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Mayo de 2006
Relationship between rupture process complexity and earthquake
size.
Authors: K. Sato and J. Mori.
Link: www.agu.org/pubs/crossref/
2006/2005JB003614.shtml
Abstract
We studied the complexity of waveforms for two shallow crustal
earthquake sequences in Japan. Using 47 earthquakes of the
2003 northern Miyagi sequence and 95 events of the 2004 mid-Niigata
sequence, we estimated the number of complex and simple P
waveforms for events with magnitudes from M3.1 to M6.6. Data
were recorded at borehole and surface stations with
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minimal site responses for close hypocentral
distances of less than 30 km. The determination of complexity
was done by counting the zero crossings of the accelerations
waveforms. Our results show that for the smaller earthquakes,
there are both simple and complex events but that all of the
larger earthquakes are complex. There appears to be a threshold
size over which the earthquakes always become complex. For
both data sets this threshold is M4.0 to M4.5. We attribute
this difference in complexity with earthquake size as an indication
that larger earthquake grow with a complex ruptures that may
be indicative of stochastic processes. There do not appear
to be large earthquakes that grow smoothly to their final
size.
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Mayo de 2006
Ionospheric GPS total electron content (TEC) disturbances
triggered by the 26 December 2004 Indian Ocean tsunami.
Authors: Jann-Yenq Liu, Yi-Ben Tsai et al.
Link: www.agu.org/pubs/crossref/
2006/2005JA011200.shtml
Abstract
Tsunami ionospheric disturbances (TIDs) of the 26 December
2004 M w 9.3 Sumatra earthquake are detected by the total
electron content (TEC) of
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ground-based receivers of the global positioning system (GPS)
in the Indian Ocean area. It is found that the tsunami waves
triggered atmospheric disturbances near the sea surface, which
then traveled upward with an average velocity of about 730
m/s (2700 km/hr) into the ionosphere and significantly disturbed
the electron density within it. Results further show that
the TIDs, which have maximum height of about 8.6–17.2
km, periods of 10–20 min, and horizontal wavelengths
of 120–240 km, travel away from the epicenter with an
average horizontal speed of about 700 km/hr (190 m/s) in the
ionosphere.
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Mayo de 2006
Increase of shear wave velocity before the 1998 eruption
of Merapi volcano (Indonesia)
Authors: U. Wegler, B.-G. Lühr, R. Snieder and A. Ratdomopurbo.
Link: www.agu.org/pubs/crossref/
2006/2006GL025928.shtml
Abstract
We infer temporal changes in the elastic
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properties of the edifice of Merapi volcano (Java, Indonesia)
before its eruption in 1998 by analyzing multiply scattered
elastic waves excited by a repeatable controlled seismic source.
A pre-eruptive increase of shear wave velocity, which correlates
well with pre-eruptive seismicity and dome-growth is revealed.
The method can be used as a "pressure-gauge" for
pressure changes inside of volcanoes, because increasing pressures
in rocks are known to cause proportionally increasing elastic
wave velocities.
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Mayo de 2006
A statistical investigation of preearthquake ionospheric
anomaly.
Authors: J. Y. Liu, Y. I. Chen et al.
Link: www.agu.org/pubs/crossref/
2006/2005JA011333.shtml
Abstract
Empirical evidence of the preearthquake ionospheric anomalies
(PEIAs) is reported by statistically investigating the relationship
between
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variations of the plasma frequency at the ionospheric F2
peak foF2 and 184 earthquakes with magnitude M = 5.0 during
1994–1999 in the Taiwan area. The PEIA, defined as the
abnormal decrease more than about 25% in the ionospheric foF2
during the afternoon period, 1200–1800 LT, significantly
occurs within 5 days before the earthquakes. Moreover, the
odds of earthquakes with PEIA increase with the earthquake
magnitude but decrease with the distance from the epicenter
to the ionosonde station. These results indicate that the
PEIA is energy related.
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Mayo de 2006
Spiking modular neural networks: A neural network modeling
approach for hydrological processes
Authors: K Parasuraman, A Elshorbagy et al.
Link:
www.agu.org/pubs/crossref/
2006/2005WR004317.shtml
Abstract
Artificial Neural Networks (ANNs) have been widely used for
modeling hydrological processes that are embedded with high
nonlinearity in both spatial and temporal scales. The input-output
functional relationship does not remain the same over the
entire modeling domain, varying at different spatial and temporal
scales. In this study, a novel neural network model called
the spiking modular neural networks (SMNNs) is proposed. An
SMNN consists of an input layer, a spiking layer, and an associator
neural network layer. The modular nature of the SMNN helps
in finding domain-dependent relationships. The
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performance
of the model is evaluated using two distinct case studies. The
first case study is that of streamflow modeling, and the second
case study involves modeling of eddy covariance-measured evapotranspiration.
Two variants of SMNNs were analyzed in this study. The first
variant employs a competitive layer as the spiking layer, and
the second variant employs a self-organizing map as the spiking
layer. The performance of SMNNs is compared to that of a regular
feed forward neural network (FFNN) model. Results from the study
demonstrate that SMNNs performed better than FFNNs for both
the case studies. Results from partitioning analysis reveal
that, compared to FFNNs, SMNNs are effective in capturing the
dynamics of high flows. In modeling evapotranspiration, it is
found that net radiation and ground temperature alone can be
used to model the evaporation flux effectively. The SMNNs are
shown to be effective in discretizing the complex mapping space
into simpler domains that can be learned with relative ease.
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Mayo de 2006
Detailed imaging of tectonic structures by multiscale Earth
resistivity tomographies: The Colfiorito normal faults (central
Italy)
Authors: I. Diaferia, M. Barchi et al.
Link: www.agu.org/pubs/crossref/
2006/2006GL025828.shtml
Abstract
This study is aimed to show how cost effective geophysical
surveying, integrated with detailed knowledge of the local
geology, can give a significant contribution to image the
subsurface structure of complex areas of active faulting.
The combined inversion of a data set consisting of a large
scale tomography and a target-dependent
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multiresolution dipole-dipole measurements, the multiscale
approach, is shown to yield detailed structural information
and to infer some characteristics of the fault zone. The operating
procedure, applied to the Colfiorito fault system, allowed
to differentiate the electrical signatures of the crossed
faults. Minor faults show sharp lateral variations generated
by a contact between different geological formations, while
the major faults are also characterized by a decrease in resistivity
values distributed on the fault damage zones. A close interpretation
was supported by detailed geological surface data. Besides,
quantitative correlations between in situ resistivity distribution
and geological units were supported by electrical logs.
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Mayo de 2006
Equivalent hydraulic conductivity of an experimental stratigraphy:
Implications for basin-scale flow simulations
Authors: Ye Zhang, Carl W. Gable and Mark Person
Link: www.agu.org/pubs/crossref/2006/
2005WR004720.shtml
Abstract
Critical issues facing basin-scale groundwater flow models
are the estimation of representative hydraulic conductivity
for the model units and the impact of nonrepresentation of
within-unit conductivity heterogeneity on the model flow prediction.
In this study, high-resolution, fully heterogeneous basin-scale
hydraulic conductivity map is generated by scaling up an experimental
stratigraphy created by physical sedimentation processes and
by assuming increasing conductivity for increasing gray scale
(proxy for sand content). A fully heterogeneous model is created,
incorporating the complete conductivity variation. Two hydrogeologic
framework models are also created, one of coarser stratigraphic
division. A novel numerical up-scaling method is developed
to compute an equivalent conductivity for each irregularly
shaped framework model unit by conducting basin-scale flow
experiments in the fully heterogeneous model. In each experiment,
different boundary conditions are specified, subjecting the
basin to various flow conditions. To evaluate the impact of
using equivalent
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conductivity
on the prediction of basin-scale hydraulic head and groundwater
flow, the flow experiments conducted in the fully heterogenous
model are repeated in the framework models. Results indicate
that for most deposits, the behavior of the equivalent conductivity
with increasing ln(K) variance is consistent with the prediction
of an analytic-stochastic theory. The equivalent conductivity
is also insensitive to the boundary condition and the number
of flow experiments performed, indicating the possible emergence
of an effective conductivity. Although all equivalent conductivities
are full tensors, the off-diagonal term is 2-3 orders of magnitude
smaller than the diagonal terms. Ignoring the off-diagonal term
has minimal impact on the framework-model-predicted hydraulic
head and groundwater flow paths, when compared to the impact
of nonrepresentation of within-unit conductivity heterogeneity.
Under certain boundary conditions, significant head deviation
can develop within framework model units that contain trended
or strongly stratified deposits. However, the accuracy of head
prediction is improved when the length of the no-flow boundary
is increased. In a topography-driven system, progressive degradation
is observed in the prediction of basin-scale flow pattern, flow
rate, and location of recharge/discharge, when the progressively
up-scaled framework models are used. In summary, the accuracy
of the framework models is controlled by the level of stratigraphic
division, conductivity heterogeneity, and boundary conditions.
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Mayo de 2006
Optimal groundwater remediation design using an Adaptive
Neural Network Genetic Algorithm
Authors: Shengquan Yan and Barbara Minsker.
Link:
www.agu.org/pubs/crossref/2006/
2005WR004303.shtml
Abstract
Large-scale water resources optimization often involves using
time-consuming simulation models to evaluate potential water
resource designs or calibrate parameter values. Approximation
models have been proposed for improving computational efficiency
of the optimization. In most instances, multiple simulation
runs have been done prior to the optimization, which are then
used to fit an approximate model that is used during the optimization.
This paper demonstrates that this
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approach
can lead to suboptimal solutions and proposes a dynamic modeling
approach, called Adaptive Neural Network Genetic Algorithm (ANGA),
in which artificial neural networks are adaptively and automatically
trained directly within a genetic algorithm (GA) to replace
the time-consuming water resource simulation models. A dynamic
learning approach is proposed to periodically sample new solutions
both to update the ANNs and to correct the GA's convergence.
Different configurations of ANGA were tested on a hypothetical
groundwater remediation design case, and then the best configuration
was applied to a field-scale case. In these applications, ANGA
saved 85-90% percent of the simulation model calls with no loss
in accuracy of the optimal solutions. These results show that
the method has substantial promise for reducing computational
effort associated with large-scale water resources optimization.
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Mayo de 2006 - Geofísica
de la atmósfera
Modelo Neuronal Multiestación para el pronóstico
del MP10
Autores: Patricio Perez y Jorge
Reyes.
Link: Click
Aquí
Abstract
Hemos desarrollado un modelo neuronal multiestación
para pronosticar el valor máximo de la concentración
promedio de 24 horas de PM10 con un día de anticipación
y hemos aplicado la técnica al caso específico
de cinco estaciones de monitoreo en Santiago de Chile. Las
entradas del modelo son las concentraciones medidas hasta
las 7 PM en las cinco estaciones en el día actual,
junto con los valores medidos y predichos de las variables
meteorológicas. Las salidas corresponden a las máximas
concentraciones esperadas para el día siguiente en
las cinco estaciones. La concentración máxima
predicha define la calidad del aire para el día siguiente.
De acuerdo con el rango en el que cae la máxima concentración
predicha, se definen tres niveles o clases de calidad del
aire: Buena (A), Mala (B) y Crítica (C). Los parámetros
del modelo fueron ajustados utilizando datos de los años
2001 y 2002 para así pronosticar las condiciones del
año 2003. Así mismo se utilizó datos
del 2002 y del 2003 para pronosticar los valores del 2004.
El pronóstico neuronal fue comparado con los resultados
obtenidos con un modelo lineal que utilizaba las mismas entradas
y con persistencia. De acuerdo con nuestros resultados, el
modelo neuronal resulta ser el más confiable. Sin embargo,
es muy importante una correcta elección de las variables
de entrada.
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Mayo de 2006 - Atmospheric Geophysics
An integrated neural network model for PM10 forecasting
Authors: Patricio Perez and Jorge
Reyes.
Link: Click
here
Abstract
We have developed an integrated artificial neural network
model to forecast the maxima of 24 h average of PM10 concentrations
1 day in advance and we have applied it to the case of five
monitoring stations in the city of Santiago, Chile. Inputs
to the model are concentrations measured until 7 PM at the
five stations on the present day plus measured and forecast
values of meteorological variables. Outputs are the expected
maxima concentrations for the following day at the site of
the same five stations. The greatest of the concentrations
among the five forecasts defines air quality for the following
day. According to the range where the concentrations fall,
three levels or classes of air quality are defined: good (A),
bad (B) and critical (C). We have adjusted the parameters
of the models using 2001 and 2002 data to forecast 2003 conditions
and 2002 and 2003 data in order to forecast 2004 values. Forecast
values using the neural model are compared with the results
obtained with a linear model with the same input variables
and with persistence. According to the results reported here,
overall, the neural model seems more accurate, although a
good choice of input variables appears to be very important.
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Mayo de 2006
Polarization analysis and polarization filtering of three-component
signals with the time-frequency S transform
Author: C. R. Pinnegar
Link: www.blackwell-synergy.com/doi/abs/10.1111/j.1365-246X.2006.02937.x
Abstract
From basic Fourier theory, a one-component signal can be expressed
as a superposition of sinusoