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Diciembre de 2014
A friction to flow constitutive law and its application
to a 2-D modeling of earthquakes
Authors: Toshihiko Shimamoto and Hiroyuki Noda
Link: Click here
Establishment of a constitutive law from friction
to high-temperature plastic flow has long been a challenging
task for solving problems such as modeling earthquakes and plate
interactions. Here we propose an empirical constitutive law
that describes this transitional behavior using only friction
and flow parameters, with good agreements with experimental
data on halite shear zones. The law predicts steady state and
transient behaviors, including the dependence of the shear resistance
of fault on slip rate, effective normal stress, and temperature.
It also predicts a change in velocity weakening to velocity
strengthening with increasing temperature, similar to the changes
recognized for quartz and granite gouge under hydrothermal conditions.
A slight deviation from the
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steady
state friction law due to the involvement of plastic deformation
can cause a large change in the velocity dependence. We solved
seismic cycles of a fault across the lithosphere with the law
using a 2-D spectral boundary integral equation method, revealing
dynamic rupture extending into the aseismic zone and rich evolution
of interseismic creep including slow slip prior to earthquakes.
Seismic slip followed by creep is consistent with natural pseudotachylytes
overprinted with mylonitic deformation. Overall fault behaviors
during earthquake cycles are insensitive to transient flow parameters.
The friction-to-flow law merges "Christmas tree" strength
profiles of the lithosphere and rate dependency fault models used
for earthquake modeling on a unified basis. Strength profiles
were drawn assuming a strain rate for the flow regime, but we
emphasize that stress distribution evolves reflecting the fault
behavior. A fault zone model was updated based on the earthquake
modeling. |
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Diciembre de 2014
On the earthquake predictability of fault interaction
models
Authors: W. Marzocchi and D. Melini.
Link: Click here
Space-time clustering is the most striking departure
of large earthquakes occurrence process from randomness. These
clusters are usually described ex-post by a physics-based model
in which earthquakes are triggered by Coulomb stress changes
induced by other surrounding earthquakes. Notwithstanding
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the popularity of this kind of modeling, its ex-ante skill in
terms of earthquake predictability gain is still unknown. Here
we show that even in synthetic systems that are rooted on the
physics of fault interaction using the Coulomb stress changes,
such a kind of modeling often does not increase significantly
earthquake predictability. Earthquake predictability of a fault
may increase only when the Coulomb stress change induced by a
nearby earthquake is much larger than the stress changes caused
by earthquakes on other faults and by the intrinsic variability
of the earthquake occurrence process. |
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Diciembre de 2014
A methodology for near-field tsunami inundation forecasting:
Application to the 2011 Tohoku tsunami
Authors: Aditya Riadi Gusman, Yuichiro Tanioka et al
Link: Click here
Existing tsunami early warning systems in the
world can give either one or a combination of estimated tsunami
arrival times, heights, or qualitative tsunami forecasts before
the tsunami hits near-field coastlines. A future tsunami early
warning system should be able to provide a reliable near-field
tsunami inundation forecast on high-resolution topography within
a short time period. Here we describe a new methodology for
near-field tsunami inundation forecasting. In this method, a
precomputed tsunami inundation and precomputed tsunami waveform
database is required. After information about a tsunami source
is estimated, tsunami waveforms at nearshore points can be simulated
in real time. A scenario that gives the most similar tsunami
waveforms is selected as the site-specific best scenario and
the tsunami inundation from that scenario is selected as the
tsunami inundation forecast. To test the algorithm, tsunami
inundation along the Sanriku Coast is forecasted by using source
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models
for the 2011 Tohoku earthquake estimated from GPS, W phase, or
offshore tsunami waveform data. The forecasting algorithm is capable
of providing a tsunami inundation forecast that is similar to
that obtained by numerical forward modeling but with remarkably
smaller CPU time. The time required to forecast tsunami inundation
in coastal sites from the Sendai Plain to Miyako City is approximately
3?min after information about the tsunami source is obtained.
We found that the tsunami inundation forecasts from the 5?min
GPS, 5?min?W phase, 10?min?W phase fault models, and 35?min tsunami
source model are all reliable for tsunami early warning purposes
and quantitatively match the observations well, although the latter
model gives tsunami forecasts with highest overall accuracy. The
required times to obtain tsunami forecast from the above four
models are 8?min, 9?min, 14?min, and 39?min after the earthquake,
respectively, or in other words 3?min after receiving the source
model. This method can be useful in developing future tsunami
forecasting systems with a capability of providing tsunami inundation
forecasts for locations near the tsunami source area. |
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Diciembre de 2014
Spatiotemporal changes, faulting regimes, and source
parameters of induced seismicity: A case study from The Geysers
geothermal field
Authors: Patricia Martínez-Garzón, Grzegorz Kwiatek
et al
Link: Click here
The spatiotemporal, kinematic, and source characteristics
of induced seismicity occurring at different fluid injection
rates are investigated to determine the predominant physical
mechanisms responsible for induced seismicity at the northwestern
part of The Geysers geothermal field, California. We analyze
a relocated hypocenter catalog from a seismicity cluster where
significant variations of the stress tensor orientation were
previously observed to correlate with injection rates. We find
that these stress tensor orientation changes may be related
to increased pore pressure and the corresponding changes in
poroelastic stresses at reservoir depth.
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Seismic
events during peak injections tend to occur at greater distances
from the injection well, preferentially trending parallel to the
maximum horizontal stress direction. In contrast, at lower injection
rates the seismicity tends to align in a different direction which
suggests the presence of a local fault. During peak injection
intervals, the relative contribution of strike-slip faulting mechanisms
increases. Furthermore, increases in fluid injection rates also
coincide with a decrease in b values. Our observations suggest
that regardless of the injection stage, most of the induced seismicity
results from thermal fracturing of the reservoir rock. However,
during peak injection intervals, the increase in pore pressure
may likewise be responsible for the induced seismicity. By estimating
the thermal and hydraulic diffusivities of the reservoir, we confirm
that the characteristic diffusion length for pore pressure is
much greater than the corresponding length scale for temperature
and also more consistent with the spatial extent of seismicity
observed during different injection rates. |
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Diciembre de 2014
Cooling magma model for deep volcanic long-period earthquakes
Authors: Naofumi Aso and Victor C. Tsai et al
Link: Click here
Deep long-period events (DLP events) or deep
low-frequency earthquakes (deep LFEs) are deep earthquakes that
radiate low-frequency seismic waves. While tectonic deep LFEs
on plate boundaries are thought to be slip events, there have
only been a limited number of studies on the physical mechanism
of volcanic DLP events around the Moho (crust-mantle boundary)
beneath volcanoes. One reasonable mechanism capable of producing
their initial fractures is the effect of thermal stresses. Since
ascending magma diapirs tend to stagnate near the Moho, where
the vertical gradient of density is high, we suggest that cooling
magma may play an important role
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in volcanic DLP event occurrence. Assuming an initial thermal
perturbation of 400°C within a tabular magma of half width
41 m or a cylindrical magma of 74?m radius, thermal strain rates
within the intruded magma are higher than tectonic strain rates
of ~10 14 s 1 and produce a total strain of 2 × 10 4. Shear
brittle fractures generated by the thermal strains can produce
a compensated linear vector dipole mechanism as observed and potentially
also explain the harmonic seismic waveforms from an excited resonance.
In our model, we predict correlation between the particular shape
of the cluster and the orientation of focal mechanisms, which
is partly supported by observations of Aso and Ide (2014). To
assess the generality of our cooling magma model as a cause for
volcanic DLP events, additional work on relocations and focal
mechanisms is essential and would be important to understanding
the physical processes causing volcanic DLP events. |
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Diciembre de 2014
Comparing foreshock characteristics and foreshock forecasting
in observed and simulated earthquake catalogs
Authors: Yosihiko Ogata and Koichi Katsura et al
Link: Click here
In this paper, we compare the empirical results
regarding foreshocks obtained from the Japan data with results
for synthetic catalogs in order to clarify whether or not the
corresponding results are consistent with the description of
the seismicity by a superposition of background activity and
epidemic-type aftershock sequence (ETAS) models. This question
is important, because it is still controversially discussed
whether the nucleation process of large earthquakes is
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driven
by seismically cascading (ETAS type) or by aseismic accelerating
processes. To explore the foreshock characteristics, we first
applied the same clustering algorithms to real and synthetic catalogs
and analyzed the temporal, spatial, and magnitude distributions
of the selected foreshocks. Most properties are qualitatively
the same in the real data and in synthetic catalogs. However,
we find some quantitative differences particularly in the temporal
acceleration, spatial convergence, and magnitude dependence, which
also depend on the assumed synthetic catalogs. Furthermore, we
calculated forecast scores based on a single-link cluster algorithm
which could be appropriate for real-time applications. We find
that the Japan Meteorological Agency catalog yields higher scores
than all synthetic catalogs and that the ETAS models having the
same magnitude sequence as the original catalog performs better
(more close to the reality) than ETAS models with randomly picked
magnitudes. We also find that the ETAS model that takes account
of the triggering effect by small earthquakes below threshold
magnitude performs more closely to the reality. |
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Diciembre de 2014
Tohoku-Oki earthquake caused major ionospheric disturbances
at 450 km altitude over Alaska
Authors: Yu-Ming Yang, X. Meng et al
Link: Click here
Ionospheric total electron content (TEC) and
atmospheric density perturbations were derived from measurements
made from instruments on board the Gravity Recovery and Climate
Experiment (GRACE) spacecraft. At the time of the Tohoku-Oki
earthquake on 11 March 2011, the twin spacecraft were orbiting
at an altitude of ~450 km over Alaska. Significant TEC
fluctuations
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(up
to 0.6 total electron content unit (TECU; 1 TECU = 1016 el m−2),
atmospheric density perturbations (~3.6 • 10−14 kg/m3),
and sudden changes in GRACE acceleration (~4 • 10−8 m/s2)
were observed ~8 min after the arrival of seismic and infrasound
waves on the ground in Alaska, ~20 min after the Tohoku-Oki
main shock at 05:46:23 UTC. The results of the three-dimensional
ionospheric-thermospheric modeling and infrasound ray-tracing
simulations are consistent with the arrival time and physical
characteristics of the disturbances at GRACE. This is the first
time that ionospheric disturbances associated with an earthquake
are clearly attributable to perturbations at such high altitudes. |
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Diciembre de 2014
El mineral más abundante de la Tierra ya tiene,
por fin, un nombre.
Autor: José Manuel Nieves - www.abc.es
Link: Click aquí
Después de más de cincuenta años
de esfuerzos, y con la ayuda de un antiguo meteorito y rayos
X de alta energía, un grupo de científicos de
la Universidad de Las Vegas ha conseguido identificar y caracterizar
por primera vez el mineral del que está hecho el 38%
de nuestro planeta.
Se llama Bridgmanita en honor de su descubridor, el físico
Percy Bridgman, laureado con el Nobel en 1964 y pionero en la
investigación de materiales a muy altas presiones. El
nuevo mineral acaba de ser aceptado por la Comisión de
Nomenclatura y Clasificación de Nuevos Minerales (CNMNC),
de la Asociación Mineralógica Internacional.
El camino ha sido largo y difícil, pero el geólogo
Oliver Tschauner, ha conseguido por fin aclarar las condiciones
en las que, en las profundidades de nuestro planeta, se forma
el mineral más abundante de la Tierra, una mezcla extraordinariamente
densa de silicatos, hierro y magnesio - (Mg,Fe)SiO3 - que hasta
ahora había escapado a cualquier análisis científico.
Para averiguar la composición de las capas más
internas de la Tierra, los investigadores necesitan someter
los más variados materiales a presiones y temperaturas
extremas, recreando así en laboratorio las duras condiciones
que reinan en las profundidades de nuestro mundo. Durante décadas,
los geólogos han creído que la recién bautizada
Bridgmanita, una estructura de gran densidad, constituye cerca
del 38% del total de la masa terrestre, y que la propiedades
físicas y químicas de ese mineral ejercen una
gran influencia en la forma en que los diferentes elementos
y el calor fluyen a través del manto.
Pero dado que la Bridgmanita no logra sobrevivir en su viaje
hacia la superficie, nadie hasta ahora había sido capaz
de analizarla ni de probar su existencia, requisitos básicos
para que un mineral sea aceptado por la Asociación Mineralógica
Internacional.
La clave, en meteoritos
Para conseguir llevar a cabo ese análisis, Oliver Tschauner
y su equipo decidieron recurrir a los meteoritos. De hecho,
las enormes compresiones a las que sus minerales se someten
durante el impacto se parecen mucho a las que esos mismos minerales
deben soportar en las hostiles condiciones de las profundidades
del planeta, con temperaturas de cerca de 2.100 grados y presiones
que son hasta 240.000 veces superiores a las que existen al
aire libre, al nivel del mar.
Otra ventaja es que, durante el impacto, la compresión
sucede lo suficientemente rápido como para impedir que
la Bridgmanita se deshaga, tal y como ocurre cuando trata de
ascender a la superficie desde las profundidades de la Tierra
y se destruye a causa del cambio de presión. Así,
muchos meteoritos conservan fragmentos de Bridgmanita "congelados"
en su interior. Pero hasta ahora todos los intentos por analizar
esos restos en laboratorio habían terminado por estropear
el mineral o, en el mejor de los casos, por arrojar resultados
incompletos.
Por eso, el equipo de Tschauner decidió probar con otro
sistema: Un haz de rayos X de alta energía que permitieran
"penetrar" en el meteorito sin dañarlo.
El equipo examinó de esta forma una sección del
meteorito Tenham, una condrita que cayó en Australia
en 1979. En su interior, los gránulos de Bridgmanita
eran escasos y de menos de un micrómetro (una millonésima
de metro) de diámetro. Por eso los investigadores tuvieron
que usar un haz de rayos enormemente condensado.
La primera muestra natural de Bridgmanita llegó llena
de sorpresas. De hecho, contenía una cantidad inesperadamente
alta de hierro férrico, muy superior al de las muestras
sintéticas. Además, la Bridgmanita también
contenía mucho más sodio que la mayoría
de las muestras creadas en los laboratorios. De forma que, en
conjunto, se descubrió una naturaleza química
muy diferente de la que se esperaba. Algo que resultará
de la máxima importancia para los estudios que se realicen
en el futuro sobre rocas del manto terrestre.
Antes de este estudio, el conocimiento científico de
las propiedades de la Bridgmanita se basaba únicamente
en muestras sintéticas, ya que el mineral sólo
permanece estable por debajo de los 660 km de profundidad, donde
reinan presiones enormes. Al abandonar las profundidades de
la Tierra, las presiones más bajas transforman a la Bridgmanita
en minerales mucho menos densos. Algunos investigadores creen
incluso que algunas de las inclusiones de ciertos diamantes
son las marcas dejadas por la Bridgmanita al alterarse mientras
las piedras emergían a la superficie.
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Diciembre de 2014
The role of viscoelasticity in subducting plates
Authors: R. J. Farrington, L.-N. Moresi et al
Link: Click here
Subduction of tectonic plates into Earth's mantle
occurs when one plate bends beneath another at convergent plate
boundaries. The characteristic time of deformation at these
convergent boundaries approximates the Maxwell relaxation time
for olivine at lithospheric temperatures and pressures, it is
therefore by definition a viscoelastic process. While this is
widely acknowledged, the large-scale features of subduction
can, and have been, successfully reproduced assuming the plate
deforms by a viscous mechanism alone. However, the energy rates
and stress profile within convergent margins are influenced
by viscoelastic deformation. In this study, viscoelastic stresses
have been systematically introduced into numerical models of
free subduction, using both
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the
viscosity and shear modulus to control the Maxwell relaxation
time. The introduction of an elastic deformation mechanism into
subduction models produces deviations in both the stress profile
and energy rates within the subduction hinge when compared to
viscous only models. These variations result in an apparent viscosity
that is variable throughout the length of the plate, decreasing
upon approach and increasing upon leaving the hinge. At realistic
Earth parameters, we show that viscoelastic stresses have a minor
effect on morphology yet are less dissipative at depth and result
in an energy transfer between the energy stored during bending
and the energy released during unbending. We conclude that elasticity
is important during both bending and unbending within the slab
hinge with the resulting stress loading and energy profile indicating
that slabs maintain larger deformation rates at smaller stresses
during bending and retain their strength during unbending at depth. |
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Noviembre de 2014
Soil CO2 emissions as a proxy for heat and mass flow assessment,
Taupo Volcanic Zone, New Zealand
Authors: Bloomberg S., Werner C et al
Link: Click here
The quantification of heat and mass flow between
deep reservoirs and the surface is important for understanding
magmatic and hydrothermal systems. Here, we use high-resolution
measurement of carbon dioxide flux (?CO2) and heat flow at the
surface to characterize the mass (CO2 and steam) and heat released
to the atmosphere from two magma-hydrothermal systems. Our soil
gas and heat flow surveys at Rotokawa and White Island in the
Taup? Volcanic Zone, New Zealand, include over 3,000 direct
measurements of ?CO2 and soil temperature and 60 carbon isotopic
values on soil gases. Carbon dioxide flux was separated into
background and
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magmatic/hydrothermal
populations based on the measured values and isotopic characterization.
Total CO2 emission rates (?CO2) of 441 ± 84 t d- 1 and
124 ± 18 t d-1 were calculated for Rotokawa (2.9 km2) and
for the crater floor at White Island (0.3 km2), respectively.
The total CO2 emissions differ from previously published values
by +386 t d-1 at Rotokawa and +25 t d-1 at White Island, demonstrating
that earlier research underestimated emissions by 700% (Rotokawa)
and 25% (White Island). These differences suggest that soil CO2
emissions facilitate more robust estimates of the thermal energy
and mass flux in geothermal systems than traditional approaches.
Combining the magmatic/hydrothermal-sourced CO2 emission (constrained
using stable isotopes) with reservoir H2O:CO2 mass ratios and
the enthalpy of evaporation, the surface expression of thermal
energy release for the Rotokawa hydrothermal system (226 MWt)
is 10 times greater than the White Island crater floor (22.5 MWt). |
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Noviembre de 2014
Fluid-induced earthquakes with variable stress drop
Authors: O. Lengliné, L. Lamourette et al
Link: Click here
The static stress drop of an earthquake, which
quantifies the ratio of seismic slip to the size of the rupture,
is almost constant over several orders of magnitudes. Although
variations are often observed, it is difficult, however, to
attribute these variations either to a well-defined phenomenon
or simply to measurement uncertainty. In this study we analyze
the static stress drop of earthquakes that occurred during a
water circulation test in the Soultz-sous-Forêts, France,
geothermal reservoir in 2010. During this circulation test,
411
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earthquakes
were recorded, the largest event having a magnitude MD 2.3. We
show that several earthquakes in the reservoir can be combined
into groups of closely located similar repeating waveforms. We
infer that the amplitudes, and hence magnitudes, vary between
the repeaters although the waveforms and spectra are both similar
in shape. We measure similar corner frequencies for these events
despite their different magnitudes, suggesting a similar rupture
size. Our results imply that events at the same location may exhibit
stress drop variations by as much as a factor of 300. We interpret
that this variation in stress drop is caused by fluid pressure
at the interface reducing the normal stress. We also hypothesize
that the observed variations reflect a transition from stable
to unstable slip on the imaged asperities. |
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Noviembre de 2014
Laboratory constraints on models of earthquake recurrence
Authors: N. M. Beeler, Terry Tullis
Link: Click here
In this study, rock friction 'stick-slip' experiments
are used to develop constraints on models of earthquake recurrence.
Constant-rate loading of bare rock surfaces in high quality
experiments produces stick-slip recurrence that is periodic
at least to second order. When the loading rate is varied, recurrence
is approximately inversely proportional to loading rate. These
laboratory events initiate due to a slip rate-dependent process
that also determines the size of the stress drop and as a consequence,
stress drop varies weakly but systematically with loading rate.
This is especially evident in experiments where the loading
rate is changed by orders of magnitude, as is thought to be
the loading condition of naturally occurring, small repeating
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earthquakes
driven by afterslip, or low-frequency earthquakes loaded by episodic
slip. As follows from the previous studies referred to above,
experimentally observed stress drops are well described by a logarithmic
dependence on recurrence interval that can be cast as a non- linear
slip-predictable model. The fault's rate dependence of strength
is the key physical parameter. Additionally, even at constant
loading rate the most reproducible laboratory recurrence is not
exactly periodic, unlike existing friction recurrence models.
We present example laboratory catalogs that document the variance
and show that in large catalogs, even at constant loading rate,
stress drop and recurrence co-vary systematically. The origin
of this covariance is largely consistent with variability of the
dependence of fault strength on slip rate. Laboratory catalogs
show aspects of both slip and time predictability and successive
stress drops are strongly correlated indicating a 'memory' of
prior slip history that extends over at least one recurrence cycle. |
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Noviembre de 2014
A methodology for near-field tsunami inundation forecasting:
Application to the 2011 Tohoku tsunami
Authors: Aditya Riadi Gusman, Yuichiro Taniokaet al
Link: Click here
Existing tsunami early warning systems in the
world can give either one or a combination of estimated tsunami
arrival times, heights, or qualitative tsunami forecasts before
the tsunami hits near-field coastlines. A future tsunami early
warning system should be able to provide a reliable near-field
tsunami inundation forecast on high-resolution topography within
a short time period. Here we describe a new methodology for
near-field tsunami inundation forecasting. In this method, a
precomputed tsunami inundation and precomputed tsunami waveform
database is required. After information about a tsunami source
is estimated, tsunami waveforms at nearshore points can be simulated
in real time. A scenario that gives the most similar tsunami
waveforms is selected as the site-specific best scenario and
the tsunami inundation from that scenario is selected as the
tsunami inundation forecast. To test the algorithm, tsunami
inundation along the
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Sanriku
Coast is forecasted by using source models for the 2011 Tohoku
earthquake estimated from GPS, W phase, or offshore tsunami waveform
data. The forecasting algorithm is capable of providing a tsunami
inundation forecast that is similar to that obtained by numerical
forward modeling but with remarkably smaller CPU time. The time
required to forecast tsunami inundation in coastal sites from
the Sendai Plain to Miyako City is approximately 3?min after information
about the tsunami source is obtained. We found that the tsunami
inundation forecasts from the 5?min GPS, 5?min?W phase, 10?min?W
phase fault models, and 35?min tsunami source model are all reliable
for tsunami early warning purposes and quantitatively match the
observations well, although the latter model gives tsunami forecasts
with highest overall accuracy. The required times to obtain tsunami
forecast from the above four models are 8?min, 9?min, 14?min,
and 39?min after the earthquake, respectively, or in other words
3?min after receiving the source model. This method can be useful
in developing future tsunami forecasting systems with a capability
of providing tsunami inundation forecasts for locations near the
tsunami source area. |
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Octubre de 2014
Knobby terrain on ancient volcanoes as an indication of dominant
early explosive volcanism on Mars
Authors: Jun Huang and Long Xiao
Link: Click here
Determining if the mechanically weak materials
of the upper crust are products of early explosive volcanism
or generated by modification of extensive effusive lava flows
is important for understanding the geologic and thermal history
of Mars. We examined 75 recently identified preserved ancient
volcanoes, whose eruption styles are representative of early
volcanism. We describe a unique knobby terrain that is
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associated
with 17 of these volcanoes. This newly identified terrain is characterized
by a combination of high-resolution images and thermophysical
and thermal infrared hyperspectral data. The morphology characteristics
of these knobby terrains are similar to terrestrial eroded ignimbrites,
and the thermophysical properties indicate that they are composed
of unconsolidated fine-grained materials. The spectral analysis
indicates that they experienced some aqueous alteration. We explain
the knobby terrain associated with these ancient volcanoes as
the products of an early explosive volcanic phase in Martian history,
followed by subsequent modification of these deposits |
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Octubre de 2014
The Role of Geodesy in Earthquake and Volcanic Studies
Authors: Manabu Hashimoto, Richard Gross et al
Link: Click here
Geodesists from 16 countries met in July to
discuss the role of geodesy in earthquake and volcanic studies,
natural hazard assessment, and disaster mitigation. The GENAH
2014
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international
symposium took place in Matsushima, Japan, a town that suffered
greatly from the tsunami of 11 March 2011. The symposium featured
sessions on seven topics: subduction zone earthquakes, the earthquake
deformation cycle, near real-time warning, the interaction between
earthquakes and volcanoes, the impact of great earthquakes on
reference frame, geodetic techniques in volcanological research,
and natural hazards. |
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Octubre de 2014
Cooling magma model for deep volcanic long-period earthquakes
Authors: Naofumi Aso and Victor C. Tsai
Link: Click here
Deep long-period events (DLP events) or deep
low-frequency earthquakes (deep LFEs) are deep earthquakes that
radiate low-frequency seismic waves. While tectonic deep LFEs
on plate boundaries are thought to be slip events, there have
only been a limited number of studies on the physical mechanism
of volcanic DLP events around the Moho (crust-mantle boundary)
beneath volcanoes. One reasonable mechanism capable of producing
their initial fractures is the effect of thermal stresses. Since
ascending magma diapirs tend to stagnate near the Moho, where
the vertical gradient of density is high, we suggest that cooling
magma may play an important role in volcanic DLP event occurrence.
Assuming an
|
initial
thermal perturbation of 400?°C within a tabular magma of half-width
41?m or a cylindrical magma of 74?m radius, thermal strain rates
within the intruded magma are higher than tectonic strain rates
of ~?10??14?s??1, and produce a total strain of 2?×?10??4.
Shear brittle fractures generated by the thermal strains can produce
a compensated-linear-vector-dipole (CLVD) mechanism as observed,
and potentially also explain the harmonic seismic waveforms from
an excited resonance. In our model, we predict correlation between
the particular shape of the cluster and the orientation of focal
mechanisms, which is partly supported by observations of Aso and
Ide [2014]. To assess the generality of our cooling magma model
as a cause for volcanic DLP events, additional work on relocations
and focal mechanisms is essential, and would be important to understanding
the physical processes causing volcanic DLP events. |
|
Octubre de 2014
The magnitude distribution of dynamically triggered earthquakes
Authors: Stephen Hernandez, Emily E. Brodsky et al
Link: Click here
Large dynamic strains carried by seismic waves
are known to trigger seismicity far from their source region.
It is unknown, however, whether surface waves trigger only small
earthquakes, or whether they can also trigger large earthquakes.
To partially address this question, we evaluate whether current
data can distinguish between the magnitude distribution of triggered
and untriggered small earthquakes. We use a mixing
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model
approach in which total seismicity is decomposed into two classes:
"triggered" events initiated or advanced by far?field
dynamic strains and "untriggered" spontaneous events
consisting of everything else. The b?value of a mixed data set,
bMIX, is decomposed into a weighted sum of b?values of its constituent
components, bT and bU. We utilize the previously observed relationship
between triggering rate and dynamic strain amplitude to identify
the fraction of triggered events in populations of earthquakes
and then invert for bT. For Californian seismicity, data are consistent
with a single parameter Gutenberg Richter hypothesis governing
the magnitudes of both triggered and untriggered earthquakes. |
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Octubre de 2014
Dynamic triggering of microearthquakes in three geothermal/volcanic
regions of California
Authors: Chastity Aiken and Zhigang Peng
Link: Click here
Geothermal/volcanic regions are most susceptible
to local earthquake triggering by regional and remote earthquakes.
Transient stresses caused by surface waves of these earthquakes
can activate critically stressed faults. Though earthquakes
can be triggered in geothermal/volcanic regions, it is less
understood how these regions differ in their triggering responses
to distant earthquakes. We conduct a systematic survey of local
earthquakes triggered by distant earthquakes in three geothermal/volcanic
regions of California: Long Valley Caldera, Coso Geothermal
Field, and Geysers Geothermal Field. We examine waveforms of
distant earthquakes with magnitudes 5.5 occurring between 2000
and
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2012
and compute ? statistics to confirm the significance of our findings.
We find that Long Valley, Coso, and Geysers vary in triggering
frequency-2.0%, 3.8%, and 6.8% in the 12?year period, respectively-and
when compared to the triggering of deep tectonic tremors along
the Parkfield-Cholame section of San Andreas Fault (9.2% in the
12?year period). Stress triggering thresholds vary among the regions
with Long Valley having the highest of ~5 kPa and ~1 kPa for the
other regions. Because dynamic stresses from distant earthquakes
are similar in these three regions, the varying triggering behavior
likely reflects faults having a tendency to be at or near failure.
This is compatible with Geysers having a higher a value in the
Gutenberg-Richter relationship and higher geothermal productivity
than the other two regions. The observation of more frequent triggering
of tremor than microearthquakes is consistent with recent laboratory
studies on increasing triggerability with lower effective stress. |
|
Octubre de 2014
Experimental evidence for seismically initiated gas bubble
nucleation and growth in groundwater as a mechanism for coseismic
borehole water level rise and remotely triggered seismicity
Authors: Jackson B. Crews and Clay A. Cooper
Link: Click here
Changes in borehole water levels and remotely
triggered seismicity occur in response to near and distant earthquakes
at locations around the globe, but the mechanisms for these
phenomena are not well understood. Experiments were conducted
to show that seismically initiated gas bubble growth in groundwater
can trigger a sustained increase in pore fluid pressure consistent
in magnitude with observed coseismic borehole water level rise,
constituting a physically plausible mechanism for remote triggering
of secondary earthquakes through the reduction of effective
stress in critically loaded geologic faults. A portion
|
of
the CO2 degassing from the Earth's crust dissolves in groundwater
where seismic Rayleigh and P waves cause dilational strain, which
can reduce pore fluid pressure to or below the bubble pressure,
triggering CO2 gas bubble growth in the saturated zone, indicated
by a spontaneous buildup of pore fluid pressure. Excess pore fluid
pressure was measured in response to the application of 0.1-1.0?MPa,
0.01-0.30?Hz confining stress oscillations to a Berea sandstone
core flooded with initially subsaturated aqueous CO2, under conditions
representative of a confined aquifer. Confining stress oscillations
equivalent to the dynamic stress of the 28 June 1992 Mw 7.3 Landers,
California, earthquake Rayleigh wave as it traveled through the
Long Valley caldera, and Parkfield, California, increased the
pore fluid pressure in the Berea core by an average of 36?±?15?cm
and 23?±?15?cm of equivalent freshwater head, respectively,
in agreement with 41.8?cm and 34?cm rises recorded in wells at
those locations. |
|
Octubre de 2014
Bayesian confidence intervals for the magnitude of the largest
aftershock
Author: Robert Shcherbakov
Link: Click here
Aftershock sequences, which follow large earthquakes,
last hundreds of days and are characterized by well-defined
frequency-magnitude and spatio-temporal distributions. The largest
aftershocks in a sequence constitute significant hazard and
can inflict additional damage to infrastructure that is already
affected by the main shock. Therefore, the estimation of the
|
magnitude
of a possible largest aftershock in a sequence is of high importance.
In this work, a Bayesian predictive distribution and the corresponding
confidence intervals for the magnitude of the largest expected
aftershock in a sequence are derived using the framework of Bayesian
analysis and extreme value statistics. The analysis is applied
to 19 well-known aftershock sequences worldwide to construct retrospectively
the confidence intervals for the magnitude of the subsequent largest
aftershock by using the statistics of early aftershocks in the
sequences. |
|
Octubre de 2014
Localized fault slip to the trench in the 2010 Maule, Chile
Mw = 8.8 earthquake from joint inversion of high-rate GPS, teleseismic
body waves, InSAR, campaign GPS, and tsunami observations
Authors: Han Yue, Thorne Lay et al
Link: Click here
The 27 February 2010, Mw 8.8 Maule earthquake
ruptured ~500?km along the plate boundary offshore central Chile
between 34°S and 38.5°S. Establishing whether co-seismic
fault offset extended to the trench is important for interpreting
both shallow frictional behavior and potential for tsunami earthquakes
in the region. Joint inversion of high-rate GPS, teleseismic
body waves, InSAR, campaign GPS, and tsunami observations yields
a kinematic rupture model with improved resolution of slip near
the trench. Bilateral rupture expansion is resolved in our model
with relatively uniform slip of 5-10 m down-dip beneath the
|
coast
and two near-trench high-slip patches with >12?m displacements.
The peak slip is ~17?m at a depth of ~15 km on the central megathrust,
located ~200?km north from the hypocenter and overlapping the
rupture zone of the 1928?M ~8 event. The up-dip slip is ~16?m
near the trench. Another shallow near-trench patch is located
~150?km southwest of the hypocenter, with a peak slip of 12?m.
Checkerboard resolution tests demonstrate that correctly modeled
tsunami data are critical to resolution of slip near the trench,
with other data sets allowing, but not requiring slip far offshore.
Large interplate aftershocks have a complementary distribution
to the co-seismic slip pattern, filling in gaps or outlining edges
of large-slip zones. Two clusters of normal faulting events locate
seaward along the plate motion direction from the localized regions
of large near-trench slip, suggesting that proximity of slip to
the trench enhanced extensional faulting in the underthrusting
plate. |
|
Octubre de 2014
Investigating the Origin of Seismic Swarms
Authors: Aladino Govoni,cLuigi Passarelli, Thomas Braun
et al
Link: Click here
According to the U.S. Geological Survey's Earthquake
Hazards Program, a seismic swarm is “a localized surge
of earthquakes, with no one shock being conspicuously larger
than all other shocks of the swarm. They might occur in a variety
of geologic environments and are not known to be indicative
of any change in the long-term seismic risk of the region in
which they occur”.
|
“A
seismic swarm is by definition a localized surge of earthquakes,
with no one shock being conspicuously larger than all other shocks
of the swarm. Seismic swarms typically last longer than more typical
earthquake sequences that consist of a main shock followed by
significantly smaller aftershocks. Seismic swarms occur in a variety
of geologic environments. They are not known to be indicative
of any change in the long-term seismic risk of the region in which
they occur.” (Link) |
|
Octubre de 2014
Observing coseismic gravity change from the Japan Tohoku-Oki
2011 earthquake with GOCE gravity gradiometry
Authors: Martin J. Fuchs, Johannes Bouman et al
Link: Click here
The Japan Tohoku-Oki earthquake (9.0 Mw) of
11 March 2011 has left signatures in the Earth's gravity field
that are detectable by data of the Gravity field Recovery and
Climate Experiment (GRACE) mission. Because the European Space
Agency's (ESA) satellite gravity mission Gravity field and steady-state
Ocean Circulation Explorer (GOCE)-launched in 2009-aims at high
spatial resolution, its measurements could complement the GRACE
information on coseismic gravity changes, although time-variable
gravity was not foreseen as goal of the GOCE mission. We modeled
the coseismic earthquake geoid signal and converted this signal
to vertical gravity gradients at GOCE satellite altitude. We
combined
|
the
single gradient observations in a novel way reducing the noise
level, required to detect the coseismic gravity change, subtracted
a global gravity model, and applied tailored outlier detection
to the resulting gradient residuals. Furthermore, the measured
gradients were along-track filtered using different gradient bandwidths
where in the space domain Gaussian smoothing has been applied.
One-year periods before and after earthquake occurrence have been
compared with the modeled gradients. The comparison reveals that
the earthquake signal is well above the accuracy of the vertical
gravity gradients at orbital height. Moreover, the obtained signal
from GOCE shows a 1.3 times higher amplitude compared with the
modeled signal. Besides the statistical significance of the obtained
signal, it has a high spatial correlation of ~0.7 with the forward
modeled signal. We conclude therefore that the coseismic gravity
change of the Japan Tohoku-Oki earthquake left a statistically
significant signal in the GOCE measured gravity gradients. |
|
Octubre de 2014
Hidden aftershocks of the 2011 Mw 9.0 Tohoku, Japan earthquake
imaged with the back-projection method
Authors: Eric Kiser and Miaki Ishii
Link: Click here
The first 25?hours of the aftershock sequence
following the March 11, 2011 Mw 9.0 Tohoku, Japan earthquake
are investigated using a back-projection method. In total, 600
aftershocks are imaged during this time period. These aftershocks
are distributed over a 500 by 300?km area, and include many
events in the outer rise.
|
The
back-projection events are compared with the JMA catalogue, which
is composed of earthquakes recorded by local seismic networks
in Japan. Surprisingly, half of the back-projection events are
not found in the JMA catalogue. These events cluster near the
Japan Trench and in the outer rise, and fill in gaps in the spatial
distribution of the early aftershock sequence where large mainshock
slip is thought to have occurred. These results show that the
JMA magnitude of completeness is very high near the trench following
the 2011 Tohoku mainshock, and earthquakes as large as magnitude
6.8 went undetected by local seismic networks. |
|
Octubre de 2014
The ULF/ELF electromagnetic radiation before the 11 March
2011 Japanese earthquake
Authors: K. Ohta, J. Izuts, A. Schekotov et al
Link: Click here
The ULF/ELF short-term electromagnetic precursor
is discovered for the disastrous Japan earthquake (EQ) occurred
on 11 March 2011. This analysis is based on the records measured
by search coil magnetometers located at Nakatsugawa (geographic
coordinates; 35.42°N, 137.55°E), Shinojima (34.67°N,
137.01°E), and
|
Izu
(34.64°N, 137.01°E) of the Chubu University network. The
data of these magnetometers are extensively used to analyze the
ULF/ELF seismo-atmospheric radiation. It is then found that the
ULF/ELF atmospheric radio emission is reliably detected on 6 March
before the main shock on 11 March, probably as a precursory signature
of the EQ. Further confirmation on its seismic origin was provided
by the observational fact that the azimuths of the radiation source
from all observation sites coincide approximately with the region
of the forthcoming EQ. |
|
Septiembre de 2014
The preparatory phase of the 2009 Mw 6.3 L'Aquila earthquake
by improving the detection capability of low-magnitude foreshocks
Authors: Monica Sugan, Aitaro Kato et al
Link: Click here
We explored the detection capability of low-magnitude
earthquakes before the 6 April 2009 Mw 6.3 L'Aquila event by
using a matched filter technique and 512 foreshocks as templates.
We analyzed continuous waveforms from 10 broadband seismic stations
in a 60 km radius
|
from
the epicenter and for ~3 months before the main shock. More than
3000 new events, mostly located on the main shock fault, were
detected to define the spatial-temporal evolution of micro-seismicity.
The foreshock sequence was active northwest of the Mw 6.3 hypocenter
in January, then migrated toward it at a speed of ~0.5?km/day
in middle of February. At that time, in a ~4?km2 patch close to
the main shock nucleation point, the cumulative number of earthquakes
gradually increased until the Mw 6.3 event. This patch, characterized
by a low b-value, played a key role in controlling the preparation
stage to the 2009 L'Aquila main rupture. |
|
Septiembre de 2014
Electric potential microelectrode for studies of electrobiogeophysics
Authors: Lars Riis Damgaardm, Nils Risgaard-Petersen
et al
Link: Click here
Spatially separated electron donors and acceptors
in sediment can be exploited by the so-called "cable bacteria."
Electric potential microelectrodes (EPMs) were constructed to
measure the electric fields that should appear when cable bacteria
conduct electrons over centimeter distances. The EPMs were needle-shaped,
shielded Ag/AgCl half-cells that were rendered insensitive to
redox-active species in the
|
environment.
Tip diameters of 40 to 100?µm and signal resolution of approximately
10??V were achieved. A test in marine sediments with active cable
bacteria showed an electric potential increase by approximately
2?mV from the sediment-water interface to a depth of approximately
20?mm, in accordance with the location and direction of the electric
currents estimated from oxygen, pH, and H2S microprofiles. The
EPM also captured emergence and decay of electric diffusion potentials
in the upper millimeters of artificial sediment in response to
changes in ion concentrations in the overlying water. The results
suggest that the EPM can be used to track electric current sources
and sinks with submillimeter resolution in microbial, biogeochemical,
and geophysical studies. |
|
Septiembre de 2014
Structures and geometries of the Tajo Basin crust, Spain:
Results of a magnetotelluric investigation compared to seismic
and thermal models
Authors: J.-P. Schmoldt, A. G. Jones et al
Link: Click here
The Tajo Basin and Betic Mountain Chain in the
south central region of the Iberian Peninsula were chosen for
investigation in the first phase of the magnetotelluric (MT)
component of the PICASSO (Program to Investigate the Convective
Alboran Sea System Overturn) project. The MT results provide
information about the electrical conductivity distribution in
previously unprobed subsurface regions, as well as complimenting
and enhancing results of prior geological and geophysical investigations
thereby enabling the
|
definition
of a petrological subsurface model and a comprehensive understanding
about the tectonic setting. Two-dimensional (2-D) inversion of
the MT data provides enhanced insight into Iberian subsurface
geology in the crust. The most striking features of the final
model are (i) a distinct vertical interface within the Variscan
basement beneath the center of the Tajo Basin that is spatially
associated with the boundary between regions with and without
substantial Alpine deformation, and (ii) a middle to lower crustal
conductive anomaly that can be related to remnants of asthenospheric
intrusion in connection with Pliocene volcanic events in the Calatrava
Volcanic Province. For the latter, effects of hydrous phases are
inferred that may originate from dehydration processes within
the subducting slab beneath Alboran Domain and Betic Mountain
Chain. |
|
Septiembre de 2014
A simple inverse method for the interpretation of pumped
flowing fluid electrical conductivity logs
Authors: R. S. Moir, A. H. Parker et al
Link: Click here
Pumped flowing fluid electrical conductivity
(FFEC) logs, also known as pumped borehole dilution testing,
is an experimentally easy-to-perform approach to evaluating
vertical variations in the hydraulic conductivity of an aquifer.
In contrast to the simplicity of the logging equipment, analysis
of the data is complex and laborious. Current methods typically
require repeated solution of the advection-dispersion equation
(ADE) for describing the flow in the borehole and comparison
with the experimental results. In this paper, we describe a
direct solution for
|
determining
borehole fluid velocity that bypasses the need for complex numerical
computation and repetitive optimization. The method rests on the
observation that, while solving the ADE for concentration profile
in the borehole (as required for modeling and combined methods)
is computationally challenging, the solution for flow distribution
along the length of the borehole given concentration data is straightforward.
The method can accommodate varying borehole diameters, and uses
the fact that multiple profiles are taken in the standard logging
approach to reduce the impact of noise. Data from both a simulated
borehole and from a field test are successfully analyzed. The
method is implemented in a spreadsheet, which is available as
supporting information material to this paper. |
|
Septiembre de 2014
The magnitude distribution of dynamically triggered earthquakes
Authors: Stephen Hernandez, Emily E. Brodsky et al
Link: Click here
Large dynamic strains carried by seismic waves
are known to trigger seismicity far from their source region.
It is unknown, however, whether surface waves trigger only small
earthquakes, or whether they can also trigger large earthquakes.
To partially address this question, we evaluate whether current
data can distinguish between the magnitude distribution of triggered
and untriggered small earthquakes. We use a mixing
|
model
approach in which total seismicity is decomposed into two classes:
"triggered" events initiated or advanced by far-field
dynamic strains and "untriggered" spontaneous events
consisting of everything else. The b-value of a mixed data set,
bMIX, is decomposed into a weighted sum of b-values of its constituent
components, bT and bU. We utilize the previously observed relationship
between triggering rate and dynamic strain amplitude to identify
the fraction of triggered events in populations of earthquakes
and then invert for bT. For Californian seismicity, data are consistent
with a single-parameter Gutenberg-Richter hypothesis governing
the magnitudes of both triggered and untriggered earthquakes. |
|
Septiembre de 2014
The 23 June 2014 Mw 7.9 Rat Islands archipelago, Alaska,
intermediate depth earthquake
Authors: Lingling Ye, Thorne Lay et al
Link: Click here
On 23 June 2014, the largest intermediate depth
earthquake (Mw 7.9) of the last 100?years ruptured within the
subducting Pacific plate about 100 km below the Rat Islands
archipelago of the Western Aleutian Islands, Alaska. The unusual
faulting orientation, strike = 206°, dip = 24°, and
|
rake=
14°, is possibly related to curvature of the underthrust slab
and high obliquity of the relative plate motions. The first ~15?s
of the rupture generated relatively weak seismic waves, followed
by strong energy release for the next 25?s. The seismic moment
is 1.0?×?1021?Nm, and slip of up to ~10 m is concentrated
within a 50 km× 50?km region. The radiated energy is 1.1
to 2.7 × 1016 J, assuming attenuation t* of 0.4 to 0.7 s.
This type of intraplate faulting can be very damaging for populated
regions above subduction zones such as Japan, Taiwan, Chile, and
Indonesia. |
|
Septiembre de 2014
Seismic moment tensor and b value variations over successive
seismic cycles in laboratory stick-slip experiments
Authors: Grzegorz Kwiatek, T. H. W. Goebel et al
Link: Click here
The formation of fault damage due to slip under
high normal stresses can rarely be monitored under in situ conditions.
To advance our understanding of microfracture processes, we
investigated stick-slip events on Westerly granite samples containing
the following: (1) a planar saw cut fault and (2) a fault developed
from a fresh fracture surface. We examined temporal changes
of seismic moment tensors and b
|
values
of acoustic emission (AE) events. During experiment on the saw
cut surface, small AEs exhibiting non-double-couple components
were observed continuously and strong AEs displaying double-couple
components were visible only when approaching the slip onsets.
Sliding on naturally fractured surfaces showed, in addition to
double-couple components, significant volumetric contributions,
especially during the interslip periods and immediately after
stick-slip events indicating substantial shear-enhanced compaction
within a relatively broad damage zone. The obtained results shed
light on how differences in fault structure control the kinematics
of microseismicity during different periods of the seismic cycle. |
|
Septiembre de 2014
REVIEWING AND VISUALIZING THE INTERACTIONS OF NATURAL HAZARDS
Authors: Joel C. Gill and Bruce D. Malamud
Link: Click here
This paper presents a broad overview, characterization
and visualization of the interaction relationships between 21
natural hazards, drawn from six hazard groups (geophysical,
hydrological, shallow Earth, atmospheric, biophysical and space
hazards). A synthesis is presented of the identified interaction
relationships between these hazards, using an accessible, visual
format particularly suited to end-users. Interactions considered
are primarily those where a primary hazard triggers or increases
the probability of secondary hazards occurring. In this paper
we do the following: (i) Identify, through a wide-ranging review
of grey- and peer-review literature, 90 interactions. (ii) Subdivide
the interactions into three levels, based on how well we can
|
characterize
secondary hazards given information about the primary hazard.
(iii) Determine the spatial overlap and temporal likelihood of
the triggering relationships occurring. (iv) Examine the relationship
between primary and secondary hazard intensities for each identified
hazard interaction and group these into five possible categories.
In this study we have synthesized, using accessible visualization
techniques, large amounts of information drawn from many scientific
disciplines. We outline the importance of constraining hazard
interactions and reinforce the importance of a holistic (or multi-hazard)
approach to natural hazard assessment. This approach allows those
undertaking research into single hazards to place their work within
the context of other hazards. It also communicates important aspects
of hazard interactions, facilitating an effective analysis by
those working on reducing and managing disaster risk within both
the policy and practitioner communities. |
|
Septiembre de 2014
Space-time model for repeating earthquakes and analysis of
recurrence intervals on the San Andreas Fault near Parkfield,
California
Authors: Shunichi Nomura, Yosihiko Ogata et al
Link: Click here
We propose a stochastic model for characteristically
repeating earthquake sequences to estimate the spatiotemporal
change in static stress loading rate. These earthquakes recur
by a cyclic mechanism where stress at a hypocenter is accumulated
by tectonic forces until an earthquake occurs that releases
the accumulated stress to a basal level. Renewal processes are
frequently used to describe this repeating earthquake mechanism.
Variations in the rate of tectonic loading due to large earthquakes
and aseismic slip transients, however, introduce nonstationary
effects into the repeating mechanism that result in nonstationary
trends in interevent times, particularly for smaller
|
magnitude
repeating events which have shorter interevent times. These trends
are also similar among repeating earthquake sites having similar
hypocenters. Therefore, we incorporate space- time structure represented
by cubic B-spline functions into the renewal model and estimate
their coefficient parameters by maximizing the integrated likelihood
in a Bayesian framework. We apply our model to 31 repeating earthquake
sequences including 824 events on the Parkfield segment of the
San Andreas Fault and estimate the spatiotemporal transition of
the loading rate on this segment. The result gives us details
of the change in tectonic loading caused by an aseismic slip transient
in 1993, the 2004 Parkfield M6 earthquake, and other nearby or
remote seismic activities. The degree of periodicity of repeating
event recurrence intervals also shows spatial trends that are
preserved in time even after the 2004 Parkfield earthquake when
time scales are normalized with respect to the estimated loading
rate. |
|
Septiembre de 2014
El Cobreloa: A geyser with two distinct eruption styles
Authors: Atsuko Namiki, Carolina Muñoz-Saez et
al
Link: Click here
We performed field measurements at a geyser
nicknamed "El Cobreloa," located in the El Tatio Geyser
Field, Northern Andes, Chile. The El Cobreloa geyser has two
distinct eruption styles: minor eruptions and more energetic
and long-lived major eruptions. Minor eruptions splash hot water
intermittently over an approximately 4 min time period. Major
eruptions begin with an eruption style similar to minor eruptions,
but then transition to a voluminous liquid water-dominated eruption,
and finally end with energetic steam discharge that continues
for approximately 1 h. We calculated eruption intervals by visual
observations, acoustic measurements, and
|
ground
temperature measurements and found that each eruption style has
a regular interval: 4 h and 40 min for major eruptions and 14
min for minor eruptions. Eruptions of El Cobreloa and geochemical
measurements suggest interaction of three water sources. The geyser
reservoir, connected to the surface by a conduit, is recharged
by a deep, hot aquifer. More deeply derived magmatic fluids heat
the reservoir. Boiling in the reservoir releases steam and hot
liquid water to the overlying conduit, causing minor eruptions,
and heating the water in the conduit. Eventually the water in
the conduit becomes warm enough to boil, leading to a steam-dominated
eruption that empties the conduit. The conduit is then recharged
by a shallow, colder aquifer, and the eruption cycle begins anew.
We develop a model for minor eruptions which heat the water in
the conduit. El Cobreloa provides insight into how small eruptions
prepare the geyser system for large eruptions. |
|
Septiembre de 2014
A new earthquake model may explain discrepancies in San Andreas
fault slip
Author: Colin Schultz
Link: Click here
Investigating the earthquake hazards of the
San Andreas fault system requires an accurate understanding
of accumulating stresses and the
|
history
of past earthquakes. Faults tend to go through an "earthquake
cycle"-locking and accumulating stress, rupturing in an earthquake,
and locking again in a well-accepted process known as "elastic
rebound." One of the key factors in preparing for California's
next "big one" is estimating the fault slip rate, the
speed at which one side of the San Andreas fault is moving past
the other. |
|
Septiembre de 2014
Tide-driven shear instability in planetary liquid cores
Authors: Alban Sauret, Michael Le Bars et al
Link: Click here
We present an experimental study on the shear
instability driven by tidal forcing in a model planetary liquid
core. The experimental setup consists of a water-filled deformable
sphere rotating around its axis and subjected to an
|
elliptical
forcing. At resonant forcing frequencies, the nonlinear self-interaction
of the excited inertial mode drives an intense and localized axisymmetric
jet. The jet becomes unstable at low Ekman number because of a
shear instability. Using particle image velocimetry measurements,
we derive a semiempirical scaling law that captures the instability
threshold of the shear instability. This mechanism is fully relevant
to planetary systems, where it constitutes a new route to generate
turbulence in their liquid cores. |
|
Septiembre de 2014
Evidence of viscoelastic deformation following the 2011 Tohoku-Oki
earthquake revealed from seafloor geodetic observation
Authors: Shun-ichi Watanabe, Mariko Sato et al
Link: Click here
The GPS/acoustic seafloor positioning has detected
significant postseismic movements after the 2011 Tohoku-Oki
earthquake (M9.0), just above the source region off the Pacific
coast of eastern Japan. In contrast to the coastal Global Navigation
Satellite Systems sites where trenchward-upward movements were
reported, the offshore sites above the main rupture zone in
|
the
northern part of the source region exhibit landward displacements
of tens of centimeters with significant subsidence from almost
3 years of repeated observations. At the sites above around the
edge of the main rupture zone, smaller amount of trench-normal
movements was found. Although the terrestrial movements were reasonably
interpreted by afterslip beneath the coastal area, these offshore
results are rather consistent with effects predicted from viscoelastic
relaxation in the upper mantle, providing definitive evidence
of its occurrence. On the other hand, the results in the southern
part of the source region imply superposition of effects from
viscoelastic relaxation and afterslip. |
|
Agosto de 2014
Intense foreshocks and a slow slip event preceded the 2014
Iquique Mw 8.1 earthquake
Authors: S. Ruiz, M. Metois et al
Link: Click here
The subduction zone in northern Chile is a well-identified
seismic gap that last ruptured in 1877. The moment magnitude
(Mw) 8.1 Iquique earthquake of 1 April 2014 broke a highly coupled
portion of this gap. To understand the seismicity preceding
this event, we studied the location and
|
mechanisms
of the foreshocks and computed Global Positioning System (GPS)
time series at stations located on shore. Seismicity off the coast
of Iquique started to increase in January 2014. After 16 March,
several Mw > 6 events occurred near the low-coupled zone. These
events migrated northward for ~50 kilometers until the 1 April
earthquake occurred. On 16 March, on-shore continuous GPS stations
detected a westward motion that we model as a slow slip event
situated in the same area where the mainshock occurred. |
|
Agosto de 2014
A simple inverse method for the interpretation of pumped
flowing fluid electrical conductivity logs
Authors: R. S. Moir, A. H. Parker et al
Link: Click here
Pumped flowing fluid electrical conductivity
(FFEC) logs, also known as pumped borehole dilution testing,
is an experimentally easy-to-perform approach to evaluating
vertical variations in the hydraulic conductivity of an aquifer.
In contrast to the simplicity of the logging equipment, analysis
of the data is complex and laborious. Current methods typically
require repeated solution of the advection-dispersion equation
(ADE) for describing the flow in the borehole and comparison
with the experimental results. In this paper, we describe a
direct solution for
|
determining
borehole fluid velocity that bypasses the need for complex numerical
computation and repetitive optimization. The method rests on the
observation that, while solving the ADE for concentration profile
in the borehole (as required for modeling and combined methods)
is computationally challenging, the solution for flow distribution
along the length of the borehole given concentration data is straightforward.
The method can accommodate varying borehole diameters, and uses
the fact that multiple profiles are taken in the standard logging
approach to reduce the impact of noise. Data from both a simulated
borehole and from a field test are successfully analyzed. The
method is implemented in a spreadsheet, which is available as
supporting information material to this paper. |
|
Agosto de 2014
Using lacustrine turbidites to illuminate past earthquakes
(Chile)
Author: JoAnna Wendel
Link: Click here
To fully understand earthquake occurrence patterns,
especially at underwater subduction
|
zones,
scientists often look to the past. Geological structures such
as turbidites—the deposits from sediment-infused, dense,
watery landslides caused by undersea earthquakes—can be good
proxies by which scientists study past earthquakes. |
|
Agosto de 2014
Self-organization of the Earth's climate system versus Milankovitch-Berger
astronomical cycles
Author: Lev A. Maslov
Link: Click here
The Late Pleistocene Antarctic temperature variation
curve is decomposed into two components: "cyclic"
and "high frequency, stochastic." For each of these
components, a mathematical model is developed which shows that
the cyclic and stochastic temperature variations are distinct,
but interconnected,
|
processes
with their own self-organization. To model the cyclic component,
a system of ordinary differential equations is written which represent
an auto-oscillating, self-organized process with constant period.
It is also shown that these equations can be used to model more
realistic variations in temperature with changing cycle length.
For the stochastic component, the multifractal spectrum is calculated
and compared to the multifractal spectrum of a critical sine-circle
map. A physical interpretation of relevant mathematical models
and discussion of future climate development within the context
of this work is given. |
|
Agosto de 2014
Varying seismic-acoustic properties of the fluctuating lava
lake at Villarrica volcano, Chile
Authors: Joshua P. Richardson, Gregory P. Waite et al
Link: Click here
Villarrica volcano outgasses through an open
lava lake, with bubbles ranging in size from submillimeter to
several meters, the largest of which produce strombolian bursting
events that are visible from the crater rim. Thousands of shallow
strombolian events identified through seismic waveform cross
correlation were found to produce discrete and repetitive long-period
seismic and infrasonic signals. We identified variations of
up to 0.7?s in seismic-acoustic arrival delay times between
April and July 2010 at a station ~750?m from the vent, which
we interpret as due to fluctuations in the level of lava lake.
|
During
time periods interpreted as having high lava lake levels, based
on reduced time delays, interevent times were also reduced, and
average seismic amplitude measurements, seismic and acoustic event
energies, and volcano acoustic-seismic ratios were all high as
compared to times when the lava lake was lower. The crater is
also a source of nearly continuous, monotonic infrasonic tremor.
We found that the peak frequency of this infrasonic tremor, typically
around 0.5-1.0 Hz, was inversely correlated with seismic-acoustic
delay times and therefore an indicator of lava lake level. We
use this correlation to propose a new model for infrasonic tremor
generation, namely, using crater geometry to approximate a Bessel
horn. We interpret the two clearest cycles of elevated seismicity
and lava lake level as due to an increase in exsolved gas, resulting
from an injection of volatile-rich magma or an overturn in a deeper
magma reservoir. |
|
Agosto de 2014
Dispersion and nonlinear effects in the 2011 Tohoku-Oki earthquake
tsunami
Authors: Tatsuhiko Saito, Daisuke Inazu et al
Link: Click here
This study reveals the roles of the wave dispersion
and nonlinear effects for the 2011 Tohoku-Oki earthquake tsunami.
We conducted tsunami simulations based on the nonlinear dispersive
equations with a high-resolution source model. The simulations
successfully reproduced the waveforms recorded in the offshore,
deep sea, and focal areas. The calculated inundation area coincided
well with the actual inundation for the Sendai Plain, which
was the widest inundation area during this event. By conducting
sets of simulations with different
|
tsunami
equations, we obtained the followings insights into the wave dispersion,
nonlinear effects, and energy dissipation for this event. Although
the wave dispersion was neglected in most studies, the maximum
amplitude was significantly overestimated in the deep sea if the
dispersion was not included. The waveform observed at the station
with the largest tsunami height (?2 m) among the deep-ocean stations
also verified the necessity of the dispersion. It is well known
that the nonlinear effects play an important role for the propagation
of a tsunami into bays and harbors. Additionally, nonlinear effects
need to be considered to accurately model later waves, even for
offshore stations. In particular, including nonlinear terms rather
than the inundation was more important when precisely modeling
the waves reflected from the coast. |
|
Agosto de 2014
A boom in boomless seismology
Author: Eric Hand
Link: Click here
Taking advantage of cheap, long-lasting sensors
developed by the oil and gas industry, seismologists are increasingly
exploring the top 10 kilometers of the crust with dense surveys
of hundreds or even thousands of seismometers.
|
What's
more, the surveys are being operated passively, which means that
ambient noise from Earth is used to illuminate the subsurface.
Passive surveys are cheaper and easier to permit than active surveys,
which rely on explosives or vibrations from special "thumper
trucks." Seismologists predict that the dense surveys will
yield precision imaging of magma chambers underneath volcanos
and earthquake rupture zones along faults. |
|
Agosto de 2014
Spherical harmonic stacking for the singlets of Earth's normal
modes of free oscillation
Authors: Benjamin F. Chao and Hao Ding
Link: Click here
We extend the spherical harmonic stacking (SHS)
method of Buland et al. (1979) for the radial (vertical) component
in the seismogram to the transverse (horizontal) components
of the displacement field. Taking advantage of the orthogonality
of the spherical harmonic functions (scalar and vectorial),
SHS isolates and accentuates the signals of individual singlets
of
|
the
Earth's normal modes of free oscillation. We apply the SHS on
the broadband Incorporated Research Institutions for Seismology
(IRIS) seismograms from up to 97 IRIS seismic stations for the
2004 Sumatra-Andaman earthquake, in experiments targeted to spheroidal
as well as toroidal modes-2S1,0S3, 2S2, 3S1, 1S3, 0T2, and 0T3.
We report the complete resolution of the singlet frequencies of
these multiplets, some for the first time, and estimate the singlets'
complex frequencies using the frequency domain autoregressive
method of Chao and Gilbert (1980). The latter contain useful information
to be used in inversions for the 3-D structure of the Earth's
interior. |
|
Agosto de 2014
Long- and short-term postseismic gravity changes of megathrust
earthquakes from satellite gravimetry
Authors: Yusaku Tanaka and Kosuke Heki
Link: Click here
Using monthly satellite gravimetry data, we
studied time-variable gravity field after three M9 class earthquakes,
the 2004 Sumatra-Andaman, 2010 Chile (Maule), and 2011 Tohoku-oki
earthquakes. The observations showed that the gravity typically
(1) decreases coseismically, (2) continues to decrease for a
few months, and (3)
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increases
over a longer period. Therefore, postseismic gravity changes have
two components with different time constants and polarities. The
mechanisms of short- and long-term postseismic gravity changes
are not as clear as coseismic changes at the moment, but might
be explained to some extent with afterslip and the Maxwell viscoelasticity,
respectively. These two components are difficult to discriminate
with surface velocity measurements because the forearc area moves
trenchward at both stages. They appear in different polarities
in gravity, making satellite gravimetry a unique tool to separate
them. |
|
Julio de 2014
Gas emissions from five volcanoes in northern Chile and implications
for the volatiles budget of the Central Volcanic Zone
Authors: G. Tamburello, T. H. Hansteen et al
Link: Click here
This study performed the first assessment of
the volcanic gas output from the Central Volcanic Zone (CVZ)
of northern Chile. We present the fluxes and compositions of
volcanic gases (H2O, CO2, H2, HCl, HF, and HBr) from five of
the most actively degassing volcanoes in this region-
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Láscar,
Lastarria, Putana, Ollagüe, and San Pedro-obtained during
field campaigns in 2012 and 2013. The inferred gas plume compositions
for Láscar and Lastarria (CO2/Stot?=?0.9-2.2; Stot/HCl
?=?1.4-3.4) are similar to those obtained in the Southern Volcanic
Zone of Chile, suggesting uniform magmatic gas fingerprint throughout
the Chilean arc. Combining these compositions with our own UV
spectroscopy measurements of the SO2 output (summing to ~1800?t?d?1
for the CVZ), we calculate a cumulative CO2 output of 1743-1988?t?d?1
and a total volatiles output of >20,200?t?d?1. |
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Julio de 2014
Multiple slow-slip events during a foreshock sequence of
the 2014 Iquique, Chile Mw 8.1 earthquake
Authors: Aitaro Kato and Shigeki Nakagawa
Link: Click here
To obtain a precise record of the foreshock
sequence before the 2014 Iquique, Chile Mw 8.1 earthquake, we
applied a matched filter technique to continuous seismograms
recorded near the source region. We newly detected about 10
times the number of seismic events listed in the routinely constructed
earthquake catalog and
|
identified
multiple sequences of earthquake migrations at speeds of 2-10?km/d,
both along strike and downdip on the fault plane, updip of the
main shock area. In addition, we found out repeating earthquakes
from the newly detected events, likely indicating aseismic slip
along the plate boundary fault during the foreshock sequence.
These observations suggest the occurrence of multiple slow-slip
events updip of the main shock area. The final slow-slip event
migrated toward the main shock nucleation point. We interpret
that several parts of the plate boundary fault perhaps experienced
slow slip, causing stress loading on the prospective largest slip
patch of the main shock rupture. |
|
Julio de 2014
Two subevents across the Japan Trench during the 7 December
2012 off Tohoku earthquake (Mw 7.3) inferred from offshore tsunami
records
Authors: Daisuke Inazu and Tatsuhiko Saito
Link: Click here
A tsunamigenic earthquake with a moment magnitude
of 7.3 occurred near the Japan Trench, off Tohoku, northeast
Japan, on 7 December 2012. Operational seismic monitoring inferred
that the earthquake was composed of doublet sources of comparable
magnitudes: the first event was reverse faulting and the second
event, which occurred 10-20 s later, was normal faulting. An
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associated
tsunami was observed at offshore stations, having an amplitude
of 10?1 to 101 cm. Inverse modeling using the observed tsunami
records reveals possible vertical seafloor deformations. We can
then constrain, in terms of the source location, that the tsunamigenic
earthquake involves two subfaults across the Japan Trench: one
is the outer trench reverse faulting as the first event and the
second is the inner trench normal faulting. The present study
shows that tsunami analysis with seismograph analyses effectively
constrains the epicenter locations, even in the case of complex
earthquakes like doublets. |
|
Julio de 2014
Multiple slow-slip events during a foreshock sequence of
the 2014 Iquique, Chile Mw 8.1 earthquake
Authors: Aitaro Kato and Shigeki Nakagawa
Link: Click here
To obtain a precise record of the foreshock
sequence before the 2014 Iquique, Chile Mw 8.1 earthquake, we
applied a matched filter technique to continuous seismograms
recorded near the source region. We newly detected about 10
times the number of seismic events listed in the routinely constructed
earthquake catalog and identified multiple sequences of earthquake
|
migrations
at speeds of 2-10?km/d, both along strike and downdip on the fault
plane, updip of the main shock area. In addition, we found out
repeating earthquakes from the newly detected events, likely indicating
aseismic slip along the plate boundary fault during the foreshock
sequence. These observations suggest the occurrence of multiple
slow-slip events updip of the main shock area. The final slow-slip
event migrated toward the main shock nucleation point. We interpret
that several parts of the plate boundary fault perhaps experienced
slow slip, causing stress loading on the prospective largest slip
patch of the main shock rupture. |
|
Julio de 2014
The 2010-2014.3 global earthquake rate increase
Authors: Tom Parsons and Eric L. Geist
Link: Click here
In light of a heightened global earthquake rate
during the first quarter of 2014 and recent studies concluding
that large earthquakes affect global seismicity for extended
periods, we revisit the question whether the temporal distribution
of global earthquakes shows clustering beyond that expected
from a time-independent Poisson process. We examine a broad
window from 1979
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to
2014.3 for M???7.0 shocks, and a narrow window for M? ??5.0 seismicity
since 2010 that has higher than average rates. We test whether
a Poisson process can be falsified at 95% confidence to assess
the degree of dependent clustering in the catalogs. If aftershocks
within at least one rupture length from main shocks/foreshocks
are filtered, then we find no evidence of global scale M???5.2-5.6
(depending on parameters) clustering since 2010 that demands a
physical explanation. There is evidence for interdependence below
this threshold that could be a consequence of catalog completeness
or a physical process. |
|
Julio de 2014
On the predictability limit of convection models of the Earth's
mantle
Authors: Léa Bello, Nicolas Coltice et al
Link: Click here
Reconstructing convective flow in the Earth's
mantle is a crucial issue for a diversity of disciplines, from
seismology to sedimentology. The common and fundamental limitation
of these reconstructions based on geodynamic modeling is the
unknown initial conditions. Because of the chaotic nature of
convection in the Earth's mantle, errors in initial conditions
grow exponentially with time and limit forecasting and hindcasting
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abilities.
In this work, we estimate for the first time the limit of predictability
of Earth's mantle convection. Following the twin experiment method,
we compute the Lyapunov time (i.e., e-folding time) for state
of the art 3-D spherical convection models, varying rheology,
and Rayleigh number. Our most Earth-like and optimistic solution
gives a Lyapunov time of 136?±?13 Myr. Rough estimates
of the uncertainties in best guessed initial conditions are around
5%, leading to a limit of predictability for mantle convection
of 95 Myr. Our results suggest that error growth could produce
unrealistic convective structures over time scales shorter than
that of Pangea dispersal. |
|
Julio de 2014
Relative water and gas permeability for gas production from
hydrate-bearing sediments
Authors: Nariman Mahabadi and Jaewon Jang
Link: Click here
Relative water and gas permeability equations
are important for estimating gas and water production from hydrate-bearing
sediments. However, experimental or numerical study to determine
fitting parameters of those equations is not available in the
literature. In this study, a pore-network model is developed
to simulate gas expansion and calculate relative water and gas
|
permeability.
Based on the simulation results, fitting parameters for modified
Stone equation are suggested for a distributed hydrate system
where initial hydrate saturations range from Sh?=?0.1 to 0.6.
The suggested fitting parameter for relative water permeability
is nw? ??2.4 regardless of initial hydrate saturation while the
suggested fitting parameter for relative gas permeability is increased
from ng?=?1.8 for Sh?=?0.1 to ng?=?3.5 for Sh? =?0.6. Results
are relevant to other systems that experience gas exsolution such
as pockmark formation due to sea level change, CO2 gas formation
during geological CO2 sequestration, and gas bubble accumulation
near the downstream of dams. |
|
Julio de 2014
El Cobreloa: A geyser with two distinct eruption styles
Authors: Atsuko Namiki , Carolina Muñoz-Saez et
al
Link: Click here
We performed field measurements at a geyser
nicknamed "El Cobreloa", located in the El Tatio Geyser
Field, Northern Andes, Chile. The El Cobreloa geyser has two
distinct eruption styles: minor eruptions, and more energetic
and long-lived major eruptions. Minor eruptions splash hot water
intermittently over an approximately 4?minute time period. Major
eruptions begin with an eruption style similar to minor eruptions,
but then transition to a voluminous liquid-water-dominated eruption,
and finally end with energetic steam discharge that continues
for approximately 1?hour. We calculated eruption intervals by
visual observations, acoustic measurements, and
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ground
temperature measurements, and found that each eruption style has
a regular interval: 4?hours and 40?minutes for major eruptions,
and ~14?minutes for minor eruptions. Eruptions of El Cobreloa
and geochemical measurements suggest interaction of three water
sources. The geyser reservoir, connected to the surface by a conduit,
is recharged by a deep, hot aquifer. More deeply derived magmatic
fluids heat the reservoir. Boiling in the reservoir releases steam
and hot liquid water to the overlying conduit, causing minor eruptions,
and heating the water in the conduit. Eventually the water in
the conduit becomes warm enough to boil, leading to a steam-dominated
eruption that empties the conduit. The conduit is then recharged
by a shallow, colder aquifer, and the eruption cycle begins anew.
We develop a model for minor eruptions which heat the water in
the conduit. El Cobreloa provides insight into how small eruptions
prepare the geyser system for large eruptions. |
|
Julio de 2014
Hydrochemical monitoring, petrological observation, and geochemical
modeling of fault healing after an earthquake
Authors: Niklas Wästeby, Alasdair Skelton et al
Link: Click here
Based on hydrochemical monitoring, petrological
observations, and geochemical modeling, we identify a mechanism
and estimate a time scale for fault healing after an earthquake.
Hydrochemical monitoring of groundwater samples from an aquifer,
which is at an approximate depth of 1200?m, was conducted over
a period of 10?years. Groundwater samples have been taken from
a borehole (HU-01) that crosses the Húsavík-Flatey
Fault (HFF) near Húsavík town, northern Iceland.
After 10?weeks of sampling, on 16 September 2002, an M 5.8 earthquake
occurred on the Grimsey Lineament, which is approximately parallel
to the HFF. This earthquake caused rupturing of a hydrological
barrier resulting in an influx of groundwater from a
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second
aquifer, which was recorded by 15-20% concentration increases
for some cations and anions. This was followed by hydrochemical
recovery. Based on petrological observations of tectonically exhumed
fault rocks, we conclude that hydrochemical recovery recorded
fault healing by precipitation of secondary minerals along fractures.
Because hydrochemical recovery accelerated with time, we conclude
that the growth rate of these minerals was controlled by reaction
rates at mineral-water interfaces. Geochemical modeling confirmed
that the secondary minerals which formed along fractures were
saturated in the sampled groundwater. Fault healing and therefore
hydrochemical recovery was periodically interrupted by refracturing
events. Supported by field and petrographic evidence, we conclude
that these events were caused by changes of fluid pressure probably
coupled with earthquakes. These events became successively smaller
as groundwater flux decreased with time. Despite refracturing,
hydrochemical recovery reached completion 8-10 years after the
earthquake. |
|
Julio de 2014
Mapping Europe's Seismic Hazard
Authors: Domenico Giardini, Jochen Wössner et al
Link: Click here
From the rift that cuts through the heart of
Iceland to the complex tectonic convergence that causes
|
frequent
and often deadly earthquakes in Italy, Greece, and Turkey to the
volcanic tremors that rattle the Mediterranean, seismic activity
is a prevalent and often life-threatening reality across Europe.
Any attempt to mitigate the seismic risk faced by society requires
an accurate estimate of the seismic hazard. |
|
Julio de 2014
The 2011 Tohoku Tsunami observed by an array of ocean bottom
electromagnetometers
Authors: Luolei Zhang, Kiyoshi Baba et al
Link: Click here
A tsunami induces secondary electromagnetic
(EM) fields of significant intensity as a result of Faraday's
law, and these EM fields can be recorded by instruments on the
seafloor. Such observations will provide parameters that are
useful in understanding tsunami generation and
|
propagation
processes. As such, EM sensors can be used as a type of tsunami
sensor. In the present study, we report the observation of tsunami-induced
EM signals by a small array of ocean bottom electromagnetometers
consisting of four stations in the northwestern Pacific Ocean.
Then, several data analysis methods are used to estimate tsunami
parameters. Referring to the theoretical relation, the wave height
is estimated from the observed magnetic and electric fields. Also,
the propagation direction of the 2011 Tohoku Tsunami is estimated
by applying analysis methods for an array and a single station. |
|
Julio de 2014
Mapping pressurized volcanic fluids from induced crustal
seismic velocity drops
Authors: F. Brenguier, M. Campillo et al
Link: Click here
A tsunami induces secondary electromagnetic
(EM) fields of significant intensity as a result of Faraday's
law, and these EM fields can be recorded by instruments on the
seafloor. Such observations will provide parameters that are
useful in understanding tsunami generation and
|
propagation
processes. As such, EM sensors can be used as a type of tsunami
sensor. In the present study, we report the observation of tsunami-induced
EM signals by a small array of ocean bottom electromagnetometers
consisting of four stations in the northwestern Pacific Ocean.
Then, several data analysis methods are used to estimate tsunami
parameters. Referring to the theoretical relation, the wave height
is estimated from the observed magnetic and electric fields. Also,
the propagation direction of the 2011 Tohoku Tsunami is estimated
by applying analysis methods for an array and a single station. |
|
Julio de 2014
Simulation of the airwave caused by the Chelyabinsk superbolide
Authors: Mikhail I. Avramenko, Igor V. Glazyrin et al
Link: Click here
Numerical simulations were carried out to model
the propagation of an airwave from the fireball that passed
over Chelyabinsk (Russia) on 15 February 2013. The airburst
of the Chelyabinsk meteoroid occurred due to its catastrophic
fragmentation in the atmosphere. Simulations of the space-time
distribution of energy deposition during the airburst were done
using a novel fragmentation model based on dimensionality considerations
and analogy to the fission chain reaction in fissile materials.
To get an estimate of the airburst energy, observed values of
the airwave arrival times to different populated localities
were retrieved from video records
|
available
on the Internet. The calculated arrival times agree well with
the observed values for all the localities. Energy deposition
in the atmosphere obtained from observations of the airwave arrival
times was found to be 460?±?60 kt in trinitrotoluene (TNT)
equivalent. We also obtained an independent estimate for the deposited
energy, kt TNT from detecting the air increment velocity due to
the wave passage in Chelyabinsk. Assuming that the energy of about
90 kt TNT was irradiated in the form of visible light and infrared
radiation, as registered with optical sensors [Yeomans and Chodas,
2013], one can value the total energy release to be about 550
kt TNT which is in agreement with previous estimates from infrasound
registration and from optical sensors data. The overpressure amplitude
and its positive phase duration in the airwave that reached the
city of Chelyabinsk were calculated to be about 2 kPa and 10?s
accordingly. |
|
Julio de 2014
Coseismic magnetization of fault pseudotachylytes: 1. Thermal
demagnetization experiments
Authors: E.C. Ferré, J.W. Geissman et al
Link: Click here
Fault pseudotachylytes form by quenching of
silicate liquids produced through coseismic frictional melting.
Here we show that in natural pseudotachylytes the main carrier
of magnetic remanence blocked in during cooling of the frictional
melt is fine grained magnetite. This confirms previous studies
on friction melt experiments. Stoichiometric magnetite, produced
during earthquakes by the breakdown of ferromagnesian silicates,
records the ambient magnetic field during seismic slip. We find
that most fault pseudotachylytes exposed in the Santa
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Rosa
Mountains, southern California, a classic pseudotachylyte locality,
acquired their natural remanent magnetization (NRM) upon cooling
of the frictional melt through the range of magnetization blocking
temperatures of the magnetite grains and this primarily constitutes
a thermal remanent magnetization (TRM). NRM intensities typical
of most pseudotachylyte veins range from 1 to 60. 10?4 Am2/kg.
A few specimens, however, contain magnetizations significantly
higher than that caused by the Earth's field as well as magnetization
directions that are highly variable over short distances. Other
magnetization processes, possibly related to coseismic electric
currents, may be involved during the seismogenic process to control
NRM acquisition. |
|
Julio de 2014
Dynamics of dikes versus cone sheets in volcanic systems
Authors: Olivier Galland, Steffi Burchardt et al
Link: Click here
Igneous sheet intrusions of various shapes,
such as dikes and cone sheets, coexist as parts of complex volcanic
plumbing systems likely fed by common sources. How they form
is fundamental regarding volcanic hazards, but yet no dynamic
model simulates and predicts satisfactorily their diversity.
Here we present scaled laboratory experiments that reproduced
dikes and cone sheets under controlled conditions. Our models
show that their formation is governed by a dimensionless ratio
(?1), which describes the shape of the magma source, and a dynamic
dimensionless ratio (?2), which compares the viscous stresses
in the flowing magma to the
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host-rock
strength. Plotting our experiments against these two numbers results
in a phase diagram evidencing a dike and a cone-sheet field, separated
by a sharp transition that fits a power law. This result shows
that dikes and cone sheets correspond to distinct physical regimes
of magma emplacement in the crust. For a given host-rock strength,
cone sheets preferentially form when the source is shallow, relative
to its lateral extent, or when the magma influx velocity (or viscosity)
is high. Conversely, dikes form when the source is deep compared
to its size, or when magma influx rate (or viscosity) is low.
Both dikes and cone sheets may form from the same source, the
shift from one regime to the other being then controlled by magma
dynamics, i.e., different values of ?2. The extrapolated empirical
dike-to-cone sheet transition is in good agreement with the occurrence
of dikes and cone sheets in various natural volcanic settings. |
|
Julio de 2014
Multiple slow-slip events during a foreshock sequence of
the 2014 Iquique, Chile Mw 8.1 earthquake
Authors: Aitaro Kato and Shigeki Nakagawa
Link: Click here
To obtain a precise record of the foreshock
sequence before the 2014 Iquique, Chile Mw 8.1 earthquake, we
applied a matched-filter technique to continuous seismograms
recorded near the source region. We newly detected about 10
times the number of seismic events listed in the routinely constructed
earthquake catalog, and identified multiple sequences of earthquake
|
migrations
at speeds of 2-10?km/day, both along-strike and down-dip on the
fault plane, up-dip of the mainshock area. In addition, we found
out repeating earthquakes from the newly detected events, likely
indicating aseismic slip along the plate boundary fault during
the foreshock sequence. These observations suggest the occurrence
of multiple slow-slip events up-dip of the mainshock area. The
final slow-slip event migrated toward the mainshock nucleation
point. We interpret that several parts of the plate boundary fault
perhaps experienced slow slip, causing stress loading on the prospective
largest slip patch of the mainshock rupture. |
|
Julio de 2014
Mapping pressurized volcanic fluids from induced crustal
seismic velocity drops
Authors: F. Brenguier, M. Campillo et al
Link: Click here
To obtain a precise record of the foreshock
sequence before the 2014 Iquique, Chile Mw 8.1 earthquake, we
applied a matched-filter technique to continuous seismograms
recorded near the source region. We newly detected about 10
times the number of seismic events listed in the routinely constructed
earthquake catalog, and identified multiple sequences of earthquake
|
migrations
at speeds of 2-10?km/day, both along-strike and down-dip on the
fault plane, up-dip of the mainshock area. In addition, we found
out repeating earthquakes from the newly detected events, likely
indicating aseismic slip along the plate boundary fault during
the foreshock sequence. These observations suggest the occurrence
of multiple slow-slip events up-dip of the mainshock area. The
final slow-slip event migrated toward the mainshock nucleation
point. We interpret that several parts of the plate boundary fault
perhaps experienced slow slip, causing stress loading on the prospective
largest slip patch of the mainshock rupture. |
|
Julio de 2014
What causes random earthquakes within a hydrothermal vent?
Author: Colin Schultz
Link: Click here
The Trans-Atlantic Geotraverse (TAG) is the
|
largest
deep sea hydrothermal vent field found anywhere on the planet.
Located on the Mid-Atlantic Ridge, a seafloor spreading center
where new oceanic crust is being created, the TAG field consists
of one active high-temperature mound, one low-temperature mound,
and seven inactive mounds. |
|
Julio de 2014
Induced electromagnetic field by seismic waves in Earth's
magnetic field
Authors: Yongxin Gao,Xiaofei Chen et al
Link: Click here
Studied in this article are the properties of
the electromagnetic (EM) fields generated by an earthquake due
to the motional induction effect, which arises from the motion
of the conducting crust across the Earth's magnetic field. By
solving the governing equations that couple the elastodynamic
equations with Maxwell equations, we derive the seismoelectromagnetic
wavefields excited by a single-point force and a double-couple
source in a full space. Two types of EM disturbances can be
generated, i.e., the coseismic EM field accompanying the seismic
wave and the independently propagating EM wave which arrives
much earlier than the seismic wave. Simulation of an Mw6.1 earthquake
shows that at a receiving location where the seismic
|
acceleration is on the order of 0.1?m/s2, the
coseismic electric and magnetic fields are on the orders of
1??V/m and 0.1?nT, respectively, agreeing with the EM data observed
in 2008 Mw6.1 Qingchuan earthquake, China, and indicating that
the motional induction effect is effective enough to generate
observable EM signal. We also simulated the EM signals observed
by Haines et al. (2007) which were called the Lorentz fields
and cannot be explained by the electrokinetic effect. The result
shows that the EM wave generated by a horizontal force can explain
the data well, suggesting that the motional induction effect
is responsible for the Lorentz fields. The motional induction
effect is compared with the electrokinetic effect, showing the
overall conclusion that the former dominates the mechanoelectric
conversion under low-frequency and high-conductivity conditions
while the latter dominates under high-frequency and low-conductivity
conditions.
|
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Julio de 2014
A turbulent, high magnetic Reynolds number experimental model
of Earth's core
Authors: Daniel S. Zimmerman, Santiago Andrés
Triana et al
Link: Click here
We present new experimental results from the
University of Maryland Three Meter Geodynamo experiment. We
drive a fully turbulent flow in water and also in sodium at
magnetic Reynolds number Rm = ??(ro?ri)2/?, up to 715 (about
half design maximum) in a spherical Couette apparatus geometrically
similar to Earth's core. We have not yet observed a self-generating
dynamo, but we study MHD effects with an externally applied
axisymmetric magnetic field. We survey a broad range of Rossby
number ?68 < Ro = ??/?o< 65 in both purely hydrodynamic
water experiments and sodium experiments with weak, nearly passive
|
applied
field. We characterize angular momentum transport and substantial
generation of internal toroidal magnetic field (the ? effect)
as a function of Ro and find a rich dependence of both angular
momentum transport and ? effect on Ro. Internal azimuthal field
generation peaks at Ro = 6 with a gain as high as 9 with weak
applied field. At this Rossby number, we also perform experiments
with significant Lorentz forces by increasing the applied magnetic
field. We observe a reduction of the ? effect, a large increase
in angular momentum transport, and the onset of new dynamical
states. The state we reach at maximum applied field shows substantial
magnetic field gain in the axial dipole moment, enhancing the
applied dipole moment. This intermittent dipole enhancement must
come from nonaxisymmetric flow and seems to be a geodynamo-style
feedback involving differential rotation and large-scale drifting
waves. |
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Julio de 2014
Three-dimensional Vp and Vp/Vs models in the Coso geothermal
area, California: Seismic characterization of the magmatic system
Authors: Qiong Zhang and Guoqing Lin
Link: Click here
We combine classic and state-of-the-art techniques
to characterize the seismic and volcanic features in the Coso
area in southern California. Seismic tomography inversions are
carried out to map the variations of Vp, Vs, and Vp/Vs beneath
Coso. The velocities in the top layers of our model are correlated
with the surface geological features. The Indian Wells Valley,
with high silica content sediment strata, shows low-velocity
anomalies up to 3 km depth, whereas the major mountain ranges,
such as the south Sierra Nevada and the Argus Range, show higher
velocities. The resulting three-dimensional velocity model is
used to improve absolute
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locations
for all local events between January 1981 and August 2011 in our
study area. We then apply similar-event cluster analysis, waveform
cross correlation, and differential time relocation methods to
improve relative event location accuracy. A dramatic sharpening
of seismicity patterns is obtained after using these methods.
We also estimate high-resolution near-source Vp/Vs ratio within
each event cluster using the differential times from waveform
cross correlation. The in situ Vp/Vs method confirms the trend
of the velocity variations from the tomographic results. An anomalous
low-velocity body with low Vp, Vs, and Vp/Vs ratios, corresponding
to the ductile behavior underlying the Coso geothermal field from
6 to 12 km depth, can be explained by the existence of frozen
felsic magmatic materials with the inclusion of water. The material
is not likely to include pervasive partial melt due to a lack
of high Vp/Vs ratios. |
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Julio de 2014
Rupture process of the 2014 Iquique Chile Earthquake in relation
with the foreshock activity
Authors: Yuji Yagi, Ryo Okuwaki et al
Link: Click here
The rupture process of the 2014 Iquique, Chile
earthquake is inverted from teleseismic P wave data applying
a novel formulation that takes into account the uncertainty
of Green's function, which has been a major error source in
waveform inversion. The estimated seismic moment is 1.5?×?1021?Nm
(Mw? =?8.1), associated with a 140?km long and 140?km wide fault
rupture along the plate interface. The source process is
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characterized
by unilateral rupture propagation. During the first 20?s, the
dynamic rupture front propagated from the hypocenter to the large
asperity located about 50?km southward, crossing a remarkably
active foreshock area at high velocity (of about 3.0?km/s), but
small and irregular seismic moment release rate. Our result may
suggest that the 20?s long initial phase was influenced by the
stress drop due to the foreshock activity near the main shock
hypocenter. Moreover, the 2?week long swarm-like foreshock activity
migrating roughly at 5?km/day toward the main shock hypocenter,
and possibly associated slow slip, contributed to the stress accumulation
prior to the Mw 8.1 megaquake. The main shock initial rupture
phase might have triggered the rupture of the large asperity,
which had large fracture energy. |
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Julio de 2014
A simple inverse method for the interpretation of pumped
flowing fluid electrical conductivity logs
Authors: R.S. Moir, A.H. Parker et al
Link: Click here
Pumped flowing fluid electrical conductivity
(FFEC) logs, also known as pumped borehole dilution testing
is an experimentally easy-to-perform approach to evaluating
vertical variations in the hydraulic conductivity of an aquifer.
In contrast to the simplicity of the logging equipment, analysis
of the data is complex and laborious. Current methods typically
require repeated solution of the Advection-Dispersion Equation
(ADE) for describing the flow in the borehole and comparison
with the experimental results. In this
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paper
we describe a direct solution for borehole fluid velocity that
bypasses the need for complex numerical computation and repetitive
optimization. The method rests on the observation that, while
solving the ADE for concentration profile in the borehole (as
required for modeling and combined methods) is computationally
challenging, the solution for flow distribution along the length
of the borehole given concentration data is straightforward. The
method can accommodate varying borehole diameters, and uses the
fact that multiple profiles are taken in the standard logging
approach to reduce the impact of noise. Data from both a simulated
borehole and from a field test are successfully analyzed. The
method is implemented in a spreadsheet, available as supplemental
material to this paper. |
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Julio de 2014
Hydrochemical monitoring, petrological observation and geochemical
modelling of fault healing after an earthquake
Authors: Niklas Wästeby, Alasdair Skelton et al
Link: Click here
Based on hydrochemical monitoring, petrological
observations and geochemical modelling, we identify a mechanism
and estimate a timescale for fault healing after an earthquake.
Hydrochemical monitoring of groundwater samples from an aquifer,
which is at an approximate depth of 1200 meters, was conducted
over a period of ten years. Groundwater samples have been taken
from a borehole (HU-01) that crosses the Húsavík-Flatey
Fault (HFF) near Húsavík town, northern Iceland.
After ten weeks of sampling, on September 16, 2002, an M 5.8
earthquake occurred on the Grimsey Lineament (GL) which is approximately
parallel to the HFF. This earthquake caused rupturing of a hydrological
barrier resulting in an influx of
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groundwater
from a second aquifer, which was recorded by 15-20% concentration
increases for some cations and anions. This was followed by hydrochemical
recovery. Based on petrological observations of tectonically-exhumed
fault rocks, we conclude that hydrochemical recovery recorded
fault healing by precipitation of secondary minerals along fractures.
Because hydrochemical recovery accelerated with time, we conclude
that the growth rate of these minerals was controlled by reaction
rates at mineral-water interfaces. Geochemical modelling confirmed
that the secondary minerals which formed along fractures were
saturated in the sampled groundwater. Fault healing and therefore
hydrochemical recovery was periodically interrupted by re-fracturing
events. Supported by field and petrographic evidence, we conclude
that these events were caused by changes of fluid pressure probably
coupled with earthquakes. These events became successively smaller
as groundwater flux decreased with time. Despite re-fracturing,
hydrochemical recovery reached completion 8-10 years after the
earthquake. |
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Julio de 2014
Tsunami source and its validation of the 2014 Iquique, Chile,
earthquake
Authors: Chao An, Ignacio Sepúlveda et al
Link: Click here
The slip distribution of the 1 April 2014 Iquique
earthquake is obtained by using the least squares inversion
of tsunami data at three Deep-Ocean Assessment and Reporting
of Tsunamis stations. Most of the slip is concentrated along
a 60 km by 40 km slip patch near the hypocenter, with magnitude
ranging from 5 to 7 m and a depth of
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23
km. The earthquake magnitude from the inversion is estimated as
Mw8.0. The slip distribution is converted into seafloor displacement
based on Okada's formula. A nonlinear shallow water equation model
is used to simulate tsunami wave propagation, and the simulated
water surface elevations are compared with the measured data at
10 tide gauges along the Chilean coast. The agreement is excellent
at gauges where the local bathymetry data are complete and the
gauges are open to the ocean; otherwise, mismatches of up to 10
min in arrival time and 1.0 m in amplitude are seen. |
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Julio de 2014
Induced electromagnetic field by seismic waves in Earth's
magnetic field
Authors: Yongxin Gao, Xiaofei Chen et al
Link: Click here
Studied in this article are the properties of
the electromagnetic (EM) fields generated by an earthquake due
to the motional induction effect, which arises from the motion
of the conducting crust across the earth's magnetic field. By
solving the governing equations that couple the elastodynamic
equations with Maxwell equations, we derive the seismoelectromagnetic
wavefields excited by a single point force and a double couple
source in a full space. Two types of EM disturbances can be
generated, i.e., the coseismic EM field accompanying the seismic
wave and the independently propagating EM wave which arrives
much earlier than the seismic wave. Simulation of an Mw6.1 earthquake
shows that at
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a
receiving location where the seismic acceleration is on the order
of 0.1?m/s2, the coseismic electric and magnetic fields are on
the orders of 1 V/m and 0.1 nT, respectively, agreeing with the
EM data observed in 2008 Mw6.1 Qingchuan earthquake, China, and
indicating that the motional induction effect is effective enough
to generate observable EM signal. We also simulated the EM signals
observed by Haines et al. [2007] which were called the Lorentz
fields and cannot be explained by the electrokinetic effect. The
result shows that the EM wave generated by a horizontal force
can explain the data well, suggesting that the motional induction
effect is responsible for the Lorentz fields. The motional induction
effect is compared with the electrokinetic effect, showing the
overall conclusion that the former dominates the mechanoelectric
conversion under low-frequency and high-conductivity conditions
while the latter dominates under high-frequency and low-conductivity
conditions. |
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Julio de 2014
Remotely triggered nonvolcanic tremor in Sumbawa, Indonesia.
Authors: F. Fuchs, M. Lupi et al
Link: Click here
We present, for the first time, evidence for
triggered tremor beneath the island of Sumbawa, Indonesia. We
show triggered tremor in response to three teleseismic earthquakes:
the Mw9.0 2011 Tohoku earthquake and two oceanic strike-slip
earthquakes (Mw 8.6 andMw8.2) offshore of Sumatra in 2012. We
constrain an apparent
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triggering
threshold of 1 mm/s ground velocity that corresponds to about
8 kPa dynamic stress. Peak tremor amplitudes of about 180 nm/s
are observed, and scale with the ground velocity induced by the
remote earthquakes. Triggered tremor responds to 45-65 s period
surface waves and predominantly correlates with Rayleigh waves,
even though the 2012 oceanic events have stronger Love wave amplitudes.
We could not locate the tremor because of minimal station coverage,
but data indicate several potential source volumes including the
Flores Thrust, the Java subduction zone, or Tambora volcano |
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Julio de 2014
3-D Seismic Surveys Explore German Petrothermal Reserves
Authors: Ewald Lüschen and Rüdiger Schulz
Link: Click here
Three-dimensional (3-D) seismic reflection
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techniques
are now an indispensable tool for locating and characterizing
sites for building geothermal power plants. The techniques not
only work when imaging layered sediments but can also pinpoint
geothermal sources in crystalline environments such as granite. |
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Julio de 2014
The 2010-2014 global earthquake rate increase
Authors: Tom Parsons and Eric L. Geist
Link: Click here
In light of a heightened global earthquake rate
during the first quarter of 2014 and recent studies concluding
that large earthquakes affect global seismicity for extended
periods, we revisit the question whether the temporal distribution
of global earthquakes shows clustering beyond that expected
from a time-independent Poisson process. We examine a broad
window from 1979
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to
2014.3 for M 7.0 shocks, and a narrow window for M 5.0 seismicity
since 2010 that has higher than average rates. We test whether
a Poisson process can be falsified at 95% confidence to assess
the degree of dependent clustering in the catalogs. If aftershocks
within at least one rupture length from main shocks/foreshocks
are filtered, then we find no evidence of global scale M 5.2-5.6
(depending on parameters) clustering since 2010 that demands a
physical explanation. There is evidence for interdependence below
this threshold that could be a consequence of catalog completeness
or a physical process. |
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Julio de 2014
Tracing the effects of an enormous meteorite impact 3 billion
years ago
Author: JoAnna Wendel
Link: Click here
The most well-known and popularized meteorite
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impact
occurred 65 million years ago and left behind the Chicxulub crater,
a feature more than 110 miles in diameter. Dust kicked up by the
impact, which lingered in the atmosphere for years, may have helped
kill all the nonavian dinosaurs along with three quarters of the
world's plant and animal species. |
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Julio de 2014
Multifaulting in a tectonic syntaxis revealed by InSAR: The
case of the Ziarat earthquake sequence (Pakistan)
Authors: B. Pinel-Puysségur, R. Grandin et al
Link: Click here
On 28-29 October 2008, within 12 h, two similar
Mw?=?6.4 strike-slip earthquakes struck Baluchistan (Pakistan),
as part of a complex seismic sequence. Interferometric Synthetic
Aperture Radar (InSAR) data reveal that the peak of surface
displacement is near the Ziarat anticline, a large active fold
affected by Quaternary strike-slip faulting. All coseismic interferograms
integrate the deformation due to both earthquakes. As their
causative faults ruptured close to each other, the individual
signals cannot be separated. According to their focal mechanisms,
each earthquake may have activated a NE-SW sinistral or a NW-SE
dextral fault segment, which leads to four possible
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scenarios
of fault orientations. A nonlinear inversion of the InSAR data
set allows rejecting two scenarios. The best slip distributions
on the two fault segments for the two remaining scenarios are
determined by linear inversion. Stress-change modeling favors
a scenario involving two abutting conjugate strike-slip faults.
Two other fault segments accommodated left-lateral strike slip
during the seismic sequence. The activated fault system includes
multiple fault segments with different orientations and little
surface expression. This may highlight, at a smaller scale, the
distributed, possibly transient character of deformation within
a broader right-lateral shear zone. It suggests that the activated
faults delineate a small tectonic block extruding and subtly rotating
within the shear zone. It occurs in the vicinity of the local
tectonic syntaxis where orogenic structures sharply turn around
a vertical axis. These mechanisms could participate in the long-term
migration of active tectonic structures within this kinematically
unstable tectonic syntaxis. |
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Julio de 2014
The 1 April 2014 Iquique, Chile, Mw 8.1 earthquake rupture
sequence
Authors: Thorne Lay, Han Yue et al
Link: Click here
On 1 April 2014, a great (Mw 8.1) interplate
thrust earthquake ruptured in the northern portion of the 1877
earthquake seismic gap in northern Chile. The sequence commenced
on 16 March 2014 with a magnitude 6.7 thrust event, followed
by thrust-faulting aftershocks that migrated northward ~40 km
over 2 weeks to near the main
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shock
hypocenter. Guided by short-period teleseismic P wave backprojections
and inversion of deepwater tsunami wave recordings, a finite-fault
inversion of teleseismic P and SH waves using a geometry consistent
with long-period seismic waves resolves a spatially compact large-slip
(~2-6.7 m) zone located ~30 km downdip and ~30?km along-strike
south of the hypocenter, downdip of the foreshock sequence. The
main shock seismic moment is 1.7?×?1021?N m with a fault
dip of 18°, radiated seismic energy of 4.5-8.4 ×1016
J, and static stress drop of ~2.5 MPa. Most of the 1877 gap remains
unbroken and hazardous. |
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Julio de 2014
Shaking up volcanoes
Authors: Stephanie G. Prejean, Matthew M. Haney et al
Link: Click here
Most volcanic eruptions that occur shortly after
a large distant earthquake do so by random chance. A few compelling
cases for earthquake-triggered eruptions exist, particularly
within 200 km of the earthquake, but this phenomenon is rare
in part
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because
volcanoes must be poised to erupt in order to be triggered by
an earthquake (1). Large earthquakes often perturb volcanoes in
more subtle ways by triggering small earthquakes and changes in
spring discharge and groundwater levels (1, 2). On page 80 of
this issue, Brenguier et al. (3) provide fresh insight into the
interaction of large earthquakes and volcanoes by documenting
a temporary change in seismic velocity beneath volcanoes in Honshu,
Japan, after the devastating Tohoku-Oki earthquake in 2011. |
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Junio de 2014
A dynamo explanation for Mercury's anomalous magnetic field
Authors: Hao Cao, Jonathan M. Aurnou et al
Link: Click here
Recent MErcury Surface, Space ENvironment, GEochemistry,
and Ranging (MESSENGER) measurements have shown that Mercury's
magnetic field is axial-dominant, yet strongly asymmetric with
respect to the equator: the field strength in the Northern Hemisphere
is approximately 3 times stronger than that in the Southern
Hemisphere. Here we show that .convective dynamo models driven
by volumetric buoyancy with north-south symmetric thermal
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boundaries
are capable of generating quasi-steady north-south asymmetric
magnetic fields similar to Mercury's. This symmetry breaking is
promoted and stabilized when the core-mantle boundary heat flux
is higher at the equator than at high latitudes. The equatorially
asymmetric magnetic field generation in our dynamo models corresponds
to equatorially asymmetric kinetic helicity, which results from
mutual excitation of two different modes of columnar convection.
Our dynamo model can be tested by future assessment of Mercury's
magnetic field from MESSENGER and BepiColombo as well as through
investigations on Mercury's lower mantle temperature heterogeneity
and buoyancy forcing in Mercury's core. |
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Junio de 2014
Simulation of the airwave caused by the Chelyabinsk superbolide
Authors: 1. Mikhail I. Avramenko, Igor V. Glazyrin et
al
Link: Click here
Numerical simulations were carried out to model
the propagation of an airwave from the fireball that passed
over Chelyabinsk (Russia) on 15 February 2013. The airburst
of the Chelyabinsk meteoroid occurred due to its catastrophic
fragmentation in the atmosphere. Simulations of the space-time
distribution of energy deposition during the airburst were done
using a novel fragmentation model based on dimensionality considerations
and analogy to the fission chain reaction in fissile materials.
To get an estimate of the airburst energy, observed values of
the airwave arrival times to different populated localities
were retrieved from video records
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available
on the Internet. The calculated arrival times agree well with
the observed values for all the localities. Energy deposition
in the atmosphere obtained from observations of the airwave arrival
times was found to be 460 ± 60 kt in trinitrotoluene (TNT)
equivalent. We also obtained an independent estimate for the deposited
energy, kt TNT from detecting the air increment velocity due to
the wave passage in Chelyabinsk. Assuming that the energy of about
90 kt TNT was irradiated in the form of visible light and infrared
radiation, as registered with optical sensors [Yeomans and Chodas,
2013], one can value the total energy release to be about 550
kt TNT which is in agreement with previous estimates from infrasound
registration and from optical sensors data. The overpressure amplitude
and its positive phase duration in the airwave that reached the
city of Chelyabinsk were calculated to be about 2 kPa and 10 s
accordingly. |