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Agosto de 2009
How will Earth's surface temperature change in future decades?
Authors: Judith L. Lean and David H. Rind
Link: Click here
Abstract
Reliable forecasts of climate change in the immediate future
are difficult, especially on regional scales, where natural
climate variations may amplify or mitigate anthropogenic warming
in ways that numerical models capture poorly. By decomposing
recent observed surface temperatures into components associated
with ENSO, volcanic and solar activity, and
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anthropogenic influences, we anticipate global
and regional changes in the next two decades. From 2009 to
2014, projected rises in anthropogenic influences and solar
irradiance will increase global surface temperature 0.15 ±
0.03°C, at a rate 50% greater than predicted by IPCC.
But as a result of declining solar activity in the subsequent
five years, average temperature in 2019 is only 0.03 ±
0.01°C warmer than in 2014. This lack of overall warming
is analogous to the period from 2002 to 2008 when decreasing
solar irradiance also countered much of the anthropogenic
warming. We further illustrate how a major volcanic eruption
and a super ENSO would modify our global and regional temperature
projections.
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Agosto de 2009
Hydrological modeling of groundwater disturbances to observed
gravity: Theory and application to Asama Volcano, Central
Japan
Authors: T. Kazama and S. Okubo
Link: Click here
Abstract
The gravity disturbance caused by groundwater is derived based
on hydrological physics by solving nonlinear hydrological
diffusion equations for three-dimensional and temporal groundwater
distributions. The gravity disturbance is then estimated by
the spatial integral of the groundwater distributions. This
approach aims to resolve the problems of previous methods
for the correction of groundwater disturbances in gravity
data, such as instrumental drift of relative
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gravimeters,
empirical estimation of gravity assuming a linear gravity response
to precipitation, and the use of lower-dimensional water transfer
models. The disturbance estimated using the proposed model is
consistent with the observed gravity change at Asama Volcano
in Central Japan during the rainy summer of 2006. The model
reproduces the rapid increase and subsequent gradual decrease
in gravity following rainfall events. The water mass within
150 m of the gravimeter is shown to dominate the observed gravity
change during precipitation. It is also demonstrated that the
use of adequately representative soil parameters is essential
in order to accurately estimate the groundwater distributions
and consequent gravity variations. This study shows that correcting
for hydrological disturbances requires a more sophisticated
model of water movements, particularly during heavy rainfall.
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Agosto de 2009
A global sensitivity analysis and Bayesian inference framework
for improving the parameter estimation and prediction of a
process-based Terrestrial Ecosystem Model
Authors: Jinyun Tang, Qianlai Zhuang et
al
Link: Click here
Abstract
A global sensitivity analysis and Bayesian inference framework
was developed for improving the parameterization and predictability
of a monthly time step process-based biogeochemistry model.
Using a Latin Hypercube sampler and an existing Terrestrial
Ecosystem Model (TEM), a set of 500,000 Monte Carlo ensemble
simulations was conducted for a black spruce forest ecosystem.
A global sensitivity analysis was then conducted to identify
the key model parameters and examine the interaction structures
among TEM parameters. Bayesian inference analysis was also
performed using these ensemble simulations and eddy flux data
of carbon, latent heat flux, and MODIS gross primary production
(GPP) to reduce the uncertainty of parameter estimation and
prediction of TEM. We found that (1) the simulated carbon
fluxes are
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mostly affected by
parameters of the maximum rate of photosynthesis (CMAX), the
half-saturation constant for CO2 uptake by plants (k c), the
half- saturation constant for Photosynthetically Active Radiation
used by plants (k i), and the change in autotrophic respiration
due to 10°C temperature increase (RHQ10); (2) the effect
of parameters on seasonal carbon dynamics varies from one parameter
to another during a year; (3) to well constrain the uncertainties
of TEM predictions and parameters using the Bayesian inference
technique, at least two different fluxes of NEP, GPP, and ecosystem
respiration (RESP) are required; and (4) different assumptions
of the error structures of the flux data used in the Bayesian
inference analysis result in different uncertainty bounds of
the posterior parameters and model predictions. We further found
that, using the Bayesian framework and eddy flux and satellite
data, the uncertainty of simulated carbon fluxes has been remarkably
reduced. The developed global sensitivity analysis and Bayesian
framework could further be used to analyze and improve the predictability
and parameterization of relatively coarse time step biogeochemistry
models when the eddy flux and satellite data are available for
other terrestrial ecosystems. |
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Agosto de 2009
Roaming zones of precipitation on ancient Mars as recorded
in valley networks
Authors: Monica R. T. Hoke, Brian M. Hynek
et al
Link: Click here
Abstract
The 10 largest valley networks in the Terra Sabaea, Arabia
Terra, and Meridiani Planum regions of Mars were mapped, crater
age dated, and analyzed by geomorphology, stream order, and
drainage density to understand changes in fluvial erosion
during early Martian history. All of these networks demonstrate
characteristics consistent with formation by precipitation.
Many appear to be in different stages of preservation, with
both highly eroded and pristine valleys sometimes appearing
in the same network. In some cases, the valley network morphologies
andcrater age dating indicate multiple periods of formation.
The results from this research place
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precipitation-driven
formation of the Martian valley networks in Terra Sabaea, Arabia
Terra, and Meridiani Planum in the late Noachian and early Hesperian
epochs (~3.6-3.8 billion years ago). Our age estimates do not
extend into earlier or later Martian history, and the spread
in these ages indicates they did not all form or cease formation
at the same time. The difference in age between the oldest and
youngest valley networks in Terra Sabaea, Arabia Terra, and
Meridiani Planum analyzed in this work is ~210 ± 50 Ma.
Within this range are valley networks that have distinctly separate
ages and those that appear to be coeval. This research suggests
the late Noachian and early Hesperian were characterized by
roaming zones of precipitation that occurred during either continuously
warmer and thicker atmospheric conditions or intermittently
clement conditions, with precipitation occasionally returning
to previously rainy regions and overall continuing near Meridiani
Planum longer than in Terra Sabaea. |
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Agosto de 2009
Is the Martian water table hidden from radar view?
Authors: W. M. Farrell, J. J. Plaut et
al
Link: Click here
Abstract
Mars may possess a global sub-surface groundwater table as
an integral part of its current hydrological system. However,
the Mars Advanced Radar for Subsurface and Ionospheric Sounding
(MARSIS) onboard the Mars Express (MEx)
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spacecraft
has yet to make a definitive detection of such a body of liquid
water. In this work, we quantify the conditions that would allow
a detection of a deep aquifer and demonstrate that the lack
of radar detection does not uniquely rule out the presence of
such a body. Specifically, if the overlying crustal material
has a conductivity above ~10-5 S/m (equivalent to a loss tanget
of 0.008), a radar echo from an aquifer could be sufficiently
attenuated by the intervening medium to prevent its detection
by MARSIS. As such, the lack of direct detection by MARSIS-a
"null result"-does not rule out the possibility of
the water table's existence. |
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Agosto de 2009
Broadband seismic wave radiation process of the 2000 western
Tottori, Japan, earthquake revealed from wavelet domain inversion
Authors: Wataru Suzuki, Tomotaka Iwata
et al
Link: Click here
Abstract
We develop a new inversion method to obtain the low- and high-frequency
wave radiation processes whose border is 1 Hz. In this method,
we adopt a consecutive approach to evaluate the fitness between
the observed and synthetic strong motions and to synthesize
the waveforms. A new feature of our inversion is an evaluation
of the fitness of broadband strong motions, including the
high-frequency range, using the wavelet coefficient of velocity
waveforms based on the frequency-dependent characteristics
of seismic waves. Another new feature is the introduction
into the source inversion of a hybrid Green's function,
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which is constructed
from theoretical and empirical Green's functions for the low-
and high-frequency ranges, respectively. In order to reach the
global minimum by nonlinear inversion, we introduce a multiscale
approach for the low-frequency inversion. After a synthetic
test, the inversion method was applied to the 2000 western Tottori
earthquake for three frequency bands. The low-frequency inversion
(0.0625-1 Hz) reveals that the asperity extends from the southeast
of the hypocenter to the shallow part of the fault above the
hypocenter. The waves of 1-2 Hz are radiated most intensely
from the rupture starting area of the southeast asperity. On
the other hand, the higher-frequency waves of 2-4 Hz are radiated
most strongly from the edge of the southeast asperity. The inversion
result suggests a possibility that broadband strong motions
are generated from the asperities, and the high-frequency wave
radiations are particularly related to the initiation and termination
of the asperity rupture. |
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Agosto de 2009
Automated classification of visible and infrared spectra
using cluster analysis
Authors: G. A. Marzo, T. L. Roush et al
Link: Click here
Abstract
Planetary space experiments collect large volumes of data
whose scientific content requires understanding. Marzo et
al. (2006) presented an unsupervised cluster analysis scheme
that is able to reduce a spectral data set to a few clusters,
allowing for more focused and rapid evaluation of their scientific
meaning. Here, we extend the original approach to account
for the measurement
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uncertainty
and build a classification scheme. We apply the clustering technique
to the ASTER and RELAB libraries of visible and infrared spectral
reflectance. These spectral libraries are documented, allowing
assignment of a label to each spectrum reflecting its physical
and chemical properties. We assess the ability of the original
and extended approaches to identify natural clusters of the
library spectra and estimate associated uncertainties of the
results. We evaluate the scientific meaning of the derived clusters
based on the labels contained within each cluster. Once the
cluster meanings are defined, we test our classification scheme
using a training-testing approach and evaluate the accuracy
of assigning the unknown spectra to the correct cluster. |
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Agosto de 2009
A possible generation mechanism of interplanetary rotational
discontinuities
Authors: C. C. Lin, C. L. Tsai et al
Link: Click here
Abstract
In the present paper, we first examine some interplanetary
directional discontinuities with very small B n /B (<0.1)
using intraspacecraft timing method. It is found that the
velocity and magnetic field fluctuations of these directional
discontinuities satisfy the Walén relation. We suggest
that these directional discontinuities are rotational discontinuities.
In addition, we investigate the stability of interplanetary
rotational discontinuities using one-dimensional hybrid simulations
and found that rotational discontinuities with all values
of B n /B can stably
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exist in the solar
wind. In one simulation run, we find that the rotational discontinuity
(RD) is still stable when the ratio, B n /B, equals 0.0001.
Finally, from one-dimensional hybrid simulation, we further
find that the ratio is significantly reduced after interaction
with interplanetary fast shocks. There are a few mechanisms
for generation of RDs. Among them, two mechanisms are well accepted.
One is nonlinear evolution of Alfvén waves in the solar
wind, and another is magnetic reconnection near the solar surface.
For magnetic reconnection, the reconnection rate, V 1n/V A1(=
B n /B), in the magnetosphere and solar wind, is usually <0.2.
Therefore the generated RDs also have B n /B < 0.2. On the
other hand, the nonlinear evolution of Alfvén waves in
the solar wind can generate RDs at all values of B n /B, which
contradicts to the Cluster results. We suggest that interplanetary
RDs with small B n /B are likely been generated through magnetic
reconnection. |
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Agosto de 2009
Obtaining parsimonious hydraulic conductivity fields using
head and transport observations: A Bayesian geostatistical
parameter estimation approach
Authors: M. Fienen, R. Hunt et al
Link: Click here
Abstract
Flow path delineation is a valuable tool for interpreting
the subsurface hydrogeochemical environment. Different types
of data, such as groundwater flow and transport, inform different
aspects of hydrogeologic parameter values (hydraulic conductivity
in this case) which, in turn, determine flow paths. This work
combines flow and transport information to estimate a unified
set of hydrogeologic parameters using the Bayesian geostatistical
inverse approach. Parameter flexibility is allowed by using
a highly
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parameterized
approach with the level of complexity informed by the data.
Despite the effort to adhere to the ideal of minimal a priori
structure imposed on the problem, extreme contrasts in parameters
can result in the need to censor correlation across hydrostratigraphic
bounding surfaces. These partitions segregate parameters into
facies associations. With an iterative approach in which partitions
are based on inspection of initial estimates, flow path interpretation
is progressively refined through the inclusion of more types
of data. Head observations, stable oxygen isotopes (18O/16O
ratios), and tritium are all used to progressively refine flow
path delineation on an isthmus between two lakes in the Trout
Lake watershed, northern Wisconsin, United States. Despite allowing
significant parameter freedom by estimating many distributed
parameter values, a smooth field is obtained. |
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Agosto de 2009
European climate variations over the past half-millennium
reconstructed from groundwater
Authors: J. A. Corcho Alvarado, F. Barbecot
et al
Link: Click here
Abstract
Temperature reconstructions for recent centuries are the basis
of estimations of the natural variability in the climate system
before and during the onset of anthropogenic perturbation.
Here we present, for the first time, an independent and physically
based reconstruction of mean annual
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temperature over the
past half millennium obtained from groundwater in France. The
reconstructed noble gas temperature (NGT) record suggests cooler
than present climate conditions throughout the 16th-19th centuries.
Periods of warming occur in the 17th-18th and 20th century,
while cooling is reconstructed in the 19th century. A noticeable
coincidence with other temperature records is demonstrated.
Deuterium excess varies in parallel with the NGT, and indicates
variation in the seasonality of the aquifer recharge; whereas
high excess air in groundwater indicates periods with high oscillations
of the water table. |
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Agosto de 2009
Detecting hydrologic deformation using GRACE and GPS
Authors: P. Tregoning, C. Watson et al
Link: Click here
Abstract
Hydrological processes cause variations in gravitational potential
and surface deformations, both of which are detectable using
space geodetic techniques. We computed elastic deformation
using continental water load estimates derived from the Gravity
Recovery and Climate Experiment and compared to 3D deformation
estimated from
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GPS
observations. The agreement is very good in areas where large
hydrologic signals occur over broad spatial scales, with correlation
in horizontal components as high as 0.9. Agreement is also observed
at smaller scales, including across Europe. This suggests that:
a) both techniques are perhaps more accurate than previously
thought and b) a large percentage of the non-linear variations
seen in our GPS time series are most likely related to geophysical
processes rather than analysis error. Low correlation at some
sites suggests that local processes or site specific analysis
errors dominate the GPS deformation estimates rather than the
broad-scale hydrologic signals detected by GRACE. |
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Agosto de 2009
Escape of methane gas from the seabed along the West Spitsbergen
continental margin
Authors: Graham K. Westbrook, Kate E. Thatcher
et al
Link: Click here
Abstract
More than 250 plumes of gas bubbles have been discovered emanating
from the seabed of the West Spitsbergen continental margin,
in a depth range of 150-400 m, at and above the present
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upper limit of the
gas hydrate stability zone (GHSZ). Some of the plumes extend
upward to within 50 m of the sea surface. The gas is predominantly
methane. Warming of the northward-flowing West Spitsbergen current
by 1°C over the last thirty years is likely to have increased
the release of methane from the seabed by reducing the extent
of the GHSZ, causing the liberation of methane from decomposing
hydrate. If this process becomes widespread along Arctic continental
margins, tens of Teragrams of methane per year could be released
into the ocean. |
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Julio de 2009
Mapping upper mantle flow beneath French Polynesia from
broadband ocean bottom seismic observations
Authors: G. Barruol, D. Suetsugu et al
Link: Click here
Abstract
Upper mantle structures and flow related to the South Pacific
Superswell and to the associated short-lived hotspots are
investigated using seismic stations deployed in French Polynesia.
Beside island stations, broadband ocean bottom seismometers
(BBOBS) have been deployed
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between the various
Polynesian archipelagos to investigate the oceanic upper mantle
"unperturbed" by thickened crust or hotspot activity.
We investigate seismic anisotropy measured at BBOBS stations
and found it rather similar to that from island stations. This
confirms the ability of OBS to be used for anisotropy purposes
and suggests some homogeneity in the upper mantle pervasive
deformation. The two OBS installed on the head of the Society
hotspot provide different anisotropy signatures than most of
the other French Polynesian stations: one is "isotropic"
to SKS waves and the other provides a fast split direction normal
to the average value observed in this area, suggesting a more
complex mantle flow induced by the plume upwelling. |
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Julio de 2009
A brief review of "solar flare effects" on the
ionosphere
Authors: B. T. Tsurutani, O. P. Verkhoglyadova
et al
Link: Click here
Abstract
The study of solar flare effects (SFEs) on the ionosphere
is having a renaissance. The development of GPS ground and
satellite data for scientific use has opened up new means
for high time resolution research on SFEs. At present, without
continuous flare photon spectra (X rays, EUV, UV, and visible)
monitoring instrumentation, the best way to model flare spectral
changes within a flare is through ionospheric GPS studies.
Flare EUV photons can increase the total electron content
of the subsolar ionosphere by up to 30% in ~5 min. Energetic
particles (ions) of 10 keV to GeV energies are accelerated
at the flare site. Electrons with energies up to several MeV
are also created. A coronal mass ejection (CME) is launched
from the Sun at the time of the flare. Fast interplanetary
CMEs (ICMEs) have upstream shocks which accelerate ions to
~10 keV to ~10 MeV. Both sources of particles, when magnetically
connected to the Earth's magnetosphere, enter the magnetosphere
and the high-latitude and midlatitude ionosphere. Those particles
that
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precipitate
into the ionosphere cause rapid increases in the polar atmospheric
ionization, disruption of transpolar communication, and cause
ozone destruction. Complicating the picture, when the ICME reaches
the magnetosphere ~1 to 4 days later, shock compression of the
magnetosphere energizes preexisting 10-100 keV magnetospheric
electrons and ions, causing precipitation into the dayside auroral
zone (~60°-65° MLAT) ionospheres. Shock compression
can also trigger supersubstorms in the magnetotail with concomitant
energetic particle precipitation into the nightside auroral
zones. If the interplanetary sheath or ICME magnetic fields
are southwardly directed and last for several hours, a geomagnetic
storm will result. A magnetic storm is characterized by the
formation of an unstable ring current with energetic particles
in the range ~10 keV to ~500 keV. The ring current decays away
by precipitation into the middle latitude ionosphere over timescales
of ~10 h. A schematic of a time line for the above SFE ionospheric
effects is provided. Descriptions of where in the ionosphere
and in what time sequence is provided in the body of the text.
Much of the terminology presently in use describing solar, interplanetary,
magnetospheric, and ionospheric SFE-related phenomena are dated.
We suggest physics-based terms be used in the future. |
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Julio de 2009
Two-dimensional turbulence, space shuttle plume transport
in the thermosphere, and a possible relation to the Great
Siberian Impact Event
Authors: M. C. Kelley, C. E. Seyler et
al
Link: Click here
Abstract
During two solstices, Stevens et al. [2003, 2005] presented
satellite and ground-based observations that indicated that
the water vapor plume from the space shuttle's main engine
rapidly expanded and moved quickly to the
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summer pole. Once
there, the water vapor plume condensed into large noctilucent
cloud displays. Here we present a hypothesis for the plume's
rapid transport and anomalous horizontal diffusion. We argue
that this system is two-dimensionally turbulent, which has the
property of inverse cascade. This energy transport to large
scales explains the poleward transport, and the well-known system
properties explain the anomalous diffusion. Additionally, we
apply these results to the aftermath of the 1908 Great Siberian
Impact Event, when extremely bright night skies were observed
in Great Britain during the days following the impact. The impacting
object must have contained considerable ice and thus, was very
likely a comet. |
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Julio de 2009
Heating and cooling of the thermosphere by internal gravity
waves
Authors: Erdal Yigit and Alexander S. Medvedev
Link: Click here
Abstract
For the first time, estimates of heating and cooling in the
upper thermosphere due to dissipating and breaking gravity
waves (GWs) of tropospheric origin have been obtained with
a comprehensive general circulation model (GCM). A GW parameterization
specifically designed for thermospheric heights has been implemented
in
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the
CMAT2 GCM covering altitudes from the tropopause to the F 2
region, and simulations for the June solstice have been performed.
They reveal that the net thermal effect of GWs above the turbopause
is cooling. The largest (up to -170 K d-1 in a zonally and temporally
averaged sense) cooling takes place in the high latitudes of
both hemispheres near 210 km. The instantaneous values of heating
and cooling rates are highly variable, and reach up to 500 and
-3000 K d-1 in the F 2 region, respectively. Inclusion of the
GW thermal effects reduces the simulated model temperatures
by up to 200 K over the summer pole and by 100 to 170 K at other
latitudes near 210 km. |
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Julio de 2009
Solar trends and global warming
Authors: S. R. E. Benestad and G. A. Schmidt
Link: Click here
Abstract
We use a suite of global climate model simulations for the
20th century to assess the contribution of solar forcing to
the past trends in the global mean temperature. In particular,
we examine how robust different published
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methodologies are
at detecting and attributing solar-related climate change in
the presence of intrinsic climate variability and multiple forcings.
We demonstrate that naive application of linear analytical methods
such as regression gives nonrobust results. We also demonstrate
that the methodologies used by Scafetta and West (2005, 2006a,
2006b, 2007, 2008) are not robust to these same factors and
that their error bars are significantly larger than reported.
Our analysis shows that the most likely contribution from solar
forcing a global warming is 7 ± 1% for the 20th century
and is negligible for warming since 1980. |
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Julio de 2009
Dissection of trained neural network hydrologic models
for knowledge extraction
Authors: Ashu Jain and Sumant Kumar
Link: Click here
Abstract
Artificial neural networks (ANNs) are powerful tools for the
modeling and forecasting of complex engineering systems and
have been exploited by researchers to solve a variety of problems
over the last couple of decades. In spite of their proven
ability to provide superior model performance compared to
traditional modeling approaches, they have not become popular
among decision makers for operational use. It is probably
because of their perceived black box nature that does not
explain or consider the underlying physical
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processes
involved. This paper presents the results of a study aimed at
a systematic dissection of the massively parallel architectures
of trained ANN hydrologic models to determine if they learn
the underlying physical subprocesses during training. This has
been achieved using simple qualitative and quantitative techniques.
The data derived from three contrasting catchments at two different
time scales were employed to develop ANN models and test the
methodologies employed for knowledge extraction. The results
obtained in this study indicate that the number of hidden neurons
determined during training for a particular data set correspond
to certain subprocesses of the overall physical process being
modeled. It has been found that the time scale of the data employed
has an effect on optimum ANN architecture and knowledge extracted.
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Julio de 2009
Remote triggering of tremor along the San Andreas Fault
in central California
Authors: Zhigang Peng, John E. Vidale et
al
Link: Click here
Abstract
We perform a systematic survey of triggered tremor along the
San Andreas Fault in central California for the 31 teleseismic
earthquakes with Mw = 7.5 since 2001. We identify 10 teleseismic
events associated with clear triggered tremor. About 52% of
the tremor is concentrated south of Parkfield near Cholame,
where ambient tremor has been identified previously, and the
rest is widely distributed in the creeping section of the
San Andreas Fault north of Parkfield. Tremor is generally
initiated and is in phase with the Love wave particle velocity.
However, the pattern becomes complicated with the arrival
of the Rayleigh waves, and sometimes tremor
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continues after the
passage of the surface waves. We identify two cases in which
tremor is triggered during the teleseismic PKP phase. These
results suggest that while shear stress from the passage of
the Love waves plays the most important role in triggering tremor
in central California, other factors, such as dilatational stresses
from the Rayleigh and P waves, also contribute. We also examine
the ambient tremor occurrence rate before and after the teleseismic
events and find a transient increase of stacked tremor rate
during the passage of the teleseismic surface waves. This observation
implies that the occurrence time of tremor is temporally advanced
by the dynamic stresses of the teleseismic waves. The amplitude
of the teleseismic waves correlates with the occurrence of triggered
tremor, and the inferred tremor-triggering threshold is ~2-3
kPa. The relatively low triggering threshold indicates that
the effective stress at the tremor source region is very low,
most likely due to near-lithostatic fluid pressure. |
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Julio de 2009
Seismic modeling of multidimensional heterogeneity scales
of Mallik gas hydrate reservoirs, Northwest Territories of
Canada
Authors: Jun-Wei Huang, Gilles Bellefleur
et al
Link: Click here
Abstract
In hydrate-bearing sediments, the velocity and attenuation
of compressional and shear waves depend primarily on the spatial
distribution of hydrates in the pore space of the subsurface
lithologies. Recent characterizations of gas hydrate accumulations
based on seismic velocity and attenuation generally assume
homogeneous sedimentary layers and neglect effects from large-
and small-scale heterogeneities of hydrate-bearing sediments.
We present an algorithm, based on stochastic medium theory,
to construct heterogeneous multivariable models that mimic
heterogeneities of hydrate-bearing sediments at the level
of detail provided by borehole logging data. Using this algorithm,
we model some key
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petrophysical
properties of gas hydrates within heterogeneous sediments near
the Mallik well site, Northwest Territories, Canada. The modeled
density, and P and S wave velocities used in combination with
a modified Biot-Gassmann theory provide a first-order estimate
of the in situ volume of gas hydrate near the Mallik 5L-38 borehole.
Our results suggest a range of 528 to 768 × 106 m3/km2
of natural gas trapped within hydrates, nearly an order of magnitude
lower than earlier estimates which did not include effects of
small-scale heterogeneities. Further, the petrophysical models
are combined with a 3-D finite difference modeling algorithm
to study seismic attenuation due to scattering and leaky mode
propagation. Simulations of a near-offset vertical seismic profile
and cross-borehole numerical surveys demonstrate that attenuation
of seismic energy may not be directly related to the intrinsic
attenuation of hydrate-bearing sediments but, instead, may be
largely attributed to scattering from small-scale heterogeneities
and highly attenuate leaky mode propagation of seismic waves
through larger-scale heterogeneities in sediments. |
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Julio de 2009
Seven years of postseismic deformation following the 1999,
M = 7.4 and M = 7.2, Izmit-Düzce, Turkey earthquake sequence
Authors: S. Ergintav, S. McClusky et al
Link: Click here
Abstract
We report the results of nearly 7 years of postseismic deformation
measurements using continuously recorded and survey mode GPS
observations for the 1999 Izmit-Düzce earthquake sequence.
Resolvable, time-dependent postseismic changes to the preearthquake
interseismic velocity field extend at least as far as the
continuous GPS station in Ankara, ~200 km southeast of the
Izmit rupture. Seven years after the earthquake sequence,
the relative postseismic velocity across the North Anatolian
Fault (NAF) reaches ~10-12 mm/a, roughly 50% of
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the steady state interseismic
rate, with the highest postseismic velocities within 40 km of
the coseismic ruptures. We use a sequence of logarithmic time
functions to fit GPS site motions. Up to three logarithmic terms
with decay constants of 1, 150, and 3500 days are necessary
to fit all the transient motion observed at the continuous GPS
stations. The first term is required for the component of site
motion parallel to the NAF at near-field sites strongly implicating
rapid, shallow afterslip. The intermediate and longer-term postseismic
velocity components reflect more broadly distributed strain
with a symmetric double-couple pattern suggestive of either
localized, deep afterslip or viscoelastic relaxation of the
upper mantle and/or lower crust. In two areas (including the
Marmara Sea) this pattern is superimposed on north-south extension
centered on the NAF. We speculate that this extension may result
from aseismic dip slip along coseismically weakened faults,
driven by the background tectonic stress. |
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Julio de 2009
Earthquake ruptures with thermal weakening and the operation
of major faults at low overall stress levels
Authors: Hiroyuki Noda, Eric M. Dunham
et al
Link: Click here
Abstract
We model ruptures on faults that weaken in response to flash
heating of microscopic asperity contacts (within a rate-and-state
framework) and thermal pressurization of pore fluid. These
are arguably the primary weakening mechanisms on mature faults
at coseismic slip rates, at least prior to large slip accumulation.
Ruptures on strongly rate-weakening faults take the form of
slip pulses or cracks, depending on the background stress.
Self-sustaining slip pulses exist within a narrow range of
stresses: below this range, artificially nucleated ruptures
arrest; above this range,
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ruptures
are crack-like. Natural earthquakes will occur as slip pulses
if faults operate at the minimum stress required for propagation.
Using laboratory-based flash heating parameters, propagation
is permitted when the ratio of shear to effective normal stress
on the fault is 0.2-0.3; this is mildly influenced by reasonable
choices of hydrothermal properties. The San Andreas and other
major faults are thought to operate at such stress levels. While
the overall stress level is quite small, the peak stress at
the rupture front is consistent with static friction coefficients
of 0.6-0.9. Growing slip pulses have stress drops of ~3 MPa;
slip and the length of the slip pulse increase linearly with
propagation distance at ~0.14 and ~30 m/km, respectively. These
values are consistent with seismic and geologic observations.
In contrast, cracks on faults of the same rheology have stress
drops exceeding 20 MPa, and slip at the hypocenter increases
with distance at ~1 m/km. |
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Julio de 2009
Evolución de los esfuerzos en el Sistema de Fallas
de San Andrés: Intervalos de recurrencia v/s profundidad
de bloqueo.
Authors: Bridget Smith-Konter and David
Sandwell
Link: Click here
Abstract
Las más grandes rupturas a lo largo del Sistema de
Fallas de San Andrés (SFSA) se han orientado de acuerdo
con la tensión que se ha acumulado en el sector superior
de la sub-corteza. En la actualidad la tasa de acumulación
de esfuerzos en un sector específico de la falla es
bien modelado mediante las actuales mediciones geodésicas.
Se cree que los esfuerzos varían entre 0.5 y 7 MPa
por siglo y que son inversamente proporcionales a los periodos
de recurrencia de los terremotos. Por otro lado, la tensión
total acumulada en un sector específico de la falla
es difícil de modelar debido a que depende de las incertezas
asociadas a la historia de las rupturas ocurridas durante
los últimos miles de años. Nosotros simulamos
la tensión acumulada en la corteza para las condiciones
pasadas y presentes asumiendo la completa liberación
del desplazamiento durante las rupturas más grandes.
Estos resultados especulativos indican que la zona sur de
San Andrés, que no ha experimentado grandes rupturas
desde hace más de 300 años, está actualmente
acercándose al máximo nivel de tensión
que puede soportar.
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Julio de 2009
Stress evolution of the San Andreas fault system: Recurrence
interval versus locking depth
Authors: Bridget Smith-Konter and David
Sandwell
Link: Click here
Abstract
Major ruptures along the San Andreas Fault System (SAFS) are
driven by stress that has accumulated in the upper locked
portion of the crust. The present-day stress accumulation
rate on any given fault segment is fairly well resolved by
current geodetic measurements. Model stress accumulation rates
vary between 0.5 and 7 MPa per century and are inversely proportional
to earthquake recurrence intervals. In contrast, the total
accumulated stress on a given fault segment is poorly resolved
since it depends on the uncertain rupture history of each
fault over the past few thousand years. We simulate accumulated
stress at crustal depths for both past and present-day conditions
by assuming complete release of accumulated slip deficit during
major ruptures. These speculative results indicate that the
southern San Andreas, which has not ruptured in a major earthquake
in over 300 years, is currently approaching a threshold stress
level.
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Julio de 2009
Auroral footprint of Ganymede
Authors: Denis Grodent, Bertrand Bonfond
et al
Major Link: Click here
Abstract
The interaction of Ganymede with Jupiter's fast rotating magnetospheric
plasma gives rise to a current system producing an auroral
footprint in Jupiter's ionosphere, usually referred to as
the Ganymede footprint. Based on an analysis of ultraviolet
images obtained with the Hubble Space Telescope, we demonstrate
that the auroral de's footprint of the order of ~0.2 to ~1.5
GW. footprint surface matches a circular region in
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Ganymede's orbital
plane having a diameter of 8-20 RG. Temporal analysis of the
auroral power of Ganymede's footprint reveals variations of
different timescales: (1) a 5-hour timescale associated with
the periodic flapping of Jupiter's plasma sheet over Ganymede,
(2) a 10-40 minute timescale possibly associated with energetic
magnetospheric events, such as plasma injections, and (3) a
100-s timescale corresponding to quasiperiodic fluctuations,
which might relate to bursty reconnections on Ganymede's magnetopause
and/or to the recurrent presence of acceleration structures
above Jupiter's atmosphere. These three temporal components
produce an auroral power emitted at Ganymede. |
|
Julio de 2009
Analyzing the variation of Lyapunov exponents of solar
and geomagnetic activity indices during coronal mass ejections
Authors: M. Mirmomeni and C. Lucas
Link: Click here
Abstract
Hostile space weather is one of the principal threats to modern
human technology because of the dependency of our lives on
satellites. Solar coronal mass ejections, solar flares, and
high-speed solar wind streams often cause some sequences of
damaging disturbances within the Earth's magnetosphere, in
the atmosphere, and even on the Earth's surface. The time-varying
Sun, as the main source of space weather causes geomagnetic
storms and substorms in many ways with coronal mass ejections
(CME) being possibly the most harmful among them. CMEs cause
transient magnetized plasma flows as interplanetary consequences,
which drive the geomagnetic storms and substorms in the Earth's
magnetosphere. It is shown that the cyclic solar activity
has chaotic characteristics. It seems that the chaotic modeling
of solar and geomagnetic
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activity
indices is an important approach for analysis and prediction
of the solar extreme events. This is particularly useful for
engineers and designers interested in space weather prediction
and its applications. One of the most important tools for eliciting
the chaotic trends is the "Lyapunov exponents," which
is a useful measure of the stability of a dynamic system. This
paper analyzes the variation of Lyapunov exponents (LE) for
three solar and geomagnetic activity indices during CMEs: the
sunspot number, disturbance storm time, and proton temperature.
The analysis has been carried out using the technique of adaptive
LE estimation adopted from previous works; the suitable window
for which has been determined via trial and error. It is shown
that the LE of these solar and geomagnetic activity indices
varies rapidly during CMEs. The variation in LEs creates a pattern
as a precursor for the forthcoming CME. This precursor, which
is an oscillation in the values of Les, begins several steps
sooner than the CME's occurrence. Then, during the CME, the
LEs decrease to a small positive or a negative value, which
demonstrates that during an anomaly such as a CME the chaotic
characteristics of solar and geomagnetic activity indices decrease
and solar and geomagnetic activity indices follow more regular
dynamics. |
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Julio de 2009
Magnetospheric cavity modes driven by solar wind dynamic
pressure fluctuations
Authors: S. G. Claudepierre, M. Wiltberger
et al
Link: Click here
Abstract
We present results from Lyon-Fedder-Mobarry (LFM) global,
three-dimensional magnetohydrodynamic (MHD) simulations of
the solar wind-magnetosphere interaction. We use these simulations
to investigate the role that solar wind dynamic pressure fluctuations
play in the generation of magnetospheric ultra-low frequency
(ULF) pulsations. The simulations presented in this study
are driven with idealized solar wind input conditions. In
four of the simulations, we
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introduce monochromatic
ULF fluctuations in the upstream solar wind dynamic pressure.
In the fifth simulation, we introduce a continuum of ULF frequencies
in the upstream solar wind dynamic pressure fluctuations. In
this numerical experiment, the idealized nature of the solar
wind driving conditions allows us to study the magnetospheric
response to only a fluctuating upstream dynamic pressure, while
holding all other solar wind driving parameters constant. The
simulation results suggest that ULF fluctuations in the solar
wind dynamic pressure can drive magnetospheric ULF pulsations
in the electric and magnetic fields on the dayside. Moreover,
the simulation results suggest that when the driving frequency
of the solar wind dynamic pressure fluctuations matches one
of the natural frequencies of the magnetosphere, magnetospheric
cavity modes can be energized. |
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Julio de 2009
Particle dynamics description of "BGK collisions"
as a Poisson process
Authors: Marco Milla and Erhan Kudeki
Link: Click here
Abstract
The Gordeyev integral for plasma particles colliding with
neutrals is obtained using a particle dynamics formalism in
which the collisions are modeled as a discrete Poisson process.
The
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result leads to an
electron density fluctuation spectrum model for partially ionized
plasmas which is identical with the spectral model obtained
from BGK plasma kinetic equations. This isomorphism between
the Poisson process and the BGK operator is analogous to a similar
relation between the Brownian motion process and the Fokker-Planck
operator with constant coefficients. We take advantage of this
analogy to derive a collisional ISR spectrum model that takes
into account collisions with both neutrals and charged species.
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Junio de 2009
Effect of thermal refraction on heat flow near the San
Andreas Fault, Parkfield, California
Authors: Patrick M. Fulton, Demian M. Saffer
et al
Link: Click here
Abstract
Heat flow data near the San Andreas Fault (SAF) do not reveal
a near-fault anomaly as expected from frictional heat generation,
an observation interpreted to indicate that the fault slips
at a depth-averaged shear stress <20 MPa. The data also
contain large unexplained scatter, which has been a separate
major issue in the analysis of heat flow within the California
Coast Ranges. Here we use numerical models of heat conduction
to evaluate the hypothesis that thermal refraction, due to
contrasts in thermal conductivity in the subsurface, both
produces the observed
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scatter
in heat flow and as a result obscures the thermal signature
from frictional heating on a fault that supports large shear
stress during slip. Our study focuses on the region around the
San Andreas Fault Observatory at Depth (SAFOD) near Parkfield,
California. Our results show that surface heat flow is most
sensitive to the contrast between Tertiary sediments and basement
rocks and to wavelengths of basement topography of ~10 km. With
realistic thermal conductivity contrasts and a reasonable interpretation
of this geologic contact, we show that thermal refraction is
a plausible explanation for the observed heat flow scatter.
However, refraction effects are unable to mask frictional heat
generation in a manner consistent with observations. We show
that even with large refraction effects, low background heat
flow, a regional NW-SE decrease in heat flow, or nonsteady state
heat conduction, the data are most consistent with a fault that
produces little to no frictional heat. |
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Junio de 2009
A complex earthquake sequence captured by the continuous
GPS network in SW Iceland
Authors: S. Hreinsdóttir, T. Árnadóttir
et al
Link: Click here
Abstract
A complex sequence of earthquakes struck the western part
of the South Iceland Seismic Zone (SISZ) on 29 May 2008. The
sequence initiated with a M w6.3 (NEIC) earthquake in the
western part of the SISZ. Aftershocks from the earthquake
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delineate two parallel
N-S trending structures 4 km apart, in addition to activity
along an E-W zone further westward. Continuous GPS measurements
can best be explained by right-lateral strike-slip motion on
two parallel N-S trending faults, with little slip occurring
on other structures illuminated by earthquake activity. We estimate
a total moment release of M w6.2, with M w6.1 on the first rupture
and M w6.0 on the second rupture. High rate (1 Hz) CGPS data
from a near-field station suggest that the main asperity on
the Kross fault ruptured within 3 s of the initial mainshock
on the Ingólfsfjall fault. |
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Junio de 2009
Wormhole formation in dissolving fractures
Authors: P. Szymczak and A. J. C. Ladd
Link: Click here
Abstract
We investigate the dissolution of artificial fractures with
three-dimensional, pore-scale numerical simulations. The fluid
velocity in the fracture space
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was
determined from a lattice Boltzmann method, and a stochastic
solver was used for the transport of dissolved species. Numerical
simulations were used to study conditions under which long conduits
(wormholes) form in an initially rough but spatially homogeneous
fracture. The effects of flow rate, mineral dissolution rate,
and geometrical properties of the fracture were investigated,
and the optimal conditions for wormhole formation were determined.
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Junio de 2009
Are historical records sufficient to constrain ENSO simulations?
Author: Andrew T. Wittenberg
Link: Click here
Abstract
A control simulation of the GFDL CM2.1 global coupled GCM,
run for 2000 years with its atmospheric composition, solar
irradiance, and land cover held fixed at 1860 values, exhibits
strong interdecadal and intercentennial
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modulation of its
ENSO behavior. To the extent that such modulation is realistic,
it could attach large uncertainties to ENSO metrics diagnosed
from centennial and shorter records - with important implications
for historical and paleo records, climate projections, and model
assessment and intercomparison. Analysis of the wait times between
ENSO warm events suggests that such slow modulation need not
require multidecadal memory; it can arise simply from Poisson
statistics applied to ENSO's interannual time scale and seasonal
phase-locking. |
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Junio de 2009
Evidence for preservation of crustal root beneath the Proterozoic
Lapland-Kola orogen (northern Fennoscandian shield) derived
from P and S wave velocity models of POLAR and HUKKA wide-angle
reflection and refraction profiles and FIRE4 reflection transect
Authors: Tomasz Janik, Elena Kozlovskaya
et al
Link: Click here
Abstract
We present an analysis of the crust and upper mantle in the
northern Fennoscandian shield, based on new P and S wave 2-D
velocity models of the POLAR and HUKKA wide-angle reflection
and refraction profiles and results of a new seismic reflection
experiment in Finland (Finnish Reflection Experiment). The
profiles are almost collocated and crossed the Proterozoic
Lapland-Kola orogen. A substantial difference of the depth
of "the wide-angle Moho" (40-42 km) and "the
reflection Moho" (47-50 km) was found. In order to explain
this difference, we compared the velocity models to published
values of Vp and Vp/Vs for
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the
main types of lower crustal and mantle rocks. We found that
the main reason for disagreement is that the wide-angle Moho
and the reflection Moho correspond to different petrological
boundaries. In the southwest and northeast portions of the profiles,
the wide-angle Moho marks contact of either anorthositic or
granulitic lower crust with a reflective layer in the upper
mantle composed of peridotites and pyroxenites. The reflection
Moho represents the bottom of this lower layer. In the center
of the profile the wide-angle Moho marks the top of a large
eclogitic body in the upper mantle, representing a well-preserved
crustal root beneath the Lapland-Kola orogen formed because
of the collision of three former Archaean crustal blocks (terranes
or microcontinents). Lack of postorogenic tectonic collapse
suggests another mechanism for stabilization of the lithosphere
in the area. Upper mantle reflectors at depth of 65-75 km may
mark the upper boundary of the cold and mechanically strong
Archaean upper mantle wedge. Alternatively, these reflectors
may represent a top of uplifted asthenosphere that can explain
preservation of crustal root. |
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Junio de 2009
Near-field tsunami forecasting from cabled ocean bottom
pressure data
Authors: Hiroaki Tsushima, Ryota Hino et
al
Link: Click here
Abstract
We propose a method for near-field tsunami forecasting from
data acquired by cabled offshore ocean bottom tsunami meters
(OBTMs) in real time. We first invert tsunami waveforms recorded
at OBTMs to estimate the spatial distribution of initial sea-surface
displacements in the tsunami source region without making
any assumptions about fault geometry and earthquake magnitude.
Then, we synthesize the coastal tsunami waveforms from the
estimated sea-surface displacement distribution. To improve
the reliability of the tsunami forecasting, we use updated
OBTM data to repeat the forecast calculation at 1-min intervals.
We tested our
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method
by simulating the 1896 Sanriku tsunami earthquake, which caused
a devastating tsunami with maximum runup height of 38 m along
the Pacific coast of northeastern Japan. Instead of real OBTM
records, proxies were used. The simulation demonstrated that
our method provided accurate estimations of coastal arrival
times and amplitudes of the first peak of the tsunami more than
20 min before the maximum amplitude wave reached the coastal
site nearest to the source. We also applied the method to real
data of a small tsunami that was caused by a local earthquake
and successfully forecasted the tsunami at coastal tide stations.
We found that accuracy of our estimated coastal tsunami amplitudes
can be affected by the spatial relationship between the tsunami
source and the offshore observation stations. Our numerical
simulation showed that even more accurate tsunami amplitude
forecasts would be achieved by deployment of additional offshore
stations separated by a distance comparable to the trench-parallel
length of the tsunami source. |
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Junio de 2009
Dynamic modeling of slow earthquakes based on thermoporoelastic
effects and inelastic generation of pores
Authors: Takehito Suzuki and Teruo Yamashita
Link: Click here
Abstract
We numerically simulate features of slow earthquakes to understand
their generation mechanism in the framework of dynamic modeling.
Their typical features will be that the fault rupture velocity
and stress drop are markedly lower than those of ordinary
earthquakes. We assume a fault in a thermoporoelastic medium
taking account of fluid flow and inelastic creation of pores
on the fault. This paper is an extension of our studies published
in the work of Suzuki and Yamashita (2006, 2007, 2008), in
which a
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nondimensional parameter
S u was shown to play a critical role in dynamic fault rupture.
The parameter S u represents the dominance of the effect of
inelastic pore creation over that of frictional heating under
the condition of no fluid flow. However, it can be shown that
S u plays an important role even if the fluid flow is considered.
In the present study we successfully simulate slow fault rupture
growth and low stress drop, which characterize the slow earthquakes.
Critical ingredients of our modeling are assumptions of (1)
S u considerably larger than assumed for the simulation of ordinary
earthquakes, (2) fluid flow into the inelastically created pores,
and (3) initial shear stress significantly smaller than assumed
for the simulation of ordinary earthquakes. The assumption of
a large value of S u corresponds to that of a slip-resistant
and low-stress-drop zone. The fluid inflow can promote the fault
rupture in the slip-resistant zone. |
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Junio de 2009
Ability of 1-Hz GPS data to infer the source process of
a medium-sized earthquake: The case of the 2008 Iwate-Miyagi
Nairiku, Japan, earthquake
Authors: Yusuke Yokota, Kazuki Koketsu
et al
Link: Click here
Abstract
We demonstrate the ability of 1-Hz Global Positioning System
(GPS) data to infer a medium-sized (M6 class) earthquake source
process through an inversion study of the 2008 Iwate-Miyagi
Nairiku, Japan, earthquake (M~6.9). 1-Hz
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GPS
waveforms for this earthquake are compared, and show good agreement
with strong motion waveforms. Ground motions are well recorded
at a sufficient number of GPS stations in and around source
region, allowing us to perform waveform inversion using 1-Hz
GPS data only. The 1-Hz GPS inversion result shows fairly good
agreement with a joint inversion of geodetic and strong motion
data, such as the seismic moment (Mw~6.9), cumulative slip distribution,
and rupture propagation. Such agreements demonstrate that 1-Hz
GPS is able to infer the dynamic features of the rupture process
as well as permanent (or static) slip even for a M6 class medium-sized
earthquake. |
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Junio de 2009
Effects of ejecta accumulation on the crater population
of asteroid 433 Eros
Authors: C. Blitz, P. Lognonné et
al
Link: Click here
Abstract
The crater population of asteroid 433 Eros exhibits a deficit
in small crater diameters that has been suggested to result
from impact-induced seismic shaking initiating downslope movements
of regolith material, covering these small craters. As in
lunar maria, saturation equilibrium was expected to characterize
the crater population of Eros, but was surprisingly not shown
by the data set. The surface of Eros displays evidence of
burial by regolith especially for boulders, suggesting that
ejecta coverage erases the craters in addition to seismic
shaking erasure. In this work we investigate the production
and
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erasure of craters
by impact ejecta and compare derived crater size distributions
with those measured for Eros. We simulate a bombardment of Eros
by an impactor population derived from the Main Asteroid Belt
and estimate the crater and ejecta characteristics with a scaling
law, allowing ejecta to progressively create a regolith blanket.
Assuming the contribution of the ejecta blanketing process only,
we find a good agreement between the simulated and the observed
population of 250 m to 4 km diameter craters for exposure times
of 600 Ma and 400 Ma. This suggests a major impact or breakup
that occurred about 500 Ma ago, inducing a surface reset. A
mismatch for craters with a diameter smaller than ~100 m remains,
indicating that seismic shaking (or another erasure process)
is still necessary to explain their low number. Our simulations
emphasize the importance of an accurate modeling of both processes
to fully understand and interpret the small body size-frequency
crater curves. |
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Junio de 2009
An evaluation of real-time air quality forecasts and their
urban emissions over eastern Texas during the summer of 2006
Second Texas Air Quality Study field study
Authors: S. McKeen, G. Grell et al
Link: Click here
Abstract
Forecasts of ozone (O3) and particulate matter (diameter less
than 2.5 µm, PM2.5) from seven air quality forecast
models (AQFMs) are statistically evaluated against observations
collected during August and September of 2006 (49 days) through
the Aerometric Information Retrieval Now (AIRNow) network
throughout eastern Texas and adjoining states. Ensemble O3
and PM2.5 forecasts created by combining the seven separate
forecasts with equal weighting, and simple bias-corrected
forecasts, are also evaluated in terms of standard statistical
measures, threshold statistics, and variance analysis. For
O3 the models and ensemble generally show statistical skill
relative to persistence for the entire region, but fail to
predict high-O3 events in the Houston region. For PM2.5, none
of the models, or ensemble, shows statistical skill, and all
but one model have significant low bias. Comprehensive comparisons
with the full suite of chemical and aerosol measurements collected
aboard the NOAA WP-3 aircraft during the summer 2006 Second
Texas Air Quality Study and the Gulf of Mexico Atmospheric
Composition and Climate Study (TexAQS II/GoMACCS) field study
are performed to help diagnose sources of model bias at the
surface. Aircraft flights specifically designed for sampling
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of
Houston and Dallas urban plumes are used to determine model
and observed upwind or background biases, and downwind excess
concentrations that are used to infer relative emission rates.
Relative emissions from the U.S. Environmental Protection Agency
1999 National Emission Inventory (NEI-99) version 3 emissions
inventory (used in two of the model forecasts) are evaluated
on the basis of comparisons between observed and model concentration
difference ratios. Model comparisons demonstrate that concentration
difference ratios yield a reasonably accurate measure (within
25%) of relative input emissions. Boundary layer height and
wind data are combined with the observed up-wind and downwind
concentration differences to estimate absolute emissions. When
the NEI-99 inventory is modified to include observed NOy emissions
from continuous monitors and expected NOx decreases from mobile
sources between 1999 and 2006, good agreement is found with
those derived from the observations for both Houston and Dallas.
However, the emission inventories consistently overpredict the
ratio of CO to NOy. The ratios of ethylene and aromatics to
NOy are reasonably consistent with observations over Dallas,
but are significantly underpredicted for Houston. Excess ratios
of PM2.5 to NOy reasonably match observations for most models
but the organic carbon fraction of PM2.5 is significantly underpredicted,
pointing to compensating error between secondary organic aerosol
(SOA) formation and primary emissions within the models' photochemistry
and emissions. Rapid SOA formation associated with both Houston
and Dallas is inferred to occur within 1 to 3 h downwind of
the urban centers, and none of the models reproduce this feature.
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Junio de 2009
A case study of resistivity and self-potential signatures
of hydrothermal instabilities, Inferno Crater Lake, Waimangu,
New Zealand
Authors: A. Legaz, J. Vandemeulebrouck
et al
Link: Click here
Abstract
Inferno Crater Lake, Waimangu, one of the largest hot springs
in New Zealand, displays vigorous cyclic behavior in lake
level and temperature. It provides a natural small-scale laboratory
for investigating the geo-electrical signature of fluid
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flows. We measured
self-potential and electrical resistivity to see whether the
huge variations of fluid volume, approximately 60,000 m3 during
a mean cycle period of 40 days, could be detected. Electrical
resistivity measurements revealed spectacular changes over time,
with the medium becoming more conductive as the lake receded.
This result is consistent with analog models, where the vapor
phase is replaced by liquid at recession. The self-potential
survey did not detect temporal changes related to fluid movements.
This can be explained by the pH of the pore water (~2.3), which
is close to the point of zero charge of silica. |
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Junio de 2009
Sudden drop of seismic velocity after the 2004 Mw 6.6 mid-Niigata
earthquake, Japan, observed with Passive Image Interferometry
Authors: U. Wegler, H. Nakahara et al
Link: Click here
Abstract
Passive Image Interferometry (PII) is a seismological method
employing ambient seismic noise to monitor the temporal evolution
of mean shear wave velocity within the Earth. First, the elastic
Green's tensor between two seismometers is computed from the
cross correlation of seismic noise recorded during a certain
period. Second, the constructed seismograms of different time
periods are treated as earthquake multiplets, and small time
shifts in
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their
coda are used to invert a relative change in mean shear wave
velocity. When applied to the source region of the 2004 M w
= 6.6 mid-Niigata earthquake, Japan (centroid depth 5 km), we
used noise recorded at six seismometers located at a distance
of less than 25 km from the epicenter. Daily inversions during
the 2 months before and after the earthquake show a sudden drop
of mean shear wave velocity of some tenths of a percent at the
time of the mid-Niigata earthquake. Using noise in two frequency
bands, 0.1-0.5 Hz and 2-8 Hz, we find similar amplitudes for
the velocity drop, which indicates that changes are not restricted
to the shallow subsurface. Possible interpretations of this
velocity decrease are a decrease of crustal stress after the
earthquake, a nonlinear site response in the shallow subsurface
layer due to strong ground motion, or structural weakening due
to the creation of new fractures in the source area of the earthquake.
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Junio de 2009
Three-dimensional P wave attenuation and velocity upper
mantle tomography of the southern Apennines-Calabrian Arc
subduction zone
Authors: Stephen Monna, Torsten Dahm
Link: Click here
Abstract
We propose a 3-D crust-upper mantle seismic attenuation (Q
P ) model of the southern Apennines-Calabrian Arc subduction
zone together with a 3-D velocity (V P ) model. The Q P model
is calculated from relative t* using the spectral ratio method
and the V P from traveltime data. The final data set used
for the inversion of the V P model consists of 2400 traveltime
arrivals recorded by 34 short-period stations that are part
of the Italian National Seismic Network, and for the Q P model,
2178 Pn phases recorded by a subset
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of 32 stations. Traveltimes
and waveforms come from 272 intermediate-depth Calabrian slab
events. This 3-D model of attenuation, together with the 3-D
velocity model, improves our knowledge of the slab/mantle wedge
structure and can be a starting point in determining the physical
state of the asthenosphere (i.e., its temperature, the presence
of melt and/or fluids) and its relation to volcanism found in
the study area. Main features of the Q P and V P models show
that the mantle wedge/slab, in particular, the area of highest
attenuation, is located in a volume underlying the Marsili Basin.
The existence and shape of this main low-Q P (and low-V P )
anomaly points to slab dehydration and fluid/material flow,
a process that may explain the strong geochemical affinities
between the subduction-related magmas from Stromboli and Vesuvius.
Other interesting features in the models are strong lateral
variations in Q P and V P that are put in relation with known
important tectonic structures and volcanic centers in the area.
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Junio de 2009
Highly Sr radiogenic tholeiitic magmas in the latest inter-Plinian
activity of Santorini volcano, Greece
Authors: G. Vaggelli, M. Pellegrini et
al
Link: Click here
Abstract
Mineralogical, geochemical, and melt inclusion analyses have
been performed on scoria samples from a small scoria fall
deposit interbedded between the Cape Riva (21 ka) pyroclastic
deposits and the Minoan (3.6 ka) Plinian eruptions of Santorini
volcano and located near the resort of Imerovigli (25°25'14?/36°26'01?,
WGS84), close to the town of Phira. The scoria samples are
basaltic andesites with normally zoned olivine, reversely
zoned pyroxenes, and variably zoned plagioclase. Olivine contains
two types of melt inclusions. Type I inclusions, hosted in
cumulus Fo85-90 olivine, are primitive low-K basalts with
MgO 6-7 wt % and contain volatile element abundances of about
500 ppm F, 1000 ppm Cl, and 1300 ppm S. Low abundance of incompatible
elements and rare earth element patterns indicate an arc tholeiitic
composition. The
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d
18O in host olivine is low (5.3‰) and lower than the value
for the groundmass (6.5‰), whereas 87Sr/86Sr of the type
I inclusions is rather high (0.70579) and higher than the whole
rock and groundmass values (0.70465 and 0.70468, respectively).
Type II inclusions, hosted in Fo80-83 olivine rims, are calc-alkaline
andesites and have low volatile contents. Interstitial glasses
(groundmass) are similar in composition to the type II inclusions.
We propose that the tholeiitic, high Sr radiogenic, type I primitive
magma was generated by high partial melting degrees of a mid-ocean
ridge basalt (MORB)-like mantle wedge metasomatized by subducted
sediment melts and minor aqueous fluids. The input amount of
sediment melts and the degree of mantle melting decreased with
time, providing the parental magmas of the calc-alkaline, less
Sr radiogenic, type II andesitic melts. The scoria fall eruption
was fed by hybrid magma with multiple generations of olivine,
clinopyroxene, and plagioclase growth and occurred a short time
after the mixing process. This magma was probably last stored
in a small side reservoir, separated from the Cape Riva and
maybe from the Minoan reservoirs and was emplaced from a subaerial
vent close to Imerovigli by a Strombolian-type ephemeral eruption.
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Junio de 2009
Basement interpretations from airborne magnetic and gravity
data over the Lambert Rift region of East Antarctica
Authors: M. A. McLean, C. J. L. Wilson
et al
Link: Click here
Abstract
Geological exposures in the Lambert Rift region of East Antarctica
comprise scattered coastal outcrops and inland nunataks sporadically
protruding through the Antarctic ice sheet from Prydz Bay
to the southernmost end of the Prince Charles Mountains. This
study utilized airborne magnetic, gravity, and ice radar data
to interpret
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the distribution and
architecture of tectonic terranes that are largely buried beneath
the thick ice sheet. Free-air and Bouguer gravity data are highly
influenced by the subice and mantle topography, respectively.
Gravity stripping facilitated the removal of the effect of ice
and Moho, and the residual gravity data set thus obtained for
the intermediate crustal level allowed a direct comparison with
magnetic data. Interpretation of geophysical data also provided
insight into the distribution and geometry of four tectonic
blocks: namely, the Vestfold, Beaver, Mawson, and Gamburtsev
domains. These tectonic domains are supported by surface observations
such as rock descriptions, isotopic data sets, and structural
mapping. |
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Junio de 2009
Missing salts on early Mars
Authors: R. E. Milliken, W. W. Fischer
et al
Link: Click here
Abstract
Our understanding of the role of water on Mars has been profoundly
influenced over the past several years by the detection of
widespread aqueous alteration minerals. Clay minerals are
found throughout ancient Noachian terrains and sulfate salts
are abundant in younger Hesperian terrains, but these phases
are rarely found
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together
in the early Martian rock record. Full alteration assemblages
are generally not recognized at local scales, hindering our
ability to close mass balance in the ancient crust. Here we
demonstrate the dissolution of basalt and subsequent formation
of smectite results in an excess of cations that should reside
with anions such as OH-, Cl-, SO3 2-, SO4 2-, or CO3 2- in a
significant reservoir of complementary salts. Such salts are
largely absent from Noachian terrains, yet the composition and/or
fate of these 'missing salts' is critical to understanding the
oxidation state and primary atmospheric volatile involved in
crustal weathering on early Mars. |
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Junio de 2009
Viscosity of liquid ferric sulfate solutions and application
to the formation of gullies on Mars
Authors: Vincent F. Chevrier, Richard Ulrich
et al
Link: Click here
Abstract
We studied the viscosity of ferric sulfate Fe2(SO4)3 solutions
as a model for low-temperature liquids on the surface of Mars
and their implication in the formation of gullies. Viscosity
varies with temperature and concentration, ranging from 7.0
× 10-3 Pa s for 38.8 wt % at 285.15 K to 4.6 Pa s for
58.2 wt % at 260.15 K. Using the experimental results, we
built a semiempirical equation of viscosity as a function
of temperature and salt concentration, which was combined
with a numerical model to estimate the effect of these solutions
on the formation of gullies. Calculated fluid velocities ranged
from 0.5
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to 14 m s-1, in accordance
with estimates from image analyses. Turbulent flow occurs in
the majority of the conditions and is characterized by a constant
velocity (~8.5 m s-1). At very low temperature and high concentration,
the laminar regime shows reduced velocities (down to ~0.5 m
s-1). In between, a transitional regime presents high velocities,
up to 14 m s-1. Using the velocities, we determined the size
threshold for boulders to be moved by the liquid flow. Depending
on the regime, boulders of diameter inferior to 3 m (turbulent),
4 m (transition), and down to 0.5 m (laminar) are displaced.
Since laminar flow occurs only in an extremely limited range
of conditions, for low temperatures (<240 K) and supersaturated
solutions, the abundance of small boulders (~0.5 m) in gully
channels requires lower velocities and higher viscosities than
ferric sulfate solution or any other water-based liquid can
reach. This suggests an important participation of debris mixed
with the liquid phase. |
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Junio de 2009
Joint Bayesian model selection and parameter estimation
of the generalized extreme value model with covariates using
birth-death Markov chain Monte Carlo
Authors: Salaheddine El Adlouni, Taha B.
M. J. Ouarda et al
Link: Click here
Abstract
This paper describes Bayesian estimation of the parameters
of the generalized extreme value (GEV) model with covariates.
For this model the parameters of the GEV distribution are
functions of covariates, allowing for dependent parameters
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and/or
trends. A Markov chain Monte Carlo (MCMC) algorithm is generally
used to estimate the posterior distributions of the parameters
in a Bayesian framework. In this paper, the birth-death MCMC
(BDMCMC) procedure is developed in order to carry out both parameter
estimation and Bayesian model selection. The BDMCMC methods
allow the jump between models of different dimensions. The general
algorithm consists of two types of sampling steps. The first
one involves dimension-changing moves, and the second is conditional
on a fixed model. Parameters are estimated in a fully Bayesian
framework, and the model is selected by the length of time that
the MCMC chain remains in that model. Real and simulated data
sets illustrate the usefulness of the proposed methodology.
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Junio de 2009
Coseismic surface-ruptures and crustal deformations of
the 2008 Wenchuan earthquake Mw7.9, China
Authors: Ken Xiansheng Hao, Hongjun Si
et al
Link: Click here
Abstract
The irregularly distributed surface fault-ruptures of the
Wenchuan earthquake spanned over 200 km along the Longmen
Shan(LMS) fault zone. Through field investigations, we found
over 10 coseismic surface-ruptures, with maximum vertical
displacements of approximately 6 m on the Yingxiu-Beichuan
fault and 2 m on the Guanxian-Anxian fault; however, the entire
fault
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rupture movement was
still not clearly understood since high topographic areas were
inaccessible. Thus, we used interferometric synthetic aperture
radar (InSAR) satellite observations to capture whole coseismic
surface-ruptures and crustal deformations across the LMS faults.
We created a novel bi-fault-slip model to invert fault-slips
using InSAR information which yielded that thrust fault-slips
were dominant at YingXiu, Houshenggou and Bajiaomiao in the
near-epicenter segment, while the dextral fault-slips were dominant
at Pingtong and Nanba along the northeast segment. The combination
of field investigations and simulations suggested that the two
coseismic fault zones ruptured with an irregular surface distribution
accompanied by crustal deformations. |
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Junio de 2009
Extension of Gutenberg-Richter distribution to MW -1.3,
no lower limit in sight
Authors: Margaret S. Boettcher, A. McGarr
et al
Link: Click here
Abstract
With twelve years of seismic data from TauTona Gold Mine,
South Africa, we show that mining-induced earthquakes follow
the Gutenberg-Richter relation with no scale break down to
the completeness level of the catalog, at moment magnitude
M W -1.3. Events recorded during
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relatively
quiet hours in 2006 indicate that catalog detection limitations,
not earthquake source physics, controlled the previously reported
minimum magnitude in this mine. Within the Natural Earthquake
Laboratory in South African Mines (NELSAM) experiment's dense
seismic array, earthquakes that exhibit shear failure at magnitudes
as small as M W -3.9 are observed, but we find no evidence that
M W -3.9 represents the minimum magnitude. In contrast to previous
work, our results imply small nucleation zones and that earthquake
processes in the mine can readily be scaled to those in either
laboratory experiments or natural faults. |
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Mayo de 2009
Possible evidence of virtual resonance in the dayside magnetosphere
Authors: Kazue Takahashi, David Berube
et al
Link: Click here
Abstract
On day 108, 2001, the Sub-Auroral Magnetometer Network (SAMNET)
and Magnetometers along the Eastern Atlantic Seaboard for
Undergraduate Research and Education (MEASURE) magnetometer
arrays detected dayside magnetic pulsations at a common frequency
of ~15 mHz at all locations below L = 4. This global pulsation
event was associated with alignment of the interplanetary
magnetic field with the Sun-Earth axis, a condition known
to generate ultralow-frequency (ULF) waves in front of the
bow shock.
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The event occurred
during the early recovery phase of a geomagnetic storm. Magnetic
field measured by the GOES 8 geostationary satellite on the
dayside indicated elevated broadband (7-80 mHz) ULF power in
the compressional component without a strong peak at 15 mHz.
These observations suggest that the global pulsations originated
from a compressional magnetohydrodynamic eigenmode oscillation
of the inner magnetosphere stimulated by a broadband external
disturbance. The equatorial Alfvén velocity corresponding
to the toroidal frequencies that were determined with the cross-phase
analysis of SAMNET and MEASURE data showed a gradual decrease
of the velocity with L without a clear signature of a plasmapause.
The observed properties of the global pulsations are consistent
with virtual resonance in the inner magnetosphere. |
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Mayo de 2009
Hybrid modeling of the mega-tsunami runup in Lituya Bay
after half a century
Authors: Robert Weiss, Hermann M. Fritz
et al
Link: Click here
Abstract
The largest mega-tsunami dates back half a century to 10 July
1958, when almost unnoticed by the general public, an earthquake
of M w 8.3 at the Fairweather Fault triggered a rockslide
into Lituya Bay. The rockslide impact generated a giant tsunami
at the head of Lituya Bay resulting in an
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unprecedented
tsunami runup of 524 m on a spur ridge in direct prolongation
of the slide axis. A forest trim line and erosion down to bedrock
mark the largest runup in recorded history. While these observations
have not been challenged directly, they have been largely ignored
in hazard mitigation studies, because of the difficulties of
even posing - much less solving - a well-defined physical problem
for investigation. We study the mega-tsunami runup with a hybrid
modeling approach applying physical and numerical models of
slide processes of deformable bodies into a U-shaped trench
similar to the geometry found at Lituya Bay. |
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Mayo de 2009
High-frequency P-wave seismic noise driven by ocean winds
Authors: Jian Zhang, Peter Gerstoft et
al
Link: Click here
Abstract
Earth's background vibrations at frequencies below about 0.5
Hz have been attributed to ocean-wave energy coupling into
the ground and propagating as surface waves and P-waves (compressional
waves deep within the Earth). However, the origin and nature
of seismic noise
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on land at frequencies
around 1 Hz has not yet been well studied. Using array beamforming,
we analyze the seismic noise fields at two remote sites (Parkfield
and the Mojave Desert) in California, for durations of one and
six months respectively. We find that (1) the seismic background
noise at about 0.6-2 Hz consists of a significant amount of
continuous P-waves originating offshore, and (2) the power of
the P-wave noise is highly correlated with the offshore wind
speed, demonstrating that these high-frequency P-waves are excited
by distant ocean winds. Our result suggests a land-based seismological
proxy for monitoring oceanic weather. |
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Mayo de 2009
Tsunami early warning using earthquake rupture duration
Authors: Anthony Lomax and Alberto Michelini
Link: Click here
Abstract
Effective tsunami early warning for coastlines near a tsunamigenic
earthquake requires notification within 5-15 minutes. We have
shown recently that tsunamigenic earthquakes have an apparent
rupture duration, T 0, greater than about 50 s. Here we show
that T 0 gives more information on
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tsunami
importance than moment magnitude, M w, and we introduce a procedure
using seismograms recorded near an earthquake to rapidly determine
if T 0 is likely to exceed T = 50 or 100 s. We show that this
"duration-exceedance" procedure can be completed within
3-10 min after the earthquake occurs, depending on station density,
and that it correctly identifies most recent earthquakes which
produced large or devastating tsunamis. This identification
forms a complement to initial estimates of the location, depth
and magnitude of an earthquake to improve the reliability of
tsunami early warning, and, in some cases, may make possible
such warning. |
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Mayo de 2009
Seismoionospheric GPS total electron content anomalies
observed before the 12 May 2008 Mw7.9 Wenchuan earthquake
Authors: J. Y. Liu, Y. I. Chen et al
Link: Click here
Abstract
The global ionospheric map (GIM) is used to observe variations
in the total electron content (TEC) of the global positioning
system (GPS) associated with 35 M = 6.0 earthquakes that occurred
in China during the 10-year period of 1 May 1998 to 30 April
2008. The statistical result indicates that the GPS TEC above
the epicenter often pronouncedly decreases on day 3-5 before
17 M = 6.3 earthquakes. The GPS TEC of the GIM and electron
density profiles probed by six
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microsatellites of
FORMOSAT3/COSMIC (F3/C) are further employed to simultaneously
observe seismoionospheric anomalies during an M w 7.9 earthquake
near Wenchuan, China, on 12 May 2008. It is found that GPS TEC
above the forthcoming epicenter anomalously decreases in the
afternoon period of day 6-4 and in the late evening period of
day 3 before the earthquake, but enhances in the afternoon of
day 3 before the earthquake. The spatial distributions of the
anomalous and extreme reductions and enhancements indicate that
the earthquake preparation area is about 1650 km and 2850 km
from the epicenter in the latitudinal and longitudinal directions,
respectively. The F3/C results further show that the ionospheric
F 2 peak electron density, N m F 2, and height, h m F 2, significantly
decreases approximately 40% and descends about 50-80 km, respectively,
when the GPS TEC anomalously reduces. |
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Abril de 2009
Constraints on shear wave attenuation in the Earth's inner
core from an observation of PKJKP
Authors: A. Cao and B. Romanowicz
Link: Click here
Abstract
Based on the high quality broadband data from Gräfenberg
array in Germany, we recently detected a reliable PKJKP phase,
for which four kinds of evidence (travel time, slowness, back-azimuth,
and comparison with a pseudo-liquid inner core model) were
simultaneously provided. Also, for
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the
first time, a clear waveform of PKJKP was observed. This gives
us an unprecedented opportunity to put constraints on the shear
wave attenuation in the earth's inner core using body waves.
In order to minimize the potential influence of dispersion and
phase shift caused by mantle heterogeneity, we adopt an envelope
modeling approach. Our results show that the estimated Q ß
from the shear phase PKJKP is significantly larger (~315 ±
150) than that from normal mode observations. Because PKJKP
samples the deep inner core, this indicates an increase of Q
ß with depth in the inner core, in agreement with what
is generally observed for Q alfa. |
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Abril de 2009
Slab segmentation revealed by anisotropic P-wave tomography
Authors: Motoko Ishise, Kazuki Koketsu
et al
Link: Click here
Abstract
Seismic anisotropy is a useful indicator for identifying the
physical and chemical condition of the Earth's interior, such
as stress and flow fields, and in situ constituent minerals.
Using traveltime tomography, we examined three-dimensional
anisotropic P-wave velocity structure of the Kii
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Peninsula, southwest
Japan, where source regions of megathrust earthquakes along
the Nankai Trough are presumed to exist. The tomography revealed
that the Philippine Sea slab beneath the peninsula is segmented,
i.e., a high-velocity slab with E-W anisotropy obtained during
seafloor spreading is broken by an anomalous low-velocity region
with N-S anisotropy. The anomaly lies along the segmentation
boundary of two previous rupture zones, the 1944 Tonankai and
1946 Nankai earthquakes, and can be explained by the occurrence
of a fracture zone with N-S oriented fractures and fault planes
revealed by wide-angle seismic data. It is likely that this
anomalous region will form a clear segmentation boundary during
future earthquakes. |
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Abril de 2009
Triggered seismicity associated with the 1990 Nicoya, Costa
Rica, Mw = 7.0 earthquake
Authors: Susan L. Bilek, Candy E. Elliott
et al
Link: Click here
Abstract
The 25 March 1990 (M w = 7.0) subduction megathrust earthquake
that occurred offshore the Nicoya Peninsula, Costa Rica, produced
a large number of aftershocks on the subduction plate interface
as expected and preceded an unusual sequence of earthquakes
75 km inland that had two periods of significant increase,
one at 60-90 days and one near 270 days, following the main
shock. This inland sequence of events would not typically
fall within the classification of aftershocks
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given
their spatial and temporal distance, and we show here that this
sequence was likely triggered by the 25 March main shock. We
compute stress changes on representative faults within this
inland region using both a simple half-space model as well as
with a 2-D finite element model that incorporates variable rheologic
properties. The half-space model predicts a minor increase in
Coulomb stress changes and a large amount of unclamping in this
region, likely enough to cause triggering on the inland right-lateral
strike-slip faults. Models that include a viscoelastic response
also indicate stress increases that may link to triggering,
particularly related to the time delay. Earthquakes on the subduction
zone thrust along Costa Rica should be considered in hazard
assessments for the inland populated region as several sets
of strike-slip faults have been mapped in the fore-arc region.
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Abril de 2009
Utilización de una Red Neuronal para el pronóstico
de tsunamis y del runup
Autores: Shailesh Namekar, Yoshiki Yamazaki
et al
Link: Click aquí
Resumen
Este paper examina la utilización de una Red Neuronal
para modelar los procesos no lineales de pronósticos
de las formas de ondas y del runup (incremento rápido
del nivel) de los tsumanis. Utilizamos una Red Neuronal tri-capa
con una función de base radial en la capa oculta. La
capa intermedia realizaba una transformación no-lineal
del input de las formas de onda cerca de la fuente del tsunami.
El evento del 2006 ocurrido en las islas Kuril demostró
la implementación y la eficacia de la Red. La división
que se observa en la zona de subducción de Kamchatka-Kuril
dentro de una serie de sub-fallas facilita la creación
de un representativo data-set mediante un modelo de longitud
de onda no-lineal. Las formas de onda calculadas cerca de
la fuente del tsunami fueron utilizadas como el input y las
formas de onda en la lejanía junto con el runup se
utilizaron como el objetivo que debía ser capaz de
predecir la Red Neuronal mediante el Algoritmo de Retropropagación.
La Red Neuronal entrenada reprodujo correctamente la resonancia
de las ondas del tsunami y los patrones de runup afectados
por el relieve y observados en la línea costera de
Hawaii a partir de los datos de entrada extraídos de
la situación observada en las Islas Aleutinas.
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Abril de 2009
Neural network for tsunami and runup forecast
Authors: Shailesh Namekar, Yoshiki Yamazaki
et al
Link: Click here
Abstract
This paper examines the use of neural network
to model nonlinear tsunami processes for forecasting of coastal
waveforms and runup. The three-layer network utilizes a radial
basis function in the hidden, middle layer for nonlinear transformation
of input waveforms near the tsunami source. Events based on
the 2006 Kuril Islands tsunami demonstrate the implementation
and capability of the network. Division of the Kamchatka-Kuril
subduction zone into a number of subfaults facilitates development
of a representative tsunami dataset using a nonlinear long-wave
model. The computed waveforms near the tsunami source serve
as the input and the far-field waveforms and runup provide
the target output for training of the network through a back-propagation
algorithm. The trained network reproduces the resonance of
tsunami waves and the topography-dominated runup patterns
at Hawaii's coastlines from input water-level data off the
Aleutian Islands.
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Abril de 2009
Motions of gently sloping ground during earthquakes
Authors: Jean-Pierre Bardet, Fang Liu et
al
Link: Click here
Abstract
During past earthquakes, gently sloping ground surfaces have
been observed to shift laterally over areas as large as a
few square kilometers. These permanent downward motions have
been attributed to temporary and partial reductions in shear
strength of soil deposits (e.g., liquefaction), static gravity
forces, and transient earthquake shakings. These motions,
the amplitudes of which remain challenging to predict, have
been frequently modeled using Newmark sliding blocks for ground
that is statically stable and occasionally
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partially or completely loses its shear strength during earthquakes.
This paper investigates in more details the deformations of
gently sloping ground that becomes temporarily unstable under
the combined effects of gravitational forces and earthquake
shakings. Using a physical model, we propose two definitions
of earthquake contributions to the displacement of gently sloping
ground, which may include or not the effects of initial velocity.
In both cases, Monte Carlo simulations reveal that motions of
gently sloping ground are influenced by pulses of earthquake
ground velocity for a wide range of reduction in soil shear
strength, even for very severe loss of shear strength. For practical
purposes, when large soil deposits become unstable during earthquakes,
the analysis concludes that the motions of gently sloping ground
are likely influenced by earthquake ground velocity. |
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Abril de 2009
Numerical models of caldera deformation: Effects of multiphase
and multicomponent hydrothermal fluid flow
Authors: M. Hutnak, S. Hurwitz et al
Link: Click here
Abstract
Ground surface displacement (GSD) in large calderas is often
interpreted as resulting from magma intrusion at depth. Recent
advances in geodetic measurements of GSD, notably interferometric
synthetic aperture radar, reveal complex and multifaceted
deformation patterns that often require complex source models
to explain the observed GSD. Although hydrothermal fluids
have been discussed as a possible deformation agent, very
few quantitative studies addressing the effects of multiphase
flow on
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crustal mechanics
have been attempted. Recent increases in the power and availability
of computing resources allow robust quantitative assessment
of the complex time-variant thermal interplay between aqueous
fluid flow and crustal deformation. We carry out numerical simulations
of multiphase (liquid-gas), multicomponent (H2O-CO2) hydrothermal
fluid flow and poroelastic deformation using a range of realistic
physical parameters and processes. Hydrothermal fluid injection,
circulation, and gas formation can generate complex, temporally
and spatially varying patterns of GSD, with deformation rates,
magnitudes, and geometries (including subsidence) similar to
those observed in several large calderas. The potential for
both rapid and gradual deformation resulting from magma-derived
fluids suggests that hydrothermal fluid circulation may help
explain deformation episodes at calderas that have not culminated
in magmatic eruption. |
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Abril de 2009
Seismic tremor in subduction zones: Rock physics evidence
Authors: Luigi Burlini, Giulio Di Toro
et al
Link: Click here
Abstract
Episodic tremor and slip (ETS) have been correlated with rupture
phenomena in subducting oceanic lithosphere at 30-45 km depth,
where high V P/V S ratios, which suggest high-fluid pressures,
have been observed. ETS, by accommodating slip in the down-dip
portion of the
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subduction
zone, may trigger megathrust earthquakes up-dip in the locked
section. During dehydration experiments on serpentinite (typical
rock of the oceanic lithosphere) at temperatures found in nature
at 30-45 km depth (400-550°C), we observe seismic signals
in the form of acoustic emissions that closely resemble low
frequency earthquakes, seismic tremor and regular earthquakes.
Our findings support the concept that water released during
dehydration reactions increases the pore pressures and can trigger
ETS and regular earthquakes by reducing slip resistance. |
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Abril de 2009
Giant impacts on early Mars and the cessation of the Martian
dynamo
Authors: J. H. Roberts, R. J. Lillis et
al
Link: Click here
Abstract
Although Mars currently has no global dynamo-driven magnetic
field, widespread crustal magnetization provides strong evidence
that such a field existed in the past. The absence of magnetization
in the younger large Noachian basins suggests that a dynamo
operated early in
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Martian history but
stopped in the mid-Noachian. Within a 100 Ma period, 15 giant
impacts occurred coincident with the disappearance of the global
magnetic field. Here we investigate a possible causal link between
the giant impacts during the early and mid-Noachian and the
cessation of the Martian dynamo at about the same time. Using
three-dimensional spherical mantle convection models, we find
that impact heating associated with the largest basins (diameters
>2500 km) can cause the global heat flow at the core-mantle
boundary to decrease significantly (10-40%). We suggest that
such a reduction in core heat flow may have led to the cessation
of the Martian dynamo. |
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Abril de 2009
Seismic attenuation tomography of the Mariana subduction
system: Implications for thermal structure, volatile distribution,
and slow spreading dynamics
Authors: Sara H. Pozgay, Douglas A. Wiens
et al
Link: Click here
Abstract
The anelastic structure of a subduction zone can place first-order
constraints on variations in temperature and volatile content.
We investigate seismic attenuation across the western Pacific
Mariana subduction system using data from the 2003-2004 Mariana
Subduction Factory Imaging Experiment. This 11-month experiment
consisted of 20 broadband stations deployed on the arc islands
and 58 semibroadband ocean bottom seismographs deployed across
the fore arc, island arc, and back-arc spreading center. We
compute amplitude spectra for P and S arrivals from local
earthquakes and invert for the path-averaged attenuation for
each waveform along with the seismic moment and corner frequency
for each earthquake. Additionally, we investigate earthquake
source parameter assumptions and frequency-dependent exponents
(a) ranging from 0 to 0.6. Tomographic inversion of nearly
3000 t*
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estimates
(at a = 0.27) for 2-D Q P -1 and Q P /Q Sstructure shows a ~75
km wide columnar-shaped high-attenuation anomaly with Q P ~
43-60 beneath the spreading center that extends from the uppermost
mantle to ~100 km depth. A weaker high-attenuation region (Q
P ~ 56-70) occurs at depths of 50-100 km beneath the volcanic
arc, and the high-attenuation regions are connected at depths
of 75-125 km. The subducting Pacific plate is characterized
by low attenuation at depths greater than 100 km, but high attenuation
is found in the plate between 50 and 100 km depth. The fore
arc shows high attenuation near the volcanic arc and beneath
the serpentinite seamounts in the outer fore arc. Q S structure
is less well resolved than Q P because of a smaller data set,
but Q P /Q S ratios are significantly less than 2 throughout
the study region. As temperatures estimated from Q S -1 are
unusually high, we interpret the arc and wedge core anomalies
as regions of high temperature with enhanced Q -1 due to hydration
and/or melt, the slab and fore-arc anomalies as indicative of
slab-derived fluids and/or large-scale serpentinization, and
the columnar-shaped high Q P -1 anomaly directly beneath the
back-arc spreading center as indicative of a narrow region of
dynamic upwelling and melt production beneath the slow spreading
ridge axis. |
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Abril de 2009
Solar energetic particle flux enhancement as a predictor
of geomagnetic activity in a neural network-based model
Authors: F. Valach, M. Revallo et al
Link: Click here
Abstract
Coronal mass ejections (CMEs) are believed to be the principal
cause of increased geomagnetic activity. They are regarded
as being in context of a series of related solar energetic
events, such as X-ray flares (XRAs) accompanied by solar radio
bursts (RSPs) and also by solar energetic particle (SEP) flux.
Two types of the RSP events are known to be geoeffective,
namely, the RSP of type II, interpreted as the signature of
shock initiation in the solar corona, and type IV, representing
material moving upward in the corona. The SEP events causing
geomagnetic response are known
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to be produced by
CME-driven shocks. In this paper, we use the method of the artificial
neural network in order to quantify the geomagnetic response
of particular solar events. The data concerning XRAs and RSPs
II and/or IV together with their heliographic positions are
taken as the input for the neural network. There is a key question
posed in our study: can the successfulness of the neural network
prediction scheme based solely on the solar disc observations
(XRA and RSP) be improved by additional information concerning
the SEP flux? To resolve this problem, we chose the SEP events
possessing significant enhancement in the 10-h window, commencing
12 h after the generation of XRAs. In particular, we consider
the flux of high-energy protons with energies over 10 MeV. We
have used a chi-square test to demonstrate that supplying such
extra input data improves the neural network prediction scheme.
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Abril de 2009
Time evolution of the subauroral electric fields: A case
study during a sequence of two substorms
Authors: R. A. Makarevich, A. C. Kellerman
et al
Link: Click here
Abstract
The temporal evolution of the subauroral polarization stream
(SAPS) is investigated using the Doppler velocity observations
by the Unwin HF radar in conjunction with the simultaneously
observed auroral luminosity and plasma convection reversal
regions. The event under study of 14 December 2004, 1000-1600
UT, occurred during geomagnetically quiet conditions when
a sequence of two substorms separated by ~2 hours was observed
by the IMAGE satellite. It is shown that the SAPS appeared
shortly after the first substorm onset and continued to dominate
the westward plasma convection at subauroral magnetic latitudes
59°-63°S for ~6 hours. An unexpected exception occurred
near the second
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substorm
onset time when a narrow channel of weaker SAPS westward convection
became embedded in the slow eastward drifting plasma for a short
~25-min period, which was attributed to the equatorward expansion
of the dawn convection cell during the growth and expansion
phases of the second substorm. Another remarkable plasma convection
feature was observed in the same 25-min period ~5° poleward
of SAPS; this was a narrow "mirror" channel of the
eastward drifting plasma parallel to the main SAPS channel.
It is argued that the latter feature may also be caused by the
equatorward expanding dawn convection cell whose streamlines
become compressed by and aligned with the SAPS poleward edge.
A possible relationship between the SAPS intensity and its position
relative to the auroral oval is also investigated. The results
suggest that the magnetosphere-ionosphere feedback processes
within SAPS become more effective as the equatorward edge of
the eastward convection region retreats poleward with the SAPS
position much less affected by the auroral dynamics. |
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Abril de 2009
Role of entropy in magnetotail dynamics
Authors: J. Birn, M. Hesse et al
Link: Click here
Abstract
The role of entropy conservation and loss in magnetotail dynamics,
particularly in relation to substorm phases, is discussed
on the basis of MHD theory and simulations, using comparisons
with particle-in-cell (PIC) simulations for validation. Entropy
conservation appears to be a
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crucial element leading
to the formation of thin embedded current sheets in the late
substorm growth phase and the potential loss of equilibrium.
Entropy conservation also governs the accessibility of final
states of evolution and the amount of energy that may be released.
Entropy loss (in the form of plasmoids) is essential in the
earthward transport of flux tubes (bubbles, bursty bulk flows).
Entropy loss also changes the tail stability properties and
may render ballooning modes unstable and thus contribute to
cross-tail variability. We illustrate these effects through
results from theory and simulations. |
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Abril de 2009
Infrasonic jet noise from volcanic eruptions
Authors: R. S. Matoza, D. Fee et al
Link: Click here
Abstract
The lowermost section of a Vulcanian or Plinian volcanic eruption
column may be thought of as a momentum-driven, turbulent,
free-shear jet flow. We propose that large-amplitude and long-duration
infrasonic (<20 Hz) signals recorded at ranges of tens
of kilometers during powerful eruptions at Mount St. Helens,
USA, and Tungurahua, Ecuador, represent a low frequency form
of jet noise. A preliminary test of this
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hypothesis is made by comparing the observed infrasonic spectra
to the empirically-derived similarity spectra for pure-air jets.
Although the spectral shapes are in approximate agreement, the
observed volcanic signals have additional complexities not present
in the pure-air laboratory data. These features may result from
multiphase flow containing solid particles and liquid droplets,
very high temperatures, and perhaps complex crater morphology.
However, the overall similarity between the volcanic signals
and jet noise indicates that broadband infrasound measurements
at volcanoes may provide a quantitative link to eruption jet
dynamics, and would aid substantially in the remote assessment
of volcanic hazard. |
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Abril de 2009
Methods for determining soil water repellency on field-moist
samples
Authors: Louis W. Dekker, Coen J. Ritsema
et al
Link: Click here
Abstract
In this paper we describe a simple and quick method for determining
the presence of water repellency in a soil by using a small
core sampler (1.5 cm in diameter, 25 cm long) and applying
the
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water drop penetration
time (WDPT) test at different depths on the sandy soil cores.
Obtained results provide spatial distribution patterns of water
repellency in a soil profile, demonstrating seasonal changes
in repellency. An advantage of the method is that the soil is
not disturbed by the sampling. For assessment of the persistence
of water repellency in strongly to extremely water repellent
soils, and for determination of the critical soil water contents,
the WDPT test and volumetric water content determinations should
preferably be performed in the laboratory. |
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Abril de 2009
A fundamental parameters approach to calibration of the
Mars Exploration Rover Alpha Particle X-ray Spectrometer
Authors: J. L. Campbell, M. Lee et al
Link: Click here
Abstract
The detection sensitivities of the Alpha Particle X-ray Spectrometer
(APXS) instruments on the Mars Exploration Rovers for a wide
range of elements were experimentally determined in 2002 using
spectra of geochemical reference materials. A flight spare
instrument was similarly calibrated, and the calibration exercise
was then continued for this unit with an extended set of geochemical
reference materials together with pure elements
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and
simple chemical compounds. The flight spare instrument data
are examined in detail here using a newly developed fundamental
parameters approach which takes precise account of all the physics
inherent in the two X-ray generation techniques involved, namely,
X-ray fluorescence and particle-induced X-ray emission. The
objectives are to characterize the instrument as fully as possible,
to test this new approach, and to determine the accuracy of
calibration for major, minor, and trace elements. For some of
the lightest elements the resulting calibration exhibits a dependence
upon the mineral assemblage of the geological reference material;
explanations are suggested for these observations. The results
will assist in designing the overall calibration approach for
the APXS on the Mars Science Laboratory mission. |
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Abril de 2009
Can compliant fault zones be used to measure absolute stresses
in the upper crust?
Authors: E. H. Hearn, Y. Fialko et al
Link: Click here
Abstract
Geodetic and seismic observations reveal long-lived zones
with reduced elastic moduli along active crustal faults. These
fault zones localize strain from nearby earthquakes, consistent
with the response of a compliant, elastic layer. Fault zone
trapped wave studies documented a small reduction in P and
S wave velocities along the Johnson Valley Fault caused by
the 1999 Hector Mine earthquake. This reduction presumably
perturbed a permanent compliant structure associated with
the fault. The inferred changes in the fault zone compliance
may produce a measurable deformation in response to background
(tectonic) stresses. This deformation should have the same
sense as the background stress, rather than the coseismic
stress change. Here we investigate how the observed
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deformation of compliant
zones in the Mojave Desert can be used to constrain the fault
zone structure and stresses in the upper crust. We find that
gravitational contraction of the coseismically softened zones
should cause centimeters of coseismic subsidence of both the
compliant zones and the surrounding region, unless the compliant
fault zones are shallow and narrow, or essentially incompressible.
We prefer the latter interpretation because profiles of line
of sight displacements across compliant zones cannot be fit
by a narrow, shallow compliant zone. Strain of the Camp Rock
and Pinto Mountain fault zones during the Hector Mine and Landers
earthquakes suggests that background deviatoric stresses are
broadly consistent with Mohr-Coulomb theory in the Mojave upper
crust (with µ = 0.7). Large uncertainties in Mojave compliant
zone properties and geometry preclude more precise estimates
of crustal stresses in this region. With improved imaging of
the geometry and elastic properties of compliant zones, and
with precise measurements of their strain in response to future
earthquakes, the modeling approach we describe here may eventually
provide robust estimates of absolute crustal stress. |
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Abril de 2009
Locations and types of ruptures involved in the 2008 Sichuan
earthquake inferred from SAR image matching
Authors: Tomokazu Kobayashi, Youichiro
Takada et al
Link: Click here
Abstract
We have detected detailed ground displacements in the proximity
of the Longmen Shan fault zone (LMSFZ) by applying a SAR offset-tracking
method in the analysis of the 2008 Sichuan earthquake. An
elevation-dependent correction is
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indispensable
for achieving sub-meter accuracy. A sharp displacement discontinuity
with a relative motion of ~1-2 m appears over a length of 200
km along the LMSFZ, which demonstrates that the main rupture
has proceeded on the Beichuan fault (BF) among several active
faults composing the LMSFZ, and a new active fault is detected
on the northeastward extension of the BF. The rupture on the
BF is characterized by a right-lateral motion in the northeast,
while in the southwest an oblique right-lateral thrust slip
is suggested. In contrast to the northeast, where a major rupture
proceeded on the BF only, in the southwest multiple thrust ruptures
have occurred in the southeastern foot of the Pengguan massif.
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Abril de 2009
Predicting relative permeability from water retention:
A direct approach based on fractal geometry
Authors: Abdullah Cihan, John S. Tyner
et al
Link: Click here
Abstract
Commonly, a soil's relative permeability curve is predicted
from its measured water retention curve by fitting equations
that share parameters between the two curves (e.g., Brooks/Corey-Mualem
and van Genuchten-Mualem). We present a new approach to predict
relative permeability by direct application of measured soil
water retention data without any fitting procedures. The new
relative permeability model, derived from a probabilistic
fractal approach, appears in series
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form
as a function of suction and the incremental change in water
content. This discrete approach describes the drained pore space
and permeability at different suctions incorporating the effects
of both pore size distribution and connectivity among water-filled
pores. We compared the new model performance predicting relative
permeability to that of the van Genuchten-Mualem (VG-M) model
for 35 paired data sets from the Unsaturated Soil hydraulic
Database (UNSODA) and five other previously published data sets.
At the 5% level of significance, the new method predicts relative
permeabilities from the UNSODA database significantly better
(mean logarithmic root-mean-square error, LRMSE = 0.813) than
the VG-M model (LRMSE = 1.555). Each prediction of relative
permeability from the five other previously published data sets
was also significantly better. |
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Abril de 2009
A sea ice free summer Arctic within 30 years?
Authors: Muyin Wang and James E. Overland
Link: Click here
Abstract
September 2008 followed 2007 as the second sequential year
with an extreme summer Arctic sea ice extent minimum. Although
such a sea ice loss was not indicated until much later in
the century in the Intergovernmental Panel on Climate Change
4th Assessment Report, many models
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show an accelerating decline in the summer minimum sea ice extent
during the 21st century. Using the observed 2007/2008 September
sea ice extents as a starting point, we predict an expected
value for a nearly sea ice free Arctic in September by the year
2037. The first quartile of the distribution for the timing
of September sea ice loss will be reached by 2028. Our analysis
is based on projections from six IPCC models, selected subject
to an observational constraints. Uncertainty in the timing of
a sea ice free Arctic in September is determined based on both
within-model contributions from natural variability and between-model
differences. |
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Abril de 2009
Origins of the extremely warm European fall of 2006
Authors: J. Cattiaux, R. Vautard et al
Link: Click here
Abstract
The fall of 2006 was the warmest on record in Europe. So far
the origins of this seasonal extreme anomaly have not been
elucidated, but understanding them is crucial since climate
change may increase the frequency and
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amplitude
of such extreme seasons. From a statistical analysis and regional
modeling experiments we estimate the contributions of regional
atmospheric circulation and sea-surface temperatures (SST) on
the continental surface temperatures of this event. Both the
regression and the dynamical model attribute about 50% of the
land temperature anomaly to the atmospheric flow conditions,
30% to the SST warm anomaly, while the missing 20% remain unexplained.
Assuming such decomposition, the contribution of trend components
would explain about 20 to 40% of the anomaly, a proportion that
should increase in the future. |
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Abril de 2009
Decrease of intensity of ELF/VLF waves observed in the
upper ionosphere close to earthquakes: A statistical study
Authors: F. Nemec, O. Santolík et
al
Link: Click here
Abstract
We present results of a systematic study of intensity of VLF
electromagnetic waves observed by the DEMETER spacecraft in
the upper ionosphere (altitude 700 km). We focus on the detailed
analysis of the previously reported decrease of wave intensity
shortly before the main shock during the nighttime. Using
a larger set of
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data
(more than 3.5 years of measurements) and a newly developed
data processing method, we confirm the existence of a very small
but statistically significant decrease of wave intensity 0-4
hours before the time of the main shock at frequencies of about
1.7 kHz. It is shown that the decrease does not occur directly
above the earthquake epicenter but is shifted about 2° in
the westward direction. Moreover, it is demonstrated that the
decrease occurs more often close to shallower earthquakes and
close to earthquakes with larger magnitudes, as it is "intuitively"
expected, representing an additional proof of validity of the
obtained results. Finally, no dependence has been found on the
occurrence of the earthquake below the ocean or below the continents. |
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Abril de 2009
Geophysics: Hot blanket in Earth's deep crust
Author: Jean Braun
Link: Click here
Abstract
Studies of rocks from Earth's crust suggest that the lower
crust is a good thermal insulator. The knock-on effects of
this finding are many - one
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being
the crust's increased potential to generate more magma.
We know little about the temperature of the deepest parts of
Earth's continental crust, which can be 30-40 kilometres below
the surface. We cannot measure temperature directly beyond a
few kilometres down, at the bottom of deep mines or drill holes. |
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Marzo de 2009
Geomagnetic excursions: Knowns and unknowns
Author: Andrew P. Roberts
Link: Click here
Abstract
Geomagnetic excursions are short-lived episodes when Earth's
magnetic field deviates into an intermediate polarity state.
Understanding the origin, frequency, amplitude, duration,
and field behavior associated with excursions is a forefront
research area within solid earth geophysics.
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Recent
advances in excursion research are summarized here, and key
further research is suggested to resolve major unanswered questions.
Improving the global distribution of excursion records, particularly
from the southern hemisphere, obtaining high-resolution sedimentary
excursion records with good age control from sites with sedimentation
rates >10 cm/kyr, obtaining volcanic excursion records coupled
with high-precision geochronology, and estimating excursion
duration with high chronological precision will all facilitate
hypothesis testing concerning the deep earth dynamics that generate
geomagnetic excursions. |
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Marzo de 2009
Temperature-dependent thermal diffusivity of the Earth's
crust and implications for magmatism
Authors: Alan G. Whittington, Anne M. Hofmeister
et al
Link: Click here
Abstract
The thermal evolution of planetary crust and lithosphere is
largely governed by the rate of heat transfer by conduction.
The governing physical properties are thermal diffusivity
( ) and conductivity (k = C P), where denotes density and
C P denotes specific heat capacity at constant pressure. Although
for crustal rocks both and k decrease above ambient temperature,
most thermal models of the Earth's lithosphere assume constant
values for ( 1 mm2 s-1) and/or k ( 3 to 5 W m-1 K-1) owing
to the large experimental uncertainties associated with conventional
contact methods at high temperatures. Recent advances in laser-flash
analysis permit accurate ( 2 per cent) measurements on minerals
and rocks to geologically relevant temperatures. Here we provide
data from laser-flash analysis for three
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different
crustal rock types, showing that strongly decreases from 1.5-2.5
mm2 s-1 at ambient conditions, approaching 0.5 mm2 s-1 at mid-crustal
temperatures. The latter value is approximately half that commonly
assumed, and hot middle to lower crust is therefore a much more
effective thermal insulator than previously thought. Above the
quartz - phase transition, crustal is nearly independent of
temperature, and similar to that of mantle materials. Calculated
values of k indicate that its negative dependence on temperature
is smaller than that of , owing to the increase of C P with
increasing temperature, but k also diminishes by 50 per cent
from the surface to the quartz - transition. We present models
of lithospheric thermal evolution during continental collision
and demonstrate that the temperature dependence of and C P leads
to positive feedback between strain heating in shear zones and
more efficient thermal insulation, removing the requirement
for unusually high radiogenic heat production to achieve crustal
melting temperatures. Positive feedback between heating, increased
thermal insulation and partial melting is predicted to occur
in many tectonic settings, and in both the crust and the mantle,
facilitating crustal reworking and planetary differentiation. |
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Marzo de 2009
Gravity mission to launch
Author: Quirin Schiermeier
Link: Click here
Abstract
The planned launch next week of GOCE, a satellite designed
to measure Earth's gravity field
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in
unprecedented detail, will bolster the fields of geodesy, oceanography
and climate prediction, scientists hope.
Part of the European Space Agency's Living Planet Programme,
GOCE - for Gravity field and steady-state Ocean Circulation
Explorer - is scheduled for launch on 16 March from the Plesetsk
Cosmodrome in Russia, using a converted Russian SS-20 missile. |
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Marzo de 2009
The processes of underthrusting and underplating in the
geologic record: structural diversity between the Franciscan
Complex (California), the Kodiak Complex (Alaska) and the
Internal Ligurian Units (Italy)
Authors: F. Meneghini, M. Marroni et al
Link: Click here
Abstract
Existing studies on active subduction margins have documented
the wide diversity in structural style between accretionary
prisms, both in space and time. Together with physical boundary
conditions of the margins, the thickness of sedimentary successions
carried by the lower plate seems to play a key role in controlling
the deformation and fluid flow during accretion. We have tested
the influence of the subducting sedimentary section by comparing
the structural style and fluid-related structures of four
units from three fossil accretionary complexes characterized
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by similar physical conditions but different
subducting sediment thicknesses: (1) the Franciscan Complex
of California, (2) the Internal Ligurian Units of Italy and
(3) the Kodiak Complex, Alaska.
Subducting plates bearing a thick sedimentary cover generally
result in coherent accretion through polyphase deformation
represented by folding and thin thrusting events, while underplating
of sediment-starved oceanic sections results in diffuse deformation
and mélange formation. These two structural styles
can alternate through time in a single complex with a long
record of accretion such as Kodiak.
The parallel analysis of the selected analogues show that
although the volume of sediments carried by the lower plate
determines different structural styles, deformation is strongly
controlled by injection of overpressured fluids during underthrusting
and accretion. Transient hydrofracturing occurs through the
development of a system of dilatant fractures grossly parallel
to the décollement zone.
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Marzo de 2009
Fault-slip analysis in the metaophiolites of the Voltri
Massif: constraints for the tectonic evolution at the Alps/Apennine
boundary
Authors: Laura Federico, Chiara Spagnolo
et al
Link: Click here
Abstract
The metaophiolitic Voltri Massif underwent a complex tectono-metamorphic
evolution, resulting from subduction to collision events during
the Alpine orogenic cycle, and was subsequently involved in
the first stages of the Apennine Orogeny. So this is a key
area to investigate the late orogenic Alpine tectonics and
its time relationship with the first Apenninic deformation
events.
We performed detailed mapping, fault-slip analysis and study
of fault rocks in a study area located at the eastern border
of the Voltri Massif.
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Here
we recognized two main generations of reverse shear zones (RSZ).
The first one (RSZ1) is associated to mylonites and protomylonites
in greenschist to low-greenschist metamorphic facies and is
characterized by a ductile to ductile-brittle behaviour; RSZ1
display either top-to-N-NW or to-SW kinematics. The second generation
of reverse shear zones (RSZ2) is characterized by fault breccia,
crush breccia or protocataclasite; RSZ2 sense of tectonic transport
ranges from top-to-NW, -SW and -NE.
RSZ1 structures are locally folded by 10 m-scale, open chevron
folds with fold axes gently plunging to SW that likely represent
the last phases of deformation during Alpine collision.
RSZ2 structures locally reactivate RSZ1, with a more brittle
behaviour and produce a huge volume of fault rock. They are
coeval with low-greenschist to zeolite-facies metamorphic conditions.
The RSZ2 fit a transpressional regime active at the eastern
border of the Voltri Massif at least during the late Oligocene-early
Miocene. |