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Septiembre de 2015
Slip pulse and resonance of the Kathmandu basin during
the 2015 Gorkha earthquake, Nepal
Authors: J. Galetzka, D. Melgar
et al
Link: Click here
Abstract
Detailed geodetic imaging of earthquake ruptures enhances
our understanding of earthquake physics and associated ground
shaking. The 25 April 2015 moment magnitude 7.8 earthquake
in Gorkha, Nepal was the first large continental megathrust
rupture to have occurred beneath a high-rate (5-hertz) Global
Positioning System (GPS) network. We used GPS and interferometric
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synthetic aperture radar data to model the
earthquake rupture as a slip pulse ~20 kilometers in width,
~6 seconds in duration, and with a peak sliding velocity of
1.1 meters per second, which propagated toward the Kathmandu
basin at ~3.3 kilometers per second over ~140 kilometers.
The smooth slip onset, indicating a large (~5-meter) slip-weakening
distance, caused moderate ground shaking at high frequencies
(>1 hertz; peak ground acceleration, ~16% of Earth's gravity)
and minimized damage to vernacular dwellings. Whole-basin
resonance at a period of 4 to 5 seconds caused the collapse
of tall structures, including cultural artifacts.
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Septiembre de 2015
Effects of long-term fluid injection on induced seismicity parameters and maximum magnitude at northwestern The Geysers geothermal field
Authors:
Grzegorz Kwiatek, Patricia Martínez-Garzón et al
Link: Click here
Abstract
The long-term temporal and spatial changes in statistical, source and stress characteristics of one cluster of induced seismicity recorded at The Geysers geothermal field (US) are analyzed in relation to the field operations, fluid migration and constraints on the maximum likely magnitude. Two injection wells, Prati-9 and Prati-29, located in the northwestern part of the field and their associated seismicity composed of 1,776 events recorded throughout a seven-year period were analyzed. The seismicity catalog was relocated and the source characteristics including focal |
mechanisms and static source parameters were refined using first-motion polarity, spectral fitting and mesh spectral ratio analysis techniques. The source characteristics together with statistical parameters (b value) and cluster dynamics were used to investigate and understand the details of fluid migration scheme in the vicinity of injection wells. The observed temporal, spatial and source characteristics were clearly attributed to fluid injection and fluid migration towards greater depths, involving increasing pore pressure in the reservoir. The seasonal changes of injection rates were found to directly impact the shape and spatial extent of the seismic cloud. A tendency of larger seismic events to occur closer to injection wells and a correlation between the spatial extent of the seismic cloud and source sizes of the largest events was observed suggesting geometrical constraints on the maximum likely magnitude and its correlation to the average injection rate and volume of fluids present in the reservoir. |
Septiembre de 2015
The Iquique earthquake sequence of April 2014: Bayesian modeling accounting for prediction uncertainty
Authors: Z Duputel, JJR Jolivet et al
Link: Click here
Abstract
The subduction zone in northern Chile is a well-identified seismic gap that last ruptured in 1877. On 1 April 2014, this region was struck by a large earthquake following a two week long series of foreshocks. This study combines a wide range of observations, including geodetic, tsunami, and seismic data, to produce a reliable kinematic slip model of the Mw=8.1 main shock and a static slip model of the Mw=7.7 aftershock. We use a novel Bayesian modeling approach that accounts for uncertainty in the Green's functions, both static and dynamic, while avoiding nonphysical regularization. The results reveal a sharp slip zone, more compact than previously thought, located downdip of the foreshock sequence and updip of high-frequency sources inferred by back-projection analysis. Both the main shock and the
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Mw=7.7 aftershock did not rupture to the trench and left most of the seismic gap unbroken, leaving the possibility of a future large earthquake in the region. The subduction zone in northern Chile is a well-identified seismic gap that last ruptured in 1877. On 1 April 2014, this region was struck by a large earthquake following a two week long series of foreshocks. This study combines a wide range of observations, including geodetic, tsunami, and seismic data, to produce a reliable kinematic slip model of the Mw=8.1 main shock and a static slip model of the Mw=7.7 aftershock. We use a novel Bayesian modeling approach that accounts for uncertainty in the Green's functions, both static and dynamic, while avoiding nonphysical regularization. The results reveal a sharp slip zone, more compact than previously thought, located downdip of the foreshock sequence and updip of high-frequency sources inferred by back-projection analysis. Both the main shock and the Mw=7.7 aftershock did not rupture to the trench and left most of the seismic gap unbroken, leaving the possibility of a future large earthquake in the region. |
Septiembre de 2015
Time-dependent model of aseismic slip on the central San Andreas Fault from InSAR time series and repeating earthquakes
Authors: M Khoshmanesh, M Shirzaei et al
Link: Click here
Abstract
The Central segment of San Andreas Fault (CSAF) is characterized by a nearly continuous right-lateral aseismic slip. However, observations of the creep rate obtained using small characteristically repeating earthquakes (CREs) show pulses of creep along the CSAF, which may indicate spatially and temporally variable seismic hazard along the CSAF. Therefore, the goal of this study is to obtain a high-resolution time-dependent model of creep along the CSAF to examine this hypothesis. To this end, we apply a time-dependent creep modeling approach, which combines interferometric synthetic aperture radar (InSAR) surface deformation time series and observations of fault creep obtained from CREs.
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The SAR data set includes C band scenes acquired by the ERS-2 and Envisat satellites between 2003 and 2011. The resulting creep rate distribution implies a peak rate up to 32mm/yr along the central part of the CSAF. Afterslip due to the 2004 Parkfield earthquake on the southeastern segment of the CSAF is also manifest in the model, and there is clear evidence of creep pulsing along strike and depth of the CSAF. Estimated annual rate of slip deficit accumulation is equivalent to a magnitude 5.6–5.7 earthquake. Taking advantage of the time-dependence of our model, we also refine the scaling relationship, which associates the released seismic moment due to a CRE event with the amount of creep on the fault, surrounding the CRE patches. This study provides the first kinematic model of creep pulsing, constrained using geodetic and seismic data, which can enhance time-dependent seismic hazard maps and improve earthquake operational forecast models. |
Septiembre de 2015
Potential for a large earthquake near Los Angeles inferred from the 2014 La Habra earthquake
Authors: A Donnellan, LG Ludwig et al
Link: Click here
Abstract
Tectonic motion across the Los Angeles region is distributed across an intricate network of strike-slip and thrust faults that will be released in destructive earthquakes similar to or larger than the 1933M6.4 Long Beach and 1994M6.7 Northridge events. Here we show that Los Angeles regional thrust, strike-slip, and oblique faults are connected and move concurrently with measurable surface deformation, even in moderate magnitude earthquakes, as part of a
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fault system that accommodates north-south shortening and westerly tectonic escape of northern Los Angeles. The 28 March 2014 M5.1 La Habra earthquake occurred on a northeast striking, northwest dipping left-lateral oblique thrust fault northeast of Los Angeles. We present crustal deformation observation spanning the earthquake showing that concurrent deformation occurred on several structures in the shallow crust. The seismic moment of the earthquake is 24% of the total geodetic moment released. Slip within the unconsolidated upper sedimentary layer may reflect shallow release of accumulated strain on still-locked deeper structures. A future M6.1–6.7 earthquake would account for the accumulated strain. Such an event could occur on any one or several of these faults, which may not have been identified by geologic surface mapping. |
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Septiembre de 2015
The Iquique earthquake sequence of April 2014: Bayesian modeling accounting for prediction uncertainty
Authors: Z Duputel,J Jiang et al
Link: Click here
Abstract
The subduction zone in northern Chile is a well-identified seismic gap that last ruptured in 1877. On April 1, 2014, this region was struck by a large earthquake following a two-week-long series of foreshocks. This study combines a wide range of observations, including geodetic, tsunami and seismic data, to produce a reliable kinematic slip
|
model of the Mw = 8.1 mainshock and a static slip model of the Mw= 7.7 aftershock. We use a novel Bayesian modeling approach that accounts for uncertainty in the Green's functions, both static and dynamic, while avoiding non-physical regularization. The results reveal a sharp slip zone, more compact than previously thought, located downdip of the foreshock sequence and up-dip of high-frequency sources inferred by back-projection analysis. Both the mainshock and the Mw = 7.7 aftershock did not rupture to the trench and left most of the seismic gap unbroken, leaving the possibility of a future large earthquake in the region. |
Septiembre de 2015
An empirical study of the distribution of earthquakes with respect to rock type and depth Authors: Y Tal et al
Link: Click here
Abstract
Whether fault slip occurs seismically or aseismically depends on the frictional properties of the fault, which might be expected to depend on rock type and depth, as well as other factors. To examine the effect of rock type and depth on the distribution of earthquakes, we compare geologic models of the San Francisco Bay and the
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Southern California regions to the distribution of seismicity. We normalize the number of earthquakes within each rock type and depth interval by the corresponding volume to determine the earthquake density. Earthquake density is determined primarily by depth, while whether the rock is sedimentary or basement has only a secondary, depth-dependent effect on the earthquake density. At very shallow depths, there is no difference in earthquake density between sedimentary and basement rocks. The earthquake density of basement rocks increases with depth more rapidly than that of sedimentary rocks to a similar but shallower maximum. |
Septiembre de 2015
Toward the reconciliation of seismological and petrological perspectives on oceanic lithosphere heterogeneity
Authors: BLN Kennett et al
Link: Click here
Abstract
The character of the high-frequency seismic phases Po and So, observed after propagation for long distances in the oceanic lithosphere, requires the presence of scattering from complex structure in 3-D. Current models use stochastic representations of seismic structure in the oceanic lithosphere. The observations are compatible with quasi-laminate features with horizontal correlation length around 10 km and
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vertical correlation length 0.5 km, with a uniform level of about 2% variation through the full thickness of the lithosphere. Such structures are difficult to explain with petrological models, which would favor stronger heterogeneity at the base of the lithosphere associated with underplating from frozen melts. Petrological evidence mostly points to smaller-scale features than suggested by seismology. The models from the different fields have been derived independently, with various levels of simplification. Fortunately, it is possible to gently modify the seismological model toward stronger basal heterogeneity, but there remains a need for some quasi-laminate structure throughout the mantle component of the oceanic lithosphere. The new models help to bridge the gulf between the different viewpoints, but ambiguities remain. |
Septiembre de 2015
Experimental evidence of electrification processes during the 2009 L'Aquila earthquake main shock
Author:
P Nenovski
Link: Click here
Abstract
Two types of coseismic magnetic field events are simultaneously observed: transient offset events and magnetic field signal that occurred at the destructive, Mw6.1 L'Aquila earthquake main
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shock. The offset event, conventionally interpreted as a signature of piezomagnetic effects, however, could not be explained as such. In the second type of coseismic event, the transient magnetic signal starts simultaneously with the offset event and reaches amplitude of 0.8nT in the total magnetic field. The signal is a local one; its amplitude shape resembles diffusion-like form with time scale characteristics that are indicative for a source deep in the crust. The polarity of the transient signal is in the horizontal plane and nearly parallel to the L'Aquila fault strike.
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Septiembre de 2015
Rupture propagation behavior and the largest possible earthquake induced by fluid injection into deep reservoirs
Author: VS Gischig
Link: Click here
Abstract
Earthquakes caused by fluid injection into deep underground reservoirs constitute an increasingly recognized risk to populations and infrastructure. Quantitative assessment of induced seismic hazard, however, requires estimating the maximum possible magnitude earthquake that may be induced during fluid injection. Here I seek constraints on an upper limit for the largest
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possible earthquake using source-physics simulations that consider rate-and-state friction and hydromechanical interaction along a straight homogeneous fault. Depending on the orientation of the pressurized fault in the ambient stress field, different rupture behaviors can occur: (1) uncontrolled rupture-front propagation beyond the pressure front or (2) rupture-front propagation arresting at the pressure front. In the first case, fault properties determine the earthquake magnitude, and the upper magnitude limit may be similar to natural earthquakes. In the second case, the maximum magnitude can be controlled by carefully designing and monitoring injection and thus restricting the pressurized fault area. |
Septiembre de 2015
Evolution of unrest at Laguna del Maule volcanic field (Chile) from InSAR and GPS measurements, 2003 to 2014
Authors: H Le Mevel, K L Feigl et al
Link: Click here
Abstract
The Laguna del Maule (LdM) volcanic field in the southern volcanic zone of the Chilean Andes exhibits a large volume of rhyolitic material erupted during postglacial times (20–2 ka). Since 2007, LdM has experienced an unrest episode characterized by high rates of deformation. Analysis of new GPS and Interferometric Synthetic
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Aperture Radar (InSAR) data reveals uplift rates greater than 190 mm/yr between January 2013 and November 2014. The geodetic data are modeled as an inflating sill at depth. The results are used to calculate the temporal evolution of the vertical displacement. The best time function for modeling the InSAR data set is a double exponential model with rates increasing from 2007 through 2010 and decreasing slowly since 2010. We hypothesize that magma intruding into an existing silicic magma reservoir is driving the surface deformation. Modeling historical uplift at Yellowstone, Long Valley, and Three Sisters volcanic fields suggests a common temporal evolution of vertical displacement rates. |
Septiembre de 2015
On the reliability of the Spatial Scintillation Index to detect earthquake precursors in the ionosphere
Authors: F Masci et al
Link: Click here
Abstract
The scientific literature includes many reports of ionospheric phenomena that are retrospectively identified prior to seismic events. These disturbances of the Earth's ionosphere are considered to be possible precursors of the impending earthquakes. However, a causal relationship between ionospheric phenomena and earthquakes has never been definitively
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demonstrated, and attempts at identifying precursory effects in the ionosphere have been called into question by several studies. Among the candidate indicators of ionospheric precursors there is the Spatial Scintillation Index (SSI) proposed by Pulinets et al. (2007). The usefulness of this index in the search for precursory effects of earthquakes has been criticized by Thomas et al. (2012) and Masci (2013). In a recent report, Pulinets and Davidenko (2014) attempt to briefly respond to the remarks of these researchers. Here we cast doubt that Pulinets and Davidenko (2014) have shown that SSI is a reliable indicator of precursory effects of earthquakes in the ionosphere. |
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Septiembre de 2015
Detection of ionospheric disturbances driven by the 2014
Chile tsunami using GPS total electron content in New Zealand
Authors: X Zhang et al
Link: Click here
Abstract
A tsunami propagating in open ocean can produce gravity waves
and thus perturb the ionosphere. In this study, we employ
a second-order numerical difference method using GPS total
electron content observed in New Zealand to detect the ionospheric
disturbances triggered by the Chile tsunami that occurred
on 1 April 2014. We observe traveling ionospheric disturbances
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(TIDs), which have similar horizontal
velocity and direction as the tsunami waves, at different times
after the event. According to the arrival times, the latter
TIDs (about 14.5-15?h after earthquake) can be attributed to
the tsunami waves whereas the former one (about 12?h 30?min
after earthquake) could be related to other sources. This suggests
that besides the propagation velocity and direction, the arrival
time is also necessary to distinguish tsunami-driven TIDs correctly.
