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Febrero de 2010
HAARP: Generación
de estratos artificiales ionosféricos mediante la utilización
de ondas
HF de alta potencia
Autores: O. Aharonson,
A. G. Hayes et al
Link: Clic aquí
Abstract
Nosotros reportamos la primera evidencia de plasmas ionosféricos
artificiales con la suficiente densidad como para sostener
una interacción con el bombeo de rayos HF de alta potencia
producidos por el Programa de Aurora Activa de Alta-Frecuencia
(HAARP) transmitidos desde Gakona, Alaska. El proceso de ionización
HF-direccionado es iniciado cerca del segundo electrón
giroarmónico a unos 220 Km de altitud en la región
ionosférica F. Una vez que el plasma artificaial consigue
la suficiente densidad como para sostener una interacción
con el bombeo de rayos, éste desciende rápidamente
como una onda de ionización a unos 150 Km de altitud.
Sin embargo, esos iniciales estratos artificiales son dinámicos
y altamente estructurados. Esta nueva capacidad para producir
una cantidad apreciable de plasma en la atmósfera superior
abre la puerta a un nuevo régimen de propagación
de ondas de radio ionosféricas donde el plasma producido
por el transmisor domina sobre el plasma ionosférico
natural y puede eventualmente ser empleado como un componente
activo de las comunicaciones, radares y otros sistemas.
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Febrero de 2010
Creation of artificial ionospheric layers using high-power
HF waves
Authors: T. Pedersen, B. Gustavsson et
al
Link: Click here
Abstract
We report the first evidence of artificial ionospheric plasmas
reaching sufficient density to sustain interaction with a
high-power HF pump beam produced by the 3.6 MW High-Frequency
Active Auroral Program (HAARP) transmitter in Gakona, Alaska.
The HF-driven ionization process is initiated near the 2nd
electron gyroharmonic at 220 km altitude in the ionospheric
F region. Once the artificial plasma reaches sufficient density
to support interaction with the transmitter beam it rapidly
descends as an ionization wave to ~150 km altitude. Although
these initial artificial layers appear to be dynamic and highly
structured, this new ability to produce significant artificial
plasma in the upper atmosphere opens the door to a new regime
in ionospheric radio wave propagation where transmitter-produced
plasmas dominate over the natural ionospheric plasma and may
eventually be employed as active components of communications,
radar, and other systems.
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Enero de 2010
Seismic structure of the Longmen Shan region from S-wave
tomography and its relationship with the Wenchuan Ms 8.0 earthquake
on 12 May 2008, southwestern China
Authors: Yi Xu, Zhiwei Li et al
Link: Click here
Abstract
Using arrival time data, we determined seismic structure of
the Longmen Shan by S-wave tomography and studied its relations
with the Ms 8.0 earthquake in Wenchuan, southwestern China.
Our results suggest that the Longmen Shan fault belt is a
rheologic boundary between
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the eastern Tibetan
plateau and the Sichuan basin, and the deep crust of the entire
Longmen Shan is significantly thickened by ductile deformation.
The upper structure of the eastern Tibetan plateau is cored
by the high-velocity Pengguan massif. Its collision with the
Sichuan basin is the most direct reason for the seismic ruptures
on the Longmen Shan fault belt north of Wenchuan, while the
lack of earthquakes south of Wenchuan is related to the weakness
of the crust. Therefore, The Ms 8.0 earthquake occurred in a
strongly heterogeneous region. The variations in the crustal
structure largely controlled the initial rupture of the Ms 8.0
earthquake and aftershock activity. |
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Enero de 2010
Waveform inversion for localized seismic structure and
an application to D'' structure beneath the Pacific
Authors: Kenji Kawai and Robert J. Geller
Link: Click here
Abstract
In order to extract more information on localized seismic
structure from observed seismic data, we have developed and
applied a new method of waveform inversion. The calculation
of synthetic seismograms and their partial derivatives is
the key steps in such an inversion. We formulate the inverse
problem of waveform inversion for localized structure, computing
partial derivatives for the 1-D and 3-D anisotropic elastic
moduli at
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particular
depth or at points in space. Our method does not use any great
circle approximations in computing the synthetics and their
partial derivatives. In this study we invert for the vertical
dependence of the shear velocity in the lowermost mantle beneath
the Pacific using the transverse component of broadband waveforms
for the period range 8-200 s. We find 1%-1.5% velocity decreases
and increases in the zones from 400 to 500 km and from 300 to
400 km above the core-mantle boundary (CMB), respectively. In
addition, we find 0.5%-1% velocity increases and decreases in
the zones from 100 to 200 km and from 0 to 100 km above the
CMB, respectively. This is interpreted as evidence for phase
transitions between perovskite and postperovskite (although
a chemical stratification model cannot be excluded). |
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Enero de 2010
Limits of the seismogenic zone in the epicentral region
of the 26 December 2004 great Sumatra-Andaman earthquake:
Results from seismic refraction and wide-angle reflection
surveys and thermal modeling
Authors: F. Klingelhoefer, M.-A. Gutscher
et al
Link: Click here
Abstract
The 26 December 2004 Sumatra earthquake (Mw = 9.1) initiated
around 30 km depth and ruptured 1300 km of the Indo-Australian-Sunda
plate boundary. During the Sumatra-OBS (ocean bottom seismometer)
survey, a wide-angle seismic profile was acquired across the
epicentral region. A seismic velocity model was obtained from
combined travel time tomography and forward modeling. Together
with reflection seismic data from the SeaCause II cruise,
the deep structure of
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the source region
of the great earthquake is revealed. Four to five kilometers
of sediments overlie the oceanic crust at the trench, and the
subducting slab can be imaged down to a depth of 35 km. We find
a crystalline backstop 120 km from the trench axis, below the
fore-arc basin. A high-velocity zone at the lower landward limit
of the ray-covered domain, at 22 km depth, marks a shallow continental
Moho, 170 km from the trench. The deep structure obtained from
the seismic data was used to construct a thermal model of the
fore arc in order to predict the limits of the seismogenic zone
along the plate boundary fault. Assuming 100°-150°C
as its updip limit, the seismogenic zone is predicted to begin
5-30 km from the trench. The downdip limit of the 2004 rupture
as inferred from aftershocks is within the 350°-450°C
temperature range, but this limit is 210-250 km from the trench
axis and is much deeper than the fore-arc Moho. The deeper part
of the rupture occurred along the contact between the mantle
wedge and the downgoing plate. |
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Enero de 2010
Tidal triggering of earthquakes precursory to the recent
Sumatra megathrust earthquakes of 26 December 2004 (Mw 9.0),
28 March 2005 (Mw 8.6), and 12 September 2007 (Mw 8.5)
Author: Sachiko Tanaka
Link: Click here
Abstract
I observed tidal triggering of earthquakes precursory to the
three giant earthquakes occurring off Sumatra on 26 December
2004 (Mw 9.0), 28 March 2005 (Mw 8.6), and 12 September 2007
(Mw 8.5). I measured the correlation
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between
the Earth tide and earthquake occurrence in and around the focal
regions of these megathrust earthquakes. The result of statistical
analysis indicates that a high correlation appeared for several
to ten years preceding the occurrence of the large earthquakes.
The correlation vanished after the main events. The frequency
distribution of tidal phase angles in the pre-event period exhibited
a peak near the angle where the tidal shear stress is at its
maximum to accelerate the fault slip. This implies that the
high correlation observed in the pre-seismic stage is not a
stochastic chance but is likely a physical consequence of the
tidal stress change. |
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Enero de 2010
Bayesian nonparametric inference for earthquake recurrence
time distributions in different tectonic regimes
Author: R. Rotondi
Link: Click here
Abstract
The objective of this paper is to estimate the probability
distribution of the time between consecutive earthquakes by
employing two equally powerful tools: for the geology, a database
of individual seismogenic sources, and for the statistics,
assuming that the unknown distribution
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is a random measure,
an estimation method based on the stochastic simulation of Markov
chains. The resort to sophisticated instruments is motivated
by the particular situation of Italy, where a complex tectonic
model is combined with infrequent, medium-size earthquakes.
The quality and the length of the parametric catalogue of Italian
earthquakes provide for a generous data set but bring with them
problems of incompleteness and uncertainty regarding the parametrization
of the events. The pointwise estimate of the interevent time
density functions makes it possible to calculate the occurrence
probability depending on the date of the last event at different
forecasting horizons. |
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Enero de 2010
An asymmetric distribution of lakes on Titan as a possible
consequence of orbital forcing
Authors: O. Aharonson, A. G. Hayes et al
Link: Click here
Abstract
A set of lakes filled or partially filled with liquid hydrocarbon
and empty lake basins have been discovered in the high latitudes
of Saturn's moon Titan. These features were mapped by the
radar instrument on the Cassini orbiter. Here we quantify
the distribution of the lakes and basins, and show a pronounced
hemispheric asymmetry in their occurrence. Whereas significant
fractions of the northern high latitudes are covered by filled
and empty lakes, the same latitudes in the
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southern
hemisphere are largely devoid of such features. We propose that
in addition to known seasonal changes, the observed difference
in lake distribution may be caused by an asymmetry in the seasons
on Titan that results from the eccentricity of Saturn's orbit
around the Sun. We suggest that the consequent hemispheric difference
in the balance between evaporation and precipitation could lead
to an accumulation of lakes in one of Titan's hemispheres. This
effect would be modulated by, and reverse with, dynamical variations
in the orbit. We propose that much like in the Earth's glacial
cycles, the resulting vigorous hydrologic cycle has a period
of tens of thousands of years and leads to active geologic surface
modification in the polar latitudes. |
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Enero de 2010
Biogeophysics: A new frontier in Earth science research
Authors: Estella A. Atekwana and Lee D.
Slater
Link: Click here
Abstract
"Biogeophysics" is a rapidly evolving Earth science
discipline concerned with the geophysical signatures of microbial
interactions with geologic media. It spans the established
disciplines of geomicrobiology, biogeoscience, and geophysics.
Biogeophysics research in the last decade has confirmed the
potential for geophysical techniques to measure not simply
the physical and chemical properties of the subsurface, as
already well established, but also to detect microbes, microbial
growth, and microbe-mineral interactions, thus representing
a major paradigm shift in geophysical thinking. In this review
we begin by defining biogeophysics and provide a
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historical perspective.
We then consider microbial alterations of petrophysical properties
as such alteration is the source of most biogeophysical signals.
Our review then focuses on geophysical interrogation of microbial
processes, including the direct detection of microbial cells
and biofilm formation, microbial metabolic by-products, microbe-mediated
redox processes, and biogeochemical and microbe-mineral transformations.
We conclude by discussing challenges, opportunities, and potential
new applications of biogeophysics to the exploration of life
in extreme environments, e.g., the deep biosphere, the cryosphere,
and other planets. We find that published biogeophysics studies
to date are mostly observation based, presenting only empirical
relationships between microbial and geophysical variables. Future
research endeavors must focus on developing theoretical and/or
numerical models for predicting geophysical signals arising
from microbial activity. |
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Enero de 2010
Effects of lateral viscosity variations on the geoid
Authors: A. Ghosh, T. W. Becker et al
Link: Click here
We investigate the effects of lateral viscosity
variations (LVVs) on mantle circulation model predictions
of the global geoid. The present study is motivated in part
by earlier findings that LVVs due to stiff slabs in the lower
mantle have a strong influence on the geoid, and that slabs
in the lower mantle are perhaps no stronger than the ambient
mantle. However, more recently, it has been argued based on
global seismic tomography models that LVVs are a minor effect
on the geoid. In the light of these seemingly contradictory
results, we re-visit the problem of slab strength in
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the
lower mantle. We substantiate that the geoid calculated from
tomography is hardly affected by the presence of LVVs, whereas
the geoid computed from global slab models yields a poor fit
to the observed geoid when LVVs are considered. However, this
degradation of fit only occurs for very long wavelengths of
flow, indicating inherent differences between the slab and tomography
models. We also investigate the effects on the geoid due to
weak plate boundaries, strong cratonic keels, and a low viscosity
region in the D? layer due to post-perovskite. In addition to
the geoid, we attempt to fit plate motions with a circulation
model that has prescribed weak zones at plate boundaries. Motions
are matched well and, taking into account LVVs, the geoid with
appropriate surface velocity boundary conditions agrees with
the observed geoid as well as for free slip cases. |
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Enero de 2010
Can texture-based classification optimally classify soils
with respect to soil hydraulics?
Authors: Navin K. C. Twarakavi, Jirka Šimunek
et al
Link: Click here
Abstract
In the past, texture-based classification of soils has been
used for grouping soils in variably saturated water flow and
solute transport studies. Classification of soils becomes
especially important for large-scale studies where the spatial
and temporal variability in the hydraulic properties of soils
exceeds the field sampling capabilities. Although soil-texture-based
classification has been widely used, questions remain about
the validity of its use from a hydraulic perspective. In this
study, we attempt to answer the following questions: (1) what
is the optimal number of (soil hydraulic) classes that can
adequately classify the soils from a hydraulic standpoint,
and (2) how does such a classification compare to the soil
texture classification currently used? To investigate these
questions, the commonly used k-means clustering algorithm
was integrated with the ROSETTA pedotransfer functions to
predict the so-called soil hydraulic classes. The optimal
soil
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hydraulic classifications
and the associated uncertainty were estimated for numbers of
soil hydraulic classes varying from 2 to 30. It was concluded
that the optimal number of soil hydraulic classes is 12. The
optimal soil hydraulic classes were represented in a ternary
diagram called the soil hydraulic triangle. While there exist
some surprising similarities in classification between the soil
texture triangle and the soil hydraulic triangle for soils with
high sand percentages (sand >60%), the opposite is true for
soils with low sand contents. From a hydraulic standpoint, the
texture-based classification does not classify soils well when
there is a considerable impact of capillary forces. The soil
texture and hydraulic classes were analyzed for accuracy using
two databases. Compared to the soil texture classes, it was
found that the soil hydraulic classes marginally improve the
accuracy of classification. Even though the improvement is only
marginal, it was observed that the optimality of soil texture
triangle for hydraulic studies cannot be assured because of
the nonuniform distribution of data across various textural
possibilities in the two databases. As an extension of this
research, we have also estimated the average soil hydraulic
parameters for the different optimal soil hydraulic classes.
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Enero de 2010
Surface water-groundwater exchange in transitional coastal
environments by airborne electromagnetics: The Venice Lagoon
example
Authors: A. Viezzoli, L. Tosi et al
Link: Click here
Abstract
A comprehensive investigation of the mixing between salt/fresh
surficial water and groundwater in transitional environments
is an issue of paramount importance considering the ecological,
cultural, and socio-economic relevance of coastal zones. Acquiring
information, which can improve the process understanding,
is often logistically challenging, and generally
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expensive and slow in these areas. Here we investigate the capability
of airborne electromagnetics (AEM) at the margin of the Venice
Lagoon, Italy. The quasi-3D interpretation of the AEM outcome
by the spatially constrained inversion (SCI) methodology allows
us to accurately distinguish several hydrogeological features
down to a depth of about 200 m. For example, the extent of the
saltwater intrusion in coastal aquifers and the transition between
the upper salt saturated and the underlying fresher sediments
below the lagoon bottom are detected. The research highlights
the AEM capability to improve the hydrogeological characterization
of subsurface processes in worldwide lagoons, wetlands, deltas.
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Enero de 2010
Coseismic fault slip of the 2008 Mw 7.9 Wenchuan earthquake
estimated from InSAR and GPS measurements
Authors: GuangCai Feng, Eric A. Hetland
et al
Link: Click here
Abstract
We infer co-seismic fault slip during the 2008 Mw 7.9 Wenchuan
earthquake from interferometric synthetic aperture radar (InSAR)
and GPS observations of ground deformation. We use ALOS/PALSAR
data from ascending orbits on six
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tracks, and we do
not use data that are strongly affected by ionospheric perturbations.
We use a fault model composed of three planar fault segments
of the Beichuan fault, and one planar segment representing the
parallel Pengguan fault. Maximum thrust-slip is up to 6.7 m
near the surface, and occurs in two locations, near Yingxiu
in the south and Beichuan in the center of the rupture. Maximum
strike-slip is over 4 m, and occurs near Pingtong and Nanba
along the northern end of the rupture. We find that the ratio
of coseismic thrust- to strike-slip on the Beichuan fault decreases
from 1.5 to 0.7 from the SW to the NE. |
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Enero de 2010
Discrimination of a preparatory stage leading to M7 characteristic
earthquakes off Ibaraki Prefecture, Japan
Author: Shozo Matsumura
Link: Click here
Abstract
An M7.0 earthquake struck off Ibaraki Prefecture, Japan, on
8 May 2008. This event was interpreted as belonging to a characteristic
earthquake series fracturing the same asperities on the upper
boundary of the Pacific plate with a quasi-regular interval
of about 22 years. The previous M7.0 occurred in 1982, 26
years before the latest one. I examined the background microseismicity
changes preceding these two events by drawing maps of earthquake
frequency ratios between two successive periods, and I found
a significant similarity between them. This similarity could
be expressed numerically by calculating correlation
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coefficients
between the two maps. Surveying the temporal transition of the
correlation coefficients, I confirmed that the similarity appeared
only for the combination of those maps just before two characteristic
earthquakes; it never appeared in the other cases. This result
implies the existence and repetition of a typical preparatory
stage leading to the characteristic earthquake, which might
be caused by a quasi-static slip on minor asperities in a hierarchical
constitution and which forms a specific pattern of seismicity
change. This evidence makes it possible to propose a definitive
procedure for the practical prediction of the next characteristic
earthquake, which is anticipated to occur around 2030. Two kinds
of correlation coefficients (rA and rB) are calculated by treating
each seismicity change map just before the 1982 and 2008 events
as references. When both rA and rB exceed the prefixed threshold
r0, it is possible to announce the time of immediate occurrence
of the target earthquake. |
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Enero de 2010
Zero- and first-order approximations for least-squares
estimation of seismic signal with coherent and random noise
Authors: Yuriy Tyapkin, Bjorn Ursin et
al
Link: Click here
Abstract
We have proposed a new least-squares method for signal estimation
with a complicated and therefore more realistic mathematical
model of the multichannel seismic record containing random
noise and an arbitrary number of coherent noise wavetrains.
The signal and all the coherent noise wavetrains are supposed
to bear individual trace-independent waveforms being mutually
uncorrelated in time stationary stochastic processes. The
amplitudes and arrival times of these record components vary
from trace to trace in an arbitrary manner. Random noise is
assumed to be a stationary stochastic process uncorrelated
with the signal and all the coherent noise wavetrains and
from trace to trace as well. Its spectral (autocorrelation)
function is trace independent to within a scale factor, the
variance. Under certain conditions, the method may be reduced
to two successive stages, namely
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preliminary subtraction
of estimates of all the coherent noise wavetrains and final
estimation of the signal from the residual record. At both stages,
optimum weighted stacking is used with reference to the variances
of random noise and to the amplitudes and arrival times of the
corresponding coherent component. A simplified scheme and an
advanced scheme for subtracting coherent noise are proposed,
which are called the zero-order and first-order approximations,
respectively. The first scheme can be thought of as the generalization
of a conventional approach for subtracting coherent noise to
the complicated data model adopted in this paper. The second
scheme has an obvious advantage over the first scheme, since
it allows the distortions that appear when estimating and subsequently
subtracting the coherent noise wavetrains to be compensated.
A simulation on synthetic data shows the efficiency of the first-order
approximation, and it provides a qualitative and quantitative
comparison of those results with the results given by the zero-order
approximation. Also, testing the zero-order approximation exploiting
the singular value decomposition on synthetic and actual data
sets demonstrates the advantage of this method over f-k filtering
combined with subsequent straight stacking. |
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Enero de 2010
Comparación entre los arreglos Wenner y Dipolo-Dipolo
aplicados al estudio de una cavidad subterránea tridimensional
Autores: Ahmad Neyamadpour, W A T Wan Abdullah
et al
Link: Click aquí
Abstract
El objetivo de este paper consistió en comparar la
capacidad de las configuraciones Wenner y dipolo-dipolo para
delimitar una cavidad subterránea ubicada en un sistio
cercano a la Universidad de Malaya, en Malasia. Se desarrolló
un levantamiento de resistividad tridimensional a lo largo
de siete líneas paralelas utilizando los arreglos Wenner
y dipolo-dipolo. Utilizamos un algoritmo robusto de ajuste
tridimensional basado en mínimos cuadrados para realizar
la inversión de los datos de resistividad aparente.
En el modelo final, se desplegó tanto la extensión
de la anomalía en sentido vertical y horizontal. Los
resultados nos permiten afirmar que el método Wenner
es superior al arreglo dipolo-dipolo cuando lo que se desea
es determinar la distribución vertical subsuperficial
de la resistividad. Sin embargo, el arreglo dipolo-dipolo
genera un mejor ajuste de rasgos laterales subsuperficiales.
Los resultados nos muestran que combinar tanto un sondaje
Wenner con un arreglo dipolo-dipolo mediante la utilización
de un adecuado modelo de análisis de inversión
tridimensional, puede ser muy útil en aplicaciones
ingenieriles y ambientales, especialmente en las aplicaciones
relacionadas con delimitación tridimensional de cavidades
subterráneas.
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Enero de 2010
Comparison of Wenner and dipole-dipole arrays in the study
of an underground three-dimensional cavity
Authors: Ahmad Neyamadpour, W A T Wan Abdullah
et al
Link: Click here
Abstract
The objective of this paper was to compare Wenner and dipole-dipole
configurations in delineating an underground cavity at a site
near the University of Malaya, Malaysia. A three-dimensional
electrical resistivity imaging survey was carried out along
seven parallel lines using Wenner and dipole-dipole arrays.
A three-dimensional least-squares algorithm, based on the
robust inversion method, was used in the inversion of the
apparent resistivity data. In the inverted model, both the
horizontal and vertical extents of the anomalous zones were
displayed. Results indicate the superiority of the Wenner
array over the dipole-dipole array for determining the vertical
distribution of the subsurface resistivity, although the dipole-dipole
array produced a better lateral extent of the subsurface features.
The results show that the three-dimensional electrical resistivity
imaging survey using both the Wenner and dipole-dipole arrays,
in combination with an appropriate three-dimensional inversion
method and synthetic model analysis, can be highly useful
for engineering and environmental applications, especially
for underground three-dimensional cavity detection.
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Enero de 2010
Application of the resistivity/gravity joint inversion
technique for Nubian sandstone aquifer assessment on the area
located at the central part of Sinai, Egypt
Authors: S A Sultan, F A Monteiro Santos
et al
Link: Click here
Abstract
Eleven deep vertical electrical soundings of AB/2 spacing
ranging from 5 to 3000 m were carried out to investigate the
upper part of the Nubian groundwater aquifer at the central
part of Sinai, Egypt. These soundings have been jointly inverted
using the SA algorithm with 160 gravity stations
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measured in the study
area, assuming that density and resistivity contrast are represented
by coincident interfaces. One hundred and sixty magnetic stations
were executed at the same locations as gravity measurements
to estimate the depth of basement rocks. The results of the
joint interpretation indicated that the depth of the groundwater
aquifer ranges from 500 to 800 m with resistivity values ranging
from 6 to 562 O m, suggesting that the fresh water is of good
quality towards the northern part of the area. The top of the
basement, which is mainly defined by gravity and magnetic data,
lies at a depth ranging from 830 to 2788 m. The results also
show that the aquifer configuration is controlled by different
regional faults in the NNW-SSE direction. |
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Enero de 2010
Extraction of structure-based geoelectric models by hybrid
genetic algorithms
Authors: Irfan Akça and Ahmet Tugrul
Basokur
Link: Click here
Abstract
A major difficulty in electrical resistivity imaging is the
identification of the lithologic units, especially in the
sedimentary environments. The geologic interpretation generally
is realized by visual inspection of the final resistivity
section. Although sharp boundary inversion techniques based
on a local linearization could allow the delineation of interfaces
between geologic units, these techniques will succeed only
if an initial model already close to the best solution is
available. Stochastic algorithms might localize a point around
the global minimum of the misfit function; however, they are
not efficient at finding the precise solution. For this reason,
our previously published hybrid genetic algorithms, derived
from evolution theories, are used to verify structure-based
models. The geometric parameters are defined by thickness
values of the lithologic units at control points distributed
along the horizontal
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axis.
A zero thickness value indicates the nonexistence of a certain
unit at the corresponding con-trol point. An unstructured grid
composed of irregular triangles is constructed by the application
of Delaunay triangulations to represent complicated structural
boundaries. In addition, the computation time for the calculation
of model response is reduced greatly by this strategy. Because
the suggested parameterization reduces the number of unknown
parameters to a few tens and the computation time for the model
responses is reduced by the Delaunay triangulation, the implementation
of hybrid genetic algorithms for 2D problems becomes possible.
A huge number of models are generated randomly in the first
generation (a population of parameters) and then updated in
subsequent generations by the simulation of biological processes.
The suggested algorithms consist of two computational phases.
In the first stage, the physical property of each subsurface
layer is represented by a distinct resistivity value. After
some succeeding genera-tions, laterally varying resistivities
within the same lithologic unit are permitted to simulate lateral
changes in geologic conditions. |
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Enero de 2010
Defining the translational velocity of the reference frame
of Earth
Author: Donald F. Argus
Link: Click here
Abstract
Earth's centre is fundamental to geodesy and geoscience because
motions of sites on the surface are estimated relative to
it. International Terrestrial Reference Frames ITRF2000 and
ITRF2005 are defined by the centre of mass of Earth's system
(CM), consisting of solid Earth, the ice sheets, the oceans,
and the atmosphere. Satellite LAGEOS rotates about CM; satellite
laser ranging (SLR) is used to estimate the velocity of CM
relative to sites on the surface. However, ITRF2000 and ITRF2005
differ by 1.8 mm yr-1, suggesting that the velocity of CM
is constrained poorly by SLR.
In this study, we define Earth's reference frame with the
centre of mass of solid Earth (CE). Site velocities estimated
using SLR, VLBI, GPS and DORIS are corrected for a postglacial
rebound model and inverted for the rotational velocities of
the plates and the rotational and translational velocities
of the four space techniques. Because the postglacial rebound
predictions are relative to
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CE, the velocity of
CE relative to sites on the surface is estimated. Because the
input SLR site velocities are relative to CM, the output SLR
translational velocity is the velocity of CM relative to CE.
The estimated velocity of CE does not depend strongly on the
postglacial rebound model corrected for. Equal within uncertainties
and having a root mean square of 0.5 mm yr-1 are estimates of
the velocity of CE determined assuming that plate interiors
are deforming radially as predicted by three postglacial rebound
models and an estimate of the velocity of CE determined assuming
that parts of plate interiors neither beneath nor along the
margins of the late Pleistocene ice sheets are not deforming
laterally.
The velocity of CE equals within uncertainties (probability
greater than 5 per cent) the velocity of CM in ITRF2000. The
velocity of CE differs significantly (0.05 per cent probability)
from the velocity of CM in ITRF2005. Earth's reference frame
(and, we believe, ITRF's) should be defined with the tightly
constrained velocity of CE, not with the poorly constrained
velocity of CM. Because CE is believed to be moving relative
to CM no faster than 0.5 mm yr-1, the velocity of CE estimated
in this study is likely to be nearer the true velocity of CM
than is the velocity of CM estimated using SLR. |
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Enero de 2010
Time domain classification and quantification of seismic
noise in an urban environment
Authors: J. C. Groos and J. R. R. Ritter
Link: Click here
Abstract
Broad-band urban seismic noise (USN) must be considered as
a temporally and spatially non-stationary random process.
Due to the high variability of USN a single measure like the
standard deviation of a seismic noise time-series or the power
spectral density at a given frequency is not enough to characterize
a sample (time-series) of USN comprehensively.Therefore, we
use long-term spectrograms and propose an automated statistical
classification in the time domain to quantify and characterize
USN. Long-term spectrograms of up to 28 d duration are calculated
from a broad-band seismic data set recorded in the metropolitan
area of Bucharest, Romania, to identify the frequency-dependent
behaviour of the timevariable processes contributing to USN.
Based on the spectral analysis eight frequency ranges between
8 mHz and 45 Hz are selected for our proposed time domain
classification. The classification scheme identifies deviations
from the Gaussian distribution of 4-hr-long timeseries of
USN. Our classification is capable to identifyGaussian distributed
seismic noise timeseries as well as
|
time-series
dominated by transient or periodic signals using six noise classes.
Four additional noise classes are introduced to identify corrupt
time-series. The performance of the method is tested with a
synthetic data set. We also apply the statistical classification
for the data set from Bucharest in three time windows (0-4,
8-12 and 13-17 EET) at 11 d in the eight frequency ranges. Only
40 per cent of the analysed time-series are observed to be Gaussian
distributed. Most common deviations from the Gaussian distribution
(~47 per cent) are due to the influence of large-amplitude transient
signals. In all frequency ranges between 0.04 and 45 Hz significant
variations of the statistical properties of USN are observed
with daytime, indicating the broad-band human influence on USN.
We observe the human activity as a dominant influence on the
USN above and below the frequency band of ocean-generated microseism
between 0.04 and 0.6 Hz. Our time domain classification and
quantification is furthermore capable to resolve the influence
of wind on seismic noise and a known site effect variation in
the metropolitan area of Bucharest. The information about noise
amplitudes and statistical properties derived automatically
from broad-band seismic data can be used to select time windows
containing adequate data for seismic noise utilization like
H/V-studies or ambient noise tomography. |
|
Enero de 2010
Energy for air capture
Author: Andrew Dessler
Link: Click here
Abstract
The use of air capture of carbon dioxide as a method for mitigating
climate change is gaining traction in the debate surrounding
geoengineering. The potential of air capture is generally
assessed in terms of monetary cost, estimated to be about
a few hundred dollars per ton of carbon, leading to the suggestion
that the costs of air capture are equal to, or even more favourable
than, the costs of conventional mitigation efforts.
However, the ultimate physical constraint on air capture is
the energy required to power the systems. At present it takes
30-60 GJ to chemically remove one ton of carbon from the atmosphere,
comparable to the 40-70 GJ of
|
energy generated during
the production of one ton of emissions. Thus, using today's
technology, it takes at least 0.5 J of energy to capture the
emissions generated in producing 1 J of fossil fuel energy.
If the energy for capturing carbon comes from fossil fuels,
then at least a third of society's fossil fuel energy would
have to be diverted to air capture to eliminate all emissions.
Alternatively, it would require an increase in the total production
of fossil fuel energy of at least a third.
Neither option is impossible, but neither is definitively attainable,
and both would be expensive and painful for the public. And
by no means is either option obviously easier or cheaper than
conventional mitigation. Technological breakthroughs may decrease
the amount of energy required for air capture, but even capture
energies of 20 and 10 GJ per ton of carbon require significant
diversion of energy to air capture… |
|
Enero de 2010
Slow diffusive fault slip propagation following the 6 April
2009 L'Aquila earthquake, Italy
Authors: A. Amoruso, L. Crescentini et
al
Link: Click here
Abstract
Two laser strainmeters that operate at 1400-m depth, about
20 km NE of the epicenter of the 6 April 2009 magnitude-6.3
L'Aquila, Italy, earthquake, have produced a clear record
of postseismic strain. Here we show the results from the analysis
of the data related to the first few
|
days
after the event. Strain after about 1.5 days is fully consistent
with afterslip on a stationary region of the earthquake causative
fault. The preceding few-hour-long transient (whose seismic
moment history is quasi-exponential) is fully consistent with
unilateral diffusive slip propagation toward the shallower part
of the same fault. The propagation path ends where later afterslip
probably occurred. Slip propagation similar to heat diffusion
has been suggested to explain the observed scaling law between
amplitude and duration of slow earthquakes; here we give the
first observational evidence of the role and details of slow
rupture propagation. |
|
Diciembre de 2009
A severe drought event in northern China in winter 2008-2009
and the possible influences of La Niña and Tibetan
Plateau
Authors: Hui Gao and Song Yang
Link: Click here
Abstract
Severe drought occurred in northern China in winter 2008-09
and the La Niña event might have exerted a major influence
on the drought. Both correlation analysis and case investigation
indicate that severe wintertime droughts in northern China
mostly occur under La Niña conditions. The La Niña
event in 2008-2009 increased the differences in temperature
and atmospheric pressure between the Indo-Pacific Oceans and
the Asian continent and intensified the northeasterly East
Asian winter monsoon flow. In winter 2008-2009, the western
Pacific subtropical high was located farther south than normal,
associating with a southward shift of the
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Intertropical Convergence
Zone so that wet and warm flow could hardly reach northern China
from the tropical oceans. Furthermore, the La Niña event
in 2008-2009 strengthened the upstream portion of the upper
tropospheric jet stream and the descending branch of the meridional
cell over the subtropics, causing a negative divergence anomaly
to the northern side of the jet entrance. As a result, the airflow
over northern China is more convergent (divergent) at the upper
(lower) levels, unfavorable for precipitation. In winter 2008-2009,
the surface and tropospheric temperatures over the Tibetan Plateau
were above normal. As a result, the India-Burma trough was shallower
and less active, weakening the moisture transportation from
the Bay of Bengal to eastern China. This feature was possibly
related to the less snow over the plateau. Results also show
that both the Niño3.4 SST and the Tibet Plateau temperature
in the previous autumn can be considered indicators of the winter
drought in northern China. |
|
Diciembre de 2009
Time domain analysis of thin-wire antennas over lossy ground
using the reflection-coefficient approximation
Authors: M. Fernández Pantoja, A.
G. Yarovoy et al
Link: Click here
Abstract
This paper presents a procedure to extend the methods of moments
in time domain for the transient analysis of thin-wire antennas
to include those cases where the antennas are located over
a lossy half-space. This extended technique is based on the
reflection coefficient (RC) approach, which approximates the
fields incident on the ground interface as plane waves and
calculates
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the
time domain RC using the inverse Fourier transform of Fresnel
equations. The implementation presented in this paper uses general
expressions for the RC which extend its range of applicability
to lossy grounds, and is proven to be accurate and fast for
antennas located not too near to the ground. The resulting general
purpose procedure, able to treat arbitrarily oriented thin-wire
antennas, is appropriate for all kind of half-spaces, including
lossy cases, and it has turned out to be as computationally
fast solving the problem of an arbitrary ground as dealing with
a perfect electric conductor ground plane. Results show a numerical
validation of the method for different half-spaces, paying special
attention to the influence of the antenna to ground distance
in the accuracy of the results. |
|
Diciembre de 2009
Space radiation environment forecast for EGYPTSAT-2 satellite
Authors: S. W. Samwel and A. A. Hady
Link: Click here
Abstract
The space environment provides an assortment of hazards whose
ill effects can range from degraded performance up to catastrophic
loss of a spacecraft. The radiation environment is believed
to be the most significant in terms of spacecraft failures.
Hence, the present work provides a radiation analysis for
the EGYPTSAT-2 which is supposed to be launched in 2012 as
a low Earth orbit satellite in order to assist the EGYPTSAT-2
instrument team with adequate planning decisions. AE-8, AP-8,
Jet Propulsion Laboratory (JPL) model, and the cosmic ray
effects in microelectronic (CREME86) code are
|
used to estimate the
fluences of the trapped electrons and protons, solar protons,
and galactic cosmic protons, respectively. SHIELDOSE-2 code
is used for space-shielding radiation dose calculations, and
the nonionizing energy loss function is used to estimate the
nonionizing dose of space radiation. Finally, the end-of-life
solar cell performance is evaluated using the displacement damage
dose (DDD) method. The slowed down spectra emerging from the
shielding material is obtained using the Multilayered Shielding
Simulation Software (MULASSIS) code. It has been found that
the radiation environment will not impede the sensitivity of
EGYPTSAT-2 materials over the course of the baseline mission
lifetime. For 1.5 mm aluminum shielding thickness, total ionizing
dose is 2.65 × 104 rads (Si) and DDD is 7.75 × 107
MeV/g(Si) for 5 years mission length, which are less than critical
thresholds. Also, a flat glass of SiO2 sheet of thickness 0.5
mm is enough to resist the damage effect of the solar array
cells. |
|
Diciembre de 2009
Source geometry from exceptionally high resolution long
period event observations at Mt Etna during the 2008 eruption
Authors: Louis De Barros, Christopher J.
Bean et al
Link: Click here
Abstract
Sezione di Catania, Istituto Nazionale di Geofisica e Vulcanologia,
Catania, Italy
During the second half of June, 2008, 50 broadband seismic
stations were deployed on Mt Etna volcano in close proximity
to the summit, allowing us to observe seismic activity with
exceptionally high resolution. 129 long period events (LP)
with dominant frequencies ranging
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between
0.3 and 1.2 Hz, were extracted from this dataset. These events
form two families of similar waveforms with different temporal
distributions. Event locations are performed by cross-correlating
signals for all pairs of stations in a two-step scheme. In the
first step, the absolute location of the centre of the clusters
was found. In the second step, all events are located using
this position. The hypocentres are found at shallow depths (20
to 700 m deep) below the summit craters. The very high location
resolution allows us to detect the temporal migration of the
events along a dike-like structure and 2 pipe shaped bodies,
yielding an unprecedented view of some elements of the shallow
plumbing system at Mount Etna. These events do not seem to be
a direct indicator of the ongoing lava flow or magma upwelling.
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Diciembre de 2009
On the secular evolution of groundwater on Mars
Authors: Robert E. Grimm and Scott L. Painter
Link: Click here
Abstract
We modeled the subsurface transport of H2O and CO2 on Mars
in a two-dimensional pole-to-equator cross-section, starting
with sudden surface freezing representing ancient climate
change. We find that excursions to low obliquity strongly
drive ice sublimation and subsequent
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groundwater evaporation
at low latitudes. This creates a hydraulic gradient in the saturated
zone that moves water equatorward and even sublimates the base
of high-latitude ice. Eventually, all H2O is lost at latitudes
less than ~30°. A subcryospheric vadose zone may be retained
at higher latitudes, but ultimately only a few monolayers of
adsorbed water will be held. A subcryospheric phreatic zone
is preserved in the same regions only where lateral heterogeneity
restricts horizontal fluid flow. The predicted contemporary
state of Mars is drier and with groundwater-if present at all-in
different locations than previously considered. |
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Diciembre de 2009
Capture of solar wind alpha-particles by the Martian atmosphere
Authors: G. M. Chanteur, E. Dubinin et
al
Link: Click here
Abstract
Integration along He++ test-particle trajectories in the self-consistent
electromagnetic fields generated by three-dimensional hybrid
simulations of the solar wind/Mars interaction is used to
evaluate the removal of solar wind a-particles due to charge-exchange
processes with neutral species of the Martian exosphere. The
total
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removal
rate of solar wind He++ ions, transformed into either singly
ionised or neutral helium, is equal to 6.7 × 1023 s-1,
which corresponds approximately to 30% of the flux of solar
a-particles through the planetary cross-section. The deposition
rate of helium neutral atoms, created by double electronic capture
on exospheric oxygen, impacting the exobase, and penetrating
below where it can be trapped, is about 1.5 × 1023 s-1.
That means an important contribution of the solar wind source
to the helium balance of the Martian atmosphere. The implantation
of the solar helium into the Martian atmosphere shows an asymmetry
related to the orientation of the motional electric field of
the solar wind, -VSW × BIMF. |
|
Diciembre de 2009
Observed El Niño-Southern Oscillation temperature
signal in the stratosphere
Authors: Melissa Free and Dian J. Seidel
Link: Click here
Abstract
Studies of stratospheric temperature variability typically
include seasonal, quasi-biennial oscillation, solar, and volcanic
effects, but the response to El Niño-Southern Oscillation
(ENSO) is less well recognized. Modeling work suggests that
ENSO may produce effects on surface climate at high latitudes
by interaction with the polar stratosphere, yet until recently,
past work has often failed to find a statistically significant
ENSO response in polar stratospheric temperature observations.
Using zonal mean temperatures
|
from several improved
radiosonde data sets beginning in 1958, we show a significant
El Niño cooling signal in the tropical stratosphere and
warming signal in the Arctic stratosphere in winter. In the
tropical stratosphere the difference of more than 1 K between
El Niño and La Niña temperatures is similar in
magnitude to the tropospheric warming signal. The significant
signal, derived from regression analysis, of more than 4 K in
the winter Arctic stratosphere is generally largest in the lower
stratosphere and extends into the upper troposphere. The signal,
with a maximum in late winter, accounts for 14% to 25% of stratospheric
temperature variability at 100 mbar in Arctic winter in radiosonde
and reanalysis data. Satellite-derived temperatures show significant
El Niño cooling in the tropical stratosphere in boreal
winter, but the warming signal in the Arctic stratosphere is
not statistically significant in that data set. |
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Diciembre de 2009
A numerical method for investigating crystal settling in
convecting magma chambers
Authors: J. Verhoeven and J. Schmalzl
Link: Click here
Abstract
Magma chambers can be considered as thermochemically driven
convection systems. We present a new numerical method that
describes the movement of crystallized minerals in terms of
active spherical particles in a convecting magma that is represented
by an infinite Prandtl number fluid. The main part focuses
on the results we obtained. A finite volume thermochemical
convection model for two and three dimensions and a discrete
element method, which is used to model granular material,
are combined. The new model is validated with floating experiments
using particles of different densities and an investigation
of single and multiparticle settling velocities. The resulting
velocities are compared with theoretical predictions by Stokes's
law and a hindered settling function for the multiparticle
system. Two
|
fundamental convection regimes are identified
in the parameter space that is spanned by the Rayleigh number
and the chemical Rayleigh number, which is a measure for the
density of the particles. We define the T regime that is dominated
by thermal convection. Here the thermal driving force is strong
enough to keep all particles in suspension. As the particles
get denser, they start settling to the ground, which results
in a C regime. The C regime is characterized by the existence
of a sediment layer with particle-rich material and a suspension
layer with few particles. It is shown that the presence of
particles can reduce the vigor of thermal convection. In the
frame of a parameter study we discuss the change between the
regimes that is systematically investigated. We show that
the so-called TC transition fits a power law. Furthermore,
we investigate the settling behavior of the particles in vigorous
thermal convection, which can be linked to crystal settling
in magma chambers. We develop an analytical settling law that
describes the number of settled particles against time and
show that the results fit the observations from numerical
and laboratory experiments.
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Diciembre de 2009
Complex nonvolcanic tremor near Parkfield, California,
triggered by the great 2004 Sumatra earthquake
Authors: Abhijit Ghosh, John E. Vidale
et al
Link: Click here
Abstract
In several instances, the passing surface waves from large
earthquakes have ignited nonvolcanic tremor (NVT) on major
faults. Still, the mechanism of tremor and its reaction to
the dynamic stressing from various body and surface waves
is poorly understood. We examine tremor near Parkfield, California,
beneath the San Andreas fault triggered by the Mw 9.2, 2004
Sumatra earthquake. The prolonged shaking produces the
|
richest
and the most varied observations of dynamically triggered tremor
to date. The tremor appears in at least three distinct locations
and shows activity pulsing with encouraging stress, as has been
observed in other cases. The greatest amount of triggering and
tremor modulation accompanies the long-period Love waves. Rayleigh
waves, on the other hand, appear to be less effective in exciting
tremor sources. Also, at times, the tremor stops before the
surface waves are complete, at other times it continues quivering
after the waves have passed. While tremor is found to be sensitive
to small stress changes, there are times when stresses of comparable
magnitudes do not trigger noticeable tremor. Some tremors in
this NVT sequence appear to be associated with the passage of
P waves, which is unusual and surprising given the small stresses
they impart. |
|
Noviembre de 2009
Coeval compressional deformation and volcanism in the central
Andes, case studies from northern Chile (23°S-24°S)
Authors: Gabriel González, José
Cembrano et al
Link: Click here
Abstract
In this contribution we examine the relationship between active
compression and construction of Pleistocene volcanoes in the
present-day magmatic arc of the central Andes (23°S-24°S).
Deformation produced several N-S striking, ~40
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km long subparallel
ridges. These ridges formed by folding of Pliocene ignimbrites
and upper Pliocene and Pleistocene lavas; they are asymmetrical
in profile and have a gentle back limb and steeper frontal limb.
Andesitic monogenetic volcanoes show a close spatial relationship
with the ridges; some volcanoes are on the hinge zone, whereas
others lay on the sides of the ridges. We interpret this spatial
pattern as a result of magma storage and migration along a system
of subhorizontal reservoirs and reverse faults. Magma reservoirs
probably formed along flat portions of reverse faults between
ramp structures that serve as episodic magma transport pathways.
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Noviembre de 2009
Global terrestrial water storage capacity and flood potential
using GRACE
Authors: J. T. Reager and J. S. Famiglietti
Link: Click here
Abstract
Terrestrial water storage anomaly from the Gravity Recovery
and Climate Experiment (GRACE) and precipitation observations
from the Global Precipitation Climatology Project (GPCP) are
applied at the regional scale to show the usefulness of a
remotely sensed, storage-based flood potential method. Over
the GRACE record
|
length,
instances of repeated maxima in water storage anomaly that fall
short of variable maxima in cumulative precipitation suggest
an effective storage capacity for a given region, beyond which
additional precipitation must be met by marked increases in
runoff or evaporation. These saturation periods indicate the
possible transition to a flood-prone situation. To investigate
spatially and temporally variable storage overflow, a monthly
storage deficit variable is created and a global map of effective
storage capacity is presented for possible use in land surface
models. To highlight a flood-potential application, we design
a monthly global flood index and compare with Dartmouth Flood
Observatory flood maps. |
|
Noviembre de 2009
Influence of gas hydrate morphology on the seismic velocities
of sands
Authors: Jeffrey A. Priest, Emily V. L.
Rees et al
Link: Click here
Abstract
This paper reports the results of a series of resonant column
tests on specimens where gas hydrate has been formed in sands
using an "excess water" technique. In these specimens
the amount of hydrate formed is restricted by the amount of
gas in the specimen and with an excess of water being present
in the pore space. Results of resonant column tests carried
out to determine compressional and shear wave velocities suggest
that gas hydrate formed in this way are frame supporting.
In contrast, the
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behavior observed
in sands where the hydrate is formed from finite water where
the remaining pore space is saturated with methane gas, termed
in this paper the "excess gas" method, exhibits a
cementing behavior, while tetrahydrofuran-hydrate sands or where
the hydrate is formed from dissolved methane within the pore
water, exhibit a pore-filling behavior for hydrate saturations
less than 40%. For sands where the hydrate is formed using the
excess water method, much larger volumes of hydrate are required
before a significant increase in shear wave velocity occurs,
although increases in compressional wave velocity are seen at
lower hydrate contents. These results suggest that hydrate interaction
with the sediment is strongly dependent on morphology, and that
natural hydrate may exhibit contrasting seismic signatures depending
upon the geological environment in which it forms. |
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Noviembre de 2009
Very low frequency earthquakes related to small asperities
on the plate boundary interface at the locked to aseismic
transition
Authors: Yoshihiro Ito, Kazushige Obara
et al
Link: Click here
Abstract
Very low frequency (VLF) seismic signals observed in southwestern
Japan are evidently radiated from shear slips on the upper
surface of the subducting Philippine Sea Plate. We used grid
moment tensor inversion and centroid moment tensor inversion
to calculate 242 moment tensor solutions with moment magnitudes
between 3.1 and 3.8 from continuous seismograms recorded over
a 5 year period by using a very dense broadband seismic network.
At least 5-10 sequences of repetitive activity were observed
|
during
the 5 years. The VLF events formed clustered distributions along
the 35 km isodepth contour of the subducting plate surface.
The nodal planes (which dip landward) of moment tensor solutions
of the VLF events reflected the configuration of the subducting
plate interface. The slip vectors were consistent with the direction
of movement of the subducting plate; the dip and strike of the
slip vectors clearly reflected the configuration of the upper
surface of the subducting plate. We found that the rates of
seismic moment release per unit area associated with five major
VLF clusters were very similar, although both the seismic magnitudes
and sizes of the clusters varied considerably. The rate of seismic
moment released from detectable VLF sources was 0.1% of the
rates of short-term slow slip events, suggesting that the source
areas occupied only 0.1% of the fault segment on which the short-term
slow slip events occurred. |
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Noviembre de 2009
The 2006-2007 Kuril Islands great earthquake sequence
Authors: Thorne Lay, Hiroo Kanamori et
al
Link: Click here
Abstract
The southwestern half of a ~500 km long seismic gap in the
central Kuril Island arc subduction zone experienced two great
earthquakes with extensive preshock and aftershock sequences
in late 2006 to early 2007. The nature of seismic coupling
in the gap had been uncertain due to the limited historical
record of prior large events and the presence of distinctive
upper plate, trench and outer rise structures relative to
adjacent regions along the arc that have experienced repeated
great interplate earthquakes in the last few centuries. The
intraplate region seaward of the seismic gap had several shallow
compressional events during the preceding decades (notably
an MS 7.2 event on 16 March 1963), leading to speculation
that the interplate fault was seismically coupled. This issue
was partly resolved by failure of the shallow portion of the
interplate megathrust in an MW = 8.3 thrust event on 15 November
2006. This event ruptured ~250 km along the seismic gap, just
northeast of the great 1963 Kuril Island (Mw = 8.5) earthquake
rupture zone. Within minutes of the thrust event, intense
earthquake activity commenced beneath the outer wall of the
trench seaward of the interplate rupture, with the larger
events having normal-faulting mechanisms. An unusual double
band of interplate and intraplate aftershocks
|
developed. On 13 January
2007, an MW = 8.1 extensional earthquake ruptured within the
Pacific plate beneath the seaward edge of the Kuril trench.
This event is the third largest normal-faulting earthquake seaward
of a subduction zone on record, and its rupture zone extended
to at least 33 km depth and paralleled most of the length of
the 2006 rupture. The 13 January 2007 event produced stronger
shaking in Japan than the larger thrust event, as a consequence
of higher short-period energy radiation from the source. The
great event aftershock sequences were dominated by the expected
faulting geometries; thrust faulting for the 2006 rupture zone,
and normal faulting for the 2007 rupture zone. A large intraplate
compressional event occurred on 15 January 2009 (Mw = 7.4) near
45 km depth, below the rupture zone of the 2007 event and in
the vicinity of the 16 March 1963 compressional event. The fault
geometry, rupture process and slip distributions of the two
great events are estimated using very broadband teleseismic
body and surface wave observations. The occurrence of the thrust
event in the shallowest portion of the interplate fault in a
region with a paucity of large thrust events at greater depths
suggests that the event removed most of the slip deficit on
this portion of the interplate fault. This great earthquake
doublet demonstrates the heightened seismic hazard posed by
induced intraplate faulting following large interplate thrust
events. Future seismic failure of the remainder of the seismic
gap appears viable, with the northeastern region that has also
experienced compressional activity seaward of the megathrust
warranting particular attention. |
|
Noviembre de 2009
Seismic tremor at the 9°50'N East Pacific Rise eruption
site
Authors: P. W. Monigle, D. R. Bohnenstiehl
et al
Link: Click here
Abstract
Ocean bottom seismic observations within the 9°50'N region
of the East Pacific Rise indicate persistent, low-amplitude
tremor activity throughout the October 2003 through February
2007 period of monitoring. These signals exhibit either monochromatic
or polychromatic spectral characteristics, with a ~6 Hz fundamental
frequency and up to two harmonics. Individual events cannot
be correlated between nearby (<1 km) stations, implying
the presence of multiple, small-amplitude sources positioned
within the shallow crust. Tremor exhibits a semidiurnal periodicity,
with some stations recording activity during times of increasing
tidal extension and others detecting tremor signals during
times of increasing compression. The amplitude, duration,
|
and
rate of activity also correlate positively with fortnightly
changes in the amplitude of the tides. These spatiotemporal
patterns are consistent with tremor generation in response to
tidally modulated fluid flow within a network of shallow cracks.
Tremor energy flux is spatially and temporally heterogeneous;
however, there are extended periods of greater and lesser activity
that can be tracked across portions of the array. Despite their
shallow crustal origin, changes in tremor amplitude and spectral
character occur in the months prior to a major microearthquake
swarm and inferred seafloor spreading event on 22 January 2006,
with an increase in the degree of correlation between tremor
activity and tidal strain in the weeks leading up to this event.
After the spreading event, two eruption-surviving stations near
the axis continue to show high rates of tremor activity, whereas
these signals are suppressed at the single station recovered
from the near-axis flanks. This off-axis quiescence may result
from the dike-induced closing of cracks or perhaps from the
emplacement of impermeable flows near the station. |
|
Noviembre de 2009
Mapping Curie temperature depth in the western United States
with a fractal model for crustal magnetization
Authors: Claire Bouligand, Jonathan M.
G. Glen et al
Link: Click here
Abstract
We have revisited the problem of mapping depth to the Curie
temperature isotherm from magnetic anomalies in an attempt
to provide a measure of crustal temperatures in the western
United States. Such methods are based on the estimation of
the depth to the bottom of magnetic sources, which is assumed
to correspond to the temperature at which rocks lose their
spontaneous magnetization. In this study, we test and apply
a method based on the spectral analysis of magnetic anomalies.
Early spectral analysis methods assumed that crustal magnetization
is a completely uncorrelated function of position. Our method
incorporates a more realistic representation where magnetization
has a fractal distribution defined by three independent parameters:
the depths to the top and bottom of
|
magnetic sources and
a fractal parameter related to the geology. The predictions
of this model are compatible with radial power spectra obtained
from aeromagnetic data in the western United States. Model parameters
are mapped by estimating their value within a sliding window
swept over the study area. The method works well on synthetic
data sets when one of the three parameters is specified in advance.
The application of this method to western United States magnetic
compilations, assuming a constant fractal parameter, allowed
us to detect robust long-wavelength variations in the depth
to the bottom of magnetic sources. Depending on the geologic
and geophysical context, these features may result from variations
in depth to the Curie temperature isotherm, depth to the mantle,
depth to the base of volcanic rocks, or geologic settings that
affect the value of the fractal parameter. Depth to the bottom
of magnetic sources shows several features correlated with prominent
heat flow anomalies. It also shows some features absent in the
map of heat flow. Independent geophysical and geologic data
sets are examined to determine their origin, thereby providing
new insights on the thermal and geologic crustal structure of
the western United States. |
|
Noviembre de 2009
Source duration of deep very low frequency earthquakes
in western Shikoku, Japan
Authors: Takanori Matsuzawa, Kazushige
Obara et al
Link: Click here
Abstract
The source durations of very low frequency earthquakes in
western Shikoku, Japan were estimated. Nineteen very low frequency
earthquakes were located in this region between
|
14
and 17 March 2007 that migrated southward with tremor activity.
The source durations of four Mw 3.5-3.8 very low frequency earthquakes
were estimated by comparing simulated and observed seismograms
to be 12-18 s. The ratio of seismic moment and source duration
is 1-5 × 1013 Nm/s. This value is several to ten times
larger than those in southern Kii, Japan. Such a ratio difference
may reflect the nature of very low frequency earthquake between
these regions. If a stress drop of 0.003-3 MPa is assumed, then
the slip propagation velocity of very low frequency earthquakes
ranges from several tens to several hundreds of m/s. |
|
Noviembre de 2009
An automatic monitoring system for nonvolcanic tremors
in southwest Japan
Authors: Naoki Suda, Ryoko Nakata et al
Link: Click here
Abstract
The recent development of a system for exchanging and distributing
seismic waveform data over high-speed networks enables seismic
events to be monitored in real time throughout Japan. In the
present study, we have developed an automatic real-time monitoring
system for deep nonvolcanic tremors in southwest Japan. The
system automatically detects the occurrence of nonvolcanic
tremors and determines their hypocenters in real time. In
addition, the system creates image files for the detected
tremor activities and makes them accessible via the
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World Wide Web. To
detect tremors we carry out a two-step numerical hypothesis
test, in which the first step is to test whether two given envelope
seismograms are correlated and the second step is to test whether
an event occurs using the results of the first test. This two-step
test is applied to real-time data every 2 min. Once an event
is detected, we can regard time lags that generate maximum cross
correlations as the travel time differences, which we use for
hypocenter determination. Although this procedure detects nontremor
signals, most of them can be rejected using several criteria.
Since the start of monitoring in 2006, the system has worked
well for detecting a wide variety of tremor activities. Results
from the present system will contribute to understanding the
stress relaxation process in the transition zone between the
locked and stable slip zones of the subducting Philippine Sea
plate interface. |
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Noviembre de 2009
Infrasound observation of the apparent North Korean nuclear
test of 25 May 2009
Authors: Il-Young Che, Tae Sung Kim et
al
Link: Click here
Abstract
On 25 May 2009, a seismic event (mb 4.6) was recorded from
a source in northeastern North Korea, close to the location
of a previous seismic event on 9 October 2006. Both events
have been declared to be nuclear tests by North Korea. For
the more recent test, five seismo-acoustic arrays in South
Korea recorded epicentral infrasonic
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signals.
The signals are characterized by amplitudes from 0.16 to 0.35
microbar and dominant frequencies between 0.8 and 4.3 Hz. Celerities
determined for the arrivals suggest that most of the infrasonic
energy travelled as a stratospheric phase. Based on observed
stratospheric amplitudes, the epicentral infrasonic energy was
estimated to be equivalent to that expected from 3.0 tons of
high explosives detonated on the surface. We conclude that this
small energy estimate is due to the atmospheric coupling from
the strong surface ground motion rather than the direct transfer
of explosion energy to the air. This relatively small infrasonic
to seismic energy ratio could be used to distinguish the event
from a common surface explosion. |
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Noviembre de 2009
Computer-generated global map of valley networks on Mars
Authors: Wei Luo and T. F. Stepinski
Link: Click here
Abstract
The presence of valley networks (VN) on Mars suggests that
early Mars was warmer and wetter than present. However, detailed
geomorphic analyses of individual networks have not led to
a consensus regarding their origin. An additional line of
evidence can be provided by the global pattern of dissection
on Mars, but the currently available global map of VN, compiled
from Viking images, is incomplete and outdated. We created
an updated map of VN by using a computer algorithm that parses
topographic data and recognizes valleys by their morphologic
signature.
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This computer-generated
map was visually inspected and edited to produce the final updated
map of VN. The new map shows an increase in total VN length
by a factor of 2.3. A global map of dissection density, D, derived
from the new VN map, shows that the most highly dissected region
forms a belt located between the equator and mid-southern latitudes.
The most prominent regions of high values of D are the northern
Terra Cimmeria and the Margaritifer Terra where D reaches the
value of 0.12 km-1 over extended areas. The average value of
D is 0.062 km-1, only 2.6 times lower than the terrestrial value
of D as measured in the same fashion. These relatively high
values of dissection density over extensive regions of the planet
point toward precipitation-fed runoff erosion as the primary
mechanism of valley formation. Assuming a warm and wet early
Mars, peculiarity of the global pattern of dissection is interpreted
in the terms of climate controlling factors influenced by the
topographic dichotomy. |
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Noviembre de 2009
Pronóstico en tiempo real posterior a un terremoto
dañino
Autores: Warner Marzocchi y Anna Maria
Lombardi
Link: Click aquí
Abstract
Aquí describimos los resultados de un posible experimento
de pronóstico de terremotos en tiempo real durante
una emergencia sísmica. El día 6 de abril de
2009 un terremoto de magnitud 6.3 Mw devastó la ciudad
de L'Aquila en Italia, cuasando cientos de muertos y una gran
devastación. Inmediatamente después de este
evento, nosotros comenzamos a emitir un pronóstico
diario de terremotos para la región destinado para
la Agencia Italiana de Protección Civil. Los pronósticos
se basaron en un modelo estocático que combina la distribución
de Gutenberg-Richter con la ley de potencia de decaimiento
espaciotemporal de inducción de terremotos. Los resultados
del primer mes de observación posterior al terremoto
de L'Aquila exhibe un buen ajuste entre lo pronosticado y
lo observado, indiciando que es un objetivo realista intentar
conseguir pronósticos confiables. Nuestra experiencia
con este experimento demuestra una urgente necesidad de conectar
los pronósticos probabilísticos con las decisiones,
de modo que antes de las crisis se establezcan protocolos
cuantitativos y transparentes que apoyen las decisiones.
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Noviembre de 2009
Real-time forecasting following a damaging earthquake
Authors: Warner Marzocchi and Anna Maria
Lombardi
Link: Click here
Abstract
We describe the results of a prospective, real-time earthquake
forecast experiment made during a seismic emergency. A Mw
6.3 earthquake struck the city of L'Aquila, Italy on April
6, 2009, causing hundreds of deaths and vast damage. Immediately
following this event, we began producing daily earthquake
forecasts for the region, and we provided these forecasts
to Civil Protection - the agency responsible for managing
the emergency. The forecasts are based on a stochastic model
that combines the Gutenberg-Richter distribution of earthquake
magnitudes and power-law decay in space and time of triggered
earthquakes. The results from the first month following the
L'Aquila earthquake exhibit a good fit between forecasts and
observations, indicating that accurate earthquake forecasting
is now a realistic goal. Our experience with this experiment
demonstrates an urgent need for a connection between probabilistic
forecasts and decision-making in order to establish - before
crises - quantitative and transparent protocols for decision
support.
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Octubre de 2009
Social science in a water observing system
Authors: John B. Braden, Daniel G. Brown
et al
Link: Click here
Abstract
We set forth an argument for the integration of social science
research with natural science and engineering research in
major research infrastructure investments addressing water
science. A program of integrated observation of
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water resources offers
great opportunities to address several environmental "grand
challenges" identified by the National Research Council,
including climate variability, institutions and resource use,
and land use dynamics, and their importance for hydrologic forecasting.
We argue that such a program has the potential to advance both
water science and the contributing disciplines. However, to
realize this potential, it is essential to recognize that social
science requires critical infrastructure funding on the scale
of advanced research facilities in the natural sciences and
engineering. |
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Octubre de 2009
Global climate imprint on seismic noise
Authors: Eléonore Stutzmann, Martin
Schimmel et al
Link: Click here
Abstract
In the absence of earthquakes, oceanic microseisms are the
strongest signals recorded by seismic stations. Using the
GEOSCOPE global seismic network, we show that the secondary
microseism spectra have global characteristics that depend
on the station latitude and on the season. In both hemispheres,
noise amplitude is larger during local winter, and close to
the equator, noise amplitude is stable over the year. There
is an excellent correlation between microseism amplitude variations
over the year and changes in the highest wave areas. Considering
the polarization of the secondary
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microseisms,
we show that stations in the Northern Hemisphere and close to
the equator record significant changes of the secondary microseism
source azimuth over the year. During Northern Hemisphere summer,
part or all of the sources are systematically located farther
toward the south than during winter. Stations in French Guyana
(MPG) and in Algeria (TAM) record microseisms generated several
thousand kilometers away in the South Pacific Ocean and in the
Indian Ocean, respectively. Thus, secondary microseism sources
generated by ocean waves which originate in the Southern Hemisphere
can be recorded by Northern Hemisphere stations when local sources
are weak. We also show, considering a station close to Antarctica,
that primary and secondary microseism noise amplitudes are strongly
affected by changes of the sea ice floe and that sources of
these microseisms are in different areas. Microseism recording
can therefore be used to monitor climate changes. |
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Octubre de 2009
Joint inversion for Vp, Vs, and Vp/Vs at SAFOD, Parkfield,
California
Authors: Haijiang Zhang, Clifford Thurber
et al
Link: Click here
Abstract
We refined the three-dimensional (3-D) Vp, Vs and Vp/Vs models
around the San Andreas Fault Observatory at Depth (SAFOD)
site using a new double-difference (DD) seismic tomography
code (tomoDDPS) that simultaneously solves for earthquake
locations and all three velocity models using both absolute
and differential P, S, and S-P times. This new method is able
to provide a more robust Vp/Vs model than that from the original
DD tomography code (tomoDD), obtained simply by dividing Vp
by Vs. For the new inversion, waveform cross-correlation times
for earthquakes from 2001 to 2002 were also used, in addition
to arrival times from earthquakes and explosions in the region.
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The Vp values extracted
from the model along the SAFOD trajectory match well with the
borehole log data, providing in situ confirmation of our results.
Similar to previous tomographic studies, the 3-D structure around
Parkfield is dominated by the velocity contrast across the San
Andreas Fault (SAF). In both the Vp and Vs models, there is
a clear low-velocity zone as deep as 7 km along the SAF trace,
compatible with the findings from fault zone guided waves. There
is a high Vp/Vs anomaly zone on the southwest side of the SAF
trace that is about 1-2 km wide and extends as deep as 4 km,
which is interpreted to be due to fluids and fractures in the
package of sedimentary rocks abutting the Salinian basement
rock to the southwest. The relocated earthquakes align beneath
the northeast edge of this high Vp/Vs zone. We carried out a
2-D correlation analysis for an existing resistivity model and
the corresponding profiles through our model, yielding a classification
that distinguishes several major lithologies. |
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Octubre de 2009
Self-similar slip pulses during rate-and-state earthquake
nucleation
Authors: Allan M. Rubin and Jean-Paul Ampuero
Link: Click here
Abstract
For a wide range of conditions, earthquake nucleation zones
on rate- and state-dependent faults that obey either of the
popular state evolution laws expand as they accelerate. Under
the "slip" evolution law, which experiments show
to be the more relevant law for nucleation, this expansion
takes the form of a unidirectional slip pulse. In numerical
simulations these pulses often tend to approach, with varying
degrees of robustness, one of a few styles of self-similar
behavior. Here we obtain an approximate self-
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similar
solution that accurately describes slip pulses growing into
regions initially sliding at steady state. In this solution
the length scale over which slip speeds are significant continually
decreases, being inversely proportional to the logarithm of
the maximum slip speed Vmax, while the total slip remains constant.
This slip is close to Dc(1-a/b)-1, where Dc is the characteristic
slip scale for state evolution and a and b are the parameters
that determine the sensitivity of the frictional strength to
changes in slip rate and state. The pulse has a "distance
to instability" as well as a "time to instability,"
with the remaining propagation distance being proportional to
(1-a/b)-2 [ln(Vmax bg/Dc)]-1, where bg is the background state
into which the pulse propagates. This solution provides a reasonable
estimate of the total slip for pulses growing into regions that
depart modestly from steady state. |
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Octubre de 2009
Magmatic processes in the Alaska subduction zone by combined
3-D b value imaging and targeted seismic tomography
Authors: T. van Stiphout, E. Kissling et
al
Link: Click here
Abstract
We combine two complementary seismological methods to study
the deep roots of arc volcanism in the Alaskan subduction
zone: targeted seismic body wave tomography and imaging of
the relative size distribution (b value) of earthquakes. For
the tomography we apply a staggered inversion scheme, starting
with the minimum 1-D velocity model, progressing to a 3-D
velocity model. Inversions are based on traveltime data from
about 5500 events (1971-2002) with approximately 100,000 P
and 50,000 S high-quality arrivals, allowing us to resolve
the 3-D
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velocity field on
a 20 × 20 km grid. For the b value imaging we have introduced
a new 3-D sampling approach to map the b values on surfaces
in the Wadati-Benioff zone (WBZ). Our b value study is based
on 12,474 relocated events of magnitude of completeness =2.4
and depth >40 km. We observe anomalously high b values at
depths around 100 km, appearing locally concentrated beneath
active subduction volcanoes. We believe that these zones are
associated with dehydration of the slab and subsequent magma
generation. The analysis of VP/VS ratios likewise shows indications
of the presence of fluids: significantly higher VP/VS ratios
rising in columns from the top of the WBZ at 100 km depth below
active volcanic centers. On the basis of the joint interpretation,
we propose that our images track fluids from their genesis at
100 km depth at the top of the subducting plate to the bottom
of the crust below volcanoes. |
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Octubre de 2009
Energetic particle evidence for magnetic filaments in Jupiter's
magnetotail
Authors: Matthew E. Hill, Dennis K. Haggerty
et al
Link: Click here
Abstract
Jupiter's immense magnetotail was uniquely traversed and observed
in situ to 2562 Jovian radii (RJ = 71,492 km) for the first
time by the Pluto-bound New Horizons spacecraft during the
first half of 2007. We show evidence that energetic ions with
sulfur-rich composition traceable to Io's volcanism originated
150 ± 40 RJ antisunward of the planet and were recurrently
released and funneled by the magnetic field down the magnetotail.
Ions detected at New Horizons by the Pluto Energetic Particle
Spectrometer Science
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Investigation
(PEPSSI) instrument showed unmistakable signatures of velocity
dispersion. We performed a survey of the unambiguous dispersion
events observed by PEPSSI and provide a quantitative description
of each event's dispersion characteristics. Energetic ions are
sensitive probes of the magnetic field structure in the magnetotail;
so, for a case study of one of the most interesting events,
beginning on day of year 118, 28 April 2007, we are also able
to estimate small-scale features of the magnetotail. Our observations
(which include energies above ~2 keV/nuc and total energies
below ~1 MeV), combined with straightforward simulations of
particle flow, are consistent with narrow spatial structures,
or filaments, on the order of 5 RJ wide in the ~400 RJ wide
> 9000 RJ long magnetotail; the Solar Wind Around Pluto plasma
instrument measurements show coherent structures on a much larger
scale (~500 RJ) in the same region. |
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Octubre de 2009
Cold and dry processes in the Martian Arctic: Geomorphic
observations at the Phoenix landing site and comparisons with
terrestrial cold desert landforms
Authors: Joseph S. Levy, James W. Head
et al
Link: Click here
Abstract
We analyze Surface Stereo Imager observations of rocks, sediments,
and permafrost-related landforms in the vicinity of the Phoenix
lander, comparing the imaged features to analogous
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examples of physical weathering and periglacial
processes observed in the Antarctic Dry Valleys. Observations
at the Phoenix landing site of pitted rocks, "puzzle
rocks" undergoing in-situ breakdown, perched clasts,
and thermal contraction crack polygon morphologies strikingly
similar to terrestrial sublimation polygons, all strongly
suggest that stable (non-churning) permafrost processes dominate
the Phoenix landing site. Morphological evidence suggests
that cold-desert processes, in the absence of wet active-layer
cryoturbation, and largely driven by sublimation of buried
ice (either pore ice, excess ice, or both) are shaping the
landscape.
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Octubre de 2009
West Antarctic Rift System in the Antarctic Peninsula
Authors: Graeme Eagles, Robert D. Larter
et al
Link: Click here
Abstract
Decades after the recognition of the West Antarctic Rift System,
and in spite of its global importance, the location and nature
of the plate boundary it formed at are unknown east of the
Byrd Subglacial Basin. Alternative constructions of the circuit
of South Pacific plate boundaries suggest the
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presence
of either a transcurrent plate boundary or a continuation of
the extensional rift system. We identify George VI Sound, a
curved depression separating Alexander Island from Palmer Land,
as the easternmost basin of a rift system that terminated at
a triple junction with the Antarctic Peninsula subduction zone.
The history of the triple junction's third, transform, arm suggests
extension started around 33.5-30 Ma. A more speculatively identified
basin further west may have formed earlier during the same episode
of rifting, starting around 43 Ma. Proposals of earlier Cenozoic
relative motion between East and West Antarctica cannot be verified
from this region. |
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Octubre de 2009
Fore-arc deformation controls frequency-size distribution
of megathrust earthquakes in subduction zones
Authors: Matthias Rosenau and Onno Oncken
Link: Click here
Abstract
Seismotectonic deformation in subduction zones seems to follow
rather simple spatiotemporal patterns with fore-arc basins
overlying the areas of large slip during quasiperiodic megathrust
earthquakes. To study the possible coupling between long-term
deformation and earthquake behavior, we use compressive granular
wedges overlying a rate- and state-dependent frictional interface
as analogue models of subduction zone fore arcs overlying
a seismogenic megathrust. For different seismogenic zone geometries,
we analyze deformation time series with respect to the accumulation
of permanent strain and frequency-size distributions of episodic
slip events equivalent to great (M > 8) earthquakes.
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We observe that permanent
deformation in the wedges localizes at the periphery of unstable
slip at depth over tens of simulated seismic cycles. For updip-limited
seismogenic zone models, this leads to structural wedge segmentation
characterized by an elastic domain overlying the zone of unstable
basal slip. Along with the evolution of segmentation the frequency-size
distributions of episodic slip events develop from more random,
Gutenberg-Richter-like events (b value ~0.6) toward more periodic,
characteristic events (b value <0.1). Corresponding coefficients
of variation (C v ) of recurrence intervals decrease from C
v ˜ 0.6 in deforming wedges to C v ˜ 0.3 in segmented
wedges. From the experiments we thus infer a positive feedback
between fore-arc tectonics and megathrust seismogenesis which
brings the system from a stochastic to a more deterministic
state. Our experimental observations imply that the quasiperiodic
recurrence of great subduction earthquakes evident from existing
earthquake records is a long-term feature intrinsically related
to the seismotectonic segmentation of the fore-arc wedges. |
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Octubre de 2009
Fore-arc deformation controls frequency-size distribution
of megathrust earthquakes in subduction zones
Authors: Matthias Rosenau and Onno Oncken
Link: Click here
Abstract
Seismotectonic deformation in subduction zones seems to follow
rather simple spatiotemporal patterns with fore-arc basins
overlying the areas of large slip during quasiperiodic megathrust
earthquakes. To study the possible coupling between long-term
deformation and earthquake behavior, we use compressive granular
wedges overlying a rate- and state-dependent frictional interface
as analogue models of subduction zone fore arcs overlying
a seismogenic megathrust. For different seismogenic zone geometries,
we analyze deformation time series with respect to the accumulation
of permanent strain and frequency-size distributions of episodic
slip events equivalent to great (M > 8) earthquakes. We
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observe
that permanent deformation in the wedges localizes at the periphery
of unstable slip at depth over tens of simulated seismic cycles.
For updip-limited seismogenic zone models, this leads to structural
wedge segmentation characterized by an elastic domain overlying
the zone of unstable basal slip. Along with the evolution of
segmentation the frequency-size distributions of episodic slip
events develop from more random, Gutenberg-Richter-like events
(b value ~0.6) toward more periodic, characteristic events (b
value <0.1). Corresponding coefficients of variation (C v
) of recurrence intervals decrease from C v ˜ 0.6 in deforming
wedges to C v ˜ 0.3 in segmented wedges. From the experiments
we thus infer a positive feedback between fore-arc tectonics
and megathrust seismogenesis which brings the system from a
stochastic to a more deterministic state. Our experimental observations
imply that the quasiperiodic recurrence of great subduction
earthquakes evident from existing earthquake records is a long-term
feature intrinsically related to the seismotectonic segmentation
of the fore-arc wedges. |
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Octubre de 2009
Did the Zipingpu Reservoir trigger the 2008 Wenchuan earthquake?
Authors: Shemin Ge, Mian Liu et al
Link: Click here
Abstract
The devastating May 2008 Wenchuan earthquake (Mw 7.9) resulted
from thrust of the Tibet Plateau on the Longmen Shan fault
zone, a consequence of the Indo-Asian continental collision.
Many have speculated on the role played by the Zipingpu Reservoir,
impounded in 2005 near the epicenter,
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in triggering the
earthquake. This study evaluates the stress changes in response
to the impoundment of the Zipingpu Reservoir and assesses their
impact on the Wenchuan earthquake. We show that the impoundment
could have changed the Coulomb stress by -0.01 to 0.05 MPa at
locations and depth consistent with reported hypocenter positions.
This level of stress change has been shown to be significant
in triggering earthquakes on critically stressed faults. Because
the loading rate on the Longmen Shan fault is <0.005 MPa/yr,
we thus suggest that the Zipingpu Reservoir potentially hastened
the occurrence of the Wenchuan earthquake by tens to hundreds
of years. |
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Octubre de 2009
A mechanistic model for the spectral induced polarization
of clay materials
Authors: P. Leroy and A. Revil
Link: Click here
Abstract
Water-saturated clay-rich media exhibit low-frequency (1 Hz
to 1 MHz) effective conductivity and effective permittivity
dispersions that are the consequence of both the polarization
of the mineral/water interface coating the surface of the
grains and the Maxwell-Wagner polarization. These low-frequency
properties are modeled by combining (1) a complexation model
of the surface properties of clay minerals (kaolinite, illite,
and smectite), (2) a polarization model of the Stern layer
(the inner portion of the electrical double layer coating
the surface of the minerals), and (3) a macroscopic model
comprising the electrochemical polarization of the grains
and the contribution of the Maxwell-Wagner effect. The macroscopic
model is based on the differential
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effective
medium theory. It includes a convolution product with the grain
size distribution. For kaolinite, the diffuse layer occupies
a small fraction of the pore space and is considered as part
of the surface of the grains. This is due to the low specific
surface area of kaolinite. In the case of illite and smectite,
the situation is different. Because of the high specific surface
areas of these minerals, the diffuse layer occupies a large
fraction of the pore space and is considered as part of the
pore space and is described using a Donnan equilibrium model.
We obtain excellent comparisons between various experimental
data reported in the literature and our model. Then, we considered
low-porosity (compacted or cemented) clay rocks and shales.
Here too, we obtained a good agreement between the data and
the predictions of a model based on a volume-averaging approach.
We also note that at very low frequencies (<1 Hz), another
polarization mechanism exists that is not reproduced by our
model. We believe that this polarization corresponds to a nonlinear
membrane polarization contribution. |
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Octubre de 2009
Limitations of the modeling of geomagnetically induced
currents in the South African power network
Authors: Chigomezyo M. Ngwira, Lee-Anne
McKinnell et al
Link: Click here
Abstract
Geomagnetically induced currents (GICs) are known to affect
electric power systems in both the midlatitude and high-latitude
regions. Monitoring of GICs in the southern African electrical
power grid first started in 1998 with the installation of
the Electric Power Research Institute Sunburst
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monitoring system.
Recent research efforts in South Africa have shown that the
modeling of GICs is effectively improved by the use of a multilayered
ground conductivity model and a modified set of network coefficients.
This paper reports on an investigation into the reliability
of a new GIC model versus the distance between the magnetometer
stations and the GIC measuring site using recent developments
within this field and the South African context. Results show
that the modeling of GICs degrades with increasing distance
between the geomagnetic observation station and the GIC site
and that the newly developed GIC model is only appropriate for
the specific geomagnetic station and GIC site pair. |
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Octubre de 2009
Estimating allowable carbon emission for CO2 concentration
stabilization using a GCM-based Earth system model
Authors: Toru Miyama and Michio Kawamiya
Link: Click here
Abstract
For atmospheric CO2 concentration stabilization, we projected
allowable carbon emission with an Earth system model based
on a general circulation model. Our calculations on centennial
timescale in various scenarios reveal how
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saturation
with respect to CO2 and climate-carbon cycle feedback reduce
natural carbon uptake, and hence allowable emission. In 450
ppm stabilization scenario, for example, climate-carbon cycle
feedback reduces the accumulative allowable carbon emission
until year 2300 from 1248 to 980 PgC. The Emission at the year
2050 is about the half of the year 2000 level for the SP450
scenario. Terrestrial carbon cycle is especially susceptible
to climate-carbon cycle feedback, and is a significant source
of projection uncertainty. Our model responds nonlinearly to
CO2 and climate, suggesting process-based models are indispensable
tool for future climate-carbon cycle projections. |
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Octubre de 2009
On the role of groundwater and soil texture in the regional
water balance: An investigation of the Nebraska Sand Hills,
USA
Authors: Tiejun Wang, Erkan Istanbulluoglu
et al
Link: Click here
Abstract
Partitioning of precipitation between evapotranspiration and
runoff at the basin scale is primarily controlled by climate
and basin characteristics. Here we use the Budyko hypothesis
to investigate the impacts of soil texture and groundwater
(e.g., baseflow) on annual and long-term mean annual water
balances of basins in a semiarid region located in and around
the Sand Hills region of Nebraska, USA. Native grasslands
are dominant across the study area with soil textures dramatically
different for the Nebraska Sand Hills (primarily sand deposits)
and the adjacent regions (mainly less permeable silt loam).
For each basin, long-term hydrologic and meteorological data
are obtained, and a baseflow index is calculated using daily
streamflow data to quantify the groundwater contribution to
streamflow. We found sound field evidence that suggests the
control of soil texture
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on mean annual water
balance and the impact of groundwater storage on interannual
variability of water balance at catchment scales, which is usually
seen at plot scales and in theoretical models. Our results show
that compared to the basins located in the adjacent regions,
the Nebraska Sand Hills basins experience much higher long-term
mean annual surface runoff ratios (R/P) and thus lower ratios
of (P-R) / P, where P is the long-term mean annual precipitation.
The high baseflow index and its positive correlation with the
ratio of R/P in the Sand Hills basins illustrate the role of
lower (higher) evapotranspiration (recharge) rates. On annual
time scales, the baseflow-dominated basins exhibit a negative
relationship between annual (P - R) / P and annual aridity index
(i.e., the ratio of annual potential evapotranspiration to annual
precipitation), which deviates from the original Budyko hypothesis.
Moreover, with decreasing baseflow index, this negative relationship
gradually transits into a positive relationship that follows
Budyko's curves. Our results suggest that soil textural differences
may strongly modify the impact of climate on regional water
balance. These findings may have important implications for
managing water resources under a fluctuating climate. |
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Octubre de 2009
Prediction of unsaturated soil hydraulic conductivity with
electrical conductivity
Authors: Claude Doussan and Stéphane
Ruy
Link: Click here
Abstract
Soil hydraulic conductivity (K) varies greatly with matric
potential (h) and exhibits a high variability at the field
scale. However, this key property for estimating water flux
in soils is difficult to measure. In contrast, soil electrical
conductivity (s) is easier to measure and is influenced by
the same parameters affecting K. We derive a simple relationship
between s and K(h) and test it
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against
laboratory and literature data. Importantly, we show that parameters
of this s-K(h) relationship can be completely determined with
accessible measurements of saturated hydraulic conductivity,
electrical conductivity of the soil solution, and clay content.
This results in K(h) estimation with a RMSE ranging between
0.4 and 0.5 for log K, i.e., of the order of most experimental
determinations of K. A further test of the s-K(h) relationship
on the large UNSODA hydraulic database shows good agreement
and the robustness of the relationship. Such a relation could
be useful in the spatial monitoring of water fluxes at the field
scale using electrical resistivity tomography if the s(h) relationship
can be obtained. |
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Octubre de 2009
IMF clock angle control of multifractality in ionospheric
velocity fluctuations
Authors: G. A. Abel, M. P. Freeman et al
Link: Click here
Abstract
We present an analysis of 8 years of meridional line-of-sight
ionospheric plasma velocity measurements from the Halley SuperDARN
radar which investigates the effect of the interplanetary
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magnetic field (IMF)
clock angle on the scaling exponents of the first three order
velocity structure functions. We only use velocity measurements
made poleward of the open/closed magnetic field line boundary
in the nightside ionosphere. The measured scaling exponents
are consistent with multifractal Kraichnan-Iroshnikov turbulence
for all clock angles but with varying intermittency that decreases
to zero during purely northward IMF conditions. We thus propose
that intermittency is inherited from the solar wind but also
discuss other possible reasons for this relationship. |
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Octubre de 2009
On the deformation cycle of a strongly
coupled plate interface: The triple earthquakes of 16 March
1963, 15 November 2006, and 13 January 2007 along the Kurile
subduction zone
Authors: M. Raeesi and K. Atakan
Link: Click here
Abstract
The great plate interface Kurile earthquake of 15 November
2006 (M w = 8.3) was immediately followed by normal aftershocks
in the outer rise of the central Kurile subduction zone. Two
months later at a distance <50 km from this event the 13
January 2007 (M w = 8.1) earthquake with a pure normal mechanism
occurred in the outer rise of the central Kurile. The 15 November
2006 earthquake ruptured an area of 280 × 150 km2 with
maximum slip of almost 7 m, while the 13 January 2007 earthquake
ruptured an area of 240 × 40 km2 with a maximum slip
of almost 14 m. These two significant events were preceded
by a major reverse earthquake on 16 March 1963 (M w = 7.7)
that took place close to the location of the 13 January 2007
earthquake. These very different earthquakes demonstrate the
different stages of
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the
deformation cycle along strongly coupled plates at the central
Kurile arc and present a rare case of stress regime reversal
over a short time span. In order to understand the details of
these three earthquakes, we inverted for the source rupture
processes of the earthquakes and delineated the location and
dimension of asperities. The role of gravity and topography
anomalies and their correlation to the asperity distribution
as well as the long-term seismicity is studied in detail. Using
trench-parallel Bouguer anomaly, we have identified the differences
in the asperities and the location of larger normal outer rise
aftershocks as well as the location of 13 January 2007 earthquake.
We also show that the long-term seismicity of the central Kurile
arc is confined to the intraslab and not the plate interface.
The static stress transfer for these earthquakes showed that
the stress on optimally oriented thrust faults is increased
in the northeastern part of the rupture area of the 15 November
2006 earthquake. Absence of major and great earthquakes to the
northeastern immediate vicinity of the rupture area of the 15
November 2006 earthquake supports the hypothesis that this area
is mature for the next strong earthquake along the Kurile arc.
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Octubre de 2009
Evidence for a 3.45-billion-year-old magnetic remanence:
Hints of an ancient geodynamo from conglomerates of South
Africa
Authors: Yoichi Usui, John A. Tarduno et
al
Link: Click here
Abstract
Paleomagnetic and rock magnetic analyses of ~3445-million-year-old
dacite conglomerate clasts and parent body rocks from the
Barberton greenstone belt, South Africa, define two dominant
components of magnetization. One component, unblocked at low
temperature, is an overprint acquired ~180 million years ago.
The other component is unblocked at high temperatures and
passes a conglomerate test, indicating that this component
is older than the depositional age
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of the conglomerate
(~3416 Ma). The high unblocking temperature component shows
scatter in the parent body rocks that can be explained by the
effects of modern lightning strikes, Archean overprinting, and
the presence of multidomain magnetic grains that are conducive
to carrying secondary magnetizations. Alternatively, this scatter
can be explained by exotic magnetization scenarios in the absence
of a dynamo, including magnetization by an external field related
to solar wind interaction with the atmosphere. Such exotic mechanisms
can be tested with the acquisition of paleointensity data. While
more scattered than paleomagnetic data recording the more recent
geomagnetic field, the high unblocking temperature component
in the dacite parent body shows some consistency, and the simplest
explanation of the data is that they reflect a geodynamo ~3445
million years ago. |
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Septiembre de 2009
Model of formation of double structure gas hydrates in
Lake Baikal based on isotopic data
Authors: Akihiro Hachikubo, Oleg Khlystov
et al
Link: Click here
Abstract
We measured the isotopic compositions of methane (C1) and
ethane (C2) of hydrate-bound gas and of dissolved gas in pore
water retrieved from bottom sediments in Lake Baikal. Both
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structure
I (sI:3%C2) and II (sII:14%C2) gas hydrates are observed in
the same sediment cores in Kukuy K-2 mud volcano. We found that
C2 dD of sI gas hydrate is larger than that of sII, whereas
C1 d 13C, C1 dD and C2 d 13C values are practically the same
in both hydrate structures. d 13C of C1 and C2 of hydrate-bound
gas are several permil smaller than those in pore water, showing
that the current pore water is not the source of gas hydrates.
These findings lead to a new model where the sII gas hydrates
were formed prior to the sI hydrates. |
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Septiembre de 2009
An updated Antarctic melt record through 2009 and its linkages
to high-latitude and tropical climate variability
Authors: Marco Tedesco and Andrew J. Monaghan
Link: Click here
Abstract
A 30-year minimum Antarctic snowmelt record occurred during
austral summer 2008-2009 according to spaceborne microwave
observations for 1980-2009. Strong positive phases of both
the El-Niño Southern Oscillation (ENSO) and the Southern
Hemisphere Annular Mode (SAM) were recorded during the months
leading up to and
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including the 2008-2009
melt season. The 30-year record confirms that significant negative
correlations exist at regional and continental scales between
austral summer melting and both the ENSO and SAM indices for
October-January. In particular, the strongest negative melting
anomalies (such as those in 2008 and 2009) are related to amplified
large-scale atmospheric forcing when both the SAM and ENSO are
in positive phases. Our results suggest that enhanced snowmelt
is likely to occur if recent positive summer SAM trends subside
in conjunction with the projected recovery of stratospheric
ozone levels, with subsequent impacts on ice sheet mass balance
and sea level trends. |
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Septiembre de 2009
Comparison of modeling methods for the determination of
effective porosities and diffusion coefficients in through-diffusion
tests
Authors:Jennifer L. Benning and David L.
Barnes
Link: Click here
Abstract
Diffusion coefficients and effective, or transport, porosities
are important parameters in the assessment of the fate and
transport of contaminants in fractured rock systems and when
contaminant transport needs to be considered in zones of low
to negligible advective flows. This study presents and discusses
various solutions, analytical and semianalytical, to Fick's
law for the
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typical
through-diffusion experiment and applies these solutions to
the results of three through-diffusion experimental data sets.
These through-diffusion experiments were conducted on two natural
breccia samples, with bulk measured porosities of approximately
13.7% and 16.8%, and one manufactured porous plate sample with
a bulk measured porosity of 40.9%, such that the samples displayed
order of magnitude differences in their diffusivities. For each
experiment, the derived effective porosities and intrinsic diffusion
coefficients for each method are compared. It was shown that
the semianalytical solution provides an improved method for
reliably estimating the effective porosity from through-diffusion
experimental data; however, there is little difference between
methods in the derived intrinsic diffusion coefficients. |
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Septiembre de 2009
Scaling of seismicity induced by nonlinear fluid-rock interaction
Authors: S. A. Shapiro and C. Dinske
Link: Click here
Abstract
Fluid injections into geothermal systems sometimes can produce
significant seismic events with magnitudes of up to 4. However,
in the case of hydraulic fracturing of hydrocarbon reservoirs,
such events occur extremely seldom. In the last case, in contrast
to the former one, the structure of rocks is being actively
destroyed (e.g., opening of tensile fractures) and the fluid-rock
interaction is strongly nonlinear (e.g., a strong increase
of permeability). What is the role of this nonlinearity? We
consider nonlinear pore pressure diffusion to explain features
of seismicity. We formulate seismicity triggering
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front. Its propagation
is sensitive to a grade of nonlinearity, to spatial dimension
of diffusion, and to the injection rate. We show that a probability
of an event with a magnitude larger than a given one increases
proportionally to the injected mass. An increase of this probability
with time is insensitive to the nonlinearity. We compare different
borehole experiments. In some cases the injection produces clearly
nonlinear impact on rocks. In others this impact is approximately
linear. We find a well agreement with our theory. We observe
an insensitivity of temporal increments of magnitude probability
to the grade of nonlinearity. These increments are controlled
by injection-rate increments. In contrast, nonlinear fluid-rock
interactions are characterized by a strong dominance of small
earthquakes. Defects activated by a nonlinear diffusion possibly
obey Gutenberg-Richter statistics with anomalous high b values.
In the case of a linear diffusion, magnitude distributions of
events are probably inherited from pre-existing fracture systems.
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Septiembre de 2009
Low frequency cultural noise
Authors: Dong-Hoon Sheen, Jin Soo Shin
et al
Link: Click here
Abstract
Abnormal cultural seismic noise is observed in the frequency
range of 0.01-0.05 Hz. Cultural noise generated by human activities
is generally observed in frequencies above 1 Hz, and is greater
in the daytime than at night. The low-frequency noise presented
in this paper exhibits a characteristic amplitude variation
and can be
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easily
identified from time domain seismograms in the frequency range
of interest. The amplitude variation is predominantly in the
vertical component, but the horizontal components also show
variations. Low-frequency noise is markedly periodic, which
reinforces its interpretation as cultural noise. Such noise
is observed world-wide, but is limited to areas in the vicinity
of railways. The amplitude variation in seismograms correlates
strongly with railway timetables, and the waveform shows a wavelength
shift associated with the Doppler effect, which indicates that
the origin of seismic background noise in the frequency range
0.01-0.05 Hz is railways. |
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Septiembre de 2009
Determination of earthquake focal depths and source time
functions in central Asia using teleseismic P waveforms
Authors: Risheng Chu, Lupei Zhu et
al
Link: Click here
Abstract
We developed a new method to determine earthquake source time
functions and focal depths. It uses theoretical Green's function
and a time-domain deconvolution with positivity constraint
to estimate the source time function from the teleseismic
P waveforms. The
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earthquake focal depth
is also determined in the process by using the time separations
of the direct P and depth phases. We applied this method to
606 earthquakes between 1990 and 2005 in Central Asia. The results
show that the Centroid Moment Tensor solutions, which are routinely
computed for earthquake larger than M5.0 globally using very
long period body and surface waves, systematically over-estimated
the source depths and durations, especially for shallow events.
Away from the subduction zone, most of the 606 earthquakes occurred
within the top 20 km of crust. This shallow distribution of
earthquakes suggests a high geotherm and a weak ductile lower
crust in the region. |
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Septiembre de 2009
Coseismic thermal pressurization can notably prolong earthquake
recurrence intervals on weak rate and state friction faults:
Numerical experiments using different constitutive equations
Authors: Yuta Mitsui and Kazuro Hirahara
Link: Click here
Abstract
We add a new perspective to component factors of earthquake
cyclicity, namely coseismic thermal pressurization (TP) within
fluid-saturated fault zones, which is pore fluid pressurization
caused by frictional heating. By using a single degree of
freedom system with a rate- and state-dependent friction law,
we show that the short-lived TP can prolong earthquake recurrence
intervals. This lengthening effect can operate even without
any notable shear heating in weak faults. Moreover, if the
maximum increase in temperature is above a certain level,
the permeability rather than the
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maximum
temperature becomes important for the lengthening effect. Lower
permeability causes longer recurrence intervals. By contrast,
narrower slip zones (more pronounced heating) do not simply
prolong recurrence intervals, although they entail higher dynamic
undershoot and energy radiation. These features do not depend
on whether the assumed evolution law is the Ruina law or the
Dieterich law. However, our results indicate that if the degree
of TP changes for each earthquake, the ideal time-predictable
model for earthquake cycles can be applicable only in the case
of faults obeying the Ruina law. Furthermore, on the basis of
the above-mentioned dependence of the interval on the permeability,
we point out that it is necessary to measure the permeability
rather than the slip zone thickness (or the increase in temperature)
in order to estimate the TP effect on long-term earthquake cycles.
Although it is currently difficult to measure the permeability
under ground, measurements should be performed in the light
of the importance of permeability in the prediction of future
seismic hazards. |
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Septiembre de 2009
Forecasting change of the magnetic field using core surface
flows and ensemble Kalman filtering
Authors: C. D. Beggan and K. A. Whaler
Link: Click here
Abstract
Accurate forecasting of the change of the Earth's internal
magnetic field over short intervals of time (e.g., less than
five years) has many applications for government, academic
and commercial users. Forecasting can be achieved by making
a number of reasonable assumptions about how the main field
interacts with the flow in the liquid outer core. In particular,
the magnetic field can be considered to be entrained in the
large scale flow along the core-mantle boundary surface over
short time periods, giving rise to measurable change at the
Earth's surface. The observed change (or secular
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variation) at or above
the surface of the Earth can thus be inverted to produce flow
models; these can be used to propagate fluid parcels threaded
by the field forwards in time to forecast the non-linear change
of the magnetic field. In addition to prediction of field change
by flow models, it would be advantageous to include observations
of the field from satellite measurements or ground-based observatories.
We therefore present a method using Ensemble Kalman Filtering
(EnKF) to produce an optimal assimilation between magnetic field
change as forecast from core flow models and direct observations
of the field. We show, by assuming a steady flow and assimilating
field observations annually, it is possible to produce a forecast
over five years with less than 30nT root mean square difference
from the 'true' field - within an assumed error budget. The
EnKF method also allows sensitivity analysis of the field models
to noise and uncertainty within the physical representation.
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Septiembre de 2009
"Choppy wave" model for nonlinear gravity waves
Authors: Frédéric Nouguier,
Charles-Antoine Guérin et al
Link: Click here
Abstract
We investigate the statistical properties of a three-dimensional
simple and versatile model for weakly nonlinear gravity waves
in infinite depth, referred to as the "choppy wave model"
(CWM). This model is analytically tractable, numerically efficient,
and robust to the inclusion of high frequencies. It is based
on horizontal rather than vertical local displacement of a
linear surface and is a priori not restricted to large wavelengths.
Under the assumption of space and time
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stationarity,
we establish the complete first- and second-order statistical
properties of surface random elevations and slopes for long-crested
as well as fully two-dimensional surfaces, and we provide some
characteristics of the surface variation rate and frequency
spectrum. We establish a relationship between the so-called
"dressed spectrum," which is the enriched wave number
spectrum of the nonlinear surface, and the "undressed"
one, which is the spectrum of the underlying linear surface.
The obtained results compare favorably with other classical
analytical nonlinear theories. The slope statistics are further
found to exhibit non-Gaussian peakedness characteristics. Compared
to observations, the measured non-Gaussian omnidirectional slope
statistics can only be explained by non-Gaussian effects and
are consistently approached by the CWM. |
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Septiembre de 2009
Possible deep fault slip preceding the 2004 Parkfield earthquake,
inferred from detailed observations of tectonic tremor
Author: David R. Shelly
Link: Click here
Abstract
Earthquake predictability depends, in part, on the degree
to which sudden slip is preceded by slow aseismic slip. Recently,
observations of deep tremor have enabled inferences of deep
slow slip even when detection by other means is not
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possible, but these
data are limited to certain areas and mostly the last decade.
The region near Parkfield, California, provides a unique convergence
of several years of high-quality tremor data bracketing a moderate
earthquake, the 2004 magnitude 6.0 event. Here, I present detailed
observations of tectonic tremor from mid-2001 through 2008 that
indicate deep fault slip both before and after the Parkfield
earthquake that cannot be detected with surface geodetic instruments.
While there is no obvious short-term precursor, I find unidirectional
tremor migration accompanied by elevated tremor rates in the
3 months prior to the earthquake, which suggests accelerated
creep on the fault ~16 km beneath the eventual earthquake hypocenter.
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Septiembre de 2009
Improved confidence in (U-Th)/He thermochronology using
the laser microprobe: An example from a Pleistocene leucogranite,
Nanga Parbat, Pakistan
Authors: J. W. Boyce, K. V. Hodges et al
Link: Click here
Abstract
The newly developed laser microprobe (U-Th)/He thermochronometer
permits, for the first time, the ability to generate precise
(U-Th)/He cooling ages for even very young (<1 Ma) samples
with a spatial resolution on the order of tens of micrometers.
This makes it possible to test the reproducibility of independent
(U-Th)/He age determinations within individual crystals, further
increasing the reliability of the method. As an example, we
apply it here to a Pleistocene granite from Nanga Parbat,
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Pakistan,
where previous constraints on the thermal history are consistent
with rapid exhumation and cooling. Twenty-one (U-Th)/He dates
determined on two monazite crystals from a single granite sample
yield a mean of 748,000 years with a ~95% confidence level of
±19,000 years. There is no discernible variation in the
distribution of (U-Th)/He ages in the cores of these crystals
and therefore no evidence for the development of substantial
diffusive-loss 4He zoning over 80% of the interior of the monazite
crystals during postcrystallization cooling of the granite.
Modeling of these data suggests that cooling at a mean rate
of ~300 K/Ma would be necessary to produce the observed ages
and the lack of a 4He gradient, which is consistent with preexisting
constraints for Nanga Parbat. Increased precision in thermochronology
permits more tightly constrained exhumation models, which should
aid geologic interpretation. |
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Septiembre
de 2009
Impacto de la geometría del megaempuje sobre la
inversión del deslizamiento cosísmico a partir
de datos geodésicos: Aplicación al terremoto
chileno de 1960.
Autores: M. S. Moreno, J. Bolte et al
Link: Click Aquí
Abstract
Nosotros analizamos el papel de la geometría del megaempuje
sobre la estimación del deslizamiento utilizando datos
del terremoto chileno de 1960 (M W = 9.5) como ejemplo. Barrientos
y Ward (1990) obtuvieron una distribución variable
para el deslizamiento aplicando un modelo de dislocación
elástica con geometría de falla plana. Su modelo
muestra zonas de deslizamiento con profundidades de 80 a 110
Km, aisladas de la zona sismogénica e interpretadas
como deslizamiento asísmico. Nosotros invertimos los
mismos datos geodésicos utilizando el Modelo de Elementos
FInitos con una geometría precisa a partir de datos
geofísicos. Se implementó el MEF Isoparamétrico
para ajustar la distribución del deslizamiento en los
elementos curvos. El deslizamiento obtenido se limitó
a la zona superficial de la interfase de la placa sugiriendo
que los sectores profundos aislados de momento fueron una
especie de artefacto de la geometría plana. Nuestro
estudio enfatiza la importancia de la geometría de
falla en la estimación del deslizamiento de los grandes
terremotos.
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Septiembre de 2009
Impact of megathrust geometry on inversion of coseismic
slip from geodetic data: Application to the 1960 Chile earthquake
Authors: M. S. Moreno,
J. Bolte et al
Link: Click here
Abstract
We analyze the role of megathrust geometry
on slip estimation using the 1960 Chile earthquake (M W =
9.5) as an example. A variable slip distribution for this
earthquake has been derived by Barrientos and Ward (1990)
applying an elastic dislocation model with a planar fault
geometry.
Their model shows slip patches at 80-110 km depth, isolated
from the seismogenic zone, interpreted as aseismic slip. We
invert the same geodetic data set using a finite element model
(FEM) with precise geometry derived from geophysical data.
Isoparametric FEM is implemented to constrain the slip distribution
of curve-shaped elements. Slip resolved by our precise geometry
model is limited to the shallow region of the plate interface
suggesting that the deep patches of moment were most likely
an artifact of the planar geometry. Our study emphasizes the
importance of fault geometry on slip estimation of large earthquakes.
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Agosto de 2009
Freshwater transport at Fimbulisen, Antarctica
Authors: Graham J. Walkden, Karen J. Heywood
et al
Link: Click here
Abstract
The intricate near-circumpolar system of fronts and currents
surrounding Antarctica isolates much of Earth's freshwater
from the saline oceans immediately north. The Antarctic Slope
Front sustains bathymetrically steered flow at the shelf break,
whereas the shallow Coastal Current travels rapidly alongside
the ice front. A hydrographic survey of the southeastern Weddell
Sea finds these two features to have merged near the narrow
(<40 km wide) continental shelf at Fimbulisen. On the prime
meridian, its Trolltunga ice tongue overshoots the shelf break
northward into this slope current. Observations on either
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side
of the ice tongue demonstrate its retarding effect on the westward-flowing
waters it overhangs and its contribution to the poorly understood
freshwater budget. From oxygen isotope ratio measurements and
referenced geostrophic shears, we find the combined glacial
meltwater and sea ice melt transport upstream of Trolltunga
at 0.6°E to account for 18.0 ± 5.8 mSv of the total
1.6 ± 0.2 Sv westward transport (Sv = 106 m3 s-1). In
Trolltunga's lee and downstream at 2.8°W, we find this figure
to ultimately increase to 23.8 ± 15.5 mSv of a total
2.8 ± 0.4 Sv transport. Each of these sections was impeded
by sea ice cover, so these estimates of westward transport are
probably lower limits. The westward glacial ice meltwater transport
of 10 ± 3 mSv at 2.8°W highlights the role that Fimbulisen
plays in preconditioning shelf waters before they reach broad
continental shelves in the southwestern Weddell Sea where they
transform to bottom waters. |
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Agosto de 2009
A new method for high-resolution characterization of hydraulic
conductivity
Authors: Inês Garcia Nobre Silva,
Dominique Weis et al
Link: Click here
Abstract
A new probe has been developed for high-resolution characterization
of hydraulic conductivity (K) in shallow unconsolidated formations.
The
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probe was recently
applied at the Macrodispersion Experiment (MADE) site in Mississippi
where K was rapidly characterized at a resolution as fine as
0.015 m, which has not previously been possible. Eleven profiles
were obtained with K varying up to 7 orders of magnitude in
individual profiles. Currently, high-resolution (0.015-m) profiling
has an upper K limit of 10 m/d; lower-resolution (˜0.4-m)
mode is used in more permeable zones pending modifications.
The probe presents a new means to help address unresolved issues
of solute transport in heterogeneous systems. |
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Agosto de 2009
Why climate sensitivity may not be so unpredictable
Authors: A. Hannart, J.-L. Dufresne et
al
Link: Click here
Abstract
Different explanations have been proposed as to why the range
of climate sensitivity predicted by GCMs has not lessened
substantially in the last decades, and subsequently if it
can be reduced. One such study (Why is climate sensitivity
so unpredictable?) addressed these questions using rather
simple theoretical considerations and reached the conclusion
that reducing
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uncertainties
on climate feedbacks and underlying climate processes will not
yield a large reduction in the envelope of climate sensitivity.
In this letter, we revisit the premises of this conclusion.
We show that it results from a mathematical artifact caused
by a peculiar definition of uncertainty used by these authors.
Applying standard concepts and definitions of descriptive statistics
to the exact same framework of analysis as Roe and Baker, we
show that within this simple framework, reducing inter-model
spread on feedbacks does in fact induce a reduction of uncertainty
on climate sensitivity, almost proportionally. Therefore, following
Roe and Baker assumptions, climate sensitivity is actually not
so unpredictable. |
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Agosto de 2009
A nudging data assimilation algorithm for the identification
of groundwater pumping
Authors: Wei-Chen Cheng, Donald R. Kendall
et al
Link: Click here
Abstract
This study develops a nudging data assimilation algorithm
for estimating unknown pumping from private wells in an aquifer
system using measured data of hydraulic head. The proposed
algorithm treats the unknown pumping as an additional sink
term in the governing equation of groundwater flow and provides
a consistent physical interpretation for pumping rate identification.
The algorithm identifies the
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unknown pumping and,
at the same time, reduces the forecast error in hydraulic heads.
We apply the proposed algorithm to the Las Posas Groundwater
Basin in southern California. We consider the following three
pumping scenarios: constant pumping rates, spatially varying
pumping rates, and temporally varying pumping rates. We also
study the impact of head measurement errors on the proposed
algorithm. In the case study we seek to estimate the six unknown
pumping rates from private wells using head measurements from
four observation wells. The results show an excellent rate of
convergence for pumping estimation. The case study demonstrates
the applicability, accuracy, and efficiency of the proposed
data assimilation algorithm for the identification of unknown
pumping in an aquifer system. |
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Agosto de 2009
Leaching systematics and matrix elimination for the determination
of high-precision Pb isotope compositions of ocean island
basalts
Authors: Inês Garcia Nobre Silva,
Dominique Weis et al
Link: Click here
Abstract
Ocean island basalts from Hawaii and Kerguelen were analyzed
for their Pb isotopic compositions to assess the effect of
acid leaching and matrix elimination by Pb anion exchange
columns on reproducibility and accuracy. Unleached samples
consistently yield Pb isotopic ratios that reflect the incorporation
of foreign material. Leaching removes up to 70-80% of the
total Pb content of the samples with corresponding weight
losses between 35 and 60%. The older and more altered Kerguelen
basalts show better external
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reproducibility
than the Hawaiian basalts, which appears to be due to the presence
in the Hawaiian samples of more radiogenic contaminants (e.g.,
seawater Pb, drilling mud, and related alteration phases). All
leached samples purified twice on anion exchange columns show
more radiogenic Pb isotopic ratios than those processed once.
The difference is larger for tholeiitic basalts (Hawaiian and
Kerguelen Plateau) than for transitional to alkalic basalts
(Kerguelen Archipelago). The small differences in measured ratios
of total procedural triplicates reflect differential elimination
of residual alteration via leaching and matrix effects. The
effectiveness of matrix elimination depends on the specific
basalt composition, and tholeiitic basalts (i.e., low Pb concentrations)
require two passes on anion exchange columns. This study shows
that all steps in sample processing are critical for achieving
accurate high-precision Pb isotopic compositions of ocean island
basalts. |