitatively and quantitatively
as statistics of extreme values and moment analysis yield remarkably
similar results. |
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Junio de 2013
Implications of high core thermal conductivity on Earth's
coupled mantle and core evolution
Authors: Takashi Nakagawa and Paul J. Tackley
Link: Click here
Abstract
We assess the effect of high thermal conductivity of Earth's
core, which was recently determined to be 2-3 times higher than
previously thought, on Earth's thermochemical-magnetic evolution
using a coupled model of simulated mantle convection and parameterized
core heat balance, following the best fit case of Nakagawa and
Tackley (2010).
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The
value of core thermal conductivity has no effect on mantle evolution.
The core-mantle boundary heat flow starts high and decreases with
time to ~13?TW, which is below the core adiabatic heat flux for
the largest thermal conductivity tested (200?W/m/K), meaning that
a purely thermal dynamo is not viable. However, gravitational
energy release and latent heat associated with inner core growth
become important in the last ~0.9?Gyr and allow continuous geodynamo
generation despite high core thermal conductivity, although we
estimate a subadiabatic region at the top of the core of the order
of hundreds of kilometers. |
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Junio de 2013
Precise relative earthquake location using surface waves
Authors: K. Michael Cleveland and Charles J. Ammon
Link: Click here
Abstract
Earthquake locations provide a fundamental tool for seismological
investigations. While dense seismic networks can provide robust
locations, accuracy and precision of these locations suffer
outside dense networks. This is particularly true in offshore
areas, where location analysis relies heavily on distant seismic
observations. We present a method for estimating precise relative
seismic source epicentroid locations using surface waves. Several
reasons, including lower velocities and strength of the signal
at distance, make use of surface waves for event location appealing.
We focus on the Panama Fracture Zone region and relocate 81
strike-slip earthquakes to produce tectonically consistent
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epicentroid locations. The resulting
pattern of earthquakes more clearly delineates recently active
regional structures than original body-wave locations. The mean
shift between the US Geological Survey National Earthquake Information
Center epicenter and our epicentroids is about 14?km (the median
is about 11?km), and typical origin time changes are generally
less than ±2?s. We find that north of 6.5°N, the plate
boundary motion is split across two roughly north-south striking
structures, the Panama and Balboa Fracture zones. For the last
36?years, slip along these two structures roughly matches slip
along the Panama Fracture Zone to the south (from 4.5°N to
6.25°N), but the Balboa Fracture zone has roughly three times
the moment than the northern Panama Fracture Zone. Our analyses
show that observed Rayleigh-wave signal-to-noise ratios for moderate-to-large
shallow earthquakes are suitable for applying the procedure and
that Rayleigh-wave observations form a self-consistent set of
constraints on the relative location of earthquake centroids. |
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Junio de 2013
Band-limited Bouguer gravity identifies new basins on the
Moon
Authors: W. E. Featherstone, C. Hirt and M. Kuhn
Link: Click here
Abstract
Spectral-domain forward modeling is used to generate topography-implied
gravity for the Moon using data from the LOLA instrument operated
on-board the LRO mission. This is subtracted from SELENE-derived
gravity to generate band-limited Bouguer gravity maps of the
Moon so as to enhance the gravitational signatures of
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anomalous
mass-densities nearer the surface. This procedure adds evidence
that two previously postulated basins on the lunar far-side, Fitzgerald-Jackson
(25°N, 191°E) and to the east of Debye (50°N, 180°E),
are indeed real. When applied over the entire lunar surface, band-limited
Bouguer gravity reveals the locations of 280 candidate basins
that have not been identified when using full-spectrum gravity
or topography alone, showing the approach to be of utility. Of
the 280 basins, 66 are classified as distinct from their band-limited
Bouguer gravity and topographic signatures, making them worthy
of further investigation. |
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Junio de 2013
The 2004 Aceh-Andaman Earthquake: early clay dehydration
controls shallow seismic rupture
Authors: Jacob Geersen, Lisa McNeill et al
Link: Click here
Abstract
The physical state of the shallow plate-boundary fault governs
the updip extent of seismic rupture during powerful subduction
zone earthquakes and thus on a first order impacts on the tsunamigenic
hazard of such events. During the 2004 Mw 9.2 Aceh-Andaman Earthquake
seismic rupture extended unusually far seaward below the accretionary
prism causing the disastrous Indian Ocean Tsunami. Here we show
that the formation of a strong bulk sediment section and a high
fluid-pressured predécollement, that likely enabled the
2004 rupture to reach the shallow plate-boundary, result from
thermally controlled diagenetic processes in the upper oceanic
basement and overlying sediments. Thickening of the sediment
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section to >2 km ~160 km seaward
of the subduction zone increases temperatures at the sediment
basement interface and triggers mineral transformation and dehydration
(e.g. smectite-illite) prior to subduction. The liberated fluids
migrate into a layer that likely host high porosity and permeability
and that is unique to the 2004 rupture area where they generate
a distinct overpressured predécollement. Clay mineral transformation
further supports processes of semi-lithification, induration of
sediments, and coupled with compaction dewatering all amplified
by the thick sediment section together strengthens the bulk sediments.
Farther south, where the 2005 Sumatra Earthquake did not include
similar shallow rupture, sediment thickness on the oceanic plate
is significantly smaller. Therefore, similar diagenetic processes
occur later and deeper in the subduction zone. Hence we propose
that shallow seismic rupture during the 2004 earthquake is primarily
controlled by the thickness and composition of oceanic plate sediments. |
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Junio de 2013
Diapiric ascent of silicic magma beneath the Bolivian Altiplano
Authors: Rodrigo del Potro, Mikel Díez et al
Link: Click here
Abstract
The vertical transport of large volumes of silicic magma, which
drives volcanic eruptions and the long-term compositional evolution
of the continental crust, is a highly debated problem. In recent
years, dyking has been favored as the main ascent mechanism,
but the structural connection between a distributed configuration
of melt-filled
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pores
in the source region and shallow magma reservoirs remains unsolved.
In the Central Andes, inversion of a new high-resolution Bouguer
anomaly data over the Altiplano-Puna Magma Body (APMB) reveals
~15 km wide, vertically elongated, low-density, 3D structures
rooted at the top of the APMB at 20 km depth. We integrate our
gravity inversion with the available geophysical, geological,
and petrological observations, and in agreement with petrological/mechanical
considerations propose that, in this region of the Andes, partially
molten granitic bodies ascend diapirically through the hot ductile
mid-upper crust. |
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Mayo de 2013
An Interactive Web Application for Visualizing Climate Data
Authors: J. Alder, S. Hostetler et al
Link: Click here
Abstract
Massive volumes of data are being created as modeling centers
from around the world finalize
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their submission of climate simulations
for the Coupled Model Intercomparison Project, phase 5 (CMIP5),
in preparation for the forthcoming Intergovernmental Panel on
Climate Change (IPCC) Fifth Assessment Report (AR5). Scientists,
resource managers, and other potential users of climate data are
faced with the daunting task of analyzing, distilling, and summarizing
this unprecedented wealth of climate informati |
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Mayo de 2013
Did the Paleo-Asian Ocean between North China Block and Mongolia
Block exist during the late Paleozoic? First paleomagnetic evidence
from central-eastern Inner Mongolia, China
Authors: Pan Zhao, Yan Chen et al
Link: Click here
Abstract
The tectonic evolution of the Paleo-Asian Ocean between the
North China Block (NCB) and the Mongolia Block (MOB) is a contentious
issue, and geodynamic models remain speculative. In an effort
to puzzle out this controversy, a paleomagnetic study was carried
out on the Silurian to Permian formations in central-eastern
Inner Mongolia (China). More than 680 sedimentary and volcanic
samples were collected from 86 sites. We have established titanium-poor
magnetite and hematite as the principal magnetic carriers. Anisotropy
of the magnetic susceptibility measurements demonstrate negligible
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deformation
of the majority of study rocks with sedimentary fabrics. From
primary magnetizations, a Late Devonian and a Permian pole are
calculated for Inner Mongolia Block (IMB) at ??=?46.8°N, ??=?349.1°E,
dp?=?14.6°, dm?=?27.3° with N?=?3 and ??=?48.7°N,
??=?3.7°E, dp?=?5.2°, dm?=?9.1° with N?=?6, respectively.
Two stages of secondary magnetization are also identified probably
due to Early Permian and Early Cretaceous magmatic events. As
preliminary results, the comparison of our new paleomagnetic poles
with available data from NCB, MOB, and Siberia indicates that
(1) the paleolatitudes of IMB, NCB, and MOB are consistent between
Late Devonian and Permian, suggesting pre-Late Devonian closure
of the Paleo-Asian Ocean and further evaluation of these three
blocks as a single entity and (2) post-Permian intracontinental
deformation was significant and characterized by block rotations,
which are due to strike-slip faulting within the welded NCB-IMB-MOB
block. |
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Mayo de 2013
Montserrat geothermal system: A 3D conceptual model
Authors: G. A. Ryan, J. R. Peacock et al
Link: Click here
Abstract
We constructed a three-dimensional conceptual model of a geothermal
system on the Caribbean island of Montserrat. The model was
generated using magnetotelluric resistivity data, earthquake
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hypocenter data, and a three-dimensional
P wave velocity model, all plotted using a shared geographical
reference. The results of the study suggest a high-temperature
fracture-controlled geothermal system at the intersection of two
faults in the SW of the island. We also present a "prospectivity
index" map that represents a proxy of the spatial variation
in harvestable heat flux at 1500 m depth. The index is the product
of relative permeability around modeled faults and a proxy for
the subsurface temperature calculated using P wave velocity anomalies. |
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Mayo de 2013
Separation and location of microseism sources
Authors: Aishwarya Moni, David Craig et al
Link: Click here
Abstract
Microseisms are ground vibrations caused largely by ocean gravity
waves. Multiple spatially separate noise sources may be coincidentally
active. A method for source separation and individual wavefield
retrieval of microseisms using a single
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pair
of seismic stations is introduced, and a method of back-azimuth
estimation assuming Rayleigh wave arrivals of microseisms is described.
These methods are combined to separate and locate sources of microseisms
in a synthetic model, and then applied to field microseismic recordings
from Ireland in the North-East Atlantic. It is shown that source
separation is an important step prior to location for both accurate
microseism locations and microseisms wavefield studies. |
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Mayo de 2013
Ionospheric response to earthquakes of different magnitudes:
Larger quakes perturb the ionosphere stronger and longer
Authors: E. Astafyeva, S. Shalimov et al
Link: Click here
Abstract
Recently, it has been shown that the ionosphere is capable of
showing images of seismic fault shortly after an earthquake.
This gives rise to the idea of retrieval of seismic information
from
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ionospheric observations. As the
first step toward such inversion, here we study distinctive features
of ionospheric response to shallow earthquakes, both submarine
and inland, of moment magnitudes Mw7.2-9.1. Using GPS measurements
of the ionospheric total electron content, we show that: (1) the
amplitude of coseismic total electron content variations in the
near-field is larger after more powerful earthquakes, and (2)
stronger earthquakes (M>7.9) are in general characterized by
a longer negative phase in coseismic perturbations. |
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Mayo de 2013
Magnetodynamo lifetimes for rocky, Earth-mass exoplanets
with contrasting mantle convection regimes
Authors: Joost van Summeren, Eric Gaidos et al
Link: Click here
Abstract
We used a thermal model of an iron core to calculate magnetodynamo
evolution in Earth-mass rocky planets to determine the sensitivity
of dynamo lifetime and intensity to planets with different mantle
tectonic regimes, surface temperatures, and core properties.
The heat flow at the core-mantle boundary (CMB) is derived from
numerical models of mantle convection with a viscous/pseudoplastic
rheology that captures the phenomenology of plate-like tectonics.
Our thermal evolution models predict a long-lived
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(?8?Gyr)
field for Earth and similar dynamo evolution for Earth-mass exoplanets
with plate tectonics. Both elevated surface temperature and pressure-dependent
mantle viscosity reduce the CMB heat flow but produce only slightly
longer-lived dynamos (?8-9.5?Gyr). Single-plate ("stagnant
lid") planets with relatively low CMB heat flow produce long-lived
(?10.5?Gyr) dynamos. These weaker dynamos can cease for several
billions of years and subsequently reactivate due to the additional
entropy production associated with inner core growth, a possible
explanation for the absence of a magnetic field on present-day
Venus. We also show that dynamo operation is sensitive to the
initial temperature, size, and solidus of a planet's core. These
dependencies would severely challenge any attempt to distinguish
exoplanets with plate tectonics and stagnant lids based on the
presence or absence of a magnetic field. |
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Mayo de 2013
New technique could boost researchers' view of Earth's core
Author: Colin Schultz
Link: Click here
Abstract
Studies of the Earth's core rely exclusively on remote detection
because direct access is blocked by temperature, pressure, and
thousands of kilometers of solid rock. Measurements of the magnetic
field or of seismic waves provide
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researchers with a glimpse into the
core's properties. Traditionally, to get a good look at the core,
researchers watch for earthquake-generated seismic body waves
that travel through the Earth-either passing through or reflecting
off the core-that are then picked up by a seismic wave detection
array on the other side of the planet. The requirement that stations
be properly aligned in comparison to the earthquake is sufficiently
strict that observations are often more opportunistic than many
scientists would prefer. |
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Mayo de 2013
Recent Voyager 1 data indicate that on 25 August 2012 at
a distance of 121.7 AU from the Sun, sudden and unprecedented
intensity changes were observed in anomalous and galactic cosmic
rays
Authors: W. R. Webber, F. B. McDonald et al
Link: Click here
Abstract
At the Voyager 1 (V1) spacecraft in the outer heliosphere, the
intensities of both anomalous cosmic rays and galactic cosmic
rays (GCRs) changed suddenly and decisively on 25 August (121.7
AU from the Sun). Within a matter of a few
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days,
the intensity of 1.9-2.7 MeV protons and helium nuclei had decreased
to less than 0.1 of their previous value, and eventually the intensities
decreased by factors of at least 300-500. Also, on 25 August,
the GCR protons, helium, and electrons increased suddenly in just
2 or 3 days by a factor of up to 2. The intensities of the GCR
nuclei of all energies from 2 to 400 MeV then remained essentially
constant with intensity levels and spectra that may represent
the local GCR. The suddenness of these intensity changes indicates
that V1 has crossed a well-defined boundary for energetic particles
at this time possibly related to the heliopause. |
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Mayo de 2013
The curious case of Mercury's internal structure
Authors: Steven A. Hauck II, Jean-Luc Margot et al
Link: Click here
Abstract
The recent determination of the gravity field of Mercury and
new Earth-based radar observations of the planet's spin state
afford the opportunity to explore Mercury's internal structure.
These observations provide estimates of two measures of the
radial mass distribution of Mercury: the normalized polar moment
of inertia and the fractional polar moment of inertia of the
solid portion of the planet overlying the liquid core. Employing
Monte Carlo techniques we calculate several million models of
the radial density structure of Mercury consistent with its
radius and bulk density and constrained by these moment of
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inertia parameters. We estimate that
the top of the liquid core is at a radius of 2020?±?30?km,
the mean density above this boundary is 3380?±?200?kg?m-3,
and the density below the boundary is 6980?±?280?kg?m-3.
We find that these internal structure parameters are robust across
a broad range of compositional models for the core and planet
as a whole. Geochemical observations of Mercury's surface by MESSENGER
indicate a chemically reducing environment that would favor the
partitioning of silicon or both silicon and sulfur into the metallic
core during core-mantle differentiation. For a core composed of
Fe-S-Si materials, the thermodynamic properties at elevated pressures
and temperatures suggest that an FeS-rich layer could form at
the top of the core and that a portion of it may be presently
solid. |
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Mayo de 2013
Probabilistic modeling of future volcanic eruptions at Mount
Etna
Authors: Annalisa Cappello, Giuseppe Bilotta et al
Link: Click here
Abstract
The statistical analysis of volcanic activity at Mt Etna was
conducted with the twofold aim of (1) constructing a probability
map for vent opening of future flank eruptions and (2) forecasting
the expected number of eruptive events at the summit craters.
The spatiotemporal map of new vent opening at Etna volcano is
based on the analysis of spatial locations and frequency of
flank eruptions starting from 1610. Thanks to the completeness
and accuracy of historical data over the last four centuries,
we examined in detail the spatial and temporal distribution
of flank eruptions
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showing
that effusive events follow a nonhomogenous Poisson process with
space-time varying intensities. After demonstrating the spatial
nonhomogeneity and the temporal nonstationarity of flank eruptions
at Etna, we calculated the recurrence rates (events expected per
unit area per unit time) and produced different spatiotemporal
probability maps of new vent opening in the next 1, 10 and 50?years.
These probabilistic maps have an immediate use in evaluating the
future timing and areas of Etna prone to volcanic hazards. Finally,
the results of the analysis of the persistent summit activity
during the last 110?years indicate that the hazard rate for eruptive
events is not constant with time, differs for each summit crater
of Mt Etna, highlighting a general increase in the eruptive frequency
starting from the middle of last century and particularly from
1971, when the SE crater was formed. |
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Mayo de 2013
Evolution of the Qin Mountains as part of the supercontinent
Rodinia
Author: Ernie Balcerak
Link: Click here
Abstract
The Qinling-Dabie orogenic complex, part of a large east-west
mountain range in the heart of China, plays a key role in helping
scientists understand the formation and breakup of the supercontinent
Rodinia, but the exact configuration and geodynamic history
of the Qinling-Dabie orogenic complex and the
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surrounding region are not fully
known. Contributing a piece to the puzzle, Bader et al. use uranium-thorium-lead
geochronology-both from published studies and new data-to investigate
the Neoproterozoic (1 billion years ago to 700 million years ago)
evolution of the Qinling-Dabie orogenic collage. The authors outline
a tectonic model for evolution of this orogen, placing it into
the context of the evolution of Rodinia, which formed around 1
billion years ago and broke up around 700 million years ago. Their
synthesis could help researchers gain a more complete understanding
of the geologic history of one of Earth's supercontinents. |
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Mayo de 2013
A decade of horizontal deformation from great earthquakes
Authors: P. Tregoning, R. Burgette et al
Link: Click here
Abstract
The 21st Century has seen the occurrence of 17 great earthquakes
(Mw >8), including some of the largest earthquakes ever recorded.
Numerical modeling of the earthquakes shows that nearly half
of the Earth's surface has undergone horizontal coseismic deformation
>1 mm, with the 2004 Sumatra-Andaman earthquake dominating
the global deformation field. This has important implications
for both the realization of a terrestrial reference frame and
in the interpretation of regional tectonic studies based on
GPS velocities. We show that far-field coseismic deformations
from great earthquakes will, if unaccounted for, introduce errors
in estimates of linear site velocities of at least 0.1-0.3 mm/yr
across most of
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the
surface of the Earth. The accumulated global deformation field
shows that two regions, Australia and the north Atlantic/Arctic
Ocean, have been largely undeformed by these great earthquakes,
with accumulated deformations generally <0.5 mm. Using GPS
estimates of surface deformation, we show that the majority of
the Australian continent is deforming at <0.2 mm/yr, the northern
part of New Zealand is rotating clockwise relative to the Australian
Plate with relative horizontal velocities of ?2 mm/yr, while the
southeastern coast of Australia is undergoing post-seismic relaxation
caused by the 2004 Mw = 8.1 Macquarie Ridge earthquake. The presence
of ongoing post-seismic relaxation thousands of kilometers from
plate margins violates the secular/linear assumption made in current
terrestrial reference frame definitions. These effects have significant
ramifications for regional tectonic interpretations and global
studies such as sea level rise that require reference frame accuracy
greater than this level. |
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Mayo de 2013
Ground motion prediction of realistic earthquake sources
using the ambient seismic field
Authors: M. A. Denolle, E. M. Dunham et al
Link: Click here
Abstract
Predicting accurate ground motion is critical for earthquake
hazard analysis, particularly in situations where sedimentary
basins trap and amplify seismic waves. We exploit the information
carried by the ambient seismic field to extract surface?wave
Green's functions between seismic stations and to predict long?period
ground motion from earthquakes. To do so, we modify the surface
impulse response to correct for the
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source depth and for the double?couple
focal mechanism. These corrections are derived under the assumption
that material properties in the immediate vicinity of the source
depend only on depth. Using this local 1?D assumption, we solve
the surface?wave eigenproblem and compute the fundamental?mode
displacement eigenfunctions to express the surface?wave excitation
at the source. We validate this technique, which we call the virtual
earthquake approach, by comparing computed seismograms with earthquake
waveforms from four moderate earthquakes that occur near broadband
stations in southern California. The depth and mechanism corrections
show clear improvements of the predicted ground motion relative
to the surface impulse response. |
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Abril de 2013
High-resolution bathymetry reveals contrasting landslide
activity shaping the walls of the Mid-Atlantic Ridge axial valley
Authors: Mathilde Cannat, Anne Mangeney et al
Link: Click here
Abstract
Axial valleys are found along most slow-spreading mid-ocean
ridges and are one of the most prominent topographic features
on Earth. In this paper, we present the first deep-tow swath
bathymetry for the axial valley walls of the Mid-Atlantic Ridge.
These data allow us to analyze axial valley wall morphology
with a very high resolution (0.5 to 1 m compared to ? 50 m for
shipboard multibeam bathymetry), revealing the role played by
landslides. Slow-spreading ridge axial valleys also commonly
expose mantle-derived serpentinized peridotites in the footwalls
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of
large offset normal faults (detachments). In our map of the Ashadze
area (lat. 13°N), ultramafic outcrops have an average slope
of 18° and behave as sliding deformable rock masses, with
little fragmentation. By contrast, the basaltic seafloor in the
Krasnov area (lat. 16°38?N) has an average slope of 32°
and the erosion of the steep basaltic rock faces leads to extensive
fragmentation, forming debris with morphologies consistent with
noncohesive granular flow. Comparison with laboratory experiments
suggests that the repose angle for this basaltic debris is >
25°. We discuss the interplay between the normal faults that
bound the axial valley and the observed mass wasting processes.
We propose that, along axial valley walls where serpentinized
peridotites are exposed by detachment faults, mass wasting results
in average slopes ? 20°, even in places where the emergence
angle of the detachment is larger. |
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Abril de 2013
Evidence for fluid-triggered slip in the 2009 Mount Rainier,
Washington earthquake swarm
Authors: David R. Shelly, Seth C. Moran et al
Link: Click here
Abstract
A vigorous swarm of over 1000 small, shallow earthquakes occurred
20-22 September 2009 beneath Mount Rainier, Washington, including
the largest number of events ever recorded in a single day at
Rainier since seismic stations were installed on the edifice
in 1989. Many events were only clearly recorded on one or two
stations on the edifice, or they overlapped in time with other
events, and thus only ~200 were locatable by manual phase picking.
To partially overcome this limitation, we applied waveform-based
event detection integrated with precise double-difference relative
relocation. With this procedure, detection and location goals
are accomplished in
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tandem, using cross-correlation with
continuous seismic data and waveform templates constructed from
cataloged events. As a result, we obtained precise locations for
726 events, an improvement of almost a factor of 4. These event
locations define a ~850?m long nearly vertical structure striking
NNE, with episodic migration outward from the initial hypocenters.
The activity front propagates in a manner consistent with a diffusional
process. Double-couple-constrained focal mechanisms suggest dominantly
near-vertical strike-slip motion on either NNW or ENE striking
faults, more than 30° different than the strike of the event
locations. This suggests the possibility of en echelon faulting,
perhaps with a component of fault opening in a fracture-mesh-type
geometry. We hypothesize that the swarm was initiated by a sudden
release of high-pressure fluid into preexisting fractures, with
subsequent activity triggered by diffusing fluid pressure in combination
with stress transfer from the preceding events. |
|
Abril de 2013
Is an ionospheric electron enhancement preceding the 2011
Tohoku-Oki earthquake a precursor?
Authors:Masashi Kamogawa and Yoshihiro Kakinami
Link: Click here
Abstract
Heki reported that the TEC gradually enhanced from 40?minutes
before the 2011 M9.0 off the Pacific coast of Tohoku earthquake
(Tohoku EQ) to the time when the co-seismic acoustic wave reached
the ionosphere and the TEC immediately recovered at the normal
state. This paper shows an alternative interpretation of total
electron content (TEC) variation in the ionosphere
|
associated
with the Tohoku EQ. Our interpretation is that a tsunamigenic
ionospheric hole, a wide depletion of the TEC, occurred after
the co-seismic acoustic wave reached the ionosphere and gradually
recovered at the normal state with several tens of minutes [Kakinami
et al., ]. The difference between Heki [] and Kakinami et al.
[] is attributed to the reference curves of the TEC to extract
the ionospheric variations. The former is given by the least-squares
fitting curve of the EQ day data excluding an expected precursor
period, while the latter is given by the data of the similar orbit
of global positioning system (GPS) satellite on another day. The
results suggest that variation of slant TEC is explained by the
depletion of TEC due to tsunami rather than the precursory enhancement. |
|
Abril de 2013
GEOPHYSICAL CHARACTERIZATION OF THE CHICXULUB IMPACT CRATER
Authors: S.P.S. Gulick, G.L. Christeson et al
Link: Click here
Abstract
Geophysical data indicate that the 65.5?million years ago Chicxulub
impact structure is a multi-ring basin, with three sets of semicontinuous,
arcuate ring faults and a topographic peak ring (PR). Slump
blocks define a terrace zone, which steps down from the inner
rim into the annular trough. Fault blocks underlie the PR, which
exhibits variable relief, due to target asymmetries. The central
structural uplift is >10?km, and the Moho is displaced by
1-2?km. The working hypothesis for the formation of Chicxulub
is: a 50?km radius transient cavity, lined with melt and impact
breccia, formed within 10?s of the impact,
|
and within minutes, weakened rebounding
crust rose kilometers above the surface, the transient crater
rim underwent localized deformation and collapsed into large slump
blocks, resulting in a inner rim at 70-85?km radius, and outer
ring faults at 70-130?km radius. The overheightened structural
uplift collapsed outward, buried the inner slump blocks, and formed
the PR. Most of the impact melt was ultimately emplaced as a coherent
<3 km thick melt sheet within the central basin that shallows
within the inner regions of the PR. Smaller pockets of melt flowed
into the annular trough. Subsequently, slope collapse, ejecta,
ground surge, and tsunami waves infilled the annular trough and
annular basin with sediments up to 3?km and 900?m thick, respectively.
Testing this working hypothesis requires direct observation of
the impactites, within and adjacent to the PR and central basin. |
|
Abril de 2013
Permanent fore-arc extension and seismic segmentation: Insights
from the 2010 Maule earthquake, Chile
Authors: Felipe Aron, Richard W. Allmendinger et al
Link: Click here
Abstract
Geologists have long known that young normal faults are an important
structural element of the Andean Coastal Cordillera, but their
relationship to the subduction seismic cycle is still unclear.
Some of the largest aftershocks of the 2010 Mw?8.8 Maule earthquake
in central Chile were nucleated on upper plate normal faults,
including the Mw? 6.9 and 7.0 events of the Pichilemu earthquake
sequence. We use the available coseismic GPS displacements,
moment tensor sums, and slip distribution models for the Maule
earthquake to compute the static strain and stress fields imposed
on the upper plate by slip on the subduction interface. The
extensional strains
|
calculated
from coseismic GPS and from a moment tensor sum of the Pichilemu
events have similar orientations and orders of magnitude. The
normal Coulomb stress increment (CSI) on the Pichilemu fault has
maximum positive stresses as high as 4.9?MPa. Regionally, the
Maule event produced a semi-elliptical, radial pattern of static
extension and deviatoric tension (CSI?>?1.5?MPa) along the
Coastal Cordillera enclosing the rupture area. This elliptical
pattern mimics the trends of the major upper-crustal structures.
The static deformation field produced by a great subduction earthquake
is an effective mechanism for generating permanent extension above
the seismogenic zone, reactivating suitably oriented, long-lived
normal faults. We suggest that the semi-elliptical outline of
the first-order structures along the Coastal Cordillera may define
the location of a characteristic, long-lived megathrust segment.
This observation implies a persistence at least over the Quaternary
of great subduction ruptures along the Maule segment. |
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Abril de 2013
The large-scale surface uplift in the Altiplano-Puna region
of Bolivia: A parametric study of source characteristics and
crustal rheology using finite element analysis
Authors: James Hickey, Joachim Gottsmann et al
Link: Click here
Abstract
This paper focuses on the driving mechanism behind a 70?km wide
region of ground uplift centered on Uturuncu volcano, in the
Altiplano-Puna region of southern Bolivia. We present a series
of forward models using finite element analysis to simultaneously
test for first-order parameters that help constrain a viable
model for the observed maximum line of sight uplift rate of
1-2?cm/yr between 1992 and 2006. Stresses from pressure sources
with finite geometries are solved numerically, accounting for
both homogeneous and heterogeneous mechanical rock properties
in elastic and viscoelastic
|
rheologies. Crustal heterogeneity
is constrained by seismic velocity data that indicate the presence
of a large low-velocity zone, the Altiplano-Puna magma body, at
depths of ~17?km below the surface. A viscoelastic rheology is
employed to account for time-dependent deformation and an inelastic
crust. Comparing homogeneous and heterogeneous models demonstrates
the significant impact of a mechanically weak, source-depth layer,
which alters surface displacement patterns by buffering subsurface
deformation. Elastic model results guide the source parameters
tested in the viscoelastic models and demonstrate a range of possible
causative source geometries. Our preferred model suggests that
pressurization of a magma source extending upward from the Altiplano-Puna
magma body is causing the observed surface uplift and alludes
to a continued increase in this pressure to explain both the spatial
and temporal patterns. We also demonstrate how a pressure-time
function plays a first-order role in explaining the observed temporal
deformation pattern. |
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Abril de 2013
Tidal modulation of continuous nonvolcanic seismic tremor
in the Chile triple junction region
Authors: A. Gallego, R. M. Russo et al
Link: Click here
AbstractWe located continuous seismic
tremor with coherent amplitude wave trains in the Chile ridge
subduction region (~46.5°S) in two clusters north and south
of the Chonos Archipelago, between the Chile trench and the
North Patagonian fore arc. Tremor persisted from December 2004
to February 2007 (the entire period of the Chile Ridge Subduction
Project temporary seismic deployment), and lasted >17?h on
six occasions. Tremor in the more active southern cluster reached
a maximum duration of 48?h, and we observed no more than 3 continuous
days without tremor activity. The cluster locations coincide
with the surface projections of subducted transform faults formed
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at
the Chile ridge. We also detected simultaneous, colocated low-frequency
microearthquakes with well-defined impulsive waves within the
tremor signals distributed from the surface to 40?km depth, suggesting
tremors and earthquakes are part of the same process. The periodicity
of tremor duration is strongly correlated with semidiurnal, diurnal,
and long-period tides, M2, N2, K1, O1, P1, and Mm (12.421 h, 12.000
h, 23.934 h, 25.819 h, 24.066?h, and 27.555?days, respectively).
We found a significant correlation between tremor occurrence and
Earth tides when tidal stress is calculated for the slip plane
of a right-lateral strike-slip fault with strike N95°E, which
is near parallel to subducted transform faults (N78°E) of
the Chile ridge, indicating that the very small stresses resulting
from the combination of ocean loading and solid Earth tides (~1
kPa) are sufficient to facilitate or suppress tremor production;
tremors occur when shear stresses are maximum and wane or are
low when shear stresses are minimum. |
|
Abril de 2013
Toward a formal definition of water scarcity in natural-human
systems
Authors: W. K. Jaeger, A.J. Plantinga et al
Link: Click here
Abstract
Water scarcity may appear to be a simple concept, yet it can
be difficult to define for complex natural-human systems. The
term 'water scarcity'; has been used in a variety of ways, and
it has given rise to a variety of related measurements and indices.
Clarity on such a fundamental concept is needed as the theme
of 'water, sustainability and climate'; is advanced in many
research programs. The purpose of this commentary is to highlight
key aspects of water scarcity that alternative measures such
as aggregate indices do not explicitly recognize. A general
and succinct definition of water scarcity is that it is the
marginal
|
value of a unit of water. We develop
a simple but robust definition of water scarcity and illustrate
it with examples of the many system connections involving biophysical
and socioeconomic factors. We make two main points. First, unlike
the scarcity of many other goods, water scarcity is hugely variable
across location, time, and a multitude of uses that are valued
either directly or indirectly by society. This means that precise
measures of water scarcity will often be elusive in practice,
though this is a reflection of the complex role of water in natural-human
systems, rather than a feature of scarcity per se. Secondly, scarcity
is fundamentally an anthropocentric concept and, thus, is distinguished
from the related notion of water deficit. While such an anthropocentric
perspective may seem limiting, it can encompass the vast range
of interests that society has in water. |
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Abril de 2013
Third-order resonant interaction of atmospheric gravity waves
Authors: Kai Ming Huang, Shao Dong Zhang et al
Link: Click here
Abstract
Relative to the extensive studies of third-order nonlinearity
for oceanic gravity waves, third-order interaction of atmospheric
gravity waves has drawn little attention. By numerical experiments
with a two-dimensional, fully nonlinear model, third-order resonant
excitation of atmospheric gravity waves is clearly exhibited.
The numerical results indicate that third-order interaction
can take place in the atmosphere, and significant energy exchange
occurs in interaction. Wave energy mainly transfers from the
high frequency
|
primary
wave to the excited wave, and an intense secondary wave can strengthen
this energy transfer, which is consistent with the results in
second-order interaction. In the whole process of interaction,
both the wave numbers and frequencies of the interacting waves
are in good agreement with the third-order resonant conditions.
Third-order resonance arises through direct interaction of waves
that satisfy the corresponding resonant conditions, and there
is not a second-order harmonic or an intermediate forced mode
involved in nonlinear interaction. Because gravity waves in the
middle and upper atmosphere generally have rather large amplitudes,
strong third-order interaction may frequently occur. Thus, this
nonlinearity may be regarded as a significant local source for
high frequency gravity waves in the middle and upper atmosphere. |
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Abril de 2013
Mass removal and clay mineral dehydration/rehydration in
carbonate-rich surface exposures of the 2008 Wenchuan Earthquake
fault: Geochemical evidence and implications for fault zone
evolution and coseismic slip
Authors: Jianye Chen, Xiaosong Yang et al
Link: Click here
Abstract
We report variations in the mineralogical, geochemical, and
isotopic (?13C, ?18O) composition of fault rocks sampled in
transects across the Zhaojiagou and Pingxi exposures of the
Wenchuan Earthquake or Longmenshan Fault Zone, where the gouge-rich
fault core and principal slip surface cuts through carbonate-rich
strata. Pervasive fluid infiltration was found to modify the
mineralogical and geochemical architecture of the fault zones
studied. Enrichment/depletion patterns, element partitioning,
and a very large implied volume loss
|
are quite different from those characterizing
faults in granites and clastic sedimentary rocks and can be explained
by a mass removal model involving dissolution and advective transport
enhanced by pressure solution. An increasing enrichment in smectite
observed toward the principal slip surface, a high abundance of
elements such as Ba, Mg, and F, the deposition of minerals such
as barite and fluorapatite, as well as the distinct depletion
in 13C in vein material consistently suggest reactions involving
a hydrothermal fluid originating at depth. Illitization of black
gouges, caused by coseismic frictional heating, was found to be
widespread. We propose that coseismic frictional heating along
with the action of postseismic hydrothermal fluids controlled
the transformation and distribution of smectite and illite within
the fault core of the Longmenshan Fault Zone. The coseismic dewatering
reactions are expected to have been more extensive at depth, possibly
helping generate excess pore pressure assisting dynamic slip weakening
during the Wenchuan Earthquake. |
|
Abril de 2013
Cumulative rate analysis (CURATE): A clustering algorithm
for swarm dominated catalogs
Authors: Katrina M. Jacobs,Euan G. C. Smith et al
Link: Click here
Abstract
We present a new cumulative rate (CURATE) clustering method
to identify earthquake sequences especially in regions with
swarm activity. The method identifies sequences by comparing
observed rates to an average rate. It is distinct from previous
clustering techniques in that no direct assumptions about physical
processes relating to temporal decay or earthquake-earthquake
interaction are made. Instead these assumptions are replaced
by a more general one, that earthquakes occurring within a sequence
likely share a common physical trigger, which is manifested
by a change in rate. The use of rate as the primary selection
parameter emphasizes that temporal proximity is the main commonality
among different sequence types. To investigate
|
catalog-scale
earthquake sequence characteristics, we apply the method along
with four standard (de-)clustering methods to a catalog of 4845
M ? 2.45 earthquakes from 1993 through 2007 in the Central Volcanic
Region of New Zealand. Despite the distinct focus of the method
on sequence formation, the declustered catalog of the CURATE method
sits within the suite of declustered catalogs produced by other
methods. A stochastic reconstruction based on epidemic-type aftershock
sequence parameters is also presented to test the differences
between catalogs that exclusively contain mainshock-aftershock
sequences and areas that exhibit multiple physical processes.
We test the declustered catalogs produced by all methods for a
Poisson temporal distribution and propose that this be used to
ensure reasonable selection parameters. The CURATE method will
be especially useful for identifying swarms, creating likelihoods
of the size and duration of sequences, and refining earthquake
forecasts that include swarms at regional and local scales. |
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Abril de 2013
Geodetic slip rates in the southern San Andreas Fault system:
Effects of elastic heterogeneity and fault geometry
Authors: E. O. Lindsey and Y. Fialko
Link: Click here
Abstract
We use high resolution interferometric synthetic aperture radar
and GPS measurements of crustal motion across the southern San
Andreas Fault system to investigate the effects of elastic heterogeneity
and fault geometry on inferred slip rates and locking depths.
Geodetically measured strain rates are asymmetric with respect
to the mapped traces of both the southern San Andreas and San
Jacinto faults. Two possibilities have been proposed to explain
this observation: large contrasts in crustal rigidity across
the faults, or an alternate fault geometry such as a dipping
San Andreas fault or a blind segment of the San
|
Jacinto Fault. We evaluate these
possibilities using a two-dimensional elastic model accounting
for heterogeneous structure computed from the Southern California
Earthquake Center crustal velocity model CVM-H 6.3. The results
demonstrate that moderate variations in elastic properties of
the crust do not produce a significant strain rate asymmetry and
have only a minor effect on the inferred slip rates. However,
we find that small changes in the location of faults at depth
can strongly impact the results. Our preferred model includes
a San Andreas Fault dipping northeast at 60°, and two active
branches of the San Jacinto fault zone. In this case, we infer
nearly equal slip rates of 18 ± 1 and 19 ± 2?mm/yr
for the San Andreas and San Jacinto fault zones, respectively.
These values are in good agreement with geologic measurements
representing average slip rates over the last 104-106?years, implying
steady long-term motion on these faults. |
|
Abril de 2013
Probabilistic modelling of future volcanic eruptions at Mount
Etna
Authors: Annalisa Cappello, Giuseppe Bilotta et al
Link: Click here
Abstract
The statistical analysis of volcanic activity at Mt Etna was
conducted with the twofold aim of (i) constructing a probability
map for vent opening of future flank eruptions and (ii) forecasting
the expected number of eruptive events at the summit craters.
The spatiotemporal map of new vent opening at Etna volcano is
based on the analysis of spatial locations and frequency of
flank eruptions starting from 1610. Thanks to the completeness
and accuracy of historical data over the last four centuries,
we examined in detail the spatial and temporal distribution
of flank eruptions showing that effusive events follow a non-
|
homogenous
Poisson process with space-time varying intensities. After demonstrating
the spatial non-homogeneity and the temporal non- stationarity
of flank eruptions at Etna, we calculated the recurrence rates
(events expected per unit area per unit time) and produced different
spatiotemporal probability maps of new vent opening in the next
1, 10 and 50?years. These probabilistic maps have an immediate
use in evaluating the future timing and areas of Etna prone to
volcanic hazards. Finally, the results of the analysis of the
persistent summit activity during the last 110?years indicate
that the hazard rate for eruptive events is not constant with
time, differs for each summit crater of Mt Etna, highlighting
a general increase in the eruptive frequency starting from the
middle of last century and particularly from 1971, when the SE
Crater was formed. |
|
Abril de 2013
Damage and seismic velocity structure of pulverized rocks
near the San Andreas Fault
Authors: Marieke Rempe, Thomas Mitchell et al
Link: Click here
Abstract
A combination of seismic refraction tomography, laboratory ultrasonic
velocity measurements and microstructural observations was used
to study the shallow velocity structure of a strand of the San
Andreas fault (SAF) just south of Littlerock, California. The
examined site has a strongly asymmetric damage structure with
respect to the SAF core. The conglomerates to the southwest
show little to no damage, whereas a ~100?m wide damage zone
exists to the northeast with a ~50?m wide zone of pulverized
granite adjacent to the fault core. Seismic P-wave velocities
of the damaged and pulverized granite were investigated over
a range of scales. In-situ seismic velocity imaging was performed
on three overlapping profiles normal to the SAF with lengths
of 350?m, 50?m and 25?m. In the laboratory, ultrasonic velocities
were measured
|
on centimeter- to decimeter-sized
samples taken along the in-situ profiles. The samples were also
investigated microstructurally. Micro-scale fracture damage intensifies
with increasing proximity to the fault core, allowing a sub-division
of the damage zone into several sections. Laboratory-derived velocities
in each section display varying degrees of anisotropy, and combined
with microfracture analysis suggest an evolving damage fabric.
Pulverized rocks close to the fault exhibit a preferred fault-parallel
orientation of microfractures, resulting in the lowest P-wave
velocity orientated in fault-perpendicular direction. Closest
to the fault, pulverized rocks exhibit a gouge-like fabric that
is transitional to the fault core. Comparison of absolute velocities
shows a scaling effect from field to laboratory for the intact
rocks. A similar scaling effect is absent for the pulverized rocks,
suggesting that they are dominated by micro-scale damage. Fault-parallel
damage fabrics are consistent with existing models for pulverized-rock
generation that predict strong dynamic reductions in fault-normal
stress. Our observations provide important constraints for theoretical
models and imaging fault damage properties at depth using remote
methods. |
|
Abril de 2013
Seismoelectric effects due to mesoscopic heterogeneities
Authors: Damien Jougnot, J. Germán Rubino et al
Link: Click here
Abstract
While the seismic effects of wave-induced fluid flow due to
mesoscopic heterogeneities have been studied for several decades,
the role played by these types of heterogeneities on seismoelectric
phenomena is largely unexplored. To address this issue, we have
developed a novel methodological framework which allows for
the coupling of wave-induced fluid flow, as inferred
|
through
numerical oscillatory compressibility tests, with the pertinent
seismoelectric conversion mechanisms. Simulating the corresponding
response of a water-saturated sandstone sample containing mesoscopic
fractures, we demonstrate for the first time that these kinds
of heterogeneities can produce measurable seismoelectric signals
under typical laboratory conditions. Given that this phenomenon
is sensitive to key hydraulic and mechanical properties, we expect
that the results of this pilot study will stimulate further exploration
on this topic in several domains of the Earth, environmental,
and engineering sciences. |
|
Abril de 2013
Simulations of tremor-related creep reveal a weak crustal
root of the San Andreas Fault
Authors: Kaj M. Johnson, David R. Shelly et al
Link: Click here
Abstract
Deep aseismic roots of faults play a critical role in transferring
tectonic loads to shallower, brittle crustal faults that rupture
in large earthquakes. Yet, until the recent discovery of deep
tremor and creep, direct inference of the physical properties
of lower-crustal fault roots has remained elusive. Observations
of tremor near Parkfield, CA provide the first evidence for
present-day localized slip on the deep extension of the San
Andreas Fault and triggered transient creep events. We develop
numerical simulations of fault slip to show that the spatiotemporal
evolution of triggered tremor near
|
Parkfield is consistent with triggered
fault creep governed by laboratory-derived friction laws between
depths of 20-35 km on the fault. Simulated creep and observed
tremor northwest of Parkfield nearly ceased for 20-30 days in
response to small coseismic stress changes of order 104 Pa from
the 2003 M6.5 San Simeon Earthquake. Simulated afterslip and observed
tremor following the 2004 M6.0 Parkfield earthquake show a coseismically
induced pulse of rapid creep and tremor lasting for 1 day followed
by a longer 30 day period of sustained accelerated rates due to
propagation of shallow afterslip into the lower crust. These creep
responses require very low effective normal stress of ~1 MPa on
the deep San Andreas Fault and near-neutral-stability frictional
properties expected for gabbroic lower-crustal rock. |
|
Abril de 2013
Stress tensor changes related to fluid injection at The Geysers
Geothermal Field, California
Authors: Patricia Martínez-Garzón, Marco Bohnhoff
et al
Link: Click here
Abstract
Studying variations of the stress field in reservoirs caused
by massive fluid injection is important towards an improved
understanding of geomechanical processes involved. We report
on spatio-temporal variations of the local stress tensor orientation
at The Geysers geothermal field, California. We apply two stress
inversion
|
methods
with detailed uncertainty assessments using a selection of events
recorded between 2007 and 2012. Our results clearly indicate variations
in the orientation of the principal stress axes for the reservoir
as a whole showing a normal faulting regime at the reservoir depth
between 2 and 3.7?km bounded by a strike-slip regime above and
below. Analyzing the temporal evolution of the stress tensor orientation
for a prominent seismicity cluster we observe a clear correlation
of changes in orientation for ?1-3 with the highest injection
rates. These results suggest that temporal changes in the stress
tensor orientation could contribute to characterize reservoirs
during stimulation. |
|
Abril de 2013
Geophysical characterization of the Chicxulub impact crater
Authors: S.P.S. Gulick, G.L. Christeson et al
Link: Click here
Abstract
Geophysical data indicate that the 65.5?million years ago Chicxulub
impact structure is a multi-ring basin, with three sets of semicontinuous,
arcuate ring faults and a topographic peak ring (PR). Slump
blocks define a terrace zone, which steps down from the inner
rim into the annular trough. Fault blocks underlie the PR, which
exhibits variable relief, due to target asymmetries. The central
structural uplift is >10?km, and the Moho is displaced by
1-2?km. The working hypothesis for the formation of Chicxulub
is: a 50?km radius transient cavity, lined with melt and
|
impact breccia, formed within 10?s
of the impact, and within minutes, weakened rebounding crust rose
kilometers above the surface, the transient crater rim underwent
localized deformation and collapsed into large slump blocks, resulting
in a inner rim at 70-85?km radius, and outer ring faults at 70-130?km
radius. The overheightened structural uplift collapsed outward,
buried the inner slump blocks, and formed the PR. Most of the
impact melt was ultimately emplaced as a coherent <3 km thick
melt sheet within the central basin that shallows within the inner
regions of the PR. Smaller pockets of melt flowed into the annular
trough. Subsequently, slope collapse, ejecta, ground surge, and
tsunami waves infilled the annular trough and annular basin with
sediments up to 3?km and 900?m thick, respectively. Testing this
working hypothesis requires direct observation of the impactites,
within and adjacent to the PR and central basin. |
|
Abril de 2013
Missing great earthquakes
Author: Susan E. Hough
Link: Click here
Abstract
The occurrence of three earthquakes with moment magnitude (Mw)
greater than 8.8 and six earthquakes larger than Mw 8.5, since
2004, has raised interest in the long-term global rate of great
earthquakes. Past studies have focused on the analysis of earthquakes
since 1900, which roughly marks the start of the instrumental
era in seismology. Before this time, the catalog is less complete
and magnitude estimates are more uncertain. Yet substantial
information is available for earthquakes before 1900, and the
catalog of
|
historical events is being used increasingly
to improve hazard assessment. Here I consider the catalog of
historical earthquakes and show that approximately half of all
Mw???8.5 earthquakes are likely missing or underestimated in
the 19th century. I further present a reconsideration of the
felt effects of the 8 February 1843, Lesser Antilles earthquake,
including a first thorough assessment of felt reports from the
United States, and show it is an example of a known historical
earthquake that was significantly larger than initially estimated.
The results suggest that incorporation of best available catalogs
of historical earthquakes will likely lead to a significant
underestimation of seismic hazard and/or the maximum possible
magnitude in many regions, including parts of the Caribbean.
|
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Abril de 2013
Theory of ground surface plasma wave associated with pre-earthquake
electrical charges
Author: Masafumi Fujii
Link: Click here
Abstract
It is shown theoretically that if mobile electrical charge exists
on the surface of the ground, a ground surface plasma wave is
induced by radio waves. If the electrical charges are generated
by tectonic stresses acting on crustal rocks prior to major
earthquakes, the detection of a ground surface plasma wave could
be used as a pre- earthquake electromagnetic phenomenon. The
|
ground surface plasma wave has a
dispersion relation, i.e., the relation between frequency and
wavelength, similar to that of the free-space plane wave in the
atmosphere over the radio broadcast frequency range. It allows
for a strong coupling between these two types of waves. This is
a mode of electromagnetic wave propagation that has not been previously
reported. Numerical analysis demonstrates (1) the propagation
of the ground surface plasma wave along a curved surface beyond
the line of sight, (2) anomalous scattering by ground surface
roughness, and (3) the generation of cross-polarized waves due
to the scattering. These results all agree well with radio wave
anomalies observed before large earthquakes. |
|
Marzo de 2013
Cryovolcanism on Titan: New results from Cassini RADAR and
VIMS
Authors: K. L. Mitchell, A. LeGall et al
Link: Click here
Abstract
The existence of cryovolcanic features on Titan has been the
subject of some controversy. Here we use observations from the
Cassini RADAR, including Synthetic Aperture Radar (SAR) imaging,
radiometry, and topographic data as well as compositional data
from the Visible and Infrared Mapping Spectrometer (VIMS) to
reexamine several putative cryovolcanic features on Titan in
terms of likely processes of origin (fluvial, cryovolcanic,
or other). We present evidence to support the cryovolcanic origin
of features in the
|
region
formerly known as Sotra Facula, which includes the deepest pit
so far found on Titan (now known as Sotra Patera), flow-like features
(Mohini Fluctus), and some of the highest mountains on Titan (Doom
and Erebor Montes). We interpret this region to be a cryovolcanic
complex of multiple cones, craters, and flows. However, we find
that some other previously supposed cryovolcanic features were
likely formed by other processes. Cryovolcanism is still a possible
formation mechanism for several features, including the flow-like
units in Hotei Regio. We discuss implications for eruption style
and composition of cryovolcanism on Titan. Our analysis shows
the great value of combining data sets when interpreting Titan's
geology and in particular stresses the value of RADAR stereogrammetry
when combined with SAR imaging and VIMS. |
|
Marzo de 2013
Growing Understanding of Subduction Dynamics Indicates Need
to Rethink Seismic Hazards
Authors: Gabriele Morra, Robert J. Geller et al
Link: Click here
Abstract
Tsunamigenic megathrust earthquakes, like the 2004 Sumatra-Andaman
and 2011 Tohoku events, are the most dramatic consequences of
subduction dynamics. The classical view is that megathrusts
release elastic energy due to the
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rupture of a fault with a width of
tens of kilometers in the down-dip direction and a length of hundreds
to a thousand kilometers along the trench. However, recent research,
particularly work on the Tohoku event, has suggested that the
generation of huge tsunamis may require the release of gravitational
energy as well as elastic energy [George et al., 2011]. Our growing
understanding of the role of gravitational energy in generating
tsunamis following megathrust earthquakes points to the need to
reevaluate earthquake and tsunami hazard assessments. |
|
Marzo de 2013
Using hydroacoustic stations as water column seismometers
Authors: Selda Yildiz, Karim Sabra et al
Link: Click here
Abstract
Getting seismic data from the deep oceans usually involves ocean-bottom
seismometers, but hydrophone arrays may provide a practical
alternative means of obtaining vector data. We here explore
this possibility using hydrophone stations of the International
Monitoring System (IMS), which have been used to study icebergs
and T-wave propagation among others. These
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stations
consist of three hydrophones at about the depth of the deep sound
channel in a horizontal triangle array with 2?km sides. We use
data from these stations in the very low frequency regime (0.01
- 0.05?Hz band), to demonstrate that these stations can also be
used as water column seismometers. By differencing the acoustic
pressure, we obtain vector quantities analogous to what a seismometer
would record. Comparing processed hydrophone station records of
the 2004 Great Sumatra-Andaman Earthquake with broadband seismograms
from a nearby island station, we find that the differenced hydrophones
are indeed a practical surrogate for seismometers. |
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Marzo de 2013
Recent Voyager 1 data indicate that on August 25, 2012 at
a distance of 121.7 AU from the Sun, sudden and unprecedented
intensity changes were observed in anomalous and galactic cosmic
rays
Authors: W.R. Webber and F.B. McDonald
Link: Click here
Abstract
At the Voyager 1 spacecraft in the outer heliosphere the intensities
of both anomalous cosmic rays (ACR) and galactic cosmic rays
(GCR) changed suddenly and decisively on
|
August 25th (121.7 AU from the Sun).
Within a matter of a few days, the intensity of 1.9-2.7 MeV protons
and helium nuclei had decreased to less than 0.1 of their previous
value and eventually the intensities decreased by factors of at
least 300-500. Also on August 25th the GCR protons, helium and
electrons increased suddenly in just 2 or 3 days by factors of
up to two. The intensities of the GCR nuclei of all energies from
2 to 400 MeV then remained essentially constant with intensity
levels and spectra that may represent the local GCR. The suddenness
of these intensity changes indicate that V1 has crossed a well-defined
boundary for energetic particles at this time possibly related
to the heliopause. |
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Marzo de 2013
Evidence for fluid-triggered slip in the 2009 Mount Rainier,
Washington earthquake swarm
Authors: David R. Shelly, Seth C. Moran et al
Link: Click here
Abstract
A vigorous swarm of over 1,000 small, shallow earthquakes occurred
September 20-22, 2009 beneath Mount Rainier, Washington, including
the largest number of events ever recorded in a single day at
Rainier since seismic stations were installed on the edifice
in 1989. Many events were only clearly recorded on one or two
stations on the edifice, or they overlapped in time with other
events, and thus only ~200 were locatable by manual phase picking.
To partially overcome this limitation, we applied waveform-based
event detection integrated with precise double-difference relative
relocation. With this procedure, detection and location goals
are accomplished in tandem, using cross-correlation with continuous
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seismic
data and waveform templates constructed from cataloged events.
As a result, we obtained precise locations for 726 events, an
improvement of almost a factor of 4. These event locations define
a ~850-m-long nearly vertical structure striking NNE, with episodic
migration outward from the initial hypocenters. The activity front
propagates in a manner consistent with a diffusional process.
Double-couple-constrained focal mechanisms suggest dominantly
near-vertical strike-slip motion on either NNW or ENE striking
faults, more than 30º different than the strike of the event
locations. This suggests the possibility of en echelon faulting,
perhaps with a component of fault opening in a fracture-mesh-type
geometry. We hypothesize that the swarm was initiated by a sudden
release of high-pressure fluid into pre-existing fractures, with
subsequent activity triggered by diffusing fluid pressure in combination
with stress transfer from the preceding events. |
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Marzo de 2013
Compositional instability of Earth's solid inner core
Authors: D. Gubbins, D. Alfe et al
Link: Click here
Abstract
All models that invoke convection to explain the observed seismic
variations in Earth's inner core require unstable inner core
stratification. Previous work has assumed that chemical effects
are stabilizing and focused on thermal convection, but recent
calculations indicate that the thermal conductivity at core
temperatures and pressures is so large that the inner core must
cool entirely by conduction. We examine partitioning of oxygen,
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sulfur, and silicon in binary iron
alloys and show that inner core growth results in a variable light
element concentration with time: oxygen concentration decreases,
sulfur concentration decreases initially and increases later,
and silicon produces a negligible effect to within the model errors.
The result is a net destabilizing concentration gradient. Convective
stability is measured by a Rayleigh number, which exceeds the
critical value for reasonable estimates of the viscosity and diffusivity.
Our results suggest that inner core convection models, including
the recently proposed translational mode, can be viable candidates
for explaining seismic results if the driving force is compositional. |
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Marzo de 2013
GOCE: The first seismometer in orbit around the Earth
Authors: Raphael F. Garcia, Sean Bruinsma et al
Link: Click here
Abstract
The first in situ sounding of a post-seismic infrasound wavefront
is presented, using data from the GOCE mission. The atmospheric
infrasounds following the great Tohoku earthquake (on 11 March
2011) induce variations of air density and vertical acceleration
of the GOCE platform. These signals are detected at two positions
along the GOCE orbit corresponding to a crossing and a doubling
of the infrasonic wavefront created by seismic surface waves.
Perturbations up to 11% of air density and 1.35×10-7 m/s2
of vertical acceleration are observed and modeled with two different
solid-atmosphere coupling codes. These perturbations
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are
a due to acoustic waves creating vertical velocities up to 130
m/s. Amplitudes and arrival times of these perturbations are reproduced
respectively within a factor 2, and within a 60?s time window.
Waveforms present a good agreement with observed data. The vertical
acceleration to air density perturbation ratio is higher for these
acoustic waves than for gravity waves. Combining these two pieces
of information offers a new way to distinguish between these two
wave types. This new type of data is a benchmark for the models
of solid-atmosphere coupling. Amplitude and frequency content
constrain the infrasound attenuation related to atmosphere viscosity
and thermal conductivity. Observed time shifts between data and
synthetics are ascribed to lateral variations of the seismic and
atmospheric sound velocities and to the influence of atmospheric
winds. These effects should be included in future modeling. This
validation of our modeling tools allows to specify more precisely
future observation projects. |
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Marzo de 2013
Causes and mechanisms of the 2011-2012 El Hierro (Canary
Islands) submarine eruption
Authors: Joan Martí, Virginie Pinel et al
Link: Click here
Abstract
El Hierro eruption started on 10 October 2011 after an unrest
episode that initiated on 17 July 2011. This is the first eruption
in the Canary Islands that has been tracked in real time. Although
being submarine and not directly observable, the data recorded
allowed its reconstruction and to identify its causes and mechanisms.
Seismicity, surface deformation, and petrological data indicate
that a batch of basanitic magma coming from a reservoir located
at a depth of about 25?km below the El Hierro Island was emplaced
at shallower depth creating a new reservoir about 10-12?km above,
where magma evolved until the initiation of the eruption. The
characteristics of seismicity and surface deformation suggest
that the necessary space to accumulate magma at this shallower
position, which coincides with the crust/mantle boundary
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beneath El Hierro, was created in
about 2 months by elastic deformation and magma-driven fracturing
of the crust. After this first intrusion episode, part of the
magma started to migrate laterally toward the SE for nearly 20?km,
always keeping the same depth and following a path apparently
controlled by stress barriers created by tectonic and rheological
contrasts in the upper lithosphere. This lateral migration of
magma ended with a submarine eruption at about 5?km offshore from
the southern corner of El Hierro Island. The total seismic energy
released during the unrest episode was of 8.1?×?1011?J,
and the total uplift previous to the onset of the eruption was
of 40?mm. Combining geological, geophysical, and petrological
data and numerical modeling, we propose a volcanological model
of the causes and mechanisms of El Hierro eruption that shows
how the stress distribution in the crust beneath El Hierro, which
was influenced by rheological contrasts, tectonic stresses, and
gravitational loading, controlled the movement and eruption of
magma. We also discuss the implications of this model in terms
of eruption forecast in the Canary Islands. |
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Marzo de 2013
The large-scale surface uplift in the Altiplano-Puna region
of Bolivia: A parametric study of source characteristics and
crustal rheology using finite element analysis
Authors: James Hickey, Joachim Gottsmann et al
Link: Click here
Abstract
This paper focuses on the driving mechanism behind a 70?km wide
region of ground uplift centered on Uturuncu volcano, in the
Altiplano-Puna region of southern Bolivia. We present a series
of forward models using finite element analysis to simultaneously
test for first-order parameters that help constrain a viable
model for the observed maximum line of sight uplift rate of
1-2?cm/yr between 1992 and 2006. Stresses from pressure sources
with finite geometries are solved numerically, accounting for
both homogeneous and heterogeneous mechanical rock properties
in elastic and viscoelastic
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rheologies.
Crustal heterogeneity is constrained by seismic velocity data
that indicate the presence of a large low-velocity zone, the Altiplano-Puna
magma body, at depths of ~17 km below the surface. A viscoelastic
rheology is employed to account for time-dependent deformation
and an inelastic crust. Comparing homogeneous and heterogeneous
models demonstrates the significant impact of a mechanically weak,
source-depth layer, which alters surface displacement patterns
by buffering subsurface deformation. Elastic model results guide
the source parameters tested in the viscoelastic models and demonstrate
a range of possible causative source geometries. Our preferred
model suggests that pressurization of a magma source extending
upward from the Altiplano-Puna magma body is causing the observed
surface uplift and alludes to a continued increase in this pressure
to explain both the spatial and temporal patterns. We also demonstrate
how a pressure-time function plays a first-order role in explaining
the observed temporal deformation pattern. |
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Marzo de 2013
Remotely triggered micro-earthquakes in the Larderello-Travale
Geothermal Field (Italy) following the 2012 May 20, Mw 5.9 Po-plain
earthquake
Authors: G. Saccorotti, D. Piccinini, et al
Link: Click here
Abstract
We report observations of remotely triggered earthquakes at
the Larderello-Travale Geothermal Field (Italy), following the
Mw?=?5.9 Po-Plain earthquake on 20 May 2012. Four distinct triggered
events are recognized within a short
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(~25?s) time interval accompanying
the sweeping of ~10s Rayleigh waves. Triggered sources are clustered
at depths in between 4?km and 6?km. The magnitude and distance
of the mainshock agree well with the triggering threshold previously
proposed for The Geysers, California. For three out of four earthquakes,
the Rayleigh wave dynamic stresses are mostly associated with
extensional vertical (szz) and shear (sxz) components, which range
up to 5?KPa. Once considering the structural setting of the area,
the most likely triggering mechanism involves the rupture of normal
faults which are kept close to failure by high-pressure crustal
fluids. |
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Marzo de 2013
Mapping inflation at Santorini volcano, Greece, using GPS
and InSAR
Authors: I. Papoutsis, X. Papanikolaou et al
Link: Click here
Abstract
Recent studies have indicated that for the first time since
1950, intense geophysical activity is occurring at the Santorini
volcano. Here, we present and discuss the surface deformation
associated with this activity, spanning from January 2011 to
February 2012. Analysis of satellite interferometry data was
performed using two well-established techniques, namely, Persistent
Scatterer Interferometry (PSI) and
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Small
Baseline Subset (SBAS), producing dense line-of-sight (LOS) ground
deformation maps. The displacement field was compared with GPS
observations from 10 continuous sites installed on Santorini.
Results show a clear and large inflation signal, up to 150?mm/yr
in the LOS direction, with a radial pattern outward from the center
of the caldera. We model the deformation inferred from GPS and
InSAR using a Mogi source located north of the Nea Kameni island,
at a depth between 3.3?km and 6.3?km and with a volume change
rate in the range of 12 million m3 to 24 million m3 per year.
The latest InSAR and GPS data suggest that the intense geophysical
activity has started to diminish since the end of February 2012. |
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Marzo de 2013
Real-time high-rate co-seismic displacement from ambiguity-fixed
precise point positioning: Application to earthquake early warning
Authors: Xingxing Li, Maorong Ge et al
Link: Click here
Abstract
Nowadays more and more high-rate real-time GPS data become available
that provide a great opportunity to contribute to earthquake
early warning (EEW) system in terms of capturing regional surface
displacements, as an independent information source, useful
for promptly estimating the magnitude of large
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destructive earthquake. In our study,
we demonstrate the performance of the real-time ambiguity-fixed
precise point positioning (PPP) approach using 5?Hz GPS data collected
during El Mayor-Cucapah earthquake (Mw 7.2, 4 April 2010). The
PPP-based displacements show to agree with accelerometer-based
displacement at centimeter level. The key for successfully obtaining
high precision displacements is the efficient ambiguity resolution.
PPP with ambiguity fixing can result in correct permanent co-seismic
offsets and correct recovery of moment magnitude and fault slip
inversion at levels comparable to post-processing. |
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Marzo de 2013
Tidal Modulation of Continuous Non-Volcanic Seismic Tremor
in the Chile Triple Junction Region
Authors: A. Gallego, R. M. Russo et al
Link: Click here
Abstract
We located continuous seismic tremorwith coherent amplitude
wavetrainsin the Chile Ridge subduction region (~46.5°S)in
two clusters north and south of the Chonos Archipelago, between
the Chile trench and the North Patagonian forearc. Tremor persisted
from December 2004 to February 2007 (the entire period of the
Chile Ridge Subduction Project temporary seismic deployment),
and lasted?>?17?hours on six occasions. Tremor in the more
active southern cluster reached a maximum duration of 48?hours,
and we observed no more than 3 continuous days without tremor
activity. The cluster locations coincide with the surface projections
of subducted transform faultsformed atthe Chile ridge. We also
|
detected simultaneous, co-located
low-frequency microearthquakes with well-defined impulsive waves
within the tremor signals distributed from the surface to 40?km
depth, suggesting tremors and earthquakes are part of the same
process. The periodicity of tremor duration is strongly correlated
with semi-diurnal, diurnal, and long period tides, M2, N2, K1,
O1, P1, and Mm (12.421, 12.000, 23.934, 25.819, 24.066?hrs and
27.555?days, respectively). We found a significant correlation
between tremor occurrence and earth tides when tidal stressis
calculated forthe slip plane of a right-lateral strike-slip fault
withstrikeN950E, which isnear parallelto subducted transform faults
(N780E) of the Chile Ridge,indicatingthat the very small stresses
resulting from the combination of ocean loading and solid earth
tides (~1kPa) are sufficient to facilitate or suppress tremor
production; tremors occur when shear stressesare maximum and wane
or are low whenshear stresses are minimum. |
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Marzo de 2013
Soil gas radon emissions and volcanic activity at El Hierro
(Canary Islands): The 2011-2012 submarine eruption
Authors: Germán D. Padilla, Pedro A. Hernández
et al
Link: Click here
Abstract
Soil gas Radon ( 222Rn) and Thoron ( 220Rn) activities have
been continuously measured during the period of the recent volcanic
unrest that occurred at El Hierro, Canary Islands, at two different
monitoring stations, namely HIE02 and
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HIE03.
Significant increases in soil 222Rn activity and 222Rn/220Rn ratio
from the soil were observed at both stations prior to the 2011-2012
submarine eruption off the coast of El Hierro, showing the highest
increases before the eruption onset and before the occurrence
of the strongest seismic event (M = 4.6). Statistical analysis
showed that the long-term temporal trend of filtered radon data
matched closely that of seismic energy release during the volcanic
unrest. The observed increases of 222Rn are related to the rock
fracturing processes (seismic activity) and the magmatic CO2 outflow
increase, as observed in HIE03 station. |
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Marzo de 2013
Stress State in the Largest Displacement Area of the 2011
Tohoku-Oki Earthquake
Authors: Weiren Lin, Marianne Conin et al
Link: Click here
Abstract
The 2011 moment magnitude 9.0 Tohoku-Oki earthquake produced
a maximum coseismic slip of more than 50 meters near the Japan
trench, which could result in a completely reduced stress state
in the region. We tested this hypothesis by determining the
in situ stress state of the frontal
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prism from boreholes drilled by the
Integrated Ocean Drilling Program approximately 1 year after the
earthquake and by inferring the pre-earthquake stress state. On
the basis of the horizontal stress orientations and magnitudes
estimated from borehole breakouts and the increase in coseismic
displacement during propagation of the rupture to the trench axis,
in situ horizontal stress decreased during the earthquake. The
stress change suggests an active slip of the frontal plate interface,
which is consistent with coseismic fault weakening and a nearly
total stress drop. |
|
Marzo de 2013
Deceleration in the Earth's oblateness
Authors: Minkang Cheng, Byron D. Tapley et al
Link: Click here
Abstract
For over three decades, satellite laser ranging (SLR) has recorded
the global nature of the long-wavelength mass change within
the Earth system. Analysis of the most recent time series of
30 day SLR-based estimates of Earth's dynamical oblateness,
characterized by the gravitational degree-2 zonal spherical
harmonic J2, indicates
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that
the long-term variation of J2 appears to be more quadratic than
linear in nature. The superposition of a quadratic and an 18.6
year variation leads to the “unknown decadal variation”
reported by Cheng and Tapley (2004). Although the primary trend
is expected to be linear due to global isostatic adjustment, there
is an evident deceleration (inline image) in the rate of the decrease
in J2 during the last few decades, likely due to changes in the
rate of the global mass redistribution from melting of the glaciers
and ice sheets as well as mass changes in the atmosphere and ocean. |
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Marzo de 2013
Along-strike variability of rupture duration in subduction
zone earthquakes
Authors: Maya El Hariri, Susan L. Bilek et al
Link: Click here
Abstract
Subduction zone earthquakes exhibit a wide spectrum of rupture
times that reflect conditions on the megathrust fault. Tsunami
earthquakes are examples of slower than expected ruptures that
produce anomalously large tsunamis relative to the surface-wave
magnitude. One model explaining tsunami earthquakes suggests
slip within patches of low rigidity material at shallow depths.
Heterogeneous fault conditions, such as having patches of low
rigidity material surrounded by higher strength material, should
produce heterogeneous earthquake rupture parameters. Here we
investigate along-strike variation in rupture duration for 427
shallow thrust earthquakes (Mw = 5.0–7.0) in the Peru,
Chile,
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Alaska, Tonga, Kuril, Izu, and Java-Sumatra
subduction zones to explore how heterogeneous seismic and tectonic
characteristics, such as differences in sediment type, thickness,
and roughness of subducting bathymetry, affect earthquake properties.
Earthquake source parameters, including rupture durations, are
estimated using multi-station deconvolution of teleseismic P and
SH waves to solve for earthquake source time functions, and all
events are relocated using additional depth phase information.
We classify events into shallow (?26 km) and deep (>26 km and
?61 km) groups based on the overall mean depth and focus on the
longest duration events with moment normalized rupture durations
of >1 standard deviation above the mean duration for each group.
We find long-duration events at all depths within the study regions
except Peru and Chile. We find no correlation with incoming sediment
thickness or type, and limited spatial correlation with regions
of past tsunami earthquakes, regions of observed afterslip, and
subducting bathymetric features. |
|
Marzo de 2013
Permanent fore-arc extension and seismic segmentation: Insights
from the 2010 Maule earthquake, Chile
Authors: Felipe Aron, Richard W. Allmendinger et al
Link: Click here
Abstract
Geologists have long known that young normal faults are an important
structural element of the Andean Coastal Cordillera, but their
relationship to the subduction seismic cycle is still unclear.
Some of the largest aftershocks of the 2010 Mw 8.8 Maule earthquake
in central Chile were nucleated on upper plate normal faults,
including the Mw 6.9 and 7.0 events of the Pichilemu earthquake
sequence. We use the available coseismic GPS displacements,
moment tensor sums, and slip distribution models for the Maule
earthquake to compute the static strain and stress fields imposed
on the upper plate by slip on the
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subduction
interface. The extensional strains calculated from coseismic GPS
and from a moment tensor sum of the Pichilemu events have similar
orientations and orders of magnitude. The normal Coulomb stress
increment (CSI) on the Pichilemu fault has maximum positive stresses
as high as 4.9 MPa. Regionally, the Maule event produced a semi-elliptical,
radial pattern of static extension and deviatoric tension (CSI
> 1.5 MPa) along the Coastal Cordillera enclosing the rupture
area. This elliptical pattern mimics the trends of the major upper-crustal
structures. The static deformation field produced by a great subduction
earthquake is an effective mechanism for generating permanent
extension above the seismogenic zone, reactivating suitably oriented,
long-lived normal faults. We suggest that the semi-elliptical
outline of the first-order structures along the Coastal Cordillera
may define the location of a characteristic, long-lived megathrust
segment. This observation implies a persistence at least over
the Quaternary of great subduction ruptures along the Maule segment. |
|
Marzo de 2013
Temporal changes in attenuation associated with the 2004
M6.0 Parkfield earthquake
Authors: C. M. Kelly, A. Rietbrock et al
Link: Click here
Abstract
Elevated seismic attenuation is often observed in fault zones
due to the high degree of fracturing and fluid content. However,
temporal changes in attenuation at the time of an earthquake
are poorly constrained but can give indications of fracture
damage and healing. In this study, spectral ratios between earthquakes
within repeating clusters are calculated in an attempt to resolve
temporal variations in attenuation at the time of the 2004 M6.0
Parkfield earthquake. A sharp increase in attenuation is observed
immediately after the earthquake, which then decays over the
next 2
|
years. Influences of intercluster
magnitude variations, time window length and previously reported
postseismic velocity changes are investigated. The postseismic
decay is fit by a logarithmic function. The timescale of the decay
is found to be similar to that in GPS data and ambient seismic
noise velocities following the 2004 M6.0 Parkfield earthquake.
The amplitude of the attenuation change corresponds to a decrease
of approximately 10% in Qp at the time of the earthquake. The
greatest changes are recorded on the northeast of the fault trace,
consistent with preferential damage in the extensional quadrant
behind a north-westerly propagating rupture tip. Our analysis
suggests that significant changes in seismic attenuation and hence
fracture dilatancy during coseismic rupture are limited to depths
of less than about 5 km. |
|
Marzo de 2013
Stress State in the Largest Displacement Area of the
2011 Tohoku-Oki Earthquake
Authors: Weiren Lin, Marianne Conin et al
Link: Click here
Abstract
The 2011 moment magnitude 9.0 Tohoku-Oki earthquake produced
a maximum coseismic slip of more than 50 meters near the Japan
trench, which could result in a completely reduced stress state
in the region. We tested this hypothesis by determining the
in situ stress state of the frontal
|
prism
from boreholes drilled by the Integrated Ocean Drilling Program
approximately 1 year after the earthquake and by inferring the
pre-earthquake stress state. On the basis of the horizontal stress
orientations and magnitudes estimated from borehole breakouts
and the increase in coseismic displacement during propagation
of the rupture to the trench axis, in situ horizontal stress decreased
during the earthquake. The stress change suggests an active slip
of the frontal plate interface, which is consistent with coseismic
fault weakening and a nearly total stress drop. |
Febrero de 2013
Insignificant solar-terrestrial triggering of earthquakes
Authors: Jeffrey J. Love and Jeremy N. Thomas
Link: Click here
Abstract
We examine the claim that solar-terrestrial interaction, as
measured by sunspots, solar-wind velocity, and geomagnetic activity,
might play a role in triggering earthquakes. We count the number
of earthquakes having magnitudes that exceed chosen thresholds
in calendar years, months, and days, and we order these counts
by
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the corresponding rank of annual,
monthly, and daily averages of the solar-terrestrial variables.
We measure the statistical significance of the difference between
the earthquake-number distributions below and above the median
of the solar-terrestrial averages by Xi 2 and Student tests. Across
a range of earthquake magnitude thresholds, we find no consistent
and statistically significant distributional differences. We also
introduce time lags between the solar-terrestrial variables and
the number of earthquakes, but again no statistically significant
distributional difference is found. We cannot reject the null
hypothesis of no solar-terrestrial triggering of earthquakes. |
Febrero
de 2013
Io's volcanism controls Jupiter's radio emissions
Authors: M. Yoneda, F. Tsuchiya et al
Link: Click here
Abstract
Jupiter's sodium nebula showed an enhancement in late May through
the beginning of June 2007. This means Io's volcanic activity
and the magnetosphere's plasma content increased during this
period. On the other hand, Jupiter's
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radio
emission called HOM became quiet after the sodium nebula enhancement.
The HOM emission is considered to be related to the activity of
aurorae on Jupiter. These observation results therefore suggest
that the increase in plasma supply from Io into Jupiter's magnetosphere
weakens its field-aligned current, which generates the radio emissions
and aurorae on Jupiter. By comparing our observation results to
recent model and observation results, we add supporting evidence
to the possibility that Io's volcanism controls Jupiter's magnetospheric
activity. |
Febrero
de 2013
Análisis de Razón Acumulativa (CURATE): Un algoritmo
de agrupación para catálogos dominados por enjambres
sísmicos.
Autores: Katrina M. Jacobs, Euan G. C. Smith et al
Link: Click aquí
Abstract
Nosotros presentamos un nuevo método de agrupamiento
de razón acumulativa (CURATE) para identificar secuencias
de sismos especialmente en regiones con actividad de enjambre.
El método identifica secuencias por medio de la comparación
de las tasas observadas respecto de la tasa promedio. Este método
es distinto de las técnicas de agrupamiento previas,
en las cuales no existen condicionantes directos sobre los procesos
físicos relacionados con la decadencia temporal o interacción
terremoto/ terremoto. En su lugar, estos supuestos se sustituyen
por uno más general y que afirma que los sismos que ocurren
dentro de una secuencia deben compartir un disparador físico
común, el que se manifiesta por un cambio en la tasa.
El uso de la tasa como el parámetro de selección
primaria hace hincapié en que la proximidad temporal
es el principal vínculo entre secuencias diferentes.
Para investigar las características de la secuencia sísmica
catálogo/escala, se aplica el método de (des-)
agrupamiento de cuatro conocidos métodos a un catálogo
de 4845 sismos de magnitud M 2,45 terremotos desde 1993 hasta
2007 en la Región Volcánica Central de Nueva Zelanda.
A pesar del enfoque peculiar del método de formación
de la secuencia, el catálogo desagrupado basado en el
método CURATE resulta ser satisfactorio dentro del conjunto
de catálogos desagrupados producidos por otros métodos.
Una reconstrucción estocástica basada en parámetros
de tipo secuencia epidémica de réplicas se presenta
también para poner a prueba las diferencias entre los
catálogos que contienen exclusivamente el sismo principal
y las secuencias de réplicas, junto con las zonas que
presentan múltiples procesos físicos. Probamos
los catálogos desagrupados producidos por todos los métodos
para una distribución temporal de Poisson y proponemos
que ésta se utiliza para asegurar parámetros razonables
de selección. El método CURATE será especialmente
útil para la identificación de los enjambres,
la definición de magnitudes de verosímiles y la
duración de las secuencias, junto con el perfeccionamiento
de los pronósticos sísmicos que incluyen enjambres
a escala regional y local.
|
Febrero de 2013
Cumulative rate analysis (CURATE): A clustering algorithm for
swarm dominated catalogs
Authors: Katrina M. Jacobs, Euan G. C. Smith et al
Link: Click here
Abstract
We present a new cumulative rate (CURATE) clustering method
to identify earthquake sequences especially in regions with
swarm activity. The method identifies sequences by comparing
observed rates to an average rate. It is distinct from previous
clustering techniques in that no direct assumptions about physical
processes relating to temporal decay or earthquake-earthquake
interaction are made. Instead these assumptions are replaced
by a more general one, that earthquakes occurring within a sequence
likely share a common physical trigger, which is manifested
by a change in rate. The use of rate as the primary selection
parameter emphasizes that temporal proximity is the main commonality
among different sequence types. To investigate catalog-scale
earthquake sequence characteristics, we apply the method along
with four standard (de-)clustering methods to a catalog of 4845
M 2.45 earthquakes from 1993 through 2007 in the Central Volcanic
Region of New Zealand. Despite the distinct focus of the method
on sequence formation, the declustered catalog of the CURATE
method sits within the suite of declustered catalogs produced
by other methods. A stochastic reconstruction based on epidemic-type
aftershock sequence parameters is also presented to test the
differences between catalogs that exclusively contain mainshock-aftershock
sequences and areas that exhibit multiple physical processes.
We test the declustered catalogs produced by all methods for
a Poisson temporal distribution and propose that this be used
to ensure reasonable selection parameters. The CURATE method
will be especially useful for identifying swarms, creating likelihoods
of the size and duration of sequences, and refining earthquake
forecasts that include swarms at regional and local scales.
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Febrero
de 2013
Exploring Hawaiian Volcanism
Authors: Michael P. Poland, Paul G. Okubo et al
Link: Click here
Abstract
In 1912 the Hawaiian Volcano Observatory (HVO) was established
by Massachusetts Institute of Technology professor Thomas A.
Jaggar Jr. on the island of Hawaii. Driven by the devastation
he observed while investigating the volcanic disasters of 1902
at Montagne Pelée in the
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Caribbean,
Jaggar conducted a worldwide search and decided that Hawai'i provided
an excellent natural laboratory for systematic study of earthquake
and volcano processes toward better understanding of seismic and
volcanic hazards. In the 100 years since HVO's founding, surveillance
and investigation of Hawaiian volcanoes have spurred advances
in volcano and seismic monitoring techniques, extended scientists'
understanding of eruptive activity and processes, and contributed
to development of global theories about hot spots and mantle plumes. |
Febrero de 2013
Effect of the largest foreshock (Mw 7.3) on triggering the
2011 Tohoku earthquake (Mw 9.0)
Authors: Aditya Riadi Gusman, Mitsuteru Fukuoka et al
Link: Click here
Abstract
The slip distribution of the largest foreshock that occurred
2 days before the mainshock of the 2011 Tohoku earthquake is
estimated by tsunami waveform inversion. The major slip region
was located on the down-dip side of the hypocenter,
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and the slip amounts ranged from
0.6 to 1.5 m. By assuming the rigidity of 4?×?1010 N m-2,
the seismic moment calculated from the slip distribution is 1.2?×?1020
N m (Mw 7.3). The slip distribution suggests that the largest
foreshock did not rupture the plate interface where the dynamic
rupture of the mainshock was initiated. The largest foreshock
increased the Coulomb stress (1.6-4.5 bars) on the plate interface
around the hypocenter of the mainshock. This indicates that the
2011 Tohoku earthquake was brought closer to failure by the largest
foreshock. |