Furthermore, we observe the phenomenon that the detected tsunami-driven
TIDs are superimposed upon other nontsunami-driven ionospheric
perturbations far away from the epicenter. The superimposed
TIDs eventually separate due to their different propagation
velocities. |
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Septiembre de 2015
High effusion rates of the Cordón Caulle 2011-2012
eruption (Southern Andes) and their relation with the quasi-harmonic
tremor
Authors: D Bertin, LE Lara et al et al
Link: Click here
Abstract
High effusion rates of intermediate-to-high-silica lavas seem
to be less uncommon than previously thought, in particular
during their initial eruptive
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stages. In
this study, we report satellite-based time-averaged discharge
rates for the 2011-2012 effusive phase at Cordón Caulle,
which are well correlated with the evolution of the quasi-harmonic
tremor, the most significant seismic signal after the initial
explosive stage. Such a correspondence could become a key method
for detection of the onset of effusive phases, especially in
remote and/or very cloudy areas, supplying an additional tool
for effective warnings and near-real-time hazard appraisal. |
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Septiembre de 2015
On scientific inference in geophysics and the use of numerical
simulations for scientific investigations
Authors: AJ Mannucci, BT Tsurutani et al
Link: Click here
Abstract
Scientific knowledge is acquired in geophysics generally without
the benefit of controlled experiments. In this paper, we discuss
how scientific inference based on observations occurs in geophysical
contexts. We develop a specific approach that uses approximate
simultaneity of proposed cause and effect phenomena to infer
causality. The approach applies equally well to effect phenomena
that follow the cause with a known time delay. We find that,
in general, establishing a causal relationship between two
phenomena based on simultaneity requires
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knowledge of how often simultaneity
of these phenomena occurs in the absence of causality. We then
extend the discussion to using numerical simulations in the
scientific inference process. Numerical simulations of physical
processes, because they can simulate the values of observations,
are often used to infer what physical processes are occurring
in nature. We discuss agreement between model output and observations
as a basis for inferring the physical processes underlying the
observations. We find that an important factor to consider,
which we here call the "confusion factor," is how
often it may occur that insufficient model representations of
the physical processes nevertheless lead to agreement between
model computations and observations. We suggest that models
of intermediate or low complexity may have a significant role
to play when using geophysical simulations to reach scientific
conclusions. |
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Septiembre de 2015
Realizing a terrestrial reference frame using the Global
Positioning System
Authors: BJ Haines, YE Bar-Sever et al
Link: Click here
Abstract
We describe a terrestrial reference frame (TRF) realization
based on Global Positioning System (GPS) data alone. Our approach
rests on a highly dynamic, long-arc (9 day) estimation strategy
and on GPS satellite antenna calibrations derived from
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Gravity Recovery
and Climate Experiment and TOPEX/Poseidon low Earth orbit receiver
GPS data. Based on nearly 17 years of data (1997-2013), our
solution for scale rate agrees with International Terrestrial
Reference Frame (ITRF)2008 to 0.03 ppb yr?1, and our solution
for 3-D origin rate agrees with ITRF2008 to 0.4 mm yr?1. Absolute
scale differs by 1.1 ppb (7 mm at the Earth's surface) and 3-D
origin by 8 mm. These differences lie within estimated error
levels for the contemporary TRF. |
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Septiembre de 2015
Evolution of unrest at Laguna del Maule volcanic field
(Chile) from InSAR and GPS measurements, 2003 to 2014
Authors: H Le Mevel, KL Feigl et al et al
Link: Click here
Abstract
The Laguna del Maule (LdM) volcanic field in the southern
volcanic zone of the Chilean Andes exhibits a large volume
of rhyolitic material erupted during postglacial times (20-2
ka). Since 2007, LdM has experienced an unrest episode characterized
by high rates of deformation. Analysis of new GPS and Interferometric
Synthetic
|
Aperture Radar (InSAR) data reveals
uplift rates greater than 190 mm/yr between January 2013 and
November 2014. The geodetic data are modeled as an inflating
sill at depth. The results are used to calculate the temporal
evolution of the vertical displacement. The best time function
for modeling the InSAR data set is a double exponential model
with rates increasing from 2007 through 2010 and decreasing
slowly since 2010. We hypothesize that magma intruding into
an existing silicic magma reservoir is driving the surface deformation.
Modeling historical uplift at Yellowstone, Long Valley, and
Three Sisters volcanic fields suggests a common temporal evolution
of vertical displacement rates. |
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Septiembre de 2015
Ocean robots listen for earthquake echoes
Author: Eric Hand
Link: Click here
Abstract
The discovery of large mantle plumes rising from the coremantle
boundary (...) underscores how far seismologists have come
in using earthquake waves to probe Earth's interior. Unfortunately,
two-thirds of the planet-the part covered by oceans-is a virtual
blank spot for seismology. Ocean islands with stations gathering
continuous seismic data
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are scarce,
and deploying ocean-bottom sensors from ships is time-consuming
and expensive.
Now, researchers are talking about a way to fill the data gap:
autonomous submersibles that listen for large earthquakes. Their
measurements would not match those of land-based instruments,
but "I think the way to go forward is to sacrifice quality
for quantity," says Guust Nolet, an emeritus geophysicist
at the University of Nice Sophia Antipolis in France, who described
the approach in a paper he and colleagues published last month
in Nature Communications. |
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Septiembre de 2015
Weak subduction makes great quakes
Author: Roland Bürgmann
Link: Click here
Abstract
The world's greatest earthquakes, producing catastrophic shaking
and tsunamis, occur in subduction zones. Here oceanic plates
dive below adjoining regions along megathrust faults (see
the figure). The recent magnitude ~9 megathrust
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earthquakes in Sumatra, Chile,
and Japan, with fault displacements of several tens of meters,
were stark reminders of the destructive power of these events.
On page 1213 of this issue, Hardebeck (1) uses the orientations
of fault planes of thousands of smaller earthquakes near and
above the world's megathrusts to evaluate the state of stress
driving these great events. The general conclusion made is that
all faults in subduction zones, including the megathrusts, are
unusually weak. |
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Septiembre de 2015
Stress orientations in subduction zones and the strength
of subduction megathrust faults
Author: Jeanne L. Hardebeck
Link: Click here
Abstract
Subduction zone megathrust faults produce most of the world's
largest earthquakes. Although the physical properties of these
faults are difficult to observe directly, their frictional
strength can be estimated indirectly by constraining the orientations
of the stresses that act on them. A global investigation of
stress orientations in
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subduction
zones finds that the maximum compressive stress axis plunges
systematically trenchward, consistently making an angle of 45°
to 60° with respect to the subduction megathrust fault.
These angles indicate that the megathrust fault is not substantially
weaker than its surroundings. Together with several other lines
of evidence, this implies that subduction zone megathrusts are
weak faults in a low-stress environment. The deforming outer
accretionary wedge may decouple the stress state along the megathrust
from the constraints of the free surface. |
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Septiembre de 2015
Lahar infrasound associated with Volcán Villarrica's
3 March 2015 eruption
Authors: JB Johnson et al
Link: Click here
Abstract
The paroxysmal 2015 eruption of Volcán Villarrica (Chile)
produced a 2.5?h long lahar, which descended more than 20?km
within the Rio Correntoso/Turbio drainage and destroyed two
small bridges. A three-element infrasound array 10?km from
the summit, and 4?km from the lahar's closest approach, was
used to study the
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flow's progression. Array processing
using cross-correlation lag times and semblance places constraints
on the lahar's dynamics, including detection of an initial flow
pulse that traveled from 2 to 12?km at an average speed of 38?m/s.
Subsequently, the lahar signal evolved to a relatively stationary
infrasonic tremor located 10 to 12?km from the vent and adjacent
to a topographic notch, through which sound may have preferentially
diffracted toward the recording site. This study demonstrates
the powerful capabilities of infrasound arrays for lahar study
and suggests their potential application for future hazard monitoring. |
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Agosto de 2015
Food security and sustainable resource management
Authors: D McLaughlin et al
Link: Click here
Abstract
The projected growth in global food demand until mid-century
will challenge our ability to continue recent increases in
crop yield and will have a significant impact on natural resources.
The water and land requirements of current agriculture are
significantly less than global reserves but local shortages
are common and have serious impacts on food security. Recent
increases in global trade have mitigated some of the effects
of spatial and temporal variability. However, trade has a
limited impact on low-income populations who remain dependent
on subsistence agriculture and local resources. Potential
adverse environmental impacts of increased agricultural production
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include unsustainable
depletion of water and soil resources, major changes in the
global nitrogen and phosphorous cycles, human health problems
related to excessive nutrient and pesticide use, and loss of
habitats that contribute to agricultural productivity. Some
typical case studies from China illustrate the connections between
the need for increased food production and environmental stress.
Sustainable options for decreasing food demand and for increasing
production include reduction of food losses on both the producer
and consumer ends, elimination of unsustainable practices such
as prolonged groundwater overdraft, closing of yield gaps with
controlled expansions of fertilizer application, increases in
crop yield and pest resistance through advances in biotechnology,
and moderate expansion of rain fed and irrigated cropland. Calculations
based on reasonable assumptions suggest that such measures could
meet the food needs of an increasing global population while
protecting the environment. |
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Agosto de 2015
Isolated cases of remote dynamic triggering in Canada detected
using cataloged earthquakes combined with a matched-filter
approach
Authors: B Wang, RM Harrington et al
Link: Click here
Abstract
Here we search for dynamically triggered earthquakes in Canada
following global main shocks between 2004 and 2014 with MS?
>?6, depth?<?100?km, and estimated peak ground velocity?>?0.2?cm/s.
We use the Natural Resources Canada (NRCan) earthquake catalog
to calculate ? statistical values in 1°?×?1°
bins in
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10?day windows before and after
the main shocks. The statistical analysis suggests that triggering
may occur near Vancouver Island, along the border of the Yukon
and Northwest Territories, in western Alberta, western Ontario,
and the Charlevoix seismic zone. We also search for triggering
in Alberta where denser seismic station coverage renders regional
earthquake catalogs with lower completeness thresholds. We find
remote triggering in Alberta associated with three main shocks
using a matched-filter approach on continuous waveform data.
The increased number of local earthquakes following the passage
of main shock surface waves suggests local faults may be in
a critically stressed state. |
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Agosto de 2015
Electrical properties of methane hydrate + sediment mixtures
Authors: WL Du Frane, LA Stern et al
Link: Click here
Abstract
Knowledge of the electrical properties of multicomponent systems
with gas hydrate, sediments, and pore water is needed to help
relate electromagnetic (EM) measurements to specific gas hydrate
concentration and distribution patterns in nature. Toward
this goal, we built a pressure cell capable of measuring in
situ electrical properties of multicomponent systems such
that the effects of individual components and mixing relations
can be assessed. We first established the temperature-dependent
electrical conductivity (?) of pure, single-phase methane
hydrate to be ~5 orders of magnitude lower than seawater,
a substantial contrast that can help
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differentiate
hydrate deposits from significantly more conductive water-saturated
sediments in EM field surveys. Here we report ? measurements
of two-component systems in which methane hydrate is mixed with
variable amounts of quartz sand or glass beads. Sand by itself
has low ? but is found to increase the overall ? of mixtures
with well-connected methane hydrate. Alternatively, the overall
? decreases when sand concentrations are high enough to cause
gas hydrate to be poorly connected, indicating that hydrate
grains provide the primary conduction path. Our measurements
suggest that impurities from sand induce chemical interactions
and/or doping effects that result in higher electrical conductivity
with lower temperature dependence. These results can be used
in the modeling of massive or two-phase gas-hydrate-bearing
systems devoid of conductive pore water. Further experiments
that include a free water phase are the necessary next steps
toward developing complex models relevant to most natural systems. |
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Agosto de 2015
Automatic earthquake confirmation for early warning system
Authors: HS Kuyuk, S Colombelli et al
Link: Click here
Abstract
Earthquake early warning studies are shifting real-time seismology
in earthquake science. They provide methods to rapidly assess
earthquakes to predict damaging ground shaking. Preventing
false alarms from these systems is key. Here we developed
a simple, robust algorithm, Authorizing GRound shaking for
Earthquake Early warning Systems (AGREEs), to reduce falsely
issued
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alarms. This is a network threshold-based
algorithm, which differs from existing approaches based on apparent
velocity of P and S waves. AGREEs is designed to function as
an external module to support existing earthquake early warning
systems (EEWSs) and filters out the false events, by evaluating
actual shaking near the epicenter. Our retrospective analyses
of the 2009 L'Aquila and 2012 Emilia earthquakes show that AGREEs
could help an EEWS by confirming the epicentral intensity. Furthermore,
AGREEs is able to effectively identify three false events due
to a storm, a teleseismic earthquake, and broken sensors in
Irpinia Seismic Network, Italy. |
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Julio de 2015
Three-dimensional resistivity image of the magmatic system
beneath Lastarria volcano and evidence for magmatic intrusion
in the back arc (northern Chile)
Authors: D Diaz, W Heise et al
Link: Click here
Abstract
Lazufre volcanic center, located in the central Andes, is
recently undergoing an episode of uplift, conforming one of
the most extensive deforming volcanic systems worldwide, but
its magmatic system and its connection with the observed uplift
are still poorly studied. Here we image the
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electrical
resistivity structure using the magnetotelluric method in the
surroundings of the Lastarria volcano, one of the most important
features in the Lazufre area, to understand the nature of the
magmatic plumbing, the associated fumarolic activity, and the
large-scale surface deformation. Results from 3-D modeling show
a conductive zone at 6?km depth south of the Lastarria volcano
interpreted as the magmatic heat source which is connected to
a shallower conductor beneath the volcano, showing the pathways
of volcanic gasses and heated fluid. A large-scale conductive
area coinciding with the area of uplift points at a magma intrusion
at midcrustal depth. |
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Julio de 2015
Larger aftershocks happen farther away: Nonseparability
of magnitude and spatial distributions of aftershocks
Authors: NJ van der Elst et al
Link: Click here
Abstract
Aftershocks may be driven by stress concentrations left by
the main shock rupture or by elastic stress transfer to adjacent
fault sections or strands. Aftershocks that occur within the
initial rupture may be limited in size, because the scale
of the stress concentrations should be smaller
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than the primary rupture itself.
On the other hand, aftershocks that occur on adjacent fault
segments outside the primary rupture may have no such size limitation.
Here we use high-precision double-difference relocated earthquake
catalogs to demonstrate that larger aftershocks occur farther
away than smaller aftershocks, when measured from the centroid
of early aftershock activity-a proxy for the initial rupture.
