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Marzo de 2011
On finding and using repeating seismic events in and near
China
Authors: David P. Schaff and Paul G. Richards
Link: Clic here
Abstract
A comprehensive search for repeating seismic events was performed
using cross correlation on waveforms for 17,898 seismic events
in and near China reported in the Annual Bulletin of Chinese
Earthquakes from 1985 to 2005. We define a repeat as an event
pair having a high cross correlation (=0.8) for a particular
bandwidth (0.5 to 5 Hz) and time window (5 s before P arrival
to 40 s after Lg arrival). Such events typically have hypocenters
separated by no more than about 1 km. This result enables
a determination of median
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relative location error of around
15 km for global and regional catalogs that locate events in
this study one at a time using phase picks. The maximum location
error is on the order of hundreds of kilometers. We use nonnormalized
cross-correlation values to measure relative amplitudes of event
pairs. The standard deviation is about 0.06 magnitude units,
much better than the precision of relative magnitudes for doublet
events, which we estimate for the Chinese catalog to have a
standard deviation of 0.36 magnitude units. Two thousand three
hundred and seventy-nine events out of the 17,898 or 13% of
the events in this catalog are classified as repeats, with later
years showing an increase in the percentage. About half the
repeats occur as isolated doublets, the rest as multiplets of
three or larger, up to a 92-event multiplet. Most of the events
appear to be triggered or are due to some earthquake interaction
since their recurrence intervals are significantly shorter than
would be estimated from tectonic loading. |
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Marzo de 2011
Deformation of compliant fault zones induced by nearby
earthquakes: Theoretical investigations in two dimensions
Authors: Benchun Duan, Jingqian Kang et
al
Link: Clic here
Abstract
Using spontaneous rupture models with off-fault elastoplastic
material response, we investigate the deformation of compliant
fault zones induced by nearby earthquake ruptures in a two-dimensional
plane-strain framework. We find that dynamic stresses from
nearby ruptures can produce inelastic strain along some portions
of a fault zone that experience dilatational stress changes,
if the fault zone rock is close to failure in the prestress
field. Accumulation of inelastic strain causes dramatic variations
in
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particle velocity across the fault zone, reversing
the sense of fault-parallel relative motion from retrograde
(opposite to the long-term geologic slip) to sympathetic (consistent
with the long-term slip) during the dynamic process. In the
static displacement field of a roughly parallel strike-slip
fault system, the inelastic response of a fault zone results
in sympathetic motion, while the elastic response generally
gives rise to retrograde motion. Our theoretical investigations
reveal that some deficiencies may exist in applying an elastic
inhomogeneity model to infer fault zone properties in previous
studies, including the assumption of negligible fault-normal
motion and ignorance of changes in some components of the
stress tensor. These deficiencies and possible constraints
on the in situ stress state by inelastic strain signals in
the static displacement field call for a reexamination of
existing observations of fault zone deformation induced by
recent large earthquakes.
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Marzo de 2011
Infrasonido Monotónico y resonancia de Helmholtz
en el Volcán Villarrica (Chile)
Autores: A. Goto y J. B. Johnson
Link: Clic aquí
Abstract
El infrasonido monotónico con peak estable de una frecuencia
de 0.77 Hz fue grabado en el Volcán Villarrica en enero
de 2010. Infrasonidos monotónico similares habían
sido previamente reportados en Villarrica (eg Ripepe et al.
[2010]). Mediante la utilización de infrasonido y observaciones
visuales desde una cámara suspendida se demuestra que
la fuente probable del infrasonido es la resonancia de Helmholtz
producida a partir de una cavidad con un volumen de 105 m3,
la que separa el lago de lava de convección activa
de un techo voladizo de salpicaduras. El techo (65 m de diámetro)
y la sección de ventilación (10 m) se ven limitados
a partir de observaciones de vídeo. Suponiendo una
cavidad cilíndrica se puede inferir una altura de la
cavidad de 31 metros, la que es corroborada por los registros
de vídeo de las gotas de las salpicaduras de la lava.
Las gotas necesitan de hasta 2.2 s para caer desde el techo
al lago de lava, lo que corresponde a una altura de más
de 24 m. Este valor está en buen acuerdo con la frecuencia
de resonancia observada.
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Marzo de 2011
Monotonic infrasound and Helmholtz resonance at Volcan
Villarrica (Chile)
Authors: A. Goto and J. B. Johnson
Link: Clic here
Abstract
Monotonic infrasound with stable peaked frequency of 0.77
Hz was recorded at Volcan Villarrica in January 2010. Similar
monotonic infrasound had been previously reported at Villarrica
(e.g., Ripepe et al. [2010]). Using joint infrasound and visual
observations from a suspended camera we demonstrate that the
likely source of infrasound is Helmholtz resonance produced
from a cavity with volume 105 m3 that separates the active
convecting lava lake from an overhanging spatter roof. Spatter
roof dimension (65 m diameter) and vent diameter (10 m) in
the roof are constrained from video observations. Assuming
a cylindrical cavity we infer a cavity height of 31 m that
is corroborated by video records of spatter drips. The drips
take as long as 2.2 s to fall from the roof into the lake,
corresponding to a height of more than 24 m, which is in good
agreement with the observed resonance frequency.
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Marzo de 2011
The initial phases of the 2008-2009 Mount Etna eruption:
A multidisciplinary approach for hazard assessment
Authors: A. Bonaccorso, A. Bonforte et
al
Link: Clic here
Abstract
Between 2007 and early 2008, the Istituto Nazionale di Geofisica
e Vulcanologia (INGV) monitoring networks on Etna volcano
recorded a recharging phase that climaxed with a new effusive
eruption on 13 May 2008 and lasted about 14 months. A dike-forming
intrusion was accompanied by a violent seismic swarm, with
more than 230 events recorded in the first 6 h, the largest
being ML = 3.9. In the meanwhile, marked ground deformation
was recorded by the permanent tilt and GPS networks, and sudden
changes in the summit area were detected by five continuously
recording magnetic
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stations.
Poor weather conditions did not allow direct observation of
the eruptive events, but important information was provided
by infrared satellite images that detected the start of lava
fountains from the eruptive fissure, feeding a lava flow. This
flow spread within the Valle del Bove depression, covering 6.4
km on the southeastern flank of the volcano in a few hours.
The seismicity and deformation pattern indicated that the dike-forming
intrusion was propagating northward. It produced a dry fracture
field, which generated concern for the possibility that the
eruptive fissures could expand downslope toward populated areas.
Monitoring and modeling of the multidisciplinary data, together
with the simulations of ash dispersal and lava flows, allowed
us both to infer the eruptive mechanisms and to provide correct
interpretation of the ongoing phenomena, furnishing useful information
for civil defense purposes. We describe how this approach of
feedback between monitoring and research provides critical support
to risk evaluation. |
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Marzo de 2011
Terremotos de corte y de tracción causados por inyección
de fluídos
Autores: Tomáš Fischer
y Alice Guest
Link: Clic aquí
Abstract
Aplicamos conceptos de mecánica de rocas a observaciones
sismológicas con el fin de explicar por qué
durante la inyección hidráulica se pueden observar
algunos eventos sísmicos de tracción y una cierta
deformación de corte. La presencia de componentes de
no-corte depende de la tensión diferencial y de la
orientación de la fractura con respecto a la dirección
s1. Siempre que el vector de desplazamiento es paralelo a
la tracción, se encuentran cuatro tipos de sismos de
acuerdo a la proporción de la fuerza cortante y de
los componentes de la tracción. Asumiendo una envolvente
de rotura de Griffith, se observa que pueden ocurrir eventos
híbridos que contienen componentes tanto de corte y
de resistencia a la tracción debido a fracturas en
ángulo de 22.5 ° de s1. Sostenemos que la tracción
pura de fracturas paralelas a s1 es poco probable en presencia
de fracturas naturales. La tracción de corte en eventos
de tracción también implica una baja tensión.
Al aplicar el análisis de dos conjuntos de datos diferentes
(Soultz-sous-Forêts y el Valle de algodón) se
muestra que diferentes orientaciones de las fracturas naturales
y de la tensión diferencial en las formaciones específicas
de cada región son favorables para los diferentes componentes
no-DC en la inyección de sismicidad inducida .
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Marzo de 2011
Shear and tensile earthquakes caused by fluid injection
Authors: Tomáš Fischer and
Alice Guest
Link: Clic here
Abstract
We apply rock mechanics concepts to the seismological observations
in order to explain why during hydraulic injection some events
display tensile and some shear deformation. The presence of
non-shear components depends on the differential stress and
the fracture orientation with respect to the s1 direction.
Provided the slip vector is parallel to the traction we define
four types of earthquakes according to the ratio of the shear
and tensile components. Assuming a Griffith failure envelope,
hybrid events containing both shear and tensile components
can occur for fractures striking within 22.5° of s1. We
argue that pure tensile fractures striking parallel to s1
are unlikely in the presence of natural fractures. The low
shear traction of tensile events also implies their small
stress drops. By applying the analysis to two different data
sets, Soultz-sous-Forets and Cotton Valley, we show that different
orientations of natural fractures and differential stress
in the targeted formations made each region favorable for
different non-DC components in the injection-induced seismicity.
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Marzo de 2011
Using radon as an environmental tracer for estimating groundwater
flow velocities in single-well tests
Authors: Michael Schubert, Lutz Brueggemann
et al
Link: Clic here
Abstract
Naturally occurring radon-222 was evaluated for its use in
estimating groundwater flow velocities using single-well tests.
Investigations were carried out for four different well scenarios,
which revealed the advantages and limitations of
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the
approach. On one hand, it was shown that radon is useful as
an environmental tracer because of (1) the low costs of the
method, (2) the avoidance of any artificial tracer injection
into the aquifer, (3) the immediate availability of results,
and (4) the need for only a single monitoring well. On the other
hand, several potential sources of error were identified, including
poor sampling, inadequate hydraulic connection of the well because
of a clogged screen, and an unsuitable well diameter resulting
in excessively long or short well water residence times. The
practical approach is supported by in-depth theoretical considerations.
General recommendations are presented concerning the use of
radon as an environmental tracer for groundwater flow assessment.
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Marzo de 2011
Thermal and pore fluid pressure history on the Chelungpu
fault at a depth of 1111 m during the 1999 Chi-Chi, Taiwan,
earthquake
Author: Jeen-Hwa Wang
Link: Clic here
Abstract
On 20 September 1999 the Ms7.6 Chi-Chi earthquake ruptured
the Chelungpu fault in central Taiwan. In 2005 the Taiwan
Chelungpu Fault Drilling Project was launched to drill two
deep holes (holes A and B) cutting across the fault plane.
The heat strength (= 7.0°C m), within a heated layer of
~5 mm, on the fault at hole A due to frictional faulting is
evaluated from the values of shear stress and thermal and
mechanical parameters measured from the core samples. Based
on a 1-D heat conduction equation and 2-D faulting model,
with the
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values of thermal diffusivity
evaluated within a representative temperature range, the thermal
and pore fluid pressure history at depths 1110.37-1111.34 m
in hole A is constructed. Results show that the peak temperature
at the center of the heated layer could have been higher than
1100°C during faulting, and temperature rise decreased with
increasing distance and time. The possible relationships between
the temperatures and chemical reactions of clay minerals are
discussed in detail. In the heated layer, pseudotachylites have
been formed and quartz plasticity might also have been operative
during faulting. Outside this slip zone, the temperature rise
was low and thus clay minerals were stable during faulting.
The evaluated pore fluid pressure is 22.5 MPa. This suggests
the existence of a suprahydrostatic state in the fault zone
during the earthquake. Indirect evidence of the existence of
fluids in the fault zone during faulting is described. Additionally,
rock physics is applied to interpret the observations. |
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Marzo de 2011
The 25 October 2010 Mentawai tsunami earthquake, from real-time
discriminants, finite-fault rupture, and tsunami excitation
Authors: Andrew V. Newman, Gavin Hayes
et al
Link: Clic here
Abstract
The moment magnitude 7.8 earthquake that struck offshore the
Mentawai islands in western Indonesia on 25 October 2010 created
a locally large tsunami that caused more than 400 human causalities.
We identify this earthquake as a rare
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slow-source
tsunami earthquake based on: 1) disproportionately large tsunami
waves; 2) excessive rupture duration near 125 s; 3) predominantly
shallow, near-trench slip determined through finite-fault modeling;
and 4) deficiencies in energy-to-moment and energy-to-duration-cubed
ratios, the latter in near-real time. We detail the real-time
solutions that identified the slow-nature of this event, and
evaluate how regional reductions in crustal rigidity along the
shallow trench as determined by reduced rupture velocity contributed
to increased slip, causing the 5-9 m local tsunami runup and
observed transoceanic wave heights observed 1600 km to the southeast.
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Marzo de 2011
Nucleación extendida del terremoto Mw 7.6 de
Izmit, ocurrido el año 1999.
Autores: Michel Bouchon, Hayrullah Karabulut
et al
Link: Clic aquí
Abstract
Estudios teóricos y de laboratorio sugieren que los
terremotos son precedidos por una fase deinestabilidad de
deslizamiento en desarrollo, donde la falla se desliza lentamente
antes de acelerar a la fase de rotura dinámica. Aquí
presentamos información donde se indica que uno de
los más grandes terremotos ocurridos, el de Izmit (Turquía)
del año 1999 y de magnitud de momento 7.6 Mw, fue precedido
por una señal sísmica de larga duración
originada en el hipocentro. La señal consistía
en una sucesión de repetidas explosiones sísmicas
que aceleraban en el tiempo y que mostraban un aumento del
ruido sísmico de baja frecuencia. Las observaciones
muestran que el terremoto fue precedido por una fase de deslizamiento
lento de 44 minutos de duración y que ocurrió
en la base de la corteza quebradiza. Este deslizamiento tuvo
una baja aceleración inicial, pero posteriormente consiguió
una alta aceleración dos minutos antes del terremoto.
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Marzo de 2011
Extended Nucleation of the 1999 Mw 7.6 Izmit Earthquake
Authors: Michel Bouchon, Hayrullah Karabulut
et al
Link: Click here
Abstract
Laboratory and theoretical studies suggest that earthquakes
are preceded by a phase of developing slip instability in
which the fault slips slowly before accelerating to dynamic
rupture. We report here that one of the best-recorded large
earthquakes to date, the 1999 moment magnitude (Mw) 7.6 Izmit
(Turkey) earthquake, was preceded by a seismic signal of long
duration that originated from the hypocenter. The signal consisted
of a succession of repetitive seismic bursts, accelerating
with time, and increased low-frequency seismic noise. These
observations show that the earthquake was preceded for 44
minutes by a phase of slow slip occurring at the base of the
brittle crust. This slip accelerated slowly initially, and
then rapidly accelerated in the 2 minutes preceding the earthquake.
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Febrero de 2011
Seismic swarm associated with the 2008 eruption of Kasatochi
Volcano, Alaska: Earthquake locations and source parameters
Authors: Natalia A. Ruppert, Stephanie
Prejean et al
Link: Click here
Abstract
An energetic seismic swarm accompanied an eruption of Kasatochi
Volcano in the central Aleutian volcanic arc in August of
2008. In retrospect, the first earthquakes in the swarm were
detected about 1 month prior to the eruption onset. Activity
in the swarm quickly intensified less than 48 h prior to the
first large explosion and subsequently subsided with decline
of eruptive activity. The largest earthquake measured as moment
magnitude 5.8, and a dozen additional earthquakes were larger
than magnitude 4. The swarm exhibited both tectonic and volcanic
characteristics. Its shear
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failure
earthquake features were b value = 0.9, most earthquakes with
impulsive P and S arrivals and higher-frequency content, and
earthquake faulting parameters consistent with regional tectonic
stresses. Its volcanic or fluid-influenced seismicity features
were volcanic tremor, large CLVD components in moment tensor
solutions, and increasing magnitudes with time. Earthquake location
tests suggest that the earthquakes occurred in a distributed
volume elongated in the NS direction either directly under the
volcano or within 5-10 km south of it. Following the MW 5.8
event, earthquakes occurred in a new crustal volume slightly
east and north of the previous earthquakes. The central Aleutian
Arc is a tectonically active region with seismicity occurring
in the crusts of the Pacific and North American plates in addition
to interplate events. We postulate that the Kasatochi seismic
swarm was a manifestation of the complex interaction of tectonic
and magmatic processes in the Earth's crust. Although magmatic
intrusion triggered the earthquakes in the swarm, the earthquakes
failed in context of the regional stress field. |
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Febrero de 2011
Induced polarization signatures of cations exhibiting differential
sorption behaviors in saturated sands
Authors: P. Vaudelet, A. Revil et al
Link: Click here
Abstract
Two sets of experiments were designed to understand the change
in induced polarization associated with the sorption of copper
and sodium, exhibiting distinct sorption behavior on a silica
sand. A sand column experiment was first performed to see
the change in the complex conductivity during the advective
transport of a copper sulfate solution. A second set of experiments
was done with the sand at equilibrium with various solutions
of NaCl and CuSO4. In the first experiment, the copper sulfate
solution replaced a sodium chloride solution, keeping the
electrical conductivity of the solution nearly constant. During
the passage of the copper sulfate solution, the apparent phase
angle decreased from 3 ± 0.2 to 0.5 ± 0.2 mrad,
while the magnitude of the conductivity of the sand remained
nearly constant. A quantitative model is
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proposed to explain the change
in the complex conductivity as a function of the chemistry assuming
a polarization mechanism associated with the Stern layer (the
inner part of the electrical double layer coating the water-mineral
interface). The Stern layer polarization is combined with a
complexation model describing the competitive sorption of copper
and sodium at the pore water interface. The change of the phase
lag is directly associated with the ion exchange between sodium
and copper at the surface of the silica grains. The explanation
of the observed phase differences between Na and Cu relies on
their different complexation behaviors, with Na being loosely
absorbed, while Cu forms relatively strong complexation with
both inner (monodentate) and outer sphere (bidentate) complexes.
The replacement of Cu2+ by Na+ is less favorable; therefore,
the kinetics of such a replacement is much slower than for the
opposite replacement (Na+ by Cu2+). We were able to reproduce
the changes in the phase lags at thermodynamic equilibrium near
the relaxation frequency and in the frequency domain. These
measurements and modeling results open the door to the quantitative
interpretation of spectral induced polarization data in the
field in terms of quantification of the sorption processes.
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Febrero de 2011
Probabilistic hurricane surge forecasting using parameterized
surge response functions
Authors: Jennifer L. Irish, Youn Kyung
Song et al
Link: Click here
Abstract
A method is proposed for rapidly determining probabilistic
maximum hurricane surge forecasts based on surge response
functions, available meteorological information, and joint
probability statistics. In using this method for
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Hurricane Ike, surge forecasts
prior to landfall were computed in a matter of seconds. From
a theoretical standpoint, surge response functions are scaling
laws derived from high-resolution numerical simulations. Surge
response functions allow rapid algebraic surge calculation while
guaranteeing accuracy and detail by incorporating high-resolution
computational results into their formulation. The Hurricane
Ike example presented here shows that this method has the potential
to improve evacuation planning and public early warning of hurricane
flooding by providing rapid and accurate probabilistic projections
of maximum surge. |
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Febrero de 2011
Large-scale inverse modeling with an application in hydraulic
tomography
Authors: X. Liu and P. K. Kitanidis
Link: Click here
Abstract
Inverse modeling has been widely used in subsurface problems,
where direct measurements of parameters are expensive and
sometimes impossible. Subsurface media are inherently heterogeneous
in complex ways, which implies that the number of unknowns
is usually large. Furthermore,
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technologies
such as hydraulic tomography and electric resistivity tomography
allow the collection of more indirect measurements, and at the
same time, there is an increased appreciation of the value of
detailed characterization of the subsurface media in, for example,
remediation projects. Hence, we need efficient inverse methods
that can assimilate a large volume of measurements to estimate
even larger numbers of parameters, i.e., large-scale inverse
modeling. In this paper, we present a Bayesian method that employs
a sparse formulation, and we applied this method to a laboratory
hydraulic tomography problem, where we successfully estimated
half a million unknowns that represent the hydraulic conductivity
field of the sandbox at a fine scale. The inversion took about
2 h with a single core. |
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Febrero de 2011
Un análisis estadístico de las anomalías
ionosféricas antes de la ocurrencia de 736 terremotos
de magnitud mayor o igual a 6.0 en el intervalo de años
2002 a 2010
Autores: H. Le, J. Y. Liu et al
Link: Clic aquí
Abstract
Este paper presenta un estudio estadístico de las anomalías
ionosféricas previas a los terremotos a través
de la medición del Contenido Total de Electrones (TEC)
del mapa mundial de la ionósfera. Fueron seleccionados
un total de 736 terremotos M6.0+ en el mundo durante el intervalo
de años 2002 a 2010. Primero definimos la anomalía
diaria. Luego definimos la razón de ocurrencias de
días anormales entre 1 a 21 días previos al
terremoto (PE) y también se determinan los días
background (PN). Los resultados muestran que los valores de
PE dependen de la magnitud del terremoto, la profundidad de
la fuente y el número de días previos al terremoto.
El PE es mayor para los terremotos de mayor magnitud y menor
profundidad y para días cercanos al evento. Los resultados
también muestran que la razón de ocurrencia
de la anomalía en muchos días antes de los terremotos
es notoriamente mayor que ese valor durante los días
background, especialmente para terremotos de grandes magnitudes
y cercanos a la superficie. Esos resultados indican que el
tipo de anomalía TEC en unos pocos días antes
de los terremotos está relacionado con una alta probabilidad
con el siguiente terremoto.
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Febrero de 2011
A statistical analysis of ionospheric anomalies before
736 M6.0+ earthquakes during 2002-2010
Authors: H. Le, J. Y. Liu et al
Link: Click here
Abstract
This paper presents a statistical study of the pre-earthquake
ionospheric anomaly by using the total electron content (TEC)
data from the global ionosphere map. A total of 736 M = 6.0
earthquakes in the global area during 2002-2010 are selected.
The anomaly day is first defined. Then the occurrence rates
of abnormal days for both the days within 1- 21 days prior
to the earthquakes (PE) and the background days (PN) are calculated.
The results show that the values of PE depend on the earthquake
magnitude, the earthquake source depth, and the number of
days prior to the earthquake. The PE is larger for earthquakes
with greater magnitude and lower depth and for days closer
to the earthquakes. The results also show that the occurrence
rate of anomaly within several days before the earthquakes
is overall larger than that during the background days, especially
for the large-magnitude and low-depth earthquakes. These results
indicate that the anomalous behavior of TEC within just a
few days before the earthquakes is related with the forthcoming
earthquakes with high probability.
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Febrero de 2011
Dynamic triggering: Stress modeling and a case study
Authors: Hector Gonzalez-Huizar and Aaron
A. Velasco
Link: Click here
Abstract
Changes in the static stress can trigger nearby earthquakes
that occur within a few fault lengths from the causative event.
Transient stresses caused by passage of surface waves commonly
trigger events at remote distances, yet little is documented
or understood about the processes and stresses necessary for
remote triggering. To understand the causative stresses and
environments behind remote, or dynamic, triggering, we must
decipher the stresses caused
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by
the passage of the surface waves in relation to the local stress
field and fault conditions where the triggered events occur.
In this study, we model the change in the stress field that
the passing of Rayleigh and Love waves causes on a fault plane
of arbitrary orientation relative to the direction of propagation
of the waves, and we apply a Coulomb failure criterion to calculate
the potential of these stress changes to trigger reverse, normal,
or strike-slip failure. We compare these model results with
data from dynamically triggered earthquakes in the Australian
Bowen Basin, an area with low seismicity and mapped regional
stress and that is at the margin of a stable continental craton.
Our data analysis shows that for this region, surface waves
arriving at 45° from the average strike direction are the
most likely to trigger local seismicity. This agrees with our
observations. |
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Enero de 2011
Columbus crater and other possible groundwater-fed paleolakes
of Terra Sirenum, Mars
Authors: J. J. Wray, R. E. Milliken et
al
Link: Click here
Abstract
Columbus crater in the Terra Sirenum region of the Martian
southern highlands contains light-toned layered deposits with
interbedded sulfate and phyllosilicate minerals, a rare occurrence
on Mars. Here we investigate in detail the morphology, thermophysical
properties, mineralogy, and stratigraphy of these deposits;
explore their regional context; and interpret the crater's
aqueous history. Hydrated mineral-bearing deposits occupy
a discrete ring around the walls of Columbus crater and are
also exposed beneath younger materials, possibly lava flows,
on its floor. Widespread minerals identified in the crater
include gypsum, polyhydrated and monohydrated Mg/Fe-sulfates,
and kaolinite; localized deposits consistent with montmorillonite,
Fe/Mg-
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phyllosilicates, jarosite,
alunite, and crystalline ferric oxide or hydroxide are also
detected. Thermal emission spectra suggest abundances of these
minerals in the tens of percent range. Other craters in northwest
Terra Sirenum also contain layered deposits and Al/Fe/Mg-phyllosilicates,
but sulfates have so far been found only in Columbus and Cross
craters. The region's intercrater plains contain scattered exposures
of Al-phyllosilicates and one isolated mound with opaline silica,
in addition to more common Fe/Mg-phyllosilicates with chlorides.
A Late Noachian age is estimated for the aqueous deposits in
Columbus, coinciding with a period of inferred groundwater upwelling
and evaporation, which (according to model results reported
here) could have formed evaporites in Columbus and other craters
in Terra Sirenum. Hypotheses for the origin of these deposits
include groundwater cementation of crater-filling sediments
and/or direct precipitation from subaerial springs or in a deep
(~900 m) paleolake. Especially under the deep lake scenario,
which we prefer, chemical gradients in Columbus crater may have
created a habitable environment at this location on early Mars.
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Enero de 2011
Sensitivity of orbital neutron measurements to the thickness
and abundance of surficial lunar water
Authors: D. J. Lawrence, D. M. Hurley et
al
Link: Click here
Abstract
Recent near-infrared spectral data have shown that surficial
water (H2O/OH) exists over large expanses of the lunar surface.
These results have led to a reexamination of the hydrogen
abundance sensitivity limits of orbital neutron data to detect
surficial hydrogen on the lunar surface. A wet-over-dry, two-layer
stratigraphy is modeled for the first time using neutron transport
codes. For thin layers (<30 g/cm2), the epithermal neutron
flux increases with increasing hydrogen concentration. This
behavior is in contrast to the standard behavior for a single
layer or dry-over-wet stratigraphy where
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the
epithermal neutron counting rate decreases with increasing hydrogen
concentration. These neutron transport results are applied to
a H2O/OH enhancement at Goldschmidt crater. The neutron behavior
at Goldschmidt is mostly controlled by spatial variations in
neutron absorbing elements. After accounting for variations
from neutron absorbing elements, there remain residual neutron
enhancements at Goldschmidt crater with marginal statistical
significance. If these residual enhancements are due to hydrogen,
then their magnitude implies the presence of an upper layer
with thickness of ~3-30 g/cm2 (or 1.7-17 cm for an assumed density
of 1.8 g/cm3) having an enhanced hydrogen abundance of 0.1-1
wt % water equivalent hydrogen. However, more work needs to
be done to understand systematic variations of neutron counting
rates at the 1-3% signal contrast level before a definitive
conclusion can be made that the residual neutron enhancement
at Goldschmidt crater is due to enhanced hydrogen abundances. |
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Enero de 2011
Directional wavelet analysis on the sphere: Application
to gravity and topography of the terrestrial planets
Author: Pascal Audet
Link: Click here
Abstract
The spectral relations (admittance and correlation) between
gravity and topography are often used to obtain information
on the density structure, flexural support, and heat flow
of planetary lithospheres. Mapping spatial variations in these
quantities requires spatiospectral analysis techniques. Here
we describe the application of a directional, continuous spherical
wavelet transform using a wavelet basis constructed from the
superposition of azimuthally adjacent complex Morlet wavelets,
in a manner similar to the "fan" wavelet developed
in the plane. The method is applied to gravity and topography
of the Earth, Venus, Mars, and the
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Moon. The wavelet
coefficients are used to compute isotropic and directional wavelet
autospectra and cross spectra, which are then combined to form
the admittance and correlation functions. The resulting maps
offer insights into lithospheric structure of the terrestrial
planets. In particular we show that the Earth and Venus have
uniformly low positive admittance and high correlation, whereas
Mars and the Moon display hemispherical contrasts with large
negative and anisotropic coefficients coinciding with lowlands.
As has long been known, the two largest impact basins in the
inner solar system, the South Pole-Aitken basin on the Moon
and the Hellas basin on Mars, display low positive admittance
and high correlation, indicating isostatic compensation. In
contrast, most other impact basins, particularly the Martian
and lunar mascons, show negative coefficients at low wavelet
degrees suggesting flexural support by a strong lithosphere.
These results imply that, although simple isotropic flexural
models can account for most observations, future models may
need to incorporate anisotropy as an additional parameter. |
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Enero de 2011
State of lithosphere beneath Tien Shan from petrology and
electrical conductivity of xenoliths
Authors: N. Bagdassarov, V. Batalev
et al
Link: Click here
Abstract
The shortening of Tien Shan and the evolution of its lithosphere
have been evaluated from P-T geothermobarometry of xenoliths
and from comparison of their electrical conductivity with
conductivities obtained from the inversion of magnetotelluric
(MT) data. Spinel lherzolite and granulite xenoliths found
in basaltic outcrop Ortosuu represent upper mantle and crust
beneath southern Tien Shan. The spinel lherzolite xenoliths
correspond to the lithospheric mantle close to the crust-mantle
boundary. The studied mantle xenoliths record two types of
the upper mantle processes: a low-degree partial melting (about
7-10%) and a cryptic metasomatism. The granulite xenoliths
are fragments of the lower crust captured from differing depths.
The temperature and pressure estimates of the garnet granulite
xenoliths indicate that they were derived from near the crust-mantle
boundary. P-T equilibration conditions of mafic granulites
and spinel lherzolites infer that the paleogeotherm corresponded
to the heat flux 80-85 mW m-2 about 70-66 Myr ago. The present-day
heat flux in the region is about 55-60 mW m-2. The position
of the Moho discontinuity 70-66 Myr ago was at a depth of
30-35 km, and the present depth of the Moho boundary is 55-60
km. The observed seismic P wave velocities above and below
the present-day Moho boundary
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are
7.3 and 7.9 km s-1, respectively. Elastic P velocities measured
on xenoliths samples in laboratory and extrapolated to PT conditions
of the Moho discontinuity (1.8 GPa and ~750°C) are 6.8 km
s-1 for the mafic granulite and 8.0 km s-1 for the spinel lherzolite.
The electrical conductivity of xenoliths has been measured at
0.8-1.0 GPa and in the range between 500°C and 850°C
for mafic granulites and at 1.0-1.5 GPa and from 500°C to
1050°C for spinel lherzolites. The contrast in conductivities
between mafic granulite and spinel lherzolite samples under
P-T conditions corresponding to the geotherm with the heat flux
~60 mW m-2 agrees well with the contrast of MT electrical conductivity
below and above the present-day Moho boundary at a depth of
55-60 km. Thus, the thickening of the Tien Shan lithosphere
is about 25 ± 5 km. Before the shortening of Tien Shan
about 20-30 Myr ago the lithosphere was rather hot, with a temperature
of 500°C at a 15 km depth and of 850°C at the Moho boundary.
In comparison to Tien Shan, the lithosphere beneath the neighboring
Tarim Basin was rather cold, 350°C at the depth of 15 km
and 500°C at the Moho boundary. These temperature differences
were the main factors that caused the mechanical weakening of
the Tien Shan crust and upper mantle. The modern strain rates
of the Tien Shan shortening are 10-14 × 3 to 10-15 s-1.
The total strength of the lithosphere beneath Tien Shan at present
is 8-4 × 1012 N m-1. In the past the lithosphere strength
beneath Tien Shan was ~1012 N m-1, which was more than 1000
times weaker in comparison with the lithospheric strength of
Tarim Basin at the beginning of the continental compression
in the region. |
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Enero de 2011
Capillary controls on methane hydrate distribution and
fracturing in advective systems
Authors: Hugh Daigle and Brandon Duganl
Link: Click here
Abstract
We simulate 1-D, steady, advective flow through a layered
porous medium to investigate how capillary controls on solubility
including the Gibbs-Thomson effect in fine-grained sediments
affect methane hydrate distribution in marine sediments. We
compute the increase in pore fluid pressure that results from
hydrate occluding the pore space and allow fractures to form
if the pore fluid pressure exceeds a fracture criterion. We
apply this model to Hydrate Ridge and northern Cascadia,
two field sites where hydrates have been observed preferentially
filling cm-scale, coarse-grained layers. We find
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that
at Hydrate Ridge, hydrate forms in the coarse-grained layers
reaching saturation of 90%, creating fractures through intervening
fine-grained layers after 2000 years. At northern Cascadia,
hydrate forms preferentially in the coarse-grained layers but
2 × 105 years are required to develop the observed hydrate
saturations (~20%-60%), suggesting that hydrate formation rates
may be enhanced by an additional source of methane such as in
situ methanogenesis. We develop expressions to determine the
combinations of sediment physical properties and methane supply
rates that will result in hydrate-filled coarse-grained layers
separated by hydrate-filled fine-grained layers, the conditions
necessary to fracture the fine-grained layers, and the conditions
that will lead to complete inhibition of hydrate formation as
pore space is constricted. This work illustrates how sediment
physical properties control hydrate distribution at the pore
scale and how hydrate distribution affects fracturing behavior
in marine sediments. |
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Enero de 2011
Un modelo sísmico probabilístico para el
Artico Europeo
Autores: Juerg Hauser, Kathleen M. Dyer
et al
Link: Click aquí
Abstract
El desarrollo de modelos sísmicos tridimensionales
para la corteza y el manto superior ha estado enfocado en
encontrar un modelo que proporcione el mejor ajuste de los
datos y de las observaciónes de las ligazones o constraints.
Por otro lado, la inversión aquí empleada ajusta
los datos en un sentido probabilístico y así
proporciona una medición cuantitativa de las incertezas
del modelo. Nuestro modelo probabilístico está
basado en dos fuentes de información: (1) Información
previa, la cual es independiente de los datos, y (2) Diferentes
sets de datos geofísicos incluyendo espesor de las
constraints, marcos de velocidades, datos gravitatorios,
velocidades de grupo de ondas superficiales y tiempos de viaje
de ondas de cuerpo regionales. Utilizamos un algoritmo Monte
Carlo con Cadenas de Markov (MCMC) para muestrear modelos
de la distribución previa, el set de modelos aceptables
y testearlos frente a la capacidad de generar los datos de
la distribución posterior (el conjunto de modelos que
ajustan los datos con las incertezas asignadas). La inversión
probabilística es computacionalmente costosa, pero
proporciona una imagen más completa del aspecto espacial
y nos permite combinar varios conjuntos de datos. La compleja
geología del Artico Europeo que abarca la corteza oceánica,
las regiones de la plataforma continental, las cuencas de
rift y la edad de la corteza, así como la cobertura
no uniforme de la región por los datos con distintos
grados de incertidumbre, lo convierte en un escenario difícil
para cualquier técnica de imagen y, por tanto, un entorno
ideal para demostrar las ventajas prácticas de un enfoque
probabilístico. Los mapas de profundidad hasta el basamento
y la profundidad del Moho derivados de la distribución
posterior están en buen acuerdo con los mapas publicados
anteriormente y con la interpretación de la disposición
de la tectónica regional. Se predijo que las incertidumbres
son tan importantes como los valores absolutos, se correlacionan
bien con las variaciones en la cobertura y calidad en la región.
Una ventaja práctica de nuestro modelo probabilístico
es que puede proporcionar estimaciones de las incertidumbres
de los observables debido a las incertidumbres del modelo.
Demostraremos como se puede utilizar para la formulación
de algoritmos de localización de terremotos que utilizarán
incertidumbres del modelo considerando el estimar las incertidumbres
de la ubicación.
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Enero de 2011
A probabilistic seismic model for the European Arctic
Authors: Juerg Hauser, Kathleen M. Dyer
et al
Link: Click here
Abstract
The development of three-dimensional seismic models for the
crust and upper mantle has traditionally focused on finding
one model that provides the best fit to the data while observing
some regularization constraints. In contrast to this, the
inversion employed here fits the data in a probabilistic sense
and thus provides a quantitative measure of model uncertainty.
Our probabilistic model is based on two sources of information:
(1) prior information, which is independent from the data,
and (2) different geophysical data sets, including thickness
constraints, velocity profiles, gravity data, surface wave
group velocities, and regional body wave traveltimes. We use
a Markov chain Monte Carlo (MCMC) algorithm to sample models
from the prior distribution, the set of plausible models,
and test them against the data to generate the posterior distribution,
the ensemble of models that fit the data with assigned uncertainties.
While being computationally more expensive, such a
probabilistic inversion provides a more complete picture of
solution space and allows us to combine various data sets.
The complex geology of the European Arctic, encompassing
oceanic crust, continental shelf regions, rift basins and
old cratonic crust, as well as the nonuniform coverage of
the region by data with varying degrees of uncertainty, makes
it a challenging setting for any imaging technique and, therefore,
an ideal environment for demonstrating the practical advantages
of a probabilistic approach. Maps of depth to basement and
depth to Moho derived from the posterior distribution are
in good agreement with previously published maps and interpretations
of the regional tectonic setting. The predicted uncertainties,
which are as important as the absolute values, correlate well
with the variations in data coverage and quality in the region.
A practical advantage of our probabilistic model is that it
can provide estimates for the uncertainties of observables
due to model uncertainties. We will demonstrate how this can
be used for the formulation of earthquake location algorithms
that take model uncertainties into account when estimating
location uncertainties.
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Enero de 2011
Spin Crossover in Ferropericlase at High Pressure: A Seismologically
Transparent Transition?
Authors: D. Antonangeli, J Siebert et al
Link: Click here
Abstract
Seismic discontinuities in Earth typically arise from structural,
chemical, or temperature variations with increasing depth.
The pressure-induced iron spin state transition in the lower
mantle may influence seismic wave velocities by changing
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the elasticity of iron-bearing minerals, but
no seismological evidence of an anomaly exists. Inelastic
x-ray scattering measurements on (Mg0.83Fe0.17)O-ferropericlase
at pressures across the spin transition show effects limited
to the only shear moduli of the elastic tensor. This explains
the absence of deviation in the aggregate seismic velocities
and, thus, the lack of a one-dimensional seismic signature
of the spin crossover. The spin state transition does, however,
influence shear anisotropy of ferropericlase and should contribute
to the seismic shear wave anisotropy of the lower mantle.
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Enero de 2011
Microterremotos y temblores disparados remotamente en el
Centro de California a partir del terremoto chileno Mw 8.8
del año 2010
Autores: Zhigang Peng, David P. Hill et
al
Link: Clicaquí
Abstract
Nosotros examinamos microterremotos y temblores tectónicos
ocurridos en el centro de California y que fueron inicializados
remotamente por el terremoto chileno Mw 8.8 ocurrido en el
año 2010. Parece ser que muchos de los microterremotos
ocurridos cerca del Campo Geotermal de Coso fueron gatillados
por el terremoto chileno, ocurriendo el mayor de ellos (Ml
3.5) durante la recepción de las ondas superficiales
de Love de gran amplitud. El evento principal chileno también
disparó numerosas ráfagas de temblores cerca
de la sección Parkfield-Cholame de la Falla de San
Andrés ( FSA o SAF). Los temblores disparados localmente
fueron parcialmente enmascarados en las frecuencias bajas
de las señales de terremotos disparadas regionalmente
desde Coso, pero pueden ser identificadas al aplicar filtros
pasa-altos o de coincidencias. Ambos temblores disparados
a lo largo de la FSA y el temblor de Coso Ml 3.5 son consistentes
con el modelo de falla friccional de diferentes profundidades
sobre fallas presionadas críticamente bajo el criterio
de falla de Coulomb. Sin embargo, la inducción lejana
de temblores parece ser más fase-correlacionada con
las ondas superficiales que con el disparo de sismos, así
como las diferencias de reflexión debidas a las diferencias
constitutivas entre la fragilidad, corteza sismogénica
y la subcorteza.
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Enero de 2011
Remotely triggered microearthquakes and tremor in central
California following the 2010 Mw 8.8 Chile earthquake
Authors: Zhigang Peng, David P. Hill et
al
Link: Click here
Abstract
We examine remotely triggered microearthquakes and tectonic
tremor in central California following the 2010 Mw 8.8 Chile
earthquake. Several microearthquakes near the Coso Geothermal
Field were apparently triggered, with the largest earthquake
(Ml 3.5) occurring during the large-amplitude Love surface
waves. The Chile mainshock also triggered numerous tremor
bursts near the Parkfield-Cholame section of the San Andreas
Fault (SAF). The locally triggered tremor bursts are partially
masked at lower frequencies by the regionally triggered earthquake
signals from Coso, but can be identified by applying high-pass
or matched filters. Both triggered tremor along the SAF and
the Ml 3.5 earthquake in Coso are consistent with frictional
failure at different depths on critically-stressed faults
under the Coulomb failure criteria. The triggered tremor,
however, appears to be more phase-correlated with the surface
waves than the triggered earthquakes, likely reflecting differences
in constitutive properties between the brittle, seismogenic
crust and the underlying lower crust.
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Diciembre de 2010
Strong vertical mixing of deep water of a stratified reservoir
during the Maule earthquake, central Chile (Mw 8.8)
Authors: Alberto de la Fuente, Carolina
Meruane et al
Link: Click here
Abstract
Vertical profiles of water temperature in Rapel reservoir
(central Chile, 34° 2' 23? S, 71° 35' 23? W, 110 m.a.s.l.),
recorded the hydrodynamic response of the stratified water
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column of the reservoir
produced by the 8.8 magnitude Maule earthquake (epicentre located
in 36°15'36?S, 73°14' 20?W), showing an intense vertical
mixing of deep water. The vertical mixing was characterized
by changes in the potential energy of the water column and the
eddy diffusivity of the mixing, and these values were used to
estimate the mixing efficiency of the event. It is hypothesized
that the turbulent flow that mixed the water column was mainly
induced by the oscillation of the bed, although surface gravitational
waves could also have contributed to the vertical mixing. So
far, this is the first time that the hydrodynamic response of
a reservoir during a mega-earthquake is measured and described.
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Diciembre de 2010
The 2010 Maule, Chile earthquake: Downdip rupture limit
revealed by space geodesy
Authors: Xiaopeng Tong, David Sandwell
et al
Link: Click here
Abstract
Radar interferometry from the ALOS satellite captured the
coseismic ground deformation associated with the 2010 Mw 8.8
Maule, Chile earthquake. The ALOS interferograms reveal a
sharp transition in fringe pattern at ~150 km from the trench
axis that is diagnostic of the downdip rupture limit of the
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Maule
earthquake. An elastic dislocation model based on ascending
and descending ALOS interferograms and 13 near-field 3-component
GPS measurements reveals that the coseismic slip decreases more
or less linearly from a maximum of 17 m (along-strike average
of 6.5 m) at 18 km depth to near zero at 43-48 km depth, quantitatively
indicating the downdip limit of the seismogenic zone. The depth
at which slip drops to near zero appears to be at the intersection
of the subducting plate with the continental Moho. Our model
also suggests that the depth where coseismic slip vanishes is
nearly uniform along the strike direction for a rupture length
of ~600 km. The average coseismic slip vector and the interseismic
velocity vector are not parallel, which can be interpreted as
a deficit in strike-slip moment release. |
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Diciembre de 2010
Source location of the 19 February 2008 Oregon bolide using
seismic networks and infrasound arrays
Authors: Kristoffer T. Walker, Michael
A. H. Hedlin et al
Link: Click here
Abstract
On 19 February 2008 a bolide traveled across the sky along
a southern trajectory ending in a terminal burst above Oregon.
The event was well recorded by the USArray, other seismic
networks, four infrasound arrays, and several video cameras.
We compare the results of locating the burst using these different
sensor networks. Specifically, we reverse time migrate acoustic-to-seismic
coupled signals recorded by the USArray out to 800 km range
to image the source in 2-D space and time. We also apply a
grid search over source altitude and time, minimizing the
misfit between observed
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and predicted arrival
times using 3-D ray tracing with a high-resolution atmospheric
velocity model. Our seismic and video results suggest a point
source rather than a line source associated with a hypersonic
trajectory. We compare the seismic source locations to those
obtained by using different combinations of observed infrasound
array signal back azimuths and arrival times. We find that all
locations are consistent. However, the seismic location is more
accurate than the infrasound locations due to the larger number
of seismic sensors, a more favorable seismic source-receiver
geometry, and shorter ranges to the seismometers. For the infrasound
array locations, correcting for the wind improved the accuracy,
but implementing arrival times while increasing the precision
reduced the accuracy presumably due to limitations of the source
location method and/or atmospheric velocity model. We show that
despite known complexities associated with acoustic-to-seismic
coupling, aboveground infrasound sources can be located with
dense seismic networks with remarkably high accuracy and precision.
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Diciembre de 2010
Geophysical properties within the San Andreas Fault Zone
at the San Andreas Fault Observatory at Depth and their relationships
to rock properties and fault zone structure
Authors: Tamara N. Jeppson, Kelly K. Bradbury
et al
Link: Click here
Abstract
We examine the relationships between borehole geophysical
data and physical properties of fault-related rocks within
the San Andreas Fault as determined from data from the San
Andreas Fault Observatory at Depth borehole. Geophysical logs,
cuttings data, and drilling data from the region 3- to 4-km
measured depth of the borehole encompass the active part of
the San Andreas Fault. The fault zone lies in a sequence of
deformed sandstones, siltstone, shale, serpentinite-bearing
block-in-matrix rocks, and sheared phyllitic siltstone. The
borehole geophysical logs reveal the
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presence
of a low-velocity zone from 3190 to 3410 m measured depth with
Vp and Vs values 10-30% lower than the surrounding rocks and
a 1-2 m thick zone of active shearing at 3301-3303 m measured
depth. Seven low-velocity excursions with increased porosity,
decreased density, and mud-gas kick signatures are present in
the fault zone. Geologic data on grain-scale deformation and
alteration are compared to borehole data and reveal weak correlations
and inverse relationships to the geophysical data. In places,
Vp and Vs increase with grain-scale deformation and alteration
and decrease with porosity in the fault zone. The low-velocity
zone is associated with a significant lithologic and structural
transition to low-velocity rocks, dominated by phyllosilicates
and penetratively foliated, sheared rocks. The zone of active
shearing and the regions of low sonic velocity appear to be
associated with clay-rich rocks that exhibit fine-scale foliation
and higher porosities that may be a consequence of the fault-related
shearing of foliated and fine-grained sedimentary rocks. |
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Diciembre de 2010
Effect of dust storms on the D region of the Martian ionosphere:
Atmospheric electricity
Authors: S. A. Haider, V. Sheel et al
Link: Click here
Abstract
We have used dust opacity values observed by the Thermal Emission
Imaging System onboard Mars Odyssey to estimate the effect
of dust aerosols in the D region of the Martian ionosphere.
An ion-dust aerosol model has been
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developed to calculate
ion concentrations and conductivity at midlatitudes during a
dust storm in the Southern Hemisphere. We report that the concentration
of the water cluster ions H+(H2O)n, NO2-(H2O)n, and CO3-(H2O)n
are reduced by 2 orders of magnitude in the presence of dust
aerosols. This indicates that during a dust storm, when the
optical depth changes considerably, a large hole in the ion
concentrations may appear until this anomalous condition returns
to the normal condition after a period of about a few days.
During such dust storms, the total ion conductivity is reduced
by an order of magnitude. |
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Diciembre de 2010
Space radiation risk limits and Earth-Moon-Mars environmental
models
Authors: Francis A. Cucinotta, Shaowen
Hu et al
Link: Click here
Abstract
We review NASA's short-term and career radiation limits for
astronauts and methods for their application to future exploration
missions outside of low Earth orbit. Career limits are intended
to restrict late occurring health effects and include a 3%
risk of exposure-induced death from cancer and new limits
for central nervous system and heart disease risks. Short-term
dose limits are used to prevent in-flight radiation sickness
or death through restriction of the doses to
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the
blood forming organs and to prevent clinically significant cataracts
or skin damage through lens and skin dose limits, respectively.
Large uncertainties exist in estimating the health risks of
space radiation, chiefly the understanding of the radiobiology
of heavy ions and dose rate and dose protraction effects, and
the limitations in human epidemiology data. To protect against
these uncertainties NASA estimates the 95% confidence in the
cancer risk projection intervals as part of astronaut flight
readiness assessments and mission design. Accurate organ dose
and particle spectra models are needed to ensure astronauts
stay below radiation limits and to support the goal of narrowing
the uncertainties in risk projections. Methodologies for evaluation
of space environments, radiation quality, and organ doses to
evaluate limits are discussed, and current projections for lunar
and Mars missions are described. |
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Diciembre de 2010
Seismic imaging using microseismic events: Results from
the San Andreas Fault System at SAFOD
Authors: A. Reshetnikov, S. Buske et al
Link: Click here
Abstract
We have developed a new passive seismic imaging approach that
consists of two steps. First, the hypocenter of the microseismic
event is precisely located. Second, this event is treated
as a "pseudo-active" seismic source and the reflections
within the recorded wavefield are processed by using a directional
migration algorithm in order to construct a high-resolution
image of the illuminated subsurface region.
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In this paper we demonstrate
the application of our approach to a number of microseismic
events recorded by a borehole array in the San Andreas Fault
Observatory at Depth main hole. Results obtained were high-resolution
3-D images of different SE-NW-oriented reflectors related to
the San Andreas Fault (SAF) system in the close vicinity of
the borehole. To support our approach, we compared our findings
with other active and passive seismic images and analyzed the
correlation with borehole lithology. We revealed a predominantly
satisfactory agreement for both juxtapositions. Furthermore,
the stacked image of several microearthquakes provides a spatial
characterization of the complex internal structure of the SAF,
with much higher resolution than can be obtained from surface
seismic reflection data. |
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Diciembre de 2010
Impact of a Cosmic Body Into Earth's Ocean and the Generation
of Large Tsunami Waves: Insight From Numerical Modeling
Authors: K. Wünnemann, G. S. Collins
et al
Link: Click here
Abstract
The strike of a cosmic body into a marine environment differs
in several respects from impact on land. Oceans cover approximately
70% of the Earth's surface, implying not only that oceanic
impact is a very likely scenario for future impacts but also
that most impacts in Earth's history must have happened in
marine environments. Therefore, the study of oceanic impact
is imperative in two respects: (1) to quantify the hazard
posed by future oceanic impacts, including the
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potential
threat of large impact-generated tsunami-like waves, and (2)
to reconstruct Earth's impact record by accounting for the large
number of potentially undiscovered crater structures in the
ocean crust. Reconstruction of the impact record is of crucial
importance both for assessing the frequency of collision events
in the past and for better predicting the probability of future
impact. We summarize the advances in the study of oceanic impact
over the last decades and focus in particular on how numerical
models have improved our understanding of cratering in the oceanic
environment and the generation of waves by impact. We focus
on insight gleaned from numerical modeling studies into the
deceleration of the projectile by the water, cratering of the
ocean floor, the late stage modification of the crater due to
gravitational collapse, and water resurge. Furthermore, we discuss
the generation and propagation of large tsunami-like waves as
a result of a strike of a cosmic body in marine environments.
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Diciembre de 2010
Scale-invariant stress orientations and seismicity rates
near the San Andreas Fault
Authors: Amy Day-Lewis, Mark Zoback et
al
Link: Click here
Abstract
We analyzed measurements of the direction of maximum horizontal
compressive stress as a function of depth in two scientific
research wells near the San Andreas Fault in central and southern
California. We found that the stress orientations exhibit
scale-invariant fluctuations over intervals
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from tens of cm to
several km. Similarity between the scaling of the stress orientation
fluctuations and the scaling of earthquake frequency with fault
size suggests that these fluctuations are controlled by stress
perturbations caused by slip on faults of various sizes in the
critically-stressed crust adjacent to the fault. The apparent
difference in stress scaling parameters between the two studies
wells seem to correspond to differences in the earthquake magnitude-frequency
statistics for the creeping versus locked sections of the fault
along which these two wells are located. This suggests that
stress heterogeneity adjacent to active faults like the San
Andreas may reflect variations in stresses and loading conditions
along the fault. |
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Diciembre de 2010
Coseismic gravity changes of the 2010 earthquake in central
Chile from satellite gravimetry
Authors: Kosuke Heki and Koji Matsuo
Link: Click here
Abstract
The first map of coseismic changes in gravity and geoid height
has been drawn using the data from the Gravity Recovery and
Climate Experiment (GRACE) satellites for the
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2004
Sumatra-Andaman earthquake. Here we present the second case
of coseismic gravity change observation by satellite gravimetry,
i.e., the change caused by an interplate thrust earthquake that
occurred on 27 February, 2010 in Central Chile (Mw = 8.8). Gravity
showed a negative jump with the largest drop of ~5 µgal
on the back-arc side. The observed changes agree with those
calculated assuming the realistic earth and fault parameters
inferred from coseismic displacements of Global Positioning
System (GPS) stations. Gravity in this area shows large seasonal
and inter-annual variability, and postseismic gravity changes
could be isolated only by carefully removing hydrological signals.
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Diciembre de 2010
Electromagnetic pulses generated by meteoroid impacts on
spacecraft
Authors: S. Close, P. Colestock et al
Link: Click here
Abstract
Meteoroid impacts on spacecraft are known to cause mechanical
damage, but their electrical effect on spacecraft systems
are not well characterized. Several reported spacecraft anomalies
are suggestive of an electrical failure associated with meteoroid
impact. We present a theory to explain plasma production and
subsequent electric fields
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occurring when a meteoroid
strikes a spacecraft, ionizing itself and part of the spacecraft.
This plasma, with a charge separation commensurate with different
specie mobilities, can produce a strong electromagnetic pulse
(EMP) at broad frequency spectra, potentially causing catastrophic
damage if the impact is relatively near an area with low shielding
or an open umbilical. Anomalies such as gyrostability loss can
be caused by an EMP without any detectable momentum transfer
due to small (<1 µg) particle mass. Subsequent plasma
oscillations can also emit significant power and may be responsible
for many reported satellite anomalies. The presented theory
discusses both a dust-free plasma expansion with coherent electron
oscillation and a dusty plasma expansion with macroscopic charge
separation. |
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Diciembre de 2010
Acoustic resonant oscillations between the atmosphere and
the solid earth during the 1991 Mt. Pinatubo eruption
Authors: Shingo Watada and Hiroo Kanamori
Link: Click here
Abstract
Long-period harmonic Rayleigh waves were observed on seismometers
during the 1991 Mt. Pinatubo eruption in the Philippines.
The amplitude spectrum of the Rayleigh waves shows two distinct
peaks at periods of about 230 and 270 s. In the Earth's atmosphere,
long-wavelength standing acoustic waves are bounded in a low-sound-velocity
channel
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between
the thermosphere and the ground. The Rayleigh waves and the
fundamental and first overtone of atmospheric acoustic waves
trapped in the low-sound-velocity channels have approximately
the same horizontal wavelength and frequency at periods of 230
and 270 s, respectively, i.e., the atmosphere and the solid
earth satisfy the condition for acoustic resonant oscillations.
The standing atmospheric long-wavelength acoustic waves set
off by the eruption selectively excited seismic spheroidal modes
near the resonant period through acoustic resonant coupling
and resulted in harmonic Rayleigh waves. In contrast, gravity
waves and Lamb waves (atmospheric boundary waves) do not couple
to the ground efficiently and are not easily observed as ground
disturbance on seismograms during volcanic eruptions. |
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Diciembre de 2010
Reconocimiento de patrones para predecir series temporales
sísmicas
Authors: A. Morales-Esteban, F. Martínez-Álvarez
et al
Link: Click here
Abstract
Los terremotos ocurren sin previo aviso y pueden
destruir una ciudad completa en pocos segundos, causando
numerosas muertes y pérdidas económicas.
Actualmente se están haciendo grandes esfuerzos
para desarrollar técnicas que permitan predecir
tales impredecibles desastres naturales para así
tomar medidas precautorias. En este paper se han utilizado
técnicas de agrupamiento para obtener patrones
que modelan los rasgos de series sísmicas temporales
y que pueden ayudar a predecir terremotos de tamaño
mediano a grande. En primer lugar los terremotos son
clasificados en diferentes grupos, siendo el número
de grupos óptimo desconocido. A continuación
se descubren patrones cuando ocurren terremotos medianos
a grandes. Los datos de los terremotos españoles
son provistos por el Instituto Geográfico de
España y se presentan tests estadísticos
no paramétricos, indicando que los resultados
muestran un notorio desempeño y significancia.
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Diciembre de 2010
Pattern recognition to forecast seismic time series
Authors: A. Morales-Esteban, F. Martínez-Álvarez
et al
Link: Click here
Abstract
Earthquakes arrive without previous warning and
can destroy a whole city in a few seconds, causing
numerous deaths and economical losses. Nowadays, a great
effort is being made to develop techniques
that forecast these unpredictable natural disasters
in order to take precautionary measures. In this paper,
clustering techniques are used to obtain patterns which
model the behavior of seismic temporal data and
can help to predict medium–large earthquakes. First,
earthquakes are classified into different groups and
the optimal number of groups, a priori unknown, is determined.
Then, patterns are discovered when
medium–large earthquakes happen. Results from the
Spanish seismic temporal data provided by the
Spanish Geographical Institute and non-parametric statistical
tests are presented and discussed, showing
a remarkable performance and the significance of the
obtained results.
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Diciembre de 2010
An extraordinary episode of Yellowstone caldera uplift,
2004-2010, from GPS and InSAR observations
Authors: Wu-Lung Chang, Robert B. Smith
et al
Link: Click here
Abstract
Geodetic measurements of Yellowstone ground deformation from
2006 to June 2010 reveal deceleration of the recent uplift
of the Yellowstone caldera following an unprecedented period
of uplift that began in 2004. In 2006-2008 uplift rates decreased
from 7 to 5 cm/yr and 4 to 2 cm/yr in the
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northern
and southwest caldera, respectively, and in 2009 rates further
reduced to 2 cm/yr and 0.5 cm/yr in the same areas. Elastic-dislocation
modeling of the deformation data robustly indicates an expanding
sill at ~7-10 km depth near the top of a seismically imaged,
crystallizing magma reservoir, with a 60% decrease in the volumetric
expansion rate between 2006 and 2009. Reduction of hydrothermal-volcanic
recharge from beneath the northeast caldera and seismic moment
release of the 2008 and 2010 large earthquake swarms are plausible
mechanisms for decelerating the caldera uplift and may have
influenced the change in recent caldera motion from uplift to
subsidence. |
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Diciembre de 2010
Rupture modes in laboratory earthquakes: Effect of fault
prestress and nucleation conditions
Authors: Xiao Lu, Ares J. Rosakis et al
Link: Click here
Abstract
Seismic inversions show that earthquake risetimes may be much
shorter than the overall rupture duration, indicating that
earthquakes may propagate as self-healing, pulse-like ruptures.
Several mechanisms for producing pulse-like ruptures have
been proposed, including velocity-weakening friction, interaction
of dynamic rupture with fault geometry and local heterogeneity,
and effect of bimaterial contrast. We present experimental
results on rupture mode selection in laboratory earthquakes
occurring on frictional interfaces, which were prestressed
both in compression and in shear. Our experiments demonstrate
that pulse-like ruptures can exist in the absence of a bimaterial
effect or of local
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heterogeneities. We
find a systematic variation from crack-like to pulse-like rupture
modes with both (1) decreasing nondimensional shear prestress
and (2) decreasing absolute levels of shear and normal prestress
for the same value of nondimensional shear prestress. Both pulse-like
and crack-like ruptures can propagate with either sub-Rayleigh
or supershear rupture speeds. Our experimental results are consistent
with theories of ruptures on velocity-weakening interfaces,
implying that velocity-weakening friction plays an important
role in governing the dynamic behavior of earthquake ruptures.
We show that there is no measurable fault-normal stress decrease
on the fault plane due to the nucleation procedure employed
in experiments, and hence, this is not a factor in the rupture
mode selection. We find that pulse-like ruptures correspond
to the levels of nondimensional shear prestress significantly
lower than the static friction coefficient, suggesting that
faults hosting pulse-like ruptures may operate at low levels
of prestress compared to their static strength. |
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Diciembre de 2010
Grain growth and loss of texture during annealing of alloys,
and the translation of Earth's inner core
Authors: Michael I. Bergman, Daniel J.
Lewis et al
Link: Click here
Abstract
The Earth's inner core is seismically anisotropic, with the
direction parallel to the rotation axis both fast and more
attenuating. There is also increasing evidence that the inner
core is asymmetric, with the western hemisphere exhibiting
slower direction-averaged P wave velocity, less overall attenuation,
and greater elastic anisotropy. It was recently suggested
that
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the
hemispherical variations might result from convective translation,
whereby enhanced solidification in the western hemisphere leads
to a net eastward translation of inner core material, with melting
occurring in the east. Annealing accompanies this eastward movement.
This study examines experimentally a previously unobserved sequence
of grain growth and loss of texture during the annealing of
a directionally solidified alloy. The growth of newly nucleated
grains results because the original grains that resulted from
directional solidification have a high energy associated with
intragranular interphase boundaries, and because the minor element
has a very low solubility in the primary phase so that a more
traditional sequence of coarsening is not possible. This supplies
a physical mechanism for the loss of texture that is suggested
seismically. |
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Diciembre de 2010
Dispersive tsunami of the 2010 Chile earthquake recorded
by the high-sampling-rate ocean-bottom pressure gauges
Authors: Tatsuhiko Saito, Takanori Matsuzawa
et al
Link: Click here
Abstract
Cabled ocean-bottom pressure gauges deployed offshore Japan
recorded the pressure change associated with the 2010 Chile
earthquake tsunami over a wider frequency range than that
of coastal tide gauges or hydrophones. Although it
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was difficult to recognize
the dispersive features in the original records, the spectrograms
clearly showed wave dispersion. A low-frequency tsunami (~1
mHz) arrived after an elapsed time of 24 hr from the earthquake
origin time, while a high-frequency tsunami (~9 mHz) arrived
after an elapsed time of 48 hr. The arrival times can be explained
by assuming a constant water depth of 4 km. However, the calculated
waveform does not correctly reproduce the number of wave packets
appearing in the observations. Cabled ocean-bottom pressure
gauges deployed offshore can record broadband tsunami signals
reflecting the wide-wavenumber-range spatial spectrum for sea-bottom
deformation caused by the earthquake. |
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Diciembre de 2010
The seismoacoustic wavefield: a new paradigm in studying
geophysical phenomena
Authors: Stephen J. Arrowsmith, Jeffrey
B. Johnson et al
Link: Click here
Abstract
The field of seismoacoustics is emerging as an important discipline
in its own right, owing to the value of colocated seismic
and infrasound arrays that sample elastic energy propagating
in both the solid Earth and the atmosphere. The fusion of
seismic and infrasonic data provides unique constraints for
studying a broad range of topics including the source physics
of natural and man-made events, interaction of mechanical
waves in Earth's crust and atmosphere, source location and
characterization, and inversion of atmospheric and shallow
subsurface properties.
This review article traces the seismoacoustic
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wavefield
from source to receiver. Beginning at the source, we review
the latest insights into the physics of natural and anthropogenic
sources that have arisen from the analysis of seismoacoustic
data. Next, a comparative review of 3-D models of the atmosphere
and solid Earth and the latest algorithms for modeling the propagation
of mechanical waves through these media provides the framework
for a discussion of the seismoacoustic path. The optimal measurement
of seismic and acoustic waves, including a discussion of instrumentation,
as well as of array configurations and regional networks, is
then outlined. Finally, we focus on broad research applications
where the analysis of seismoacoustic data is starting to yield
important new results, such as in the field of nuclear explosion
monitoring. This review is intended to provide a primer on the
field of seismoacoustics for seismologists or acousticians,
while also providing a more general review of what constraints
seismoacoustics can uniquely provide for understanding geophysical
phenomena. |
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Diciembre de 2010
Spatial characterization of the hydraulic conductivity
using direct-push injection logging
Authors: Steven C. Lessoff, Uwe Schneidewind
et al
Link: Click here
Abstract
Detailed information on the spatial structure of hydraulic
conductivity (K) is important for understanding and predicting
groundwater flow and transport. Direct-push injection logging
(DPIL) is a promising technology for rapid measurement of
K in unconsolidated formations. This technology was used to
gain information on the highly heterogeneous aquifer at the
Lauswiesen test site in Germany. Using a large body of DPIL
and direct-push slug testing measurements, we
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characterize the structure
of K on scales not previously possible. Two new applications
of DPIL are put forward: (1) use of raw DPIL measurements of
relative conductivity Kr to characterize the spatial distribution
of K and (2) transformation of Kr measurements to K values based
on their statistical moments. The DPIL results are compatible
to those obtained using more conventional methodologies. The
main achievement of the methodology is the possibility to delineate
deterministic aquifer subunits as well as the identification
of the statistical parameters of the log conductivity for each
subunit. In particular, the horizontal integral scale I, a parameter
affecting solute transport, is difficult and costly to identify
using other approaches. Nevertheless, further studies are needed
to clarify questions on low Kr measurements and the nature of
the relationship between Kr and K. |
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Noviembre de 2010
Slip distribution of the 2003 Tokachi-oki Mw 8.1 earthquake
from joint inversion of tsunami waveforms and geodetic data
Authors: F. Romano, A. Piatanesi et al
Link: Click here
Abstract
We study the 2003 Mw 8.1 Tokachi-oki earthquake, a great interplate
event that occurred along the southwestern Kuril Trench and
generated a significant tsunami. To determine the earthquake
slip distribution, we perform the first joint inversion of
tsunami waveforms measured by tide gauges and of coseismic
displacement measured both by GPS stations and three ocean
bottom pressure gauges (PG) for this event. The resolution
of the different data sets on the slip distribution is assessed
by means of several
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checkerboard
tests. Results show that tsunami data constrain the slip distribution
offshore, whereas GPS data constrain the slip distribution in
the onshore zone. The three PG data only coarsely constrain
the offshore slip, indicating that denser networks should be
installed close to subduction zones. Combining the three data
sets significantly improves the inversion results. Joint inversion
of the 2003 Tokachi-oki earthquake data leads to maximum slip
values (~6 m) confined at depths greater than ~25 km, between
30 and 80 km northwest of the hypocenter, with a patch of slip
(3 m) in the deepest part of the source (~50 km depth). Slip
values are very low (=1 m) updip from the hypocenter. Furthermore,
the rupture does not extend on the plate interface off Akkeshi.
As a significant back slip amount (~4 m) has accumulated there
since the last 1952 earthquake, this segment could rupture during
the next large interplate event along the Kuril Trench. |
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Noviembre de 2010
Role of seepage forces on seismicity triggering
Author: Alexander Y. Rozhko
Link: Click here
Abstract
Borehole fluid injection is commonly used for geological sequestration
of carbon dioxide, underground storage of natural gas, waste
injections, and during stimulations and development of geothermal
and hydrocarbon reservoirs. Typically, the injection process
induces significant seismicity, with some earthquakes as large
as magnitude four. Induced seismicity has also been observed
around producing hydrocarbon boreholes. Recently, it has been
argued that some induced seismicity data can be explained
by a highly nonlinear fluid diffusion
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mechanism or by the
propagation of fluid pressure pulses. The nature of the nonlinearity
and the mechanisms by which a pressure pulse can trigger seismicity
are still uncertain. In this paper I show that the same spatiotemporal
variation of seismicity can be explained and predicted by linear
diffusion coupled to deformation of a linear poroelastic medium.
By calculating the propagation of Coulomb Yielding Stress (CYS)
perturbation with time, it is demonstrated that seismicity can
be triggered by this perturbation. The change of CYS along the
diffusion front is caused by seepage forces, which are body
forces generated by fluid pressure gradients, and can explain
induced seismicity during borehole fluid injection and extraction.
Using published experimental data, I demonstrate how the spatiotemporal
distribution of fluid-induced seismic events can be used for
reservoir modeling and characterization. |
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Noviembre de 2010
Stochastic estimation of aquifer geometry using seismic
refraction data with borehole depth constraints
Authors: Jinsong Chen, Susan S. Hubbard
et al
Link: Click here
Abstract
We develop a Bayesian model to invert surface seismic refraction
data with depth constraints from boreholes for characterization
of aquifer geometry and apply it to seismic and borehole data
sets collected at the contaminated Oak Ridge National Laboratory
site in Tennessee. Rather than the traditional approach of
first inverting the seismic arrival times for seismic velocity
and then using that information to aid in the spatial interpolation
of wellbore data, we jointly invert seismic first arrival
time data and wellbore-based information, such as depths of
key lithological boundaries. We use a staggered-grid finite
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difference
algorithm with second-order accuracy in time and fourth-order
accuracy in space to model seismic full waveforms and use an
automated method to pick the first arrival times. We use Markov
Chain Monte Carlo methods to draw many samples from the joint
posterior probability distribution, on which we can estimate
the key interfaces and their associated uncertainty as a function
of horizontal location and depth. We test the developed method
on both synthetic and field case studies. The synthetic studies
show that the developed method is effective at rigorous incorporation
of multiscale data and the Bayesian inversion reduces uncertainty
in estimates of aquifer zonation. Applications of the approach
to field data, including two surface seismic profiles located
620 m apart from each other, reveal the presence of a low-velocity
subsurface zone that is laterally persistent. This geophysically
defined feature is aligned with the plume axis, suggesting it
may serve as an important regional preferential flow pathway.
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Noviembre de 2010
Quantitative earthquake forecasts resulting from static
stress triggering
Authors: S. Hainzl, G. B. Brietzke et al
Link: Click here
Abstract
In recent years, the triggering of earthquakes has been discussed
controversially with respect to the underlying mechanisms
and the capability to evaluate the resulting seismic hazard.
Apart from static stress interactions, other mechanisms including
dynamic stress transfer have been proposed to be part of a
complex triggering
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process. Exploiting
the theoretical relation between long-term earthquake rates
and stressing rate, we demonstrate that static stress changes
resulting from an earthquake rupture allow us to predict quantitatively
the aftershock activity without tuning specific model parameters.
These forecasts are found to be in excellent agreement with
all first-order characteristics of aftershocks, in particular,
(1) the total number, (2) the power law distance decay, (3)
the scaling of the productivity with the main shock magnitude,
(4) the foreshock probability, and (5) the empirical Båth
law providing the maximum aftershock magnitude, which supports
the conclusion that static stress transfer is the major mechanism
of earthquake triggering. |
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Noviembre de 2010
A study of acoustic propagation from a large bolide in
the atmosphere with a dense seismic network
Authors: Michael A. H. Hedlin, Doug Drob
et al
Link: Click here
Abstract
A large meteor entered the atmosphere above northeastern Oregon
on 19 February 2008 at 530 PST. Several hundreds of broadband
seismic stations in the U.S. Pacific Northwest recorded acoustic-to-seismic
coupled signals from this event. The travel times of the first
arriving energy are consistent with a terminal explosion source
model, suggesting that the large size of the explosion masked
any signals associated with a continuous line source along
its supersonic trajectory. Several infrasound arrays in North
America also recorded this event. Both the
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seismic and infrasound data have been used to locate the explosion
in 3-D space and time. Climatological atmospheric velocity models
predict that infrasound signals from sources that occur at mid-northern
latitudes in winter are usually ducted to the east due to eastward
zonal winds. In this paper, we analyze travel time picks and
use 3-D ray tracing to generate synthetic travel times based
on various atmospheric models to show that the seismic network
data instead reveal a predominant westward propagation direction.
A sudden stratospheric warming event that reversed the zonal
wind flow explains this westward propagation. The seismic data
illuminate in unprecedented spatial detail the range and azimuthal
definition of shadow zones out to a range of 500 km, suggesting
that dense seismic networks can be used to study infrasound
propagation at spatial resolutions that exceed that which can
be done with only a handful of globally distributed infrasound
arrays. |
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Noviembre de 2010
Benford's law in the natural sciences
Authors: M. Sambridge, H. Tkalcic et al
Link: Click here
Abstract
More than 100 years ago it was predicted that the distribution
of first digits of real world observations would not be uniform,
but instead follow a trend where measurements with lower first
digit (1,2,…) occur more frequently than those with higher
first digits (…,8,9). This result has long been known
but regarded largely as a mathematical curiosity and received
little attention in the natural sciences. Here we show that
the first digit rule is likely to be a widespread phenomenon
and may provide new ways to detect anomalous signals in data.
We test 15 sets of modern observations drawn from the fields
of physics, astronomy, geophysics,
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chemistry, engineering
and mathematics, and show that Benford's law holds for them
all. These include geophysical observables such as the length
of time between geomagnetic reversals, depths of earthquakes,
models of Earth's gravity, geomagnetic and seismic structure.
In addition we find it also holds for other natural science
observables such as the rotation frequencies of pulsars; green-house
gas emissions, the masses of exoplanets as well as numbers of
infectious diseases reported to the World Health Organization.
The wide range of areas where it is manifested opens up new
possibilities for exploitation. An illustration is given of
how seismic energy from an earthquake can be detected from just
the first digit distribution of displacement counts on a seismometer,
i.e., without actually looking at the details of a seismogram
at all. This led to the first ever detection of an earthquake
using first digit information alone. |
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Noviembre de 2010
Evaluación y optimización de redes sísmicas
y algoritmos para la alerta temprana de terremotos en el caso
de Estambul (Turquía)
Autores: Adrien Oth, Maren Böse et
al
Link: Clic aquí
Abstract
Los sistemas de Alerta Temprana de Terremotos (ATT o EEW)
deberían aportar confiables alertas, lo más
rápido posible y con un mínimo número
de falsas alarmas y eventos no predichos. Usando el ejemplo
de la megaciudad de Estambul y basándonos en un set
de escenarios simulados de terremotos, nosotros presentamos
un innovador punto de vista para evaluar y optimizar las redes
sísmicas de EEW, en particular en regiones con escasos
sismos registrados instrumentalmente. Nosotros mostramos que
cuando las ubicaciones de las estaciones EEW son correctamente
escogidas, su desempeño puede ser fortalecido al modificar
sus parámetros específicos que determinan la
declaración de alertas. Así mismo, a no ser
que se usen sismómetros ubicados en el fondo oceánico
o que se modifique el actual algoritmo EEW, el agregar estaciones
no mejora significativamente el desempeño.
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Noviembre de 2010
Evaluation and optimization of seismic networks and algorithms
for earthquake early warning - the case of Istanbul (Turkey)
Authors: Adrien Oth, Maren Böse et
al
Link: Click here
Abstract
Earthquake early warning (EEW) systems should provide reliable
warnings as quickly as possible with a minimum number of false
and missed alarms. Using the example of the megacity Istanbul
and based on a set of simulated scenario earthquakes, we present
a novel approach for evaluating and optimizing seismic networks
for EEW, in particular in regions with a scarce number of
instrumentally recorded earthquakes. We show that, while the
current station locations of the existing Istanbul EEW system
are well chosen, its performance can be enhanced by modifying
the parameters governing the declaration of warnings. Furthermore,
unless using ocean bottom seismometers or modifying the current
EEW algorithm, additional stations might not lead to any significant
performance increase.
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Octubre de 2010
Time-dependent volcano source monitoring using interferometric
synthetic aperture radar time series: A combined genetic algorithm
and Kalman filter approach
Authors: M. Shirzaei and T. R. Walter
Link: Click here
Abstract
Modern geodetic methods allow continuous monitoring of deformation
fields of volcanoes. The acquired data contribute significantly
to the study of the dynamics of magmatic sources prior to,
during and after eruptions and intrusions. In addition to
advancing the monitoring techniques, it is important to develop
suitable approaches to deal with deformation time series.
Here, we present, test and apply a new approach for time-dependent,
nonlinear inversion using a combination of a genetic algorithm
(GA) and a Kalman filter (KF). The GA is used in the form
presented by Shirzaei and Walter (2009), and the KF implementation
now allows for the treatment of monitoring data as a full
time series rather than
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as single time steps. This approach provides
a flexible tool for assessing unevenly sampled and heterogeneous
time series data and explains the deformation field using
time-consistent dislocation sources. Following synthetic tests,
we demonstrate the merits of time-consistent source modeling
for interferometric synthetic aperture radar (InSAR) data
available between 1992 and 2008 from the Campi Flegrei volcano
in Italy. We obtained multiple episodes of linear velocity
for the reservoir pressure change associated with a parabolic
surface deformation on the volcano. These data may be interpreted
via differential equations as a linear flux to the shallow
reservoir that provides new insight into how both the shallow
and deep reservoirs communicate beneath Campi Flegrei. The
synthetic test and case study demonstrate the robustness of
our approach and the ability to track and monitor the source
of systems with complex dynamics. It is applicable to time-dependent
optimization problems in volcanic and tectonic environments
in other tectonic environments in other areas and allows understanding
of the spatiotemporal extent of a physical process in quantitative
manner.
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Octubre de 2010
Neural network modeling and an uncertainty analysis in
Bayesian framework: A case study from the KTB borehole site
Authors: Saumen Maiti and Ram Krishna Tiwari
Link: Click here
Abstract
A new probabilistic approach based on the concept of Bayesian
neural network (BNN) learning theory is proposed for decoding
litho-facies boundaries from well-log data. We show that how
a multi-layer-perceptron neural network model can be employed
in Bayesian framework to classify changes in litho-log successions.
The method is then applied to the German Continental Deep
Drilling Program (KTB) well-log data for classification and
uncertainty estimation in the litho-facies boundaries. In
this framework, a posteriori distribution of network parameter
is estimated via the principle of Bayesian probabilistic theory,
and an objective function is minimized following the scaled
conjugate gradient optimization scheme. For the model development,
we inflict a suitable criterion, which provides probabilistic
information by emulating different combinations of synthetic
data. Uncertainty in the relationship between the data and
the model space is appropriately taken care by assuming a
Gaussian a priori distribution of networks parameters (e.g.,
synaptic weights and biases). Prior to applying the new method
to the real KTB data, we tested the proposed method on synthetic
examples to examine the sensitivity of neural network hyperparameters
in prediction. Within this
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framework, we examine
stability and efficiency of this new probabilistic approach
using different kinds of synthetic data assorted with different
level of correlated noise. Our data analysis suggests that the
designed network topology based on the Bayesian paradigm is
steady up to nearly 40% correlated noise; however, adding more
noise (~50% or more) degrades the results. We perform uncertainty
analyses on training, validation, and test data sets with and
devoid of intrinsic noise by making the Gaussian approximation
of the a posteriori distribution about the peak model. We present
a standard deviation error-map at the network output corresponding
to the three types of the litho-facies present over the entire
litho-section of the KTB. The comparisons of maximum a posteriori
geological sections constructed here, based on the maximum a
posteriori probability distribution, with the available geological
information and the existing geophysical findings suggest that
the BNN results reveal some additional finer details in the
KTB borehole data at certain depths, which appears to be of
some geological significance. We also demonstrate that the proposed
BNN approach is superior to the conventional artificial neural
network in terms of both avoiding "over-fitting" and
aiding uncertainty estimation, which are vital for meaningful
interpretation of geophysical records. Our analyses demonstrate
that the BNN-based approach renders a robust means for the classification
of complex changes in the litho-facies successions and thus
could provide a useful guide for understanding the crustal inhomogeneity
and the structural discontinuity in many other tectonically
complex regions. |
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Octubre de 2010
Precursory seismic anomalies and transient crustal deformation
prior to the 2008 Mw = 6.9 Iwate-Miyagi Nairiku, Japan, earthquake
Authors: Takao Kumazawa, Yosihiko Ogata
et al
Link: Click here
Abstract
The epidemic-type aftershock sequence (ETAS) model has been
widely used for detecting seismicity anomalies, such as quiescence
and activation during aftershock sequences of large earthquakes
and background levels of seismicity. The causes of such anomalies
are thought to be associated with spatiotemporal changes in
stress, even tiny perturbation of local stress. Here we analyze
the seismic activity during the decade prior to the 2008 Iwate-Miyagi
inland earthquake of M7.2 (Mw6.9), to fit the ETAS model to
the seismicity from various regions around the source over
northern Honshu, Japan. From the viewpoint of the analyses
with the ETAS model, we find
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northern
Honshu is divided into three areas of distinctive behaviors,
increased seismicity, decreased seismicity, and normal seismicity
relative to the ETAS prediction. As in other previously published
papers, here we hypothesize that Coulomb stress changes caused
by a few years of precursory slip of the Iwate-Miyagi earthquake
resulted in the seismicity changes in and around the 2008 source
region. The confirmed significant seismic anomalies in respective
regions are consistent with the increments of the Coulomb failure
stress of the corresponding regions that are calculated by the
assumed slow slip on the southern part of the faults of the
main shock. The local crustal deformations observed from a dense
Global Positioning System network, including a station right
above the focal fault, supports that slow slip on the fault
had been taking place for about 5 years prior to the occurrence
of the focal earthquake and suggests that the slip terminated
or migrated to down-dip extension of the fault around 1 year
before the rupture. |
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Octubre de 2010
Solar Influence on Climate
Authors: L. J. Gray, J. Beer et al
Link: Click here
Abstract
Understanding the influence of solar variability on the Earth's
climate requires knowledge of solar variability, solar-terrestrial
interactions, and the mechanisms determining the response
of the Earth's climate system. We provide a summary of our
current understanding in each of these three areas. Observations
and mechanisms for the Sun's variability are described, including
solar
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irradiance variations
on both decadal and centennial time scales and their relation
to galactic cosmic rays. Corresponding observations of variations
of the Earth's climate on associated time scales are described,
including variations in ozone, temperatures, winds, clouds,
precipitation, and regional modes of variability such as the
monsoons and the North Atlantic Oscillation. A discussion of
the available solar and climate proxies is provided. Mechanisms
proposed to explain these climate observations are described,
including the effects of variations in solar irradiance and
of charged particles. Finally, the contributions of solar variations
to recent observations of global climate change are discussed.
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Octubre de 2010
Spatiotemporal correlations of aftershock sequences
Authors: Tiago P. Peixoto, Katharina Doblhoff-Dier
et al
Link: Click here
Abstract
Aftershock sequences are of particular interest in seismic
research because they may condition seismic activity in a
given region over long time spans. Although they are typically
identified with periods of enhanced seismic activity after
a large earthquake as characterized by the Omori law, our
knowledge of the spatiotemporal correlations
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between
events in an aftershock sequence is limited. Here, we study
the spatiotemporal correlations of two aftershock sequences
form California (Parkfield and Hector Mine) using the recently
introduced concept of "recurrent" events. We find
that both sequences have very similar properties and that most
of them are captured by the space-time epidemic-type aftershock
sequence (ETAS) model if one takes into account catalog incompleteness.
However, the stochastic ETAS model does not capture those spatiotemporal
correlations that give rise to the observed distribution of
recurrent events on small spatial scales. We also find that
there is no clear evidence for stress shadows that have been
observed for smaller earthquakes. |
|
Octubre de 2010
Search for the global signature of the Martian dynamo
Authors: C. Milbury, G. Schubert et al
Link: Click here
Abstract
We analyze Mars' global magnetic field observations to gain
insight into the evolution of Mars and its paleodynamo. Statistical
properties of the magnetic field above Noachian and Hesperian
age crust are similar, suggesting that the dynamo persisted
past the Noachian. We model crustal magnetization in order
to match the large-scale features of the magnetic field. The
model is based on a spherical shell of uniform thickness that
was magnetized uniformly by an internal dipole. Crustal magnetization
is removed from the northern lowlands, the Tharsis volcanic
province, and the Hellas, Argyre, and Isidis impact basins.
The magnetic field due to the remaining
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crustal magnetization
is computed and compared with published models of the magnetic
field data. The comparison is based on the spherical harmonic
coefficients of the radial magnetic field component for the
crustal magnetization models and the models of the observations.
The correlation coefficients between the magnetization models
and the models of the observations are calculated as a function
of spherical harmonic degree. The correlations maximize for
paleopole positions that are located near the equator in the
southeast and northwest quadrants of Mars. The root-mean-square
differences of the spherical harmonic coefficients are also
calculated, and paleopole positions that minimize these differences
generally agree with the paleopoles that maximize the correlations.
The low-latitude to midlatitude paleopoles suggest that true
polar wander occurred early in Mars' history, and the polarity
of the paleopole positions suggests that at least one reversal
of the dynamo magnetic field occurred. |
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Octubre de 2010
Geomagnetic activity triggered by interplanetary shocks
Authors: C. Yue, Q. G. Zong et al
Link: Click here
Abstract
Interplanetary (IP) shocks can greatly disturb the Earth's
magnetosphere, causing the global dynamic changes in the electromagnetic
fields and the plasma. In order to investigate this, we have
systematically analyzed 106 IP shock events based on OMNI
data, GOES, and Los Alamos National Laboratory satellite observations
during 1997-2007. It is revealed that the median value of
IMF Bz keeps negative/positive prior to shock arrival and
becomes more negative/positive following the shock arrival.
The statistical analysis shows that IP shocks with southward
interplanetary magnetic field (IMF) (46%) are likely to increase
AE (AL, AU) and PC indices significantly. The amplitude of
AE index increases
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from
200 to 600 nT, AU from 100 to 200 nT, AL from 50 to 400 nT,
and PC from 1.5 to 3 approximately in 10 min, which could be
a signature of geomagnetic activity/substorms onset (or substorm
further intensification). Meanwhile, there is a strong injection
of energetic electrons in the dawn region following the shock
arrival and a strong depletion in the dusk region 30 min later,
showing a clear dawn-dusk asymmetry. On the other hand, there
is only the typical shock compression effect for IP shocks with
northward IMF (54%). The median value of AE index increased
from 80 to 150 nT, AU from 50 to 90 nT, AL index decreased from
-30 to -40 nT, and PC index increased from 0.6 to 1.2 in ~10
min following the shock arrival. Both individual cases and statistical
studies indicate that the magnetosphere-ionosphere system must
be preconditioned for a substorm-like geomagnetic activity to
be triggered by an IP shock with southward IMF impact, whereas
IP shock with northward IMF precondition shows only compression
effect. |
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Octubre de 2010
Landslide-generated tsunamis at Réunion Island
Authors: Karim Kelfoun, Thomas Giachetti
et al
Link: Click here
Abstract
Landslides that occur on oceanic volcanoes can reach the sea
and trigger catastrophic tsunamis. Réunion Island has
been the location of numerous huge landslides involving tens
to hundreds of cubic kilometers of material. We use a new
two-fluid (seawater and landslide) numerical model to estimate
the wave amplitudes and the propagation of tsunamis associated
with landslide events on Réunion Island. A 10 km3 landslide
from the eastern flank of Piton de la Fournaise volcano would
lift the water surface by
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about 150 m where
it entered the sea. The wave thus generated would reach Saint-Denis,
the capital of Réunion Island (population of about 150,000
people), in only 12 min, with an amplitude of more than 10 m,
and would reach Mauritius Island in 18 min. Although Mauritius
is located about 175 km from the impact, waves reaching its
coast would be greater than those for Réunion Island.
This is due to the initial shape of the wave, and its propagation
normal to the coast at Mauritius but generally coast-parallel
at Réunion Island. A submarine landslide of the coastal
shelf of 2 km3, would trigger a ~40 m high wave that would severely
affect the proximal coast in the western part of Réunion
Island. For a landslide of the shelf of only 0.5 km3, waves
of about 2 m in amplitude would affect the proximal coast. |
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Octubre de 2010
Corruption of accuracy and efficiency of Markov chain Monte
Carlo simulation by inaccurate numerical implementation of
conceptual hydrologic models
Authors: G. Schoups, J. A. Vrugt et al
Link: Click here
Abstract
Conceptual rainfall-runoff models have traditionally been
applied without paying much attention to numerical errors
induced by temporal integration of water balance dynamics.
Reliance on first-order, explicit, fixed-step integration
methods leads to computationally cheap simulation models that
are easy to implement. Computational speed is especially desirable
for estimating parameter and predictive uncertainty using
Markov chain Monte Carlo (MCMC) methods. Confirming earlier
work of Kavetski et al. (2003), we show here that the computational
speed of first-order, explicit, fixed-step integration methods
comes at a cost: for a case study with a spatially lumped
conceptual rainfall-runoff model, it introduces artificial
bimodality in the marginal
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posterior
parameter distributions, which is not present in numerically
accurate implementations of the same model. The resulting effects
on MCMC simulation include (1) inconsistent estimates of posterior
parameter and predictive distributions, (2) poor performance
and slow convergence of the MCMC algorithm, and (3) unreliable
convergence diagnosis using the Gelman-Rubin statistic. We studied
several alternative numerical implementations to remedy these
problems, including various adaptive-step finite difference
schemes and an operator splitting method. Our results show that
adaptive-step, second-order methods, based on either explicit
finite differencing or operator splitting with analytical integration,
provide the best alternative for accurate and efficient MCMC
simulation. Fixed-step or adaptive-step implicit methods may
also be used for increased accuracy, but they cannot match the
efficiency of adaptive-step explicit finite differencing or
operator splitting. Of the latter two, explicit finite differencing
is more generally applicable and is preferred if the individual
hydrologic flux laws cannot be integrated analytically, as the
splitting method then loses its advantage. |
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Octubre de 2010
Use of electrical imaging and distributed temperature sensing
methods to characterize surface water-groundwater exchange
regulating uranium transport at the Hanford 300 Area, Washington
Authors: Lee D. Slater, Dimitrios Ntarlagiannis
et al
Link: Click here
Abstract
We explored the use of continuous waterborne electrical imaging
(CWEI), in conjunction with fiber-optic distributed temperature
sensor (FO-DTS) monitoring, to improve the conceptual model
for uranium transport within the Columbia River corridor at
the Hanford 300 Area, Washington. We first inverted resistivity
and induced polarization CWEI data sets for distributions
of electrical resistivity and polarizability, from which the
spatial complexity of the primary hydrogeologic units was
reconstructed. Variations in the depth to the interface between
the overlying coarse-grained, high-permeability Hanford Formation
and the
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underlying finer-grained,
less permeable Ringold Formation, an important contact that
limits vertical migration of contaminants, were resolved along
~3 km of the river corridor centered on the 300 Area. Polarizability
images were translated into lithologic images using established
relationships between polarizability and surface area normalized
to pore volume (Spor). The FO-DTS data recorded along 1.5 km
of cable with a 1 m spatial resolution and 5 min sampling interval
revealed subreaches showing (1) temperature anomalies (relatively
warm in winter and cool in summer) and (2) a strong correlation
between temperature and river stage (negative in winter and
positive in summer), both indicative of reaches of enhanced
surface water-groundwater exchange. The FO-DTS data sets confirm
the hydrologic significance of the variability identified in
the CWEI and reveal a pattern of highly focused exchange, concentrated
at springs where the Hanford Formation is thickest. Our findings
illustrate how the combination of CWEI and FO-DTS technologies
can characterize surface water-groundwater exchange in a complex,
coupled river-aquifer system. |
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Octubre de 2010
Effects of multiphase methane supply on hydrate accumulation
and fracture generation
Authors: Hugh Daigle and Brandon Dugan
Link: Click here
Abstract
We simulate methane hydrate formation with multiphase flow
and free gas within the regional hydrate stability zone (RHSZ).
We find that hydrate distribution and fracture behavior are
largely determined by the phase of the methane supply. We
allow free gas to enter the RHSZ when
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porewater
salinity increases to the value required for three-phase equilibrium.
Fractures nucleate when the excess pore pressure exceeds the
vertical hydrostatic effective stress. At Hydrate Ridge, where
methane supply is dominantly free gas, hydrate saturation increases
upwards and fractures nucleate high within the RHSZ, eventually
allowing gas to vent to the seafloor. At Blake Ridge, where
methane supply is dominantly in the dissolved phase, hydrate
saturation is greatest at the base of the RHSZ; fractures nucleate
here and in some cases could propagate through the RHSZ, allowing
methane-charged water to vent to the seafloor. |
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Octubre de 2010
On the relations between fracture energy and physical observables
in dynamic earthquake models
Author: Andrea Bizzarri
Link: Click here
Abstract
We explore the relationships between the fracture energy density
(EG) and the key parameters characterizing earthquake sources,
such as the rupture velocity (vr), the total fault slip (utot),
and the dynamic stress drop (?td). We perform several numerical
experiments of three-dimensional, spontaneous, fully dynamic
ruptures developing on planar faults of finite width, obeying
different governing laws and accounting for both homogeneous
and heterogeneous friction. Our results indicate that EG behaves
differently, depending on the adopted governing law and mainly
on the rupture mode (pulselike or cracklike, sub- or supershear
regime). Subshear,
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homogeneous
ruptures show a general agreement with the theoretical prediction
of EG , but for ruptures that accelerate up to supershear speeds
it is difficult to infer a clear dependence of fracture energy
density on rupture speed, especially in heterogeneous configurations.
We see that slip pulses noticeably agree with the theoretical
prediction of EG utot2, contrarily to cracklike solutions, both
sub- and supershear and both homogeneous and heterogeneous,
which is in agreement with seismological inferences, showing
a scaling exponent roughly equal to 1. We also found that the
proportionality between EG and ?td2, expected from theoretical
predictions, is somehow verified only in the case of subshear,
homogeneous ruptures with RD law. Our spontaneous rupture models
confirm that the total fracture energy (the integral of EG over
the whole fault surface) has a power law dependence on the seismic
moment, with an exponent nearly equal to 1.13, in general agreement
with kinematic inferences of previous studies. Overall, our
results support the idea that EG should not be regarded as an
intrinsic material property. |
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Octubre de 2010
Coral evidence for earthquake recurrence and an A.D. 1390-1455
cluster at the south end of the 2004 Aceh-Andaman rupture
Authors: Aron J. Meltzner, Kerry Sieh et
al
Link: Click here
Abstract
Coral records of relative sea level change provide a history
of vertical interseismic and coseismic deformation along the
coast of northern Simeulue Island, Sumatra, and reveal details
about earthquakes in the 10th and 14th-15th centuries A.D.
along the southern end of the December 2004 Mw 9.2 Sunda megathrust
rupture. Over a 56 year period between A.D. 1390 and 1455,
northern Simeulue experienced a cluster of megathrust ruptures,
associated with total uplift that was
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considerably more
than in 2004. Uplifted corals at two sites constrain the first
event of the cluster to A.D. 1393 ± 3 and 1394 ±
2 (2s). A smaller but well-substantiated uplift occurred in
northern Simeulue in 1430 ± 3. An inferred third uplift,
in A.D. 1450 ± 3, killed all corals on the reef flats
of northern Simeulue. The amount of uplift during this third
event, though confirmed only to have exceeded 28 and 41 cm at
two sites, probably surpassed the 100 and 44 cm that occurred,
respectively, at those sites in 2004, and it was likely more
than in 2004 over all of northern Simeulue. The evidence for
past earthquake clustering combined with the inference of considerably
greater uplift in A.D. 1390-1455 than in 2004 suggests that
strain may still be stored along the southernmost part of the
2004 rupture. Interseismic subsidence rates recorded by northern
Simeulue coral microatolls have varied by up to a factor of
4 at some sites from one earthquake cycle to another. |
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Octubre de 2010
A tremor and slip event on the Cocos-Caribbean subduction
zone as measured by a global positioning system (GPS) and
seismic network on the Nicoya Peninsula, Costa Rica
Authors: Kimberly C. Outerbridge, Timothy
H. Dixonet al
Link: Click here
Abstract
In May 2007 a network of global positioning systems (GPS)
and seismic stations on the Nicoya Peninsula, of northern
Costa Rica, recorded a slow-slip event accompanied by seismic
tremor. The close proximity of the Nicoya Peninsula to the
seismogenic part of the Cocos-Caribbean subduction plate boundary
makes it a good location to study such events. Several centimeters
of southwest motion were recorded
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by
the GPS stations over a period of several days to several weeks,
and the seismic stations recorded three distinct episodes of
tremor during the same time span. Inversion of the surface displacement
data for the depth and pattern of slip on the plate interface
shows peak slip at a depth of 25-30 km, downdip of the main
seismogenic zone. Estimated temperatures here are ~250°-300°C,
lower than in other subduction zones where events of this nature
have been previously identified. There may also be a shallower
patch of slip at ~6 km depth. These results are significant
in that they are the first to suggest that slow slip can occur
at the updip transition from stick slip to stable sliding, and
that a critical temperature threshold is not required for slow
slip. Tremor and low-frequency earthquake locations are more
difficult to determine. Our results suggest they occur on or
near the plate interface at the same depth range as the deep
slow slip, but not spatially colocated. |
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Octubre de 2010
Dynamics and rapid migration of the energetic 2008-2009
Yellowstone Lake earthquake swarm
Authors: Jamie Farrell, Robert B. Smith
et al
Link: Click here
Abstract
Yellowstone National Park experienced an unusual earthquake
swarm in December-January, 2008-2009 that included rapid northward
migration of the activity at 1 km per day and shallowing of
the maximum focal depths from 12 to 2 km beneath northern
Yellowstone Lake. The
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swarm consisted of
811 earthquakes, 0.5 < MW < 4.1, aligned on a N-S 12-km-long
vertical plane of hypocenters. The largest earthquake of the
swarm had a 50% tensile crack-opening source determined by a
full waveform inversion that we interpret as a magmatic expansion
component. In addition, GPS data revealed E-W crustal extension
coincident with the swarm. Modeling of GPS and seismic data
is consistent with E-W opening of ~10 cm on a N-S striking vertical
dike. Our interpretation is that the swarm was induced by magmatic
fluid migration or propagation of a poroelastic stress pulse
along a pre-existing fracture zone. |
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Octubre de 2010
On the ionospheric application of Poynting's theorem
Author: A. D. Richmond
Link: Click here
Abstract
It has been proposed that the geomagnetic field-aligned component
of the perturbation Poynting vector above the ionosphere,
as obtained from the cross product of the electric and magnetic
perturbation fields observed on a spacecraft, may be used
to estimate the field line-integrated electromagnetic energy
dissipation in the ionosphere below. This paper clarifies
conditions under which this approximation may be either valid
or invalid. It is shown that the downward field-aligned component
of the perturbation Poynting vector can underestimate the
electromagnetic energy dissipation in regions of high ionospheric
Pedersen conductance, and it can significantly overestimate
the dissipation in regions of low
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conductance.
Local values of upward perturbation Poynting vector do not necessarily
correspond to net ionospheric generation of electromagnetic
energy along that geomagnetic field line. An Equipotential Boundary
Poynting Flux (EBPF) theorem is presented for quasi-static electromagnetic
fields as follows: when a volume of the ionosphere is bounded
on the sides by an equipotential surface and on the bottom by
the base of the conducting ionosphere, then the area integral
of the downward normal component of the perturbation Poynting
vector over the top of that volume equals the energy dissipation
within the volume. This equality does not apply to volumes with
arbitrary side boundaries. However, the EBPF theorem can be
applied separately to different components of the electric potential,
such as the large- and small-scale components. Since contours
of the small-scale component of potential tend to close over
relatively localized regions, the associated small-scale structures
of downward perturbation Poynting vector tend to be dissipated
locally. |
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Octubre de 2010
Hydrodynamic factors affecting the persistence of the Exxon
Valdez oil in a shallow bedrock beach
Authors: Yuqiang Xia, Hailong Li
Link: Click here
Abstract
We report a field study and numerical modeling of multicomponent
flow in a tidal gravel beach in Knight Island, Prince William
Sound, Alaska, where oil from the 1989 Exxon Valdez oil spill
persisted. Field measurements of water table, salinity, and
tracer (lithium) concentration were obtained for around a
week during the summer of 2008. The numerical model MARUN
was used to simulate the field observations. On the basis
of field experiments and numerical simulations, the beach
was identified to have a two-layered hydraulic structure:
a high-permeability surface layer underlain by a low-permeability
lower layer. The hydraulic conductivity was found to be 5
× 10-2 m s-1 for the surface layer and 7 × 10-6
m s-1
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for
the lower layer. The simulations reproduced the observed water
table, salinity, and lithium concentrations accurately. The
small flow entering the beach from the land side resulted in
a beach water table dropping below the interface of the two
layers. This seems to be the major reason for the presence of
oil in the lower layer. The exchange flow between the beach
and the sea due to tidal influence was ~2.12 m3 d-1 m-1. The
patterns of inflow and outflow rates showed that the maximum
seawater-groundwater exchange occurred in the middle to high
intertidal zone, which explains the persistence of oil in the
lower intertidal zone. To explore bioremediation of the beach
with nutrient amendment, a numerical simulation of nutrient
application on the beach surface was conducted, where the applied
nutrient concentration was 5,000 mg L-1. The results showed
that the nutrient concentration remaining in oiled areas after
a week was larger than 50 mg L-1, which is larger than that
needed for maximum microbial growth (2-10 mg L-1). This implies
that the bioremediation via nutrient application on the beach
surface could be adopted if nutrients were the only limiting
factor. |
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Octubre de 2010
On the resolution of shallow mantle viscosity structure
using postearthquake relaxation data: Application to the 1999
Hector Mine, California, earthquake
Authors: Fred F. Pollitz and Wayne Thatcher
Link: Click here
Abstract
Most models of lower crust/mantle viscosity inferred from
postearthquake relaxation assume one or two uniform-viscosity
layers. A few existing models possess apparently significant
radially variable viscosity structure in the shallow mantle
(e.g., the upper 200 km), but the resolution of such variations
is not clear. We use a geophysical inverse procedure to address
the resolving power of inferred shallow mantle viscosity structure
using postearthquake relaxation data. We apply this methodology
to 9 years of GPS-constrained crustal motions after the 16
October 1999 M = 7.1 Hector Mine earthquake. After application
of a differencing method to isolate the postearthquake
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signal
from the "background" crustal velocity field, we find
that surface velocities diminish from ~20 mm/yr in the first
few months to 2 mm/yr after 2 years. Viscoelastic relaxation
of the mantle, with a time-dependent effective viscosity prescribed
by a Burgers body, provides a good explanation for the postseismic
crustal deformation, capturing both the spatial and temporal
pattern. In the context of the Burgers body model (which involves
a transient viscosity and steady state viscosity), a resolution
analysis based on the singular value decomposition reveals that
at most, two constraints on depth-dependent steady state mantle
viscosity are provided by the present data set. Uppermost mantle
viscosity (depth 60 km) is moderately resolved, but deeper viscosity
structure is poorly resolved. The simplest model that explains
the data better than that of uniform steady state mantle viscosity
involves a linear gradient in logarithmic viscosity with depth,
with a small increase from the Moho to 220 km depth. However,
the viscosity increase is not statistically significant. This
suggests that the depth-dependent steady state viscosity is
not resolvably different from uniformity in the uppermost mantle.
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Octubre de 2010
The Caribbean plate: Pulled, pushed, or dragged?
Authors: S. van Benthem and R. Govers
Link: Click here
Abstract
Mechanical coupling between the lithosphere and the asthenosphere
remains a controversial topic in the geosciences. Beneath
the Caribbean plate, shear wave splitting measurements indicate
EW strain in the asthenosphere, which can be interpreted as
mantle flow driving or resisting motion of the overlying lithosphere.
Here, we constrain the average shear traction on the base
of the Caribbean plate by balancing all torques. These torques
result from body forces that act on the Caribbean (slab pull,
ridge push, lateral
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density
variations), from plate boundary friction and from basal shear
tractions. We obtain a range of physically realistic torque
solutions, which we examine further by computing the corresponding
stresses and rotations within the Caribbean plate for comparison
with observations. The deformation field for the Caribbean is
particularly sensitive to the amount of friction on intraplate
faults. Representative models have a good fit with observations
and are characterized by (1) a near-zero basal shear traction
(=0.3 MPa), (2) (lithosphere-averaged) plate boundary friction
= 10 MPa, (3) local forces due to indenters and trench pull,
(4) a net pull by the Caribbean slab and (5) intraplate fault
shear stresses on the order of tens of megapascals. We conclude
that the mechanical coupling of the Caribbean plate to the underlying
asthenosphere is small. |
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Octubre de 2010
Permeability and pore connectivity: A new model based on
network simulations
Authors: Y. Bernabé and M. Li
Link: Click here
Abstract
The purpose of this paper is to model the effect of pore size
heterogeneity and pore connectivity on permeability. Our approach
is that of conceptual modeling based on network simulations.
We simulated fluid flow through pipe networks with different
coordination numbers and different pipe radius distributions.
Following a method widely used in percolation theory, we sought
"universal" relationships (i.e., independent of
lattice type)
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between
macroscopic properties such as permeability k and porosity ,
and, pore geometry attributes such as hydraulic radius rH, coordination
number z, and so forth. Our main result was that in three-dimensional
simple cubic, FCC, and BCC networks, permeability obeyed "universal"
power laws, k (z - zc)ß, where the exponent ß is
a function of the standard deviation of the pore radius distribution
and zc = 1.5 is the percolation threshold expressed in terms
of the coordination number. Most importantly, these power law
relationships hold in a wide domain, from z close to zc to the
maximum possible values of z. A permeability model was inferred
on the basis of the power laws mentioned above. It was satisfactorily
tested by comparison with published, experimental, and microstructural
data on Fontainebleau sandstone. |
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Octubre de 2010
Seismic interaction and delayed triggering along the North
Anatolian Fault
Authors :V. Durand, M. Bouchon et al
Link: Click here
Abstract
The deformation of northwestern Turkey is the result of the
encounter of the westward extrusion of the Anatolian plate
with the north-south extension of the Aegean domain. While
the North Anatolian Fault localizes the former type of deformation
and has been the site of more large earthquakes (9 events
of magnitude =7) than any other continental fault over the
past 100 years, the extension is diffuse and is characterized
by spatial clusters of smaller earthquakes and near-
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continuous activity. We study the evolution
of seismicity along the fault and in the clusters neighboring
the fault before and after the two large earthquakes of 1999.
We observe that the un-ruptured section of the fault and the
extension clusters respond very differently to the earthquakes.
While significant aftershock activity on the fault segments
adjacent to the rupture only occurs at relatively short distance
from the rupture (=30 km), the clusters can be activated at
much larger distances (300 km). Remarkably their triggering
is not immediate after the earthquake but is delayed in time.
Their peak seismic activation may occur weeks or months after
the earthquake. This distant and delayed triggering, which
is consistent with recent GPS observations, may help resolve
some of the previously unexplained characteristics of the
1939-1999 sequence of large earthquakes along the fault.
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Octubre de 2010
Nostradamus: The radar that wanted to be a seismometer
Authors :Giovanni Occhipinti, Philippe
Dorey et al
Link: Click here
Abstract
Surface waves emitted after large earthquakes are known to
induce, by dynamic coupling, atmospheric infrasonic waves
propagating upward through the neutral and ionized atmosphere.
Those waves have been detected in the past at ionospheric
heights using a variety of techniques, such as HF Doppler
sounding or GPS receivers. The HF Doppler technique, particularly
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sensitive
to the ionospheric signature of Rayleigh waves is used here
to show ionospheric perturbations consistent with the propagation
of Rayleigh wave phases R1 and R2 following the Sumatra earthquake
on the 28 March 2005 (M = 8.6). This is in our knowledge the
first time that the phase R2 is detected by ionospheric sounding.
In addition, we prove here that the ionospheric signature of
R2 is also observed by over-the-horizon (OTH) Radar. The latter
was never used before to detect seismic signature in the ionosphere.
Adding the OTH Radar to the list of the "ionospheric seismometers"
we discuss and compare the performances of the three different
instruments mentioned above, namely HF Doppler sounding, GPS
receivers and OTH radar. |
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Octubre de 2010
Experimental insights into the scaling and variability
of local tsunamis triggered by giant subduction megathrust
earthquakes
Authors :Matthias Rosenau, Rainer Nerlich
et al
Link: Click here
Abstract
Giant subduction megathrust earthquakes of magnitude 9 and
larger pose a significant tsunami hazard in coastal regions.
In order to test and improve empirical tsunami forecast models
and to explore the susceptibility of different subduction
settings we here analyze the scaling of subduction earthquake-triggered
tsunamis in the near field and their variability related to
source heterogeneities. We base our analysis on a sequence
of 50 experimentally simulated great to giant (Mw = 8.3-9.4)
subduction megathrust earthquakes generated using an elastoplastic
analog model. Experimentally observed surface deformation
is translated to local tsunami runup
|
using
linear wave theory. We find that the intrinsic scaling of local
tsunami runup is characterized by a linear relationship to peak
earthquake slip, an exponential relationship to moment magnitude,
and an inverse power law relationship to fore-arc slope. Tsunami
variability is controlled by coseismic slip heterogeneity and
strain localization within the fore-arc wedge and is characterized
by a coefficient of variation Cv ~ 0.5. Wave breaking modifies
the scaling behavior of tsunamis triggered by the largest (Mw
> 8.5) events in subduction settings with shallow dipping
(<1-2°) fore-arc slopes, limits tsunami runup to <30
m, and reduces its variability to Cv ~ 0.2. The resulting effective
scaling relationships are validated against historical events
and numerical simulations and reproduce empirical scaling relationships.
The latter appear as robust and liberal estimates of runup up
to magnitude Mw = 9.5. A global assessment of tsunami susceptibility
suggests that accretionary plate margins are more prone to tsunami
hazard than erosive margins. |
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Octubre de 2010
Assessing the skill of satellite-based precipitation estimates
in hydrologic applications
Authors :Ming Pan, Haibin Li et al
Link: Click here
Abstract
An important application of global precipitation measurement
rainfall products is providing forcing inputs for hydrologic
applications, and the goal of this study is to assess how
skillful they are for such applications. To do that, we force
a land surface model with both satellite estimates and ground-based
measurements and test how well they can predict hydrologic
states and fluxes useful for water resource applications,
i.e., soil moisture, evapotranspiration, and river streamflow.
A number of satellite-based precipitation products ranging
from retrievals based only on microwave measurements, combined
microwave + infrared estimates,
|
to
gauge-corrected products are tested over the entire continental
United States region. As a reference to the satellite retrievals,
estimates from global and regional weather model reanalyses
(the analysis fields from these models) are tested as well.
It is found that the microwave + infrared combined estimates
can match the skill of the coarse resolution European Center
for Medium-Range Weather Forecasts global reanalysis but not
the regional National Center for Environmental Predictions reanalysis.
Gauge corrections to satellite products significantly enhance
their skill by greatly reducing the bias in hydrologic predictions,
especially over mountainous areas. Rainfall errors are shown
to have strong impact on river streamflow predictions and column
total soil moisture and relatively weak impact on near surface
moisture and evapotranspiration. River streamflow experiments
also suggest that satellite rainfall errors are highly correlated
in space within the range of one storm system and thus do not
reduce in magnitude with spatial scale (basin size). |
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