Aftershocks as large as or larger than the initiating event
nucleate almost exclusively in the outer regions of the aftershock
zone. This observation is interpreted as a signature of elastic
rebound in the earthquake catalog and can be used to improve
forecasting of large aftershocks. |
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Julio de 2015
On the reliability of the Spatial Scintillation Index to
detect earthquake precursors in the ionosphere
Authors: F Masci and JN Thomas
Link: Click here
Abstract
The scientific literature includes many reports of ionospheric
phenomena that are retrospectively identified prior to seismic
events. These disturbances of the Earth's ionosphere are considered
to be possible precursors of the impending earthquakes. However,
a causal relationship between ionospheric phenomena and earthquakes
has never been definitively
|
demonstrated
and attempts at identifying precursory effects in the ionosphere
have been called into question by several studies [see, e.g.,
Masci and Thomas, 2014; Masci et al., 2014]. Among the candidate
indicators of ionospheric precursors there is the Spatial Scintillation
Index (SSI) proposed by Pulinets et al. [2007]. The usefulness
of this index in the search for precursory effects of earthquakes
has been criticized by Thomas et al. [2012] and Masci [2013].
In a recent report, Pulinets and Davidenko [2014] attempt to
briefly respond to the remarks of these researchers. Here we
cast doubt that Pulinets and Davidenko [2014] have shown that
SSI is a reliable indicator of precursory effects of earthquakes
in the ionosphere. |
|
Julio de 2015
Gas pressure gradients in unsaturated porous media and
the assumption of infinite gas mobility
Authors: L Hou, BE Sleep et al
Link: Click here
Abstract
The assumption that gas is infinitely mobile, moving without
viscous pressure drops, is common in studies of unsaturated
flow in porous media. The objectives of this work were to
use experimental measurements to examine that assumption in
systems experiencing rapid drainage, and to explore the extent
to which observed pressure drops could be described by conventional
multiphase flow simulation tracking viscous flow in both phases.
Because many published studies have used vented columns in
an effort to equilibrate pore gas pressures with inlet gas,
an additional objective of the work was
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to use experimental measurements
to explore the ability of column vents to equilibrate pore gas
with inlet gas during dynamic drainage.
Results of the work suggest that gas pressure gradients can
be significant, and that the assumption of infinite gas mobility
is likely to be unsatisfactory in many systems where moderately
rapid saturation change occurs. While vents have the potential
to influence flow by providing additional gas inlets, experimental
results of this work show almost no impact on pore gas pressures
from a vent similar in size to those in other published studies.
An equation developed as a part of the work suggests that the
spatial slope of gas pressure with distance away from the front
during dynamic drainage is proportional to the ratio of outflow
Darcy velocity to saturated hydraulic conductivity for vertical
columns. As such, systems with more rapid saturation change
also have a greater potential to exhibit experimental artifacts
related to gas pressure gradients. |
|
Julio de 2015
Debates-Perspectives on socio-hydrology: Introduction
Author: Alberto Montanari
Link: Click here
Abstract
Science improves our knowledge of the nature and living beings.
It helps the humans to investigate the several unknowns of
the universe and generates solutions for practical problems.
In fact, science has several functions, including the attempt
to increase the quality of the human life. The role played
by science for the development of society has been recently
stressed by several institutions and scientific associations.
In the
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context of
water science, the need to "…improve our capability
to make predictions of water resources dynamics to support sustainable
societal development in a changing environment" [Montanari
et al., 2013] has been repeatedly emphasized in recent times.
In fact, the International Association of Hydrological Sciences
(IAHS) has launched in 2013 the scientific decade "Panta
Rhei - Everything flows - Change in hydrology and society"
(www.iahs.info/pantarhei). Its purpose is to promote scientific
research to seek solutions to global and local water challenges
due to changing environment and social systems, by stressing
the key role that the hydrological community plays in this respect. |
|
Julio de 2015
Automatic earthquake confirmation for early warning system
Authors: HS Kuyuk, S Colombelli et al
Link: Click here
Abstract
Earthquake early warning studies are shifting real-time seismology
in earthquake science. They provide methods to rapidly assess
earthquakes to predict damaging ground shaking. Preventing
false alarms from these systems is key. Here we developed
a simple, robust algorithm, Authorizing GRound shaking for
Earthquake Early warning Systems (AGREEs), to reduce falsely
issued
|
alarms. This is a network threshold-based
algorithm, which differs from existing approaches based on apparent
velocity of P and S waves. AGREEs is designed to function as
an external module to support existing earthquake early warning
systems (EEWSs) and filters out the false events, by evaluating
actual shaking near the epicenter. Our retrospective analyses
of the 2009 L'Aquila and 2012 Emilia earthquakes show that AGREEs
could help an EEWS by confirming the epicentral intensity. Furthermore,
AGREEs is able to effectively identify three false events due
to a storm, a teleseismic earthquake, and broken sensors in
Irpinia Seismic Network, Italy. |
|
Julio de 2015
Injection-induced seismicity: Poroelastic and earthquake
nucleation effects
Authors: P Segall and S Lu
Link: Click here
Abstract
The standard model of injection-induced seismicity considers
changes in Coulomb strength due solely to changes in pore
pressure. We consider two additional effects: full poroelastic
coupling of stress and pore pressure, and time-dependent earthquake
nucleation. We model stress and pore pressure due to specified
injection rate in a homogeneous, poroelastic medium. Stress
and pore pressure are used to compute seismicity rate through
the Dieterich (1994) model. For constant injection rate, the
time to reach a critical seismicity rate scales with t ? r2/(cfc),
where r is distance from the injector, c is hydraulic diffusivity,
and fc is a factor that depends
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on mechanical
properties, and weakly on r. The seismicity rate decays following
a peak, consistent with some observations. During injection
poroelastic coupling may increase or decrease the seismicity
rate, depending on the orientation of the faults relative to
the injector. If injection-induced stresses inhibit slip, abrupt
shut-in can lead to locally sharp increases in seismicity rate;
tapering the flux mitigates this effect. The maximum magnitude
event has been observed to occur postinjection. We suggest the
seismicity rate at a given magnitude depends on the nucleation
rate, the size distribution of fault segments, and if the background
shear stress is low, the time-varying volume of perturbed crust.
This leads to a rollover in frequency-magnitude distribution
for larger events, with a "corner" that increases
with time. Larger events are absent at short times, but approach
the background frequency with time; larger events occurring
post shut-in are thus not unexpected. |
|
Julio de 2015
The ups and downs of the TVZ: Geodetic observations of
deformation around the Taupo Volcanic Zone, New Zealand
Authors:IJ Hamling, S Hreinsdottir et al
Link: Click here
Abstract
The 300?km long Taupo Volcanic Zone (TVZ) formed as a result
of back-arc rifting associated with subduction of the Pacific
Plate beneath the Australian Plate, with current extension
rates of 8-15?mm?yr?1. Using GPS and interferometric
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synthetic aperture radar (InSAR)
data, collected by the European (ESA) and Japanese (JAXA) space
agencies, we present ground deformation observations from 2003
to 2011. Both InSAR and vertical GPS data show widespread subsidence
across the central TVZ at rates of up to 20?mm?yr?1. Using simple
elastic dislocation models to represent the contraction of a
sill like body at 6?km depth, we predict an annual volume change
of 0.011-0.016?km3 beneath the central TVZ. We suggest that
the majority of the observed subsidence is a result of the cooling
and subsequent contraction of magma within the shallow crust. |
|
Julio de 2015
Earthquake magnitude calculation without saturation from
the scaling of peak ground displacement
Authors: D Melgar, BW Crowell et al
Link: Click here
Abstract
GPS instruments are noninertial and directly measure displacements
with respect to a global reference frame, while inertial sensors
are affected by systematic offsets-primarily tilting-that
adversely impact integration to displacement. We study the
magnitude scaling properties of peak ground displacement (PGD)
from high-rate GPS
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networks
at near-source to regional distances (~10-1000 km), from earthquakes
between Mw6 and 9. We conclude that real-time GPS seismic waveforms
can be used to rapidly determine magnitude, typically within
the first minute of rupture initiation and in many cases before
the rupture is complete. While slower than earthquake early
warning methods that rely on the first few seconds of P wave
arrival, our approach does not suffer from the saturation effects
experienced with seismic sensors at large magnitudes. Rapid
magnitude estimation is useful for generating rapid earthquake
source models, tsunami prediction, and ground motion studies
that require accurate information on long-period displacements. |
|
Julio de 2015
What processes control the chemical compositions of arc
front stratovolcanoes?
Authors: S J Turner and CH Langmuir
Link: Click here
Abstract
Arc front stratovolcanoes have global chemical systematics
that constrain processes at convergent margins. Positive correlations
exist for arc averages among "fluid mobile," "high
field strength," and "large ion lithophile"
elements. 143Nd/144Nd and 87Sr/86Sr from rear-arc lavas lacking
subduction signature align with the oceanic "mantle array,"
and correlate with arc front 143Nd/144Nd. Most chemical parameters
(but not isotopes) also correlate well with crustal thickness
and slightly less well with the slab thermal parameter, but
not with the depth of the slab nor model slab surface temperatures.
Successful models of arc volcanism should account for these
global regularities. Two distinct models can quantitatively
account for the
|
observations-different extents
of melting of the mantle wedge caused by variations in wedge
thermal structure, or varying contributions from the subducting
slab owing to variations in the slab thermal structure. Both
successful model scenarios require a significant flux of melted
ocean crust to the mantle source of all volcanic arcs. The wedge
melting model has constant contributions from ocean crust, sediment,
and mantle wedge to lavas globally, while the slab model varies
slab contributions with slab temperature. The wedge melting
model fit improves by incorporating convergence rate and slab
dip, which should affect the wedge thermal structure; the slab
model is not supported by a similar analysis. The wedge model
also more easily accommodates the isotope data. The two models
predict different primary H2O contents, with large variations
in H2O for the wedge model, and relatively constant H2O for
the slab model. An evaluation of the effects of varying sediment
compositions on arc lavas will benefit from considering the
very different consequences of the two models. |
|
Julio de 2015
An electrical conductivity model for fractal porous media
Authors: Wei Wei, J Cai et al
Link: Click here
Abstract
Archie's equation is an empirical electrical conductivity-porosity
model that has been used to predict the formation factor of
porous rock for more than 70?years. However, the physical
interpretation of its parameters, e.g., the cementation exponent
m, remains questionable.
|
In this study,
a theoretical electrical conductivity equation is derived based
on the fractal characteristics of porous media. The proposed
model is expressed in terms of the tortuosity fractal dimension
(DT), the pore fractal dimension (Df), the electrical conductivity
of the pore liquid, and the porosity. The empirical parameter
m is then determined from physically based parameters, such
as DT and Df. Furthermore, a distinct interrelationship between
DT and Df is obtained. We find a reasonably good match between
the predicted formation factor by our model and experimental
data. |
|
Julio de 2015
A new physics-based modeling approach for tsunami-ionosphere
coupling
Authors: X Meng, A Komjathy et al
Link: Click here
Abstract
Tsunamis can generate gravity waves propagating upward through
the atmosphere, inducing total electron content (TEC) disturbances
in the ionosphere. To capture this process, we have implemented
tsunami-generated gravity waves into the Global Ionosphere-Thermosphere
Model (GITM) to construct a three-dimensional physics-based
model WP (Wave Perturbation)-GITM. WP-GITM takes tsunami wave
properties, including
|
the wave height,
wave period, wavelength, and propagation direction, as inputs
and time-dependently characterizes the responses of the upper
atmosphere between 100?km and 600?km altitudes. We apply WP-GITM
to simulate the ionosphere above the West Coast of the United
States around the time when the tsunami associated with the
March 2011 Tohuku-Oki earthquke arrived. The simulated TEC perturbations
agree with Global Positioning System observations reasonably
well. For the first time, a fully self-consistent and physics-based
model has reproduced the GPS-observed traveling ionospheric
signatures of an actual tsunami event. |
|
Junio de 2015
Low slip rates and multi-millennial return times for Mw
7 earthquake faults in southern Calabria (Italy)
Authors: PAC Galli et al
Link: Click here
Abstract
The Calabrian Arc is the epicentral region of 1/3 of the strongest
earthquakes of Italy (Mw=7.0). These are confined within a
narrow peninsula which is the emerging portion of a slab-related
accretionary wedge, and all occurred in the past four centuries.
Therefore, here, more than anywhere in Italy, historical seismicity
alone is not sufficient for
|
seismic hazard
assessment. We carried out geological and paleoseismological
studies in southern Calabria that allowed characterizing the
seismogenic behavior of the Cittanova fault which was responsible
for one of the most catastrophic earthquakes to ever occur in
Europe, the Mw 7.0 February 5, 1783 event. We have found out
conclusive evidence for four Holocene earthquakes prior to 1783,
with a recurrence time longer (~3.2 kyr) than the other Apennine
faults (0.3-2.4 kyr). We have also estimated a robust slip-rate
for the late Upper Pleistocene (0.6 mm/yr), and an extension-rate
(0.4 mm/yr) that could reflect the residual back-arc opening
of the Tyrrhenian basin. |
|
Junio de 2015
Lower plate deformation at the Chile Triple Junction from
the paleomagnetic record (45°30'-46° S)
Authors: Y Lagabrielle, J Bourgois et al
Link: Click here
Abstract
During the CTJ cruise, geophysical surveys were conducted
between 45°S and 48°S, in the region of the Chile
Triple Junction (CTJ), where the Nazca and Antarctica plates
are subducting beneath the South America plate. Near the CTJ,
the South Chile Rise (SCR), which separates the Nazca and
Antarctica lower plates, consists of three spreading segments
trending ~N160°, separated by a series of parallel fracture
zones. The active spreading centers of the three segments
consist of grabens with various widths and depths,
|
bounded by steep fault scarps.
We provide robust data showing that the SCR recorded remote
and long-term effects of ridge subduction far from the subduction
front. Magnetic profiles, multibeam bathymetric and seismic
data were acquired at intervals of 13 km along a N80°E direction
across the SCR during the CTJ cruise of R/V L'Atalante. Deformation
of the oceanic lithosphere includes: (1) a segmentation of the
spreading axes along strike, (2) some ridge jumps, and (3) local
constriction and changes in trend of the fracture zone valleys.
Off-axis volcanism is observed in places that may suggest a
link with an abnormal stress field induced by ridge subduction.
The tectonic and volcanic anomalies, which occurred in response
to the subduction of the SCR1 axis may be correlated with geochemical
anomalies and slab fragmentation recognized by previous works. |
|
Junio de 2015
High-resolution magnetics reveal the deep structure of
a volcanic-arc-related basalt-hosted hydrothermal site (Palinuro,
Tyrrhenian Sea)
Authors: F Szitkar, S Petersen et al
Link: Click here
Abstract
High-resolution magnetic surveys have been acquired over the
partially sedimented Palinuro massive sulfide deposits in
the Aeolian volcanic arc, Tyrrhenian Sea. Surveys flown close
to the seafloor using an autonomous underwater vehicle (AUV)
show that the volcanic-arc-related basalt-hosted hydrothermal
site is associated with zones of lower magnetization. This
observation reflects the alteration of basalt affected by
hydrothermal circulation and/or the progressive accumulation
of
|
a nonmagnetic
deposit made of hydrothermal and volcaniclastic material and/or
a thermal demagnetization of titanomagnetite due to the upwelling
of hot fluids. To discriminate among these inferences, estimate
the shape of the nonmagnetic deposit and the characteristics
of the underlying altered area-the stockwork-we use high-resolution
vector magnetic data acquired by the AUV Abyss (GEOMAR) above
a crater-shaped depression hosting a weakly active hydrothermal
site. Our study unveils a relatively small nonmagnetic deposit
accumulated at the bottom of the depression and locked between
the surrounding volcanic cones. Thermal demagnetization is unlikely
but the stockwork extends beyond the limits of the nonmagnetic
deposit, forming lobe-shaped zones believed to be a consequence
of older volcanic episodes having contributed in generating
the cones. |
|
Junio de 2015
Active volcanism on Venus in the Ganiki Chasma rift zone
Authors: E V Shalygin, WJ Markiewicz et al
Link: Click here
Abstract
Venus is known to have been volcanically resurfaced in the
last third of solar system history and to have undergone a
significant decrease in volcanic activity a few hundred million
years ago. However, fundamental questions remain: Is Venus
still volcanically active today, and if so,
|
where and in what geological and
geodynamic environment. Here we show evidence from the Venus
Express Venus Monitoring Camera for transient bright spots that
are consistent with the extrusion of lava flows that locally
cause significantly elevated surface temperatures. The very
strong spatial correlation of the transient bright spots with
the extremely young Ganiki Chasma, their similarity to locations
of rift-associated volcanism on Earth, provide strong evidence
for their volcanic origin and suggests that Venus is currently
geodynamically active. |
|
Junio de 2015
Automatic earthquake confirmation for early warning system
Authors: HS Kuyuk, S Colombelli et al
Link: Click here
Abstract
Earthquake early warning studies are shifting real-time seismology
in earthquake science. They provide methods to rapidly assess
earthquakes to predict damaging ground shaking. Preventing
false alarms from these systems is key. Here, we developed
a simple, robust algorithm, Authorizing GRound shaking for
Earthquake Early warning Systems, (AGREEs) to reduce falsely
issued
|
alarms. This
is a network-threshold-based algorithm, which differs from existing
approaches based on apparent velocity of P- and S-waves. AGREEs
is designed to function as an external module to support existing
earthquake early warning systems (EEWS) and filters out the
false events, by evaluating actual shaking near the epicenter.
Our retrospective analyses of the 2009 L'Aquila and 2012 Emilia
earthquakes show that AGREEs could help an EEWS by confirming
the epicentral intensity. Furthermore, AGREEs is able to effectively
identify three false events due to a storm, a teleseismic earthquake,
and broken sensors in Irpinia Seismic Network, Italy. |
|
Junio de 2015
3-D resistivity image of the magmatic system beneath Lastarria
volcano and evidence for magmatic intrusion in the back arc
(Northern Chile)
Authors: D Díaz, W Heise et al
Link: Click here
Abstract
Lazufre volcanic center, located in the central Andes, is
recently undergoing an episode of uplift, conforming one of
the most extensive deforming volcanic systems worldwide, but
its magmatic system and its connection with the observed uplift
is still poorly studied. Here we image the electrical
|
resistivity structure using the
magnetotelluric method in the surroundings of the Lastarria
volcano, one of the most important features in the Lazufre area,
to understand the nature of the magmatic plumbing, the associated
fumarolic activity and the large scale surface deformation.
Results from 3-D modeling show a conductive zone at 6 km depth
south of the Lastarria volcano interpreted as the magmatic heat
source which is connected to a shallower conductor beneath the
volcano, showing the pathways of volcanic gasses and heated
fluid. A large scale conductive area coinciding with the area
of uplift points at a magma intrusion at mid crustal depth. |
|
Junio de 2015
A New Physics-based Modeling Approach for Tsunami-Ionosphere
Coupling
Authors: X Meng, A Komjathy et al
Link: Click here
Abstract
Tsunamis can generate gravity waves propagating upward through
the atmosphere, inducing total electron content (TEC) disturbances
in the ionosphere. To capture this process, we have implemented
tsunami-generated gravity waves into the Global Ionosphere-Thermosphere
Model (GITM) to construct a three-dimensional physics-based
model WP (Wave Perturbation)-GITM. WP-
|
GITM takes
tsunami wave properties, including the wave height, wave period,
wavelength, and propagation direction, as inputs, and time-dependently
characterizes the responses of the upper atmosphere between
100 km and 600 km altitudes. We apply WP-GITM to simulate the
ionosphere above the West Coast of the United States around
the time when the tsunami associated with the March 2011 Tohuku-Oki
earthquke arrived. The simulated TEC perturbations agree with
Global Positioning System (GPS) observations reasonably well.
For the first time, a fully self-consistent and physics-based
model has reproduced the GPS-observed traveling ionospheric
signatures of an actual tsunami event. |
|
Junio de 2015
Delineation of connectivity structures in 2-D heterogeneous
hydraulic conductivity fields
Authors: AR Thyukhova, W Kinzelbach et al
Link: Click here
Abstract
Connectivity is a critical aquifer property controlling anomalous
transport behavior at large scales. But connectivity cannot
be easily defined in a continuous field based on information
of the hydraulic conductivity alone. We conceptualize it as
a connecting structure – a connected subset of a continuous
hydraulic conductivity field that consists of paths of least
hydraulic resistance. We
|
develop a simple and robust numerical
method to delineate the connectivity structure using information
of the hydraulic conductivity field only. First, the topology
of the connectivity structure is determined by finding the path(s)
of least resistance between two opposite boundaries. And second,
a series of connectivity structures are created by inflating
and shrinking the individual channels. Finally, we apply this
methodology to different heterogeneous fields. We show that
our method captures the main flow channels as well as the pathways
of early time solute arrivals. We find our method informative
to study connectivity in 2D heterogeneous hydraulic conductivity
fields. This article is protected by copyright. All rights reserved. |
|
Junio de 2015
Active volcanism on Venus in the Ganiki Chasma rift zone
Authors: EV Shalygin, WJ Markiwicz et al
Link: Click here
Abstract
Venus is known to have been volcanically resurfaced in the
last third of solar system history and to have undergone a
significant decrease in volcanic activity a few hundred million
years ago. However, fundamental questions remain: Is Venus
still volcanically active today, and if so,
|
where and
in what geological and geodynamic environment? Here we show
evidence from the Venus Express Venus Monitoring Camera for
transient bright spots that are consistent with the extrusion
of lava flows that locally cause significantly elevated surface
temperatures. The very strong spatial correlation of the transient
bright spots with the extremely young Ganiki Chasma, their similarity
to locations of rift-associated volcanism on Earth, provide
strong evidence for their volcanic origin and suggests that
Venus is currently geodynamically active. |
|
Junio de 2015
Earthquakes induced by fluid injections
Author: Francois H. Cornet
Link: Click here
Abstract
In the early 1960s, the U.S. Army unintentionally triggered
some seismic activity by injecting waste fluids into the basement
rock beneath the Rocky Mountain Arsenal, near Denver, Colorado.
It is now recognized that an increase in the pressure applied
by the fluids that fill fractures and faults at depth balances
progressively the normal stress exerted through the rock on
these fractures but leaves the shear stress supported by these
surfaces unchanged. According to friction laws, when the fluid
pressure gets too high and the effective normal stress gets
too low, shear motion starts. This motion is generally considered
to be mostly seismogenic, that is, to be the source of earthquakes
that may reach in some localities magnitudes larger than 5,
or even 6, as was observed upon filling the Koyna Dam in India
in the early 1960s. Today, the phenomenon of fluid-induced
seismicity has become a societal concern wherever injection
of large quantity of fluids at depth is involved. On page
1224 of this issue, however, Guglielmi et al. report that
water injected in a natural fault at a depth of 282 m generates
nonseismic motion. That is, the ground displacements take
place at slow velocities (4 µm/s), with only very small
microseismic activity...
|
Junio de 2015
Earthquakes induced by fluid injections
Authors: Yves Guglielmi, Frédéric Cappa et al
Link: Click here
Abstract
Anthropogenic fluid injections are known to induce earthquakes.
The mechanisms involved are poorly understood, and our ability
to assess the seismic hazard associated with geothermal energy
or unconventional hydrocarbon production remains limited.
We directly measure fault slip and seismicity induced by fluid
injection into a natural fault. We observe highly dilatant
and slow [~4 micrometers per second (µm/s)] aseismic
slip associated with a 20-fold increase of permeability, which
transitions to faster slip (~10 µm/s) associated with
reduced dilatancy and micro-earthquakes. Most aseismic slip
occurs within the fluid-pressurized zone and obeys a rate-strengthening
friction law xx with v0 = 0.1 µm/s. Fluid injection
primarily triggers aseismic slip in this experiment, with
micro-earthquakes being an indirect effect mediated by aseismic
creep.
Injection of fluid into the crust can generate seismicity,
with source mechanisms similar to natural earthquakes and
occasionally large enough to cause damage (1–6). Induced
seismicity is generally regarded as a manifestation of the
effective stress principle, where the increase in pore pressure
reduces the effective normal stress and brings a ...
|
|
Mayo de 2015
Investigating the final seismic swarm before the Iquique-Pisagua
2014 Mw 8.1 by comparison of continuous GPS and seismic foreshock
data
Authors: J Bedford, M Moreno et al
Link: Click here
Abstract
Preexisting networks of seismometers and continuous GPS in
Northern Chile successfully captured surface motions and seismicity
leading up to the 1 April Mw 8.1. Here we compare continuous
GPS (cGPS) with predictions of seismic dislocations for the
final foreshock swarm, beginning with the 16 March Mw 6.7.
|
Results show
that the cumulative cGPS motion can be largely explained by
seismic slip because evolutions of cGPS positions for most stations
stay within the ranges of seismic predictions (given sensible
ranges of assumed source errors). However, cGPS motions between
18-21 and 25-31 March outpace seismic predictions, supporting
the existence of aseismic transients that were most probably
the afterslip from preceding bursts of seismicity. A parameter
search reveals that the 16 March Mw 6.7 cGPS displacements can
be recreated with a fault plane significantly rotated anticlockwise
from the strike of the plate interface, suggesting that failure
was on a structure other than the plate interface. |
|
Mayo de 2015
Origin of transient self-potential signals associated with
very long period seismic pulses observed during the 2000 activity
of Miyakejima volcano
Authors: O Kuwano, S Yoshida et al
Link: Click here
Abstract
Origin of the previously reported transient geoelectrical
(self-potential, SP) signals in the Miyakejima 2000 activity,
that repeatedly occurred concurrently with very long period
(VLP) seismic pulses, was investigated. SP waveforms stacked
across repeated VLP events showed a step-like rise followed
by a gradual decay at all stations spread over the island
of 8?km diameter. Within a realistic range of hydrological
diffusivity, the short time constants of the SP signals cannot
be explained by the electrokinetic effect caused by
|
fluid flow within a limited volume,
proposed earlier as a fluid injection hypothesis. On the other
hand, poroelasticity predicts an island-wide distributed flow
field to occur almost instantaneously upon VLP events due to
the step of strain field imposed by the mechanical event. We
propose that the observed SP signals resulted from the streaming
current by this island-wide flow field. Our quantitative model,
assuming a vertical tensile crack as a mechanical source, which
has been suggested by preceding seismic studies, can explain
the time constants and the amplitude of the SP signals (both
spatial pattern and absolute amplitude), within a reasonable
range of rock properties and the scalar moment of the mechanical
source (VLP event). Location and attitude of the mechanical
source were well constrained by grid search and are consistent
with those estimated earlier from other types of data. |
|
Mayo de 2015
Revised interpretation of recent InSAR signals observed
at Llaima volcano (Chile)
Authors: D Remy, Y Chen et all
Link: Click here
Abstract
We analyzed C band and L band interferometric synthetic aperture
radar (InSAR) data acquired from 2003 to 2011 to search for
volcanic deformations at Llaima volcano, Southern Andes (38.69°S,
71.73°W). There, specific environmental conditions (steep
slopes, snow- or ice-capped summit, dense vegetation cover,
and strong tropospheric artifacts) and limited amount of radar
data available make it challenging to accurately
|
measure ground surface displacement
with InSAR. To overcome these difficulties, we first performed
a careful analysis of the water vapor variations using Medium-Resolution
Imaging Spectrometer and Moderate Resolution Imaging Spectroradiometer
near-infrared water vapor products and then we inverted wrapped
interferograms for both topographic correlated phase delays
and a simple model source strength. In the light of our results,
we conclude that there is no detectable ground displacement
related to a deep magmatic source for the 2003-2011 period and
that most of the fringes observed in the interferograms were
produced by tropospheric delays. |
|
Mayo de 2015
Bubbles attenuate elastic waves at seismic frequencies:
First experimental evidence
Authors: N Tisato, B Quintal et al
Link: Click here
Abstract
The migration of gases from deep to shallow reservoirs can
cause damageable events. For instance, some gases can pollute
the biosphere or trigger explosions and eruptions. Seismic
tomography may be employed to map the accumulation of subsurface
bubble-bearing fluids to help mitigating such hazards. Nevertheless,
how gas bubbles modify seismic waves is still unclear. We
show that saturated rocks strongly
|
attenuate
seismic waves when gas bubbles occupy part of the pore space.
Laboratory measurements of elastic wave attenuation at frequencies
<100?Hz are modeled with a dynamic gas dissolution theory
demonstrating that the observed frequency-dependent attenuation
is caused by wave-induced-gas-exsolution-dissolution (WIGED).
This result is incorporated into a numerical model simulating
the propagation of seismic waves in a subsurface domain containing
CO2-gas bubbles. This simulation shows that WIGED can significantly
modify the wavefield and illustrates how accounting for this
physical mechanism can potentially improve the monitoring and
surveying of gas bubble-bearing fluids in the subsurface. |
|
Mayo de 2015
Coseismic compression/dilatation and viscoelastic uplift/subsidence
following the 2012 Indian Ocean earthquakes quantified from
satellite gravity observations
Authors: S Han, J Sauber et al
Link: Click here
Abstract
The 2012 Indian Ocean earthquake sequence (Mw 8.6, 8.2) is
a rare example of great strike-slip earthquakes in an intraoceanic
setting. With over a decade of Gravity Recovery and Climate
Experiment (GRACE) data, we were able to measure and model
the unanticipated large coseismic and postseismic gravity
changes of
|
these events. Using the approach
of normal mode decomposition and spatial localization, we computed
the gravity changes corresponding to five moment tensor components.
Our analysis revealed that the gravity changes are produced
predominantly by coseismic compression and dilatation within
the oceanic crust and upper mantle and by postseismic vertical
motion. Our results suggest that the postseismic positive gravity
and the postseismic uplift measured with GPS within the coseismic
compressional quadrant are best fit by ongoing uplift associated
with viscoelastic mantle relaxation. Our study demonstrates
that the GRACE data are suitable for analyzing strike-slip earthquakes
as small as Mw 8.2 with the noise characteristics of this region. |
|
Mayo de 2015
An increase in the rate of global mean sea level rise since
2010
Authors: S Yi, W Sun et al
Link: Click here
Abstract
The global mean sea level (GMSL) was reported to have dropped
5?mm due to the 2010/2011 La Niña and have recovered
in 1?year. With longer observations, it is shown that the
GMSL went further up to a total amount of 11.6?mm by the end
|
of 2012,
excluding the 3.0?mm/yr background trend. A reconciled sea level
budget, based on observations by Argo project, altimeter, and
gravity satellites, reveals that the true GMSL rise has been
masked by El Niño-Southern Oscillation-related fluctuations
and its rate has increased since 2010. After extracting the
influence of land water storage, it is shown that the GMSL has
been rising at a rate of 4.4?±?0.5?mm/yr for more than
3?years, due to an increase in the rate of both land ice loss
and steric change. |
|
Mayo de 2015
Estimating hydraulic conductivity of fractured rocks from
high-pressure packer tests with an Izbash's law-based empirical
model
Authors: Y Chen, S Hu et al
Link: Click here
Abstract
High-pressure packer test (HPPT) is an enhanced constant head
packer test for characterizing the permeability of fractured
rocks under high-pressure groundwater flow conditions. The
interpretation of the HPPT data, however, remains difficult
due to the transition of flow conditions in the conducting
structures and the hydraulic fracturing-induced permeability
enhancement in the tested rocks. In this study, a number of
HPPTs were performed in the sedimentary and intrusive rocks
located at 450 m depth in central Hainan Island. The obtained
Q-P curves were divided into a laminar flow phase (I), a non-Darcy
flow phase
|
(II), and a hydraulic fracturing
phase (III). The critical Reynolds number for the deviation
of flow from linearity into phase II was 25?66. The flow of
phase III occurred in sparsely to moderately fractured rocks,
and was absent at the test intervals of perfect or poor intactness.
The threshold fluid pressure between phases II and III was correlated
with RQD and the confining stress. An Izbash's law-based analytical
model was employed to calculate the hydraulic conductivity of
the tested rocks in different flow conditions. It was demonstrated
that the estimated hydraulic conductivity values in phases I
and II are basically the same, and are weakly dependent on the
injection fluid pressure, but it becomes strongly pressure dependent
as a result of hydraulic fracturing in phase III. The hydraulic
conductivity at different test intervals of a borehole is remarkably
enhanced at highly fractured zone or contact zone, but within
a rock unit of weak heterogeneity, it decreases with the increase
of depth. |
|
Mayo de 2015
The Yellowstone magmatic system from the mantle plume to
the upper crust
Authors: Hsin-Hua Huang, Fan-Chi Lin et al
Link: Click here
Abstract
The Yellowstone supervolcano is one of the largest active
continental silicic volcanic fields in the world. An understanding
of its properties is key to enhancing our knowledge of volcanic
mechanisms and corresponding risk. Using a joint local and
teleseismic earthquake P-wave
|
seismic inversion,
we revealed a basaltic lower-crustal magma body that provides
a magmatic link between the Yellowstone mantle plume and the
previously imaged upper-crustal magma reservoir. This lower-crustal
magma body has a volume of 46,000 cubic kilometers, ~4.5 times
that of the upper-crustal magma reservoir, and contains a melt
fraction of ~2%. These estimates are critical to understanding
the evolution of bimodal basaltic-rhyolitic volcanism, explaining
the magnitude of CO2 discharge, and constraining dynamic models
of the magmatic system for volcanic hazard assessment. |
|
Mayo de 2015
Dynamics of wind-affected volcanic plumes: The example
of the 2011 Cordón Caulle eruption, Chile
Authors: C. Bonadonna, M. Pistolesi et al
Link: Click here
Abstract
The 2011 Cordón Caulle eruption represents an ideal
case study for the characterization of long-lasting plumes
that are strongly affected by wind. The climactic phase lasted
for about 1?day and was classified as subplinian with plumes
between ~9 and 12?km above the vent and mass flow rate (MFR)
on the order of ~107?kg?s?1. Eruption intensity fluctuated
during the first 11?days with MFR values between 106 and 107?kg?s?1.
This activity was followed by several months of low-intensity
plumes with MFR?<?106?kg?s?1. Plume dynamics and rise were
strongly affected by wind during the whole eruption with negligible
upwind spreading and
|
sedimentation. The plumes that
developed on 4-6 and 20-22 June can be described as transitional,
i.e., plumes showing transitional behavior between strong and
weak dynamics, while the wind clearly dominated the rise height
on all the other days resulting in the formation of weak plumes.
Individual phases of the eruption range between Volcanic Explosivity
Indices (VEIs) 3 and 4, while the cumulative deposit related
to 4-7 June 2011 is associated with VEIs 4 and 5. Crosswind
cloud and deposit dispersal of the first few days are best described
by a linear combination of gravitational spreading and turbulent
diffusion, with velocities between 1 and 10 m?s?1. Downwind
cloud velocity for the same days is best described by a linear
combination of gravitational spreading and wind advection, with
velocities between 17 and 45?m?s?1. Results show how gravitational
spreading can be significant even for subplinian and small-moderate
eruptions strongly advected by wind and with low Richardson
number and low MFR. |
|
Mayo de 2015
Triggering of earthquake swarms following the 2011 Tohoku
megathrust earthquake
Authors: Koji Umeda, Koichi Asamori et al
Link: Click here
Abstract
Earthquake swarms, often interpreted to result from fluids
invading the brittle seismogenic zone, have seismicity patterns
that are significantly different from an aftershock sequence.
Following the Mw 9.0 Tohoku-Oki earthquake, an unusual, shallow
normal-faulting swarm sequence occurred near the Pacific coast
in the southeast Tohoku district. An integrated approach combining
geophysical and geochemical methods was utilized to establish
the presence of aqueous fluids around the seismic source region
and their derivation. Magnetotelluric inversion defined an
anomalous conductor with a width of 20?km and clearly visible
to depths of more than 20?km, extending to the base of the
crust. Independent geophysical observations, including seismic,
strongly support the suggestion that fluid-filled porous materials
and fluids associated with slab dehydration are present in
the convergent plate boundary. In order to provide geochemical
|
constraints
on the source of the fluids triggering the swarm activity, new
helium isotope data were acquired from gas and water samples
around the seismic source region. The observed 3He/4He ratios
in these samples are significantly lower than the atmospheric
value of 1.4?×?10?6, indicating that the mantle helium
contribution is less than 10% of the total helium. Assuming
the fluid-triggered swarm activity, the plausible explanations
for the generation of fluids are limited to the following: (1)
sediment porosity reduction and from smectite-illite and opal-quartz
reactions in the subducting deep sea sediments, (2) metamorphism
of fore-arc basin sediments, sedimentary, and/or volcanic rocks
detached from the plate, or (3) dehydration reactions in the
subducted oceanic crust and/or hydrated mantle below the fore-arc
mantle wedge. Geophysical and geochemical findings suggest metamorphic
fluids produced by isothermal decompression of altered sediments
accompanying uplift and exhumation. Owing to continued fluid
production at depths of ~20?km, the fluids migrate into the
seismic source region. The swarm sequence would have been triggered
by stress changes associated with the Tohoku-Oki earthquake,
enhanced by vertical metamorphic fluid expulsion from the reaction
zone. |
|
Mayo de 2015
The Yellowstone Caldera
Authors: P. Tizzani, M. Battaglia et al et al
Link: Click here
Abstract
We studied the Yellowstone caldera geological unrest between
1977 and 2010 by investigating temporal changes in differential
Interferometric Synthetic Aperture Radar (InSAR), precise
spirit leveling and gravity measurements. The analysis of
the 1992-2010 displacement time series, retrieved by applying
the SBAS InSAR technique, allowed the identification of three
areas of deformation: (i) the Mallard Lake (ML) and Sour Creek
(SC) resurgent domes, (ii) a region close to the Northern
Caldera Rim (NCR), and (iii) the eastern Snake River Plain
(SRP). While the eastern SRP shows a signal related to tectonic
deformation, the other two regions are influenced by the caldera
unrest. We removed the tectonic
|
signal from the InSAR displacements,
and we modeled the InSAR, leveling, and gravity measurements
to retrieve the best fitting source parameters. Our findings
confirmed the existence of different distinct sources, beneath
the brittle- ductile transition zone, which have been intermittently
active during the last three decades. Moreover, we interpreted
our results in the light of existing seismic tomography studies.
Concerning the SC dome, we highlighted the role of hydrothermal
fluids as the driving force behind the 1977-1983 uplift; since
1983-1993 the deformation source transformed into a deeper one
with a higher magmatic component. Furthermore, our results support
the magmatic nature of the deformation source beneath ML dome
for the overall investigated period. Finally, the uplift at
NCR is interpreted as magma accumulation, while its subsidence
could either be the result of fluids migration outside the caldera
or the gravitational adjustment of the source from a spherical
to a sill-like geometry. |
|
Mayo de 2015
Potential for larger earthquakes in the East San Francisco
Bay Area due to the direct connection between the Hayward
and Calaveras Faults
Authors: E. Chaussard, R. Bürgmann et al
Link: Click here
Abstract
The Hayward and Calaveras Faults, two strike-slip faults of
the San Andreas System located in the East San Francisco Bay
Area, are commonly considered independent structures for seismic
hazard assessment. We use Interferometric Synthetic Aperture
RADAR to show that surface creep on the Hayward Fault continues
15?km
|
farther
south than previously known, revealing new potential for rupture
and damage south of Fremont. The extended trace of the Hayward
Fault, also illuminated by shallow repeating micro-earthquakes,
documents a surface connection with the Calaveras Fault. At
depths greater than 3-5?km, repeating micro-earthquakes located
10?km north of the surface connection highlight the 3-D wedge
geometry of the junction. Our new model of the Hayward and Calaveras
Faults argues that they should be treated as a single system
with potential for earthquake ruptures generating events with
magnitudes greater than 7, posing a higher seismic hazard to
the East San Francisco Bay Area than previously considered. |
|
Mayo de 2015
Probing the underbelly of a supervolcano
Authors: Nikolai M. Shapiro and Ivan Koulakov
Link: Click here
Abstract
Human civilization remains vulnerable to volcanic eruptions.
For example, the moderate eruption of Eyjafjallajökull
volcano in Iceland in 2010 was responsible for the total disruption
of air traffic in Europe for several days. The largest eruptions
known in human history (such as that of Mount Tambora, Indonesia,
in 1815) ejected enormous volumes of volcanic material, ranging
from 25 to 150 km3, and caused serious worldwide climate changes,
leading to huge loss of life even in countries located far
from volcanoes. Even greater eruptions that have spewed out
more than 1000
|
km3 of ash and volcanic gases
into the atmosphere have occurred in the recent geological past.
The ash of such supereruptions covered huge areas, polluted
the atmosphere, and caused notable climate changes throughout
the world with marked effects on the biosphere. Evaluating whether
such strong volcanic eruptions will occur in the future requires
an understanding of the geological processes and physical mechanisms
that led to them. Such an understanding can be gained from studies
of the volcanic systems known to produce these supereruptions
in the near past. On page 773 of this issue, Huang et al. present
a new seismic tomography study of the crust and the uppermost
mantle beneath the Yellowstone volcanic field that provides
insights into the functioning of supervolcanoes. |
|
Abril de 2015
Magma and fluid migration at Yellowstone Caldera in the
last three decades inferred from InSAR, leveling, and gravity
measurements
Authors: P. Tizzani, M. Battaglia et al
Link: Click here
Abstract
We studied the Yellowstone caldera geological unrest between
1977 and 2010 by investigating temporal changes in differential
Interferometric Synthetic Aperture Radar (InSAR), precise
spirit leveling and gravity measurements. The analysis of
the 1992-2010 displacement time series, retrieved by applying
the SBAS InSAR technique, allowed the identification of three
areas of deformation: (i) the Mallard Lake (ML) and Sour Creek
(SC) resurgent domes, (ii) a region close to the Northern
Caldera Rim (NCR), and (iii) the eastern Snake River Plain
(SRP). While the eastern SRP shows a signal related to tectonic
deformation, the other two regions are influenced
|
by
the caldera unrest. We removed the tectonic signal from the
InSAR displacements, and we modeled the InSAR, leveling, and
gravity measurements to retrieve the best fitting source parameters.
Our findings confirmed the existence of different distinct sources,
beneath the brittle- ductile transition zone, which have been
intermittently active during the last three decades. Moreover,
we interpreted our results in the light of existing seismic
tomography studies. Concerning the SC dome, we highlighted the
role of hydrothermal fluids as the driving force behind the
1977-1983 uplift; since 1983-1993 the deformation source transformed
into a deeper one with a higher magmatic component. Furthermore,
our results support the magmatic nature of the deformation source
beneath ML dome for the overall investigated period. Finally,
the uplift at NCR is interpreted as magma accumulation, while
its subsidence could either be the result of fluids migration
outside the caldera or the gravitational adjustment of the source
from a spherical to a sill-like geometry. |
|
Abril de 2015
Deep recycling of oceanic asthenosphere material during
subduction
Authors: Lijun Liu and Quan Zhou
Link: Click here
Abstract
Uncertainties in the origin and composition of oceanic asthenosphere
lead to different views on its temporal evolution upon subduction.
We investigate the evolution of asthenosphere material during
subduction using high-resolution geodynamic models. In contrast
to some earlier models suggesting that limited amount of asthenosphere
material can be entrained during
|
subduction, we find that much
of the subslab mantle (>100 km thick) could recycle into
the deep mantle following the slab, even if this mantle layer
remains buoyant and less viscous during entrainment. Our results
support the hypothesis that observed trench-parallel subslab
seismic anisotropy is a downward continuation of the anisotropic
asthenosphere. The entrainment of sometimes buoyant asthenosphere
material provides a new mechanism for reducing slab dip angle
which is consistent the shallower Pacific slab underneath Japan
than that farther south. Episodic release of entrained buoyant
materials during subduction can also explain enigmatic intraplate
volcanism, such as the Changbaishan volcano in Northeast China. |
|
Abril de 2015
Historical bathymetric charts and the evolution of Santorini
submarine volcano, Greece
Authors: A. B. Watts, P. Nomikou et al
Link: Click here
Abstract
Historical bathymetric charts are a potential resource for
better understanding the dynamics of the seafloor and the
role of active processes, such as submarine volcanism. The
British Admiralty, for example, have been involved in lead
line measurements of seafloor depth since the early 1790s.
Here, we report on an analysis of historical charts in the
region of Santorini volcano, Greece. Repeat lead line surveys
in 1848, late 1866, and 1925-1928 as well as multibeam swath
bathymetry surveys in 2001 and 2006 have been used to document
changes in seafloor depth. These data reveal that the flanks
of the Kameni
|
Islands, a dacitic
dome complex in the caldera center, have shallowed by up to
?175 m and deepened by up to ?80 m since 1848. The largest shallowing
occurred between the late 1866 and 1925-1928 surveys and the
largest deepening occurred during the 1925-1928 and 2001 and
2006 surveys. The shallowing is attributed to the emplacement
of lavas during effusive eruptions in both 1866-1870 and 1925-1928
at rates of up to 0.18 and 0.05 km3 a?1, respectively. The deepening
is attributed to a load-induced viscoelastic stress relaxation
following the 1866-1870 and 1925-1928 lava eruptions. The elastic
thickness and viscosity that best fits the observed deepening
are 1.0 km and ?1016 Pa s, respectively. This parameter pair,
which is consistent with the predictions of a shallow magma
chamber thermal model, explains both the amplitude and wavelength
of the historical bathymetric data and the present day rate
of subsidence inferred from InSAR analysis. |
|
Abril de 2015
Reconciling subduction dynamics during Tethys closure with
large-scale Asian tectonics: Insights from numerical modeling
Authors: F.A. Capitanio, A. Replumaz et al
Link: Click here
Abstract
We use three-dimensional numerical models to investigate the
relation between subduction dynamics and large-scale tectonics
of continent interiors. The models show how the balance between
forces at the plate margins such as subduction, ridge push,
and far-field forces, controls the coupled plate margins and
interiors evolution. Removal of part of the slab by lithospheric
break-off during subduction destabilizes the convergent margin,
forcing migration of the subduction zone, whereas in the upper
plate large-scale lateral extrusion, rotations, and back-arc
stretching ensue. When external forces are modeled, such as
ridge push and far-field forces, indentation increases, with
large
|
collisional
margin advance and thickening in the upper plate. The balance
between margin and external forces leads to similar convergent
margin evolutions, whereas major differences occur in the upper
plate interiors. Here, three strain regimes are found: large-scale
extrusion, extrusion and thickening along the collisional margin,
and thickening only, when negligible far-field forces, ridge
push, and larger far-field forces, respectively, add to the
subduction dynamics. The extrusion tectonics develops a strong
asymmetry toward the oceanic margin driven by large-scale subduction,
with no need of preexisting heterogeneities in the upper plate.
Because the slab break-off perturbation is transient, the ensuing
plate tectonics is time-dependent. The modeled deformation and
its evolution are remarkably similar to the Cenozoic Asian tectonics,
explaining large-scale lithospheric faulting and thickening,
and coupling of indentation, extrusion and extension along the
Asian convergent margin as a result of large-scale subduction
process. |
|
Abril de 2015
Accumulation of permanent deformation during earthquake
cycles on reverse faults
Link: Click here
Abstract
A two-dimensional viscoelastic finite element model with a
discrete stick-slip thrust fault is used to investigate tectonic
strain accumulation and release associated with repetitive
earthquakes. Results show that although interseismic and coseismic
motions are opposite in sign, they generally do not completely
cancel each other over
|
a seismic cycle, as normally presumed
in elastic rebound theory. Complete elastic strain release adjacent
to dip-slip faults is inhibited by gravity, so that permanent
deformation accumulates in the lithosphere with time. In the
models, this permanent deformation is observed at the surface
as large-scale upwarping of a hanging wall anticline and depression
of an adjacent footwall foreland basin. The results are supported
by observation of similar features adjacent to reverse faults
in active collisional mountain belts and along the coastal margin
of some modern subduction zones. |
|
Abril de 2015
Deep recycling of oceanic asthenosphere material during
subduction
Authors: Lijun Liu and Quan Zhou
Link: Click here
Abstract
Uncertainties in the origin and composition of oceanic asthenosphere
lead to different views on its temporal evolution upon subduction.
We investigate the evolution of asthenosphere material during
subduction using high-resolution geodynamic models. In contrast
to some earlier models suggesting that limited amount of asthenosphere
material can be entrained during
|
subduction,
we find that much of the subslab mantle (>100?km thick) could
recycle into the deep mantle following the slab, even if this
mantle layer remains buoyant and less viscous during entrainment.
Our results support the hypothesis that observed trench-parallel
subslab seismic anisotropy is a downward continuation of the
anisotropic asthenosphere. The entrainment of sometimes buoyant
asthenosphere material provides a new mechanism for reducing
slab dip angle which is consistent the shallower Pacific slab
underneath Japan than that farther south. Episodic release of
entrained buoyant materials during subduction can also explain
enigmatic intraplate volcanism, such as the Changbaishan volcano
in Northeast China. |
|
Abril de 2015
Postearthquake relaxation evidence for laterally variable
viscoelastic structure and water content in the Southern California
mantle
Author: Fred F. Pollitz
Link: Click here
Abstract
I reexamine the lower crust and mantle relaxation following
two large events in the Mojave Desert: the 1992 M7.3 Landers
and 1999 M7.1 Hector Mine, California, earthquakes. Time series
from continuous GPS sites out to 300 km from the ruptures
are used to constrain models of postseismic relaxation. Crustal
motions in the Mojave Desert region are elevated above background
for several years following each event. To account for broadscale
relaxation of the lower crust and mantle, the Burgers body
model is employed, involving transient and steady state viscosities.
Joint afterslip/postseismic relaxation modeling of the GPS
time series up to one
|
decade following
the Hector Mine earthquake reveals a significant rheological
contrast between a northwest trending "southwest domain"
(that envelopes the San Andreas fault system and western Mojave
Desert) and an adjacent "northeast domain" (that envelopes
the Landers and Hector Mine rupture areas in the central Mojave
Desert). The steady state viscosity of the northeast domain
mantle asthenosphere is inferred to be ?4 times greater than
that of the southwest domain. This pattern is counter to that
expected for regional heat flow, which is higher in the northeast
domain, but it is explicable by means of a nonlinear rheology
that includes dependence on both strain rate and water concentration.
I infer that the southwest domain mantle has a relatively low
steady state viscosity because of its high strain rate and water
content. The relatively low mantle water content of the northeast
domain is interpreted to result from the continual extraction
of water through igneous and volcanic activity over the past
?20 Myr. |
|
Abril de 2015
Exploring the feasibility of a nationwide earthquake early
warning system in Italy
Authors: M. Picozzi, A. Zollo
Link: Click here
Abstract
When accompanied by appropriate training and preparedness
of a population, Earthquake Early Warning Systems (EEWS) are
effective and viable tools for the real-time reduction of
societal exposure to seismic events in metropolitan areas.
The Italian Accelerometric Network, RAN, which consists of
about 500 stations installed over all the active seismic zones,
as well as many cities and strategic infrastructures in Italy,
has the potential to serve as a nationwide early warning system.
In this work, we present a feasibility study for a nationwide
EEWS in Italy obtained by the integration of the RAN and the
software platform
|
PRobabilistic
and Evolutionary early warning SysTem (PRESTo). The performance
of the RAN-PRESTo EEWS is first assessed by testing it on real
strong motion recordings of 40 of the largest earthquakes that
have occurred during the last 10 years in Italy. Furthermore,
we extend the analysis to regions that did not experience earthquakes
by considering a nationwide grid of synthetic sources capable
of generating Gutenberg-Richter sequences corresponding to the
one adopted by the seismic hazard map of the Italian territory.
Our results indicate that the RAN-PRESTo EEWS could theoretically
provide for higher seismic hazard areas reliable alert messages
within about 5 to 10?s and maximum lead times of about 25?s.
In case of large events (M?>?6.5), this amount of lead time
would be sufficient for taking basic protective measures (e.g.,
duck and cover, move away from windows or equipment) in tens
to hundreds of municipalities affected by large ground shaking. |
|
Abril de 2015
Seismic fatigue failure may have triggered the 2014 Mw7.9
Rat Islands earthquake
Author: Masatoshi Miyazawa
Link: Click here
Abstract
Seismic waves propagating from large earthquakes cause global
transient stress changes capable of triggering other earthquakes
at great distances. The study of such remote and dynamic triggering
phenomena provides a better understanding of the mechanisms
that generate earthquakes. I introduce an integrated seismicity
model to stochastically evaluate the time intervals
|
of consecutive
earthquakes at global scales, making it possible to detect a
pair of earthquakes possibly related to each other. I show a
Mw7.9 intermediate-depth earthquake that occurred in the Rat
Islands in 2014 is inferred to have been associated with a sequence
of distant large (Mw?6.5) earthquakes originating from the Kermadec
Islands. The passage of seismic surface waves from the Kermadec
events that produced small stress changes varying within at
most 10?Pa at the hypocenter, probably caused a reduction in
the fault's strength by cyclic fatigue and eventually triggered
its failure during their passage. |
|
Marzo de 2015
Multiple scattering from icequakes at Erebus volcano, Antarctica:
Implications for imaging at glaciated volcanoes
Authors: J. Chaput, M. Campillo et al
Link: Click here
Abstract
We examine seismic coda from an unusually dense deployment
of over 100 short-period and broadband seismographs in the
summit region of Mount Erebus volcano on a network with an
aperture of approximately 5 km. We investigate the energy-partitioning
properties of the seismic wavefield generated by thousands
of small icequake sources originating on the upper volcano
and use them to estimate Green's functions via coda cross
correlation. Emergent coda seismograms suggest that this locale
should be particularly amenable to such methods. Using a small
aperture subarray, we find that
|
modal
energy partition between S and P wave energy between 1 and 4
Hz occurs in just a few seconds after event onset and persists
for tens of seconds. Spatially averaged correlograms display
clear body and surface waves that span the full aperture of
the array. We test for stable bidirectional Green's function
recovery and note that good symmetry can be achieved at this
site even with a geographically skewed distribution of sources.
We estimate scattering and absorption mean free path lengths
and find a power law decrease in mean free path between 1.5
and 3.3 Hz that suggests a quasi-Rayleigh or Rayleigh-Gans scattering
situation. Finally, we demonstrate the existence of coherent
backscattering (weak localization) for this coda wavefield.
The remarkable properties of scattered seismic wavefields in
the vicinity of active volcanoes suggests that the abundant
small icequake sources may be used for illumination where temporal
monitoring of such dynamic structures is concerned. |
|
Marzo de 2015
Vertical collapse origin of Las Cañadas caldera
(Tenerife, Canary Islands) revealed by 3-D magnetotelluric
inversion
Authors: P. Piña-Varas, J. Ledo et al
Link: Click here
Abstract
Tenerife island geology is one of the most complexes of the
Canaries archipelago. This complexity is evidenced by the
existing controversy regarding the lateral or vertical collapse
origin of the Las Cañadas caldera. The resistivity
structure of the Las Cañadas caldera has been determined
|
by the 3-D inversion
of 188 broadband magnetotelluric data. The resistivity distribution
obtained in the final model shows clear evidences of the presence
of a vertical structure under the Teide, associated to the buried
northern wall of the caldera. Additionally, the characteristics
of the main resistivity structure, a ring-shaped low-resistivity
body (<10 m) interpreted as a hydrothermal clay alteration
cap, would point out the presence of a handwall for the Icod
Valley lateral landslide located under the Teide, but not in
the southern caldera wall (current wall). All these support
the vertical collapse hypothesis to explain the origin of the
Las Cañadas caldera. |
|
Marzo de 2015
Investigating multiple fault rupture at the Salar del Carmen
segment of the Atacama Fault System (northern Chile): Fault
scarp morphology and knickpoint analysis
Authors: Oktawian Ewiak, Pia Victor et al
Link: Click here
Abstract
This study presents a new geomorphological approach to investigate
the past activity and potential seismic hazard of upper crustal
faults at the Salar del Carmen segment of the Atacama Fault
System in the northern Chile forearc. Our contribution is
based on the analysis of a large set of topographic profiles
and allows extrapolating fault analysis from a few selected
locations to distances of kilometers along strike of the fault.
We detected subtle changes in the fault scarp geometry which
may represent the number of paleoearthquakes experienced by
the structure and extracted the cumulative and last incremental
|
displacement
along strike of the investigated scarps. We also tested the
potential of knickpoints in channels crossing the fault scarps
as markers for repeated fault rupture and proxies for seismic
displacement. The number of paleoearthquakes derived from our
analysis is 2-3, well in agreement with recent paleoseismological
investigations, which suggest 2-3 earthquakes (Mw = 6.5-6.7)
at the studied segments. Knickpoints record the number of events
for about 55% of the analyzed profile pairs. Only few knickpoints
represent the full seismic displacement, while most retain only
a fraction of the displacement. The along-strike displacement
distributions suggest fault growth from the center toward the
tips and linkage of individual ruptures. Our approach also improves
the estimation of paleomagnitudes in case of multiple fault
rupture by allowing to quantify the last increment of displacement
separately. Paleomagnitudes calculated from total segment length
and the last increment of displacement (Mw = 6.5-7.1) are in
agreement with paleoseismological results. |
|
Marzo de 2015
Rare dynamic triggering of remote M>=5.5 earthquakes
from global catalog analysis
Authors: Christopher W. Johnson, Roland Bürgmann et al
Link: Click here
Abstract
Probing the effects of a transient stress on the timing of
an earthquake occurrence is necessary for understanding the
remote interaction of large-magnitude events. Global catalog
data containing 35?years of M???5.5 earthquakes allow us to
explore for periods of enhanced or suppressed seismic activity.
We consider 113?M? ??7.5 main shocks between 1977 and 2012
and focus on seismic activity on time scales from seconds
to days following these main shocks. We search for evidence
of dynamic triggering of large-magnitude events similar to
the previously observed global increase during the first few
days following the
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2012
M8.6 Indian Ocean main shock. We restrict the analysis to regions
of elevated strain during the passage of surface waves. Using
a threshold of 0.1 microstrain (~3?kPa) and a temporal window
of ±1?year, we stack daily seismicity rate curves using
the exclusion-zone declustered M>=5.5 catalog events in order
to resolve deviations from the background rate. Our results
do not indicate a significant change in activity for at least
10?days when considering the collective set of 113 main shocks
and subsets at M8.0 and M8.5 thresholds. The results also do
not indicate immediate triggering of M>=5.5 events. We do
find two instances of increased seismicity in the elevated strain
region within 10?days. These increases are subsequent to two
main shocks, the 1977?M8.3 and 2012?M8.6, both located in the
Indian Ocean. We conclude that a global change in M>=5.5
earthquake rates following a transient stress from distant earthquakes
is a rare occurrence. |
|
Marzo de 2015
Episodic tremor and slip near the Japan Trench prior to
the 2011 Tohoku-Oki earthquake
Authors: Yoshihiro Ito, Ryota Hino et al
Link: Click here
Abstract
Change in the rates of aseismic deformation prior to large
earthquakes is a major area of interest in earthquake physics.
Here we present evidence that episodic tremor and slip occurred
in the shallow subduction zone within the source region of
the 2011 Tohoku-Oki earthquake prior to the
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main shock.
Ocean bottom seismometers near the Japan Trench recorded some
excitations in amplitude of ambient noise level accompanying
both the 2008 and 2011 slow slip events. The observed signals
show that low frequencies of 5-8 Hz dominated, suggesting that
the excitations were due to small low-frequency tremors accompanying
the slow slip events. The largest amplitude tremor was observed
just before the 2011 event. The estimated sources of tremors
were possibly distributed within the coseismic slip area of
the 2011 event, suggesting the shallow plate-boundary thrust
near the trench is a general location of slow earthquakes. |
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Marzo de 2015
Quantification of volcanic cloud-top heights and thicknesses
using A-train observations for the 2008 Chaitén eruption
Authors: A T Prata, S T Siems et al
Link: Click here
Abstract
New evidence of vertically thin (<400 m), low-level (<10
km) volcanic ash clouds, as well as confirmation of previously
reported high-level (>10 km) ash clouds, from the May 2008
Chaitén eruption in southern Chile is presented. A-trainremote
sensors were used to measure high resolution volcanic cloud-top
heights (VCTHs) during the explosive phase (2-10 May 2008)
of the eruption. Ash clouds were identified using a reverse
absorption technique applied to hyperspectral measurements
taken by the Atmospheric InfraRed Sounder (AIRS). Once identified,
heights and thicknesses were derived
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from
the Cloud-Aerosol LIdar with Orthogonal Polarisation (CALIOP)
instrument. As these two instruments are part of the same constellation
of satellites, coincident retrievals are routinely possible.
Collocation of the data allowed for detection of volcanic ash
within CALIOP profiles. Using a simple thresholding algorithm,
VCTH and thickness were derived from the CALIOP profiles. A
total of 12 VCTH measurements, ranging from 3.7 to 16.6 km,
have been derived. Back trajectories from the Hybrid Single
Particle Lagrangian Integrated Trajectory (HYSPLIT) dispersion
model were used as a check on volcanic origin of the detected
ash clouds. Ensemble forward trajectories were generated to
demonstrate how the new data could be used to improve Volcanic
Ash Advisory Centre (VAAC) operations. The findings reported
here demonstrate several cases where low-level ash was not recorded
previously, and include observations of thin ash clouds at large
distances (~cpo 4000 km) from the volcano. |
|
Marzo de 2015
Static Coulomb stress-based Southern California earthquake
forecasts: A pseudoprospective test
Authors: Anne Strader and David D. Jackson
Link: Click here
Abstract
Many studies support the hypothesis that where earthquakes
occur, recent changes in resolved Coulomb stress tend to be
positive. How about the converse hypothesis, that where resolved
Coulomb stress recently increased, earthquakes are more likely
to occur? Successful earthquake forecasting by Coulomb stress
changes requires the converse. To test this, we calculated
stress everywhere in our study area, not just at earthquake
locations. We modeled stress accumulation in Southern California
since 1812 both from the elastic effect of slip below locked
faults and from M???5 "source" earthquakes up to
any given date. To minimize the effect of secondary aftershocks
not directly related to the
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source earthquakes,
we measured seismicity using a gridded binary map: each 0.1°?×?0.1°
cell is "activated" if containing one or more test
events ("receiver" earthquakes) of M???2.8. We then
constructed an empirical relationship between resolved Coulomb
stress and activation rate within regions with similar stress
values, defining probabilities of activated cells during the
"test" period, within 11?years of the M7.1 Hector
Mine earthquake. We found that Coulomb stress reliably indicates
future earthquake locations at the 95% confidence interval.
However, smoothed seismicity forecasts outperformed Coulomb
forecasts in some areas with large earthquakes due to aftershock
clustering. Most earthquakes tend to nucleate in areas with
Coulomb stress changes greater than 0.5?MPa or less than ?0.5?MPa.
Within areas with increased Coulomb stress from older earthquakes,
fewer earthquakes occurred than anticipated. After reducing
stress uncertainty impact, Coulomb rate-and-state forecasts
may also improve upon statistical earthquake forecasts. |
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Marzo de 2015
Novel analysis of a sudden ionospheric disturbance using
Schumann resonance measurements
Authors: Micha Dyrda, Andrzej Kulak et al
Link: Click here
Abstract
A spherical cavity between Earth and the lower ionosphere
forms a global resonator for Extremely Low Frequency electromagnetic
waves. Constant thunderstorm activity leads to the formation
of a resonance field in the cavity, known as the Schumann
resonance. Solar flare generated
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Sudden
Ionospheric Disturbances (SID) modify the ionosphere affecting
the ground-based radio communication systems. They are also
expected to modify radiowave propagation in the cavity. In this
paper, the Schumann Resonance spectral decomposition method
is used for the first time to study the cavity resonance frequencies
during the SID accompanying a strong X2.1 solar flare. We analyzed
rapid changes in the frequencies and Q factors of the first
five resonance modes using a 5?min timescale. The observed frequency
shifts were compared to the ionizing solar flare fluxes in the
UV, X-ray, and high-energy ? rays. |
|
Marzo de 2015
On the stress dependence of the earthquake b value
Author: Christopher H. Scholz
Link: Click here
Abstract
Laboratory experiments have shown that the b value in the
size distribution of acoustic emission events decreases linearly
with differential stress. There have been a number of observations
that indicate that this relation may also hold for earthquakes.
Here using a simple frictional strength model for
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stresses in
the continental lithosphere combined with earthquake b values
measured as a function of depth in a wide variety of tectonic
regions, we verify and calibrate that relation, finding b=1.23±0.06
(0.0012±0.0003), where the stress difference is in megapascal.
For subduction zones, we find that b value correlates linearly
with the slab pull force and with the net reduction of plate
interface normal force, both of which also indicate a negative
linear relation between b value and differential stress. |
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Marzo de 2015
Stress drops for intermediate-depth intraslab earthquakes
beneath Hokkaido, northern Japan: Differences between the
subducting oceanic crust and mantle events
Authors: Saeko Kita and Kei Katsumata
Link: Click here
Abstract
Spatial variations in the stress drop for 1726 intermediate-depth
intraslab earthquakes were examined in the subducting Pacific
plate beneath Hokkaido, using precisely relocated hypocenters,
the corner frequencies of events, and detailed determined
geometry of the upper interface of the Pacific plate. The
results show that median stress drop for intraslab earthquakes
generally increases with an increase in depth from 10 to 157
Mpa at depths of 70-300 km. More specifically,
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median stress
drops for events in the oceanic crust decrease (9.9-6.8 MPa)
at depths of 70-120 km and increase (6.8-17 MPa) at depths of
120-170 km, whereas median stress drop for events in the oceanic
mantle decrease (21.6-14.0 MPa) at depths of 70-170 km, where
the geometry of the Pacific plate is well determined. The increase
in stress drop with depth in the oceanic crust at depths of
120-170 km, for which several studies have shown an increase
in velocity, can be explained by an increase in the velocity
and a decrease in the water content due to the phase boundary
with dehydration in the oceanic crust. Stress drops for events
in the oceanic mantle were larger than those for events in the
oceanic crust at depths of 70-120 km. Differences in both the
rigidity of the rock types and in the rupture mechanisms for
events between the oceanic crust and mantle could be causes
for the stress drop differences within a slab. |
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Marzo de 2015
The Swarm Initial Field Model for the 2014 geomagnetic
field
Authors: Nils Olsen, Gauthier Hulot et al
Link: Click here
Abstract
Data from the first year of ESA's Swarm constellation mission
are used to derive the Swarm Initial Field Model (SIFM), a
new model of the Earth's magnetic field and its time variation.
In addition to the conventional magnetic field observations
provided by each of the three Swarm satellites, explicit advantage
is taken of the
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constellation
aspect by including east-west magnetic intensity gradient information
from the lower satellite pair. Along-track differences in magnetic
intensity provide further information concerning the north-south
gradient. The SIFM static field shows excellent agreement (up
to at least degree 60) with recent field models derived from
CHAMP data, providing an initial validation of the quality of
the Swarm magnetic measurements. Use of gradient data improves
the determination of both the static field and its secular variation,
with the mean misfit for east-west intensity differences between
the lower satellite pair being only 0.12?nT. |
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Marzo de 2015
Slow stick slip of antigorite serpentinite under hydrothermal
conditions as a possible mechanism for slow earthquakes
Authors: Keishi Okazaki and Ikuo Katayama
Link: Click here
Abstract
Slow earthquakes, characterized by a different scaling law
to regular earthquakes, have been detected at the hydrated
plate interface in the subduction zones, but the generating
mechanism of them remains almost unexplored. Frictional experiments
on antigorite serpentinite under hydrothermal conditions are
conducted to assess
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the distinct
scaling law of slow earthquakes. Slow stick-slip was observed
at temperatures that were close to the dehydration temperature
of antigorite, which is resulted by the localized dehydration
of serpentine in the shear zone. The occurrence of slow stick-slip
is consistent with the temperature range found in the corner
of the mantle wedge in SW Japan and Cascadia, where slow earthquakes
occur. The laboratory slow stick-slip shows a similar scaling
law of slow earthquakes, but distinct from that of regular earthquakes.
We propose that the shear-induced dehydration of the serpentine
play an important role for the generation of slow earthquakes. |
|
Marzo de 2015
Coping with earthquakes induced by fluid injection
Authors: A. McGarr, B. Bekins et al
Link: Click here
Abstract
Large areas of the United States long considered geologically
stable with little or no detected seismicity have recently
become seismically active. The increase in earthquake activity
began in the mid-continent starting in 2001 and has continued
to rise. In 2014, the rate of occurrence of earthquakes with
magnitudes (M) of 3 and greater
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in
Oklahoma exceeded that in California (see the figure). This
elevated activity includes larger earthquakes, several with
M > 5, that have caused significant damage. To a large extent,
the increasing rate of earthquakes in the mid-continent is due
to fluid-injection activities used in modern energy production.
We explore potential avenues for mitigating effects of induced
seismicity. Although the United States is our focus here, Canada,
China, the UK, and others confront similar problems associated
with oil and gas production, whereas quakes induced by geothermal
activities affect Switzerland, Germany, and others. |
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Marzo de 2015
New jitters over megaquakes in Himalayas
Author: Priyanka Pulla
Link: Click here
Abstract
Seismologists worried about the prospect of a massive earthquake
in the shadow of the Himalayas, where it could devastate cities
such as Kathmandu and Delhi, have long cast a wary glance
at an eerily calm region called the central seismic gap (CSG).
A massive earthquake in
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southwestern
Tibet in 1505 C.E., researchers proposed a decade ago, relieved
enough strain to quiet that stretch of the restive Himalayas.
But new findings now suggest that the 1505 temblor was smaller
than thought and was just one of a cluster of potent quakes
to rattle the region within a few centuries. If so, major quakes
in the Himalayas, unlike in many other seismic hot spots, may
not relieve enough strain to forestall later quakes—meaning
that authorities must gird for a megaearthquake anywhere at
any time. |
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Febrero de 2015
Three-dimensional dynamic rupture simulations across interacting
faults: The Mw7.0, 2010, Haiti earthquake
Authors: R. Douilly, H. Aochi et al
Link: Click here
Abstract
The mechanisms controlling rupture propagation between fault
segments during a large earthquake are key to the hazard posed
by fault systems. Rupture initiation on a smaller fault sometimes
transfers to a larger fault, resulting in a significant event
(e.g., 2002 M7.9 Denali USA and 2010 M7.1 Darfield New Zealand
earthquakes). In other cases rupture is constrained to the
initial fault and does not transfer to nearby faults, resulting
in events of more moderate magnitude. This was the case of
the 1989 M6.9 Loma Prieta and 2010 M7.0 Haiti
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earthquakes
which initiated on reverse faults abutting against a major strike-slip
plate boundary fault but did not propagate onto it. Here we
investigate the rupture dynamics of the Haiti earthquake, seeking
to understand why rupture propagated across two segments of
the Léogâne fault but did not propagate to the
adjacent Enriquillo Plantain Garden Fault, the major 200?km
long plate boundary fault cutting through southern Haiti. We
use a finite element model to simulate propagation of rupture
on the Léogâne fault, varying friction and background
stress to determine the parameter set that best explains the
observed earthquake sequence, in particular, the ground displacement.
The two slip patches inferred from finite fault inversions are
explained by the successive rupture of two fault segments oriented
favorably with respect to the rupture propagation, while the
geometry of the Enriquillo fault did not allow shear stress
to reach failure. |
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Febrero de 2015
Complex inner core of the Earth: The last frontier of global
seismology
Author: Hrvoje Tkalcic*
Link: Click here
Abstract
The days when the Earth's inner core (IC) was viewed as a
homogeneous solid sphere surrounded by the liquid outer core
(OC) are now behind us. Due to a limited number of data sampling
the IC and a lack of experimentally controlled conditions
in the deep Earth studies, it has been difficult to scrutinize
competitive hypotheses in this active area of research. However,
a number of new concepts linking IC structure and dynamics
has been proposed lately to explain different types of seismological
observations. A common denominator of recent observational
work on the IC is increased complexity seen in IC physical
properties such as its isotropic and anisotropic structure,
attenuation, inner core boundary (ICB) topography, and its
rotational dynamics. For example, small-scale features have
been observed to exist as a widespread phenomenon in the uppermost
inner core, probably superimposed on much longer-scale features.
The characterization of small-scale features sheds light on
the nature of the
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solidification
process and helps in understanding seismologically observed
hemispherical dichotomy of the IC. The existence of variations
in the rate and level of solidification is a plausible physical
outcome in an environment where vigorous compositional convection
in the OC and variations in heat exchange across the ICB may
control the process of crystal growth. However, further progress
is hindered by the fact that the current traveltime data of
PKIKP waves traversing the IC do not allow discriminating between
variations in isotropic P wave velocity and velocity anisotropy.
Future studies of attenuation in the IC might provide crucial
information about IC structure, although another trade-off exists—that
of the relative contribution of scattering versus viscoelastic
attenuation and the connection with the material properties.
Future installations of dense arrays, cross paths of waves that
sample the IC, and corresponding array studies will be a powerful
tool to image and clearly distinguish between viscoelastic and
scattering attenuation, and isotropic- and anisotropic-heterogeneity
related effects on traveltimes of core-sensitive body waves.
This will then inevitably contribute to a better understanding
of what the IC is made of, how it solidifies and how it contributes
to the generation and dynamics of the geomagnetic field. |
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Febrero de 2015
Fault slip distribution of the 2014 Iquique, Chile, earthquake
estimated from ocean-wide tsunami waveforms and GPS data
Authors: Aditya Riadi Gusman, Satoko Murotani et al
Link: Click here
Abstract
We applied a new method to compute tsunami Green's functions
for slip inversion of the 1 April 2014 Iquique earthquake
using both near-field and far-field tsunami waveforms. Inclusion
of the effects of the elastic loading of seafloor, compressibility
of seawater, and the geopotential
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variation
in the computed Green's functions reproduced the tsunami traveltime
delay relative to long-wave simulation and allowed us to use
far-field records in tsunami waveform inversion. Multiple time
window inversion was applied to tsunami waveforms iteratively
until the result resembles the stable moment rate function from
teleseismic inversion. We also used GPS data for a joint inversion
of tsunami waveforms and coseismic crustal deformation. The
major slip region with a size of 100?km?×?40?km is located
downdip the epicenter at depth ~28 km, regardless of assumed
rupture velocities. The total seismic moment estimated from
the slip distribution is 1.24?×?1021?N?m (Mw 8.0). |
|
Febrero de 2015
From quiescence to unrest: 20 years of satellite geodetic
measurements at Santorini volcano, Greece
Authors: Michelle M. Parks, James D. P. Moore et al
Link: Click here
Abstract
Periods of unrest at caldera-forming volcanic systems characterized
by increased rates of seismicity and deformation are well
documented. Some can be linked to eventual eruptive activity,
while others are followed by a return to quiescence. Here
we use a 20?year record of interferometric synthetic aperture
radar (InSAR) and GPS measurements from Santorini volcano
to further our understanding of geodetic signals at a caldera-forming
volcano during the periods of both quiescence and unrest,
with measurements spanning a phase of quiescence and slow
subsidence (1993–2010), followed by a phase of unrest
(January 2011 to April 2012) with caldera-wide inflation and
seismicity. Mean InSAR velocity maps from 1993–2010 indicate
an average subsidence rate of ~6?mm/yr over the southern half
of the intracaldera island Nea Kameni. This subsidence can
be accounted for by a combination of thermal contraction of
the 1866–1870 lava flows and load-induced relaxation
of the substrate. For the period of unrest, we use a joint
inversion technique to convert InSAR
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measurements
from three separate satellite tracks and GPS observations from
10 continuous sites into a time series of subsurface volume
change. The optimal location of the inflating source is consistent
with previous studies, situated north of Nea Kameni at a depth
of ~4?km. However, the time series reveals two distinct pressure
pulses. The first pulse corresponds to a volume change (?V)
within the shallow magma chamber of (11.56?±?0.14)?×?106?m3,
and the second pulse has a V of (9.73?± ?0.10)?×?106?m3.
The relationship between the timing of these pulses and microseismicity
observations suggests that these pulses may be driven by two
separate batches of magma supplied to a shallow reservoir. We
find no evidence suggesting a change in source location between
the two pulses. The decline in the rates of volume change at
the end of both pulses and the observed lag of the deformation
signal behind cumulative seismicity, suggest a viscoelastic
response. We use a simple model to show that two separate pulses
of magma intruding into a shallow magma chamber surrounded by
a viscoelastic shell can account for the observed temporal variation
in cumulative volume change and seismicity throughout the period
of unrest. Given the similarities between the geodetic signals
observed here and at other systems, this viscoelastic model
has potential use for understanding behavior at other caldera
systems. |
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Febrero de 2015
Investigating multiple fault rupture at the Salar del Carmen
segment of the Atacama Fault System (northern Chile): Fault
scarp morphology and knickpoint analysis
Authors: Oktawian Ewiak, Pia Victor et al
Link: Click here
Abstract
This study presents a new geomorphological approach to investigate
the past activity and potential seismic hazard of upper crustal
faults at the Salar del Carmen segment of the Atacama Fault
System in the northern Chile forearc. Our contribution is
based on the analysis of a large set of topographic profiles
and allows extrapolating fault analysis from a few selected
locations to distances of kilometers along strike of the fault.
We detected subtle changes in the fault scarp geometry which
may represent the number of paleoearthquakes experienced by
the structure and extracted the cumulative and last incremental
displacement along strike of the investigated
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scarps.
We also tested the potential of knickpoints in channels crossing
the fault scarps as markers for repeated fault rupture and proxies
for seismic displacement. The number of paleoearthquakes derived
from our analysis is 2-3, well in agreement with recent paleoseismological
investigations, which suggest 2-3 earthquakes (Mw=6.5-6.7) at
the studied segments. Knickpoints record the number of events
for about 55% of the analyzed profile pairs. Only few knickpoints
represent the full seismic displacement, while most retain only
a fraction of the displacement. The along-strike displacement
distributions suggest fault growth from the center toward the
tips and linkage of individual ruptures. Our approach also improves
the estimation of paleomagnitudes in case of multiple fault
rupture by allowing to quantify the last increment of displacement
separately. Paleomagnitudes calculated from total segment length
and the last increment of displacement (Mw=6.5-7.1) are in agreement
with paleoseismological results. |
|
Febrero de 2015
Rapid slip-deficit rates at the eastern margin of the Tibetan
plateau prior to the 2008 Mw 7.9 Wenchuan Earthquake
Authors: T. Ben Thompson, Andreas Plesch et al
Link: Click here
Abstract
The Longmen Shan is the steepest topographic front at the
India-Asia collision zone and the site of the Mw 7.9 Wenchuan
earthquake. Here, to explain the interseismic GPS velocities
across the greater Longmen Shan region, we develop a boundary
element model including earthquake
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cycle effects,
topography, the westward dipping Beichuan fault, and a ~20-km-deep,
shallowly dipping, detachment, inferred from observed afterslip
and from structural considerations. Previous analyses which
neglected the detachment and earthquake cycle effects have found
shortening rates near zero. In contrast, we find that interseismic
GPS data are consistent with a shortening rate of 5.7 ±
1.5 mm/yr and maximum surface slip-deficit rate of 9.5 ±
2.5 mm/yr. This model unifies the interpretation of geodetic
deformation throughout the earthquake cycle and suggests that
the Longmen Shan is an active fold-and-thrust belt with of Wenchuan-like
recurrence intervals as short as 600 years. |
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Febrero de 2015
Application of tc*Pd in Earthquake Early Warning
Authors: Po-Lun Huang, Ting-Li Lin et al
Link: Click here
Abstract
Rapid assessment of damage potential and size of an earthquake
at the station is highly demanded for onsite earthquake early
warning
|
(EEW). We study
the application of ?c*Pd for its estimation on the earthquake
size using 123 eventrecorded by the borehole stations of KiK-net
in Japan. The new type of earthquake size determined by ?c*Pd
is more related to the damage potential. We find that ?c*Pd
provides another parameter to measure the size of earthquake
and the threshold to warn strong ground motion. |
|
Febrero de 2015
Real-time eruption forecasting using the material Failure
Forecast Method with a Bayesian approach
Authors: A. Boué, P. Lesage et al
Link: Click here
Abstract
Many attempts for deterministic forecasting of eruptions and
landslides have been performed using the material Failure
Forecast Method (FFM). This method consists in adjusting an
empirical power law on precursory patterns of seismicity or
deformation.Until now, most of the studies have presented
hindsight forecasts based on complete time series of precursors,
and do not evaluate the ability of the method for carrying
out real-time forecasting with partial precursory sequences.
In this study, we present a rigorous approach of the FFM designed
for real-time applications on
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volcano-seismic
precursors. We use a Bayesian approach based on the FFM theory
and an automatic classification of seismic events. The probability
distributions of the data deduced from the performance of this
classification are used as input. As output, it provides the
probability of the forecast time at each observation time before
the eruption. The spread of the a posteriori probability density
function (pdf) of the prediction time and its stability with
respect to the observation time are used as criteria to evaluate
the reliability of the forecast. We test the method on precursory
accelerations of LP seismicity prior to vulcanian explosions
at Volcán de Colima (Mexico). For explosions preceded
by a single phase of seismic acceleration, we obtain accurate
and reliable forecasts using approximately 80% of the whole
precursory sequence. It is however more difficult to apply the
method to multiple acceleration patterns. |
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Febrero de 2015
Aftershock triggering by postseismic stresses: a study
based on Coulomb-Rate-and-State models
Authors: Camilla Cattania, Sebastian Hainzl et al
Link: Click here
Abstract
The spatio-temporal clustering of earthquakes is a feature
of medium and short term seismicity, indicating that earthquakes
interact. However, controversy exists about the physical mechanism
behind aftershock triggering: static stress transfer and reloading
by postseismic processes have been proposed as explanations.
In this work, we use a Coulomb Rate-and-State model to study
the role of coseismic and postseismic stress changes on aftershocks,
and focus on two processes: creep on the mainshock fault plane
(afterslip), and secondary aftershock triggering by previous
aftershocks. We model the seismic response to Coulomb stress
changes using the
|
Dieterich constitutive
law, and focus on two events: the Parkfield, Mw=6.0 and the
Tohoku, Mw=9.0 earthquakes.
We find that modeling secondary triggering systematically improves
the maximum log-likelihood fit of the sequences. The effect
of afterslip is more subtle, and difficult to assess for near-fault
events, where model errors are largest. More robust conclusions
can be drawn for off-fault aftershocks: following the Tohoku
earthquake, afterslip promotes shallow crustal seismicity in
the Fukushima region. Simple geometrical considerations indicate
that afterslip-induced stress changes may have been significant
on trench parallel crustal fault systems following several of
the largest recorded subduction earthquakes. Moreover, the time
dependence of afterslip strongly enhances its triggering potential:
seismicity triggered by an instantaneous stress change decays
more quickly than seismicity triggered by gradual loading, and
as a result we find afterslip to be particularly important between
few weeks and few months after the mainshock. |
|
Febrero de 2015
Characterization of water content dynamics and tracer
breakthrough by 3-D electrical resistivity tomography (ERT)
under transient unsaturated conditions
Authors: Markus Wehrer and Lee D. Slater
Link: Click here
Abstract
Characterization of preferential flow and transport is still
a major challenge but may be improved employing noninvasive,
tomographic methods. In this study, 3-D time lapse electrical
resistivity tomography (ERT) was employed during infiltration
on an undisturbed, unsaturated soil core in a laboratory lysimeter.
A tracer breakthrough was conducted during transient conditions
by applying a series of short-term infiltrations, simulating
natural precipitation events. The electrical response was
quantitatively validated using data from a multicompartment
suction sampler. Water content probes were also installed
for ground-truthing of ERT responses. Water content variations
associated with an
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infiltration front dominated the electrical response observed
during individual short-term infiltration events, permitting
analysis of water content dynamics from ERT data. We found that,
instead of the application of an uncertain petrophysical function,
shape measures of the electrical conductivity response might
be used for constraining hydrological models. Considering tracer
breakthroughs, the ERT observed voxel responses from time lapse
tomograms at constant water contents in between infiltration
events were used to quantitatively characterize the breakthrough
curve. Shape parameters of the breakthrough derived from ERT,
such as average velocity, were highly correlated with the shape
parameters derived from local tracer breakthrough curves observed
in the compartments of the suction plate. The study demonstrates
that ERT can provide reliable quantitative information on both,
tracer breakthroughs and water content variations under the
challenging conditions of variable background electrical conductivity
of the pore solution and non steady-state infiltration. |
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Febrero de 2015
On the onset of ionospheric precursors 40 min before strong
earthquakes
Authors: F. Masci, J. N. Thomas et al
Link: Click here
Abstract
Heki (2011) and Heki and Enomoto (2013) claimed that anomalous,
yet similar, increases of ionospheric total electron content
(TEC) started ~40 min prior to the 2011 Tohoku-Oki, as well
as before other Mw? >?8 earthquakes. The authors concluded
that the reported TEC anomalies were likely related to the
pending earthquakes, suggesting also that TEC monitoring may
be useful for future earthquake prediction. Here we
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carefully
examine the findings of Heki (2011) and Heki and Enomoto (2013)
by performing new analyses of the same TEC data. Our interpretation
is that the 40 min onset of the ionospheric precursors is an
artifact induced by the definition of the reference line adopted
in analyzing TEC variations. We also discuss this repeatability
in the tectonic and geodynamic context of the earthquakes. By
performing a Superimposed Epoch Analysis of TEC data, we show
that, however, the TEC increase reported by Heki (2011) was
not particularly anomalous. We conclude that the TEC precursors
reported by Heki (2011) and Heki and Enomoto (2013) are not
useful for developing short-term earthquake prediction capabilities. |
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Febrero de 2015
Absolute gravity change associated with magma mass movement
in the conduit of Asama Volcano (Central Japan), revealed
by physical modeling of hydrological gravity disturbances
Authors: Takahito Kazama, Shuhei Okubo et al
Link: Click here
Abstract
The gravity signal originating from magma mass movement in
a volcanic conduit is retrieved from the hydrologically disturbed
absolute gravity data obtained at Asama Volcano (Central Japan)
in 2004, using a three-dimensional hydrological model. We
improve the hydrological model of the previous study using
realistic soil parameters and boundary conditions, to better
estimate the spatiotemporal land-water distributions and the
consequent hydrological gravity disturbances. The newly estimated
gravity disturbances agree with the absolute gravity values
observed by FG5 gravimeters in 2004-2009 within about 2.6
?Gal, by additionally accounting for the excess
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discharge
of groundwater mass associated with a sloping impermeable surface
below the discharge area. After the gravity disturbance of 20
?Gal amplitude is subtracted from the absolute gravity data
observed during the 2004 eruptive event, the gravity residual
of 5 ?Gal amplitude shows a significant decrease in synchronization
with eruptions, because the ascending magma mass in the conduit
affects the upward attraction force to the gravimeters installed
on the flank of Asama Volcano. The magma head altitude, to which
the residual gravity is converted assuming a homogeneous linear
density in the conduit, shows a comprehensive agreement of the
time variation in the magma head with those in other volcanic
observations, such as gas emission rate and earthquake frequency.
By correcting the hydrological gravity disturbances using this
hydrological model and simultaneously obtained meteorological
data in real time, spatiotemporal variations in the magma mass
can be instantaneously monitored at Asama Volcano, even before
eruptions during future volcanic events. |
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Febrero de 2015
The Swarm Initial Field Model for the 2014 geomagnetic
field
Authors: Nils Olsen, Gauthier Hulot et al
Link: Clic here
Abstract
Data from the first year of ESA's Swarm constellation mission
are used to derive the Swarm Initial Field Model (SIFM), a
new model of the Earth's magnetic field and its time variation.
In A regionalization method based on a clusteraddition to
the conventional magnetic field observations provided by each
of the three Swarm
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satellites, explicit advantage is taken of
the constellation aspect by including East-West magnetic intensity
gradient information from the lower satellite pair. Along-track
differences in magnetic intensity provide further information
concerning the North-South gradient. The SIFM static field
shows excellent agreement (up to at least degree 60) with
recent field models derived from CHAMP data, providing an
initial validation of the quality of the Swarm magnetic measurements.
Use of gradient data improves the determination of both the
static field and its secular variation, with the mean misfit
for East-West intensity differences between the lower satellite
pair being only 0.12 nT.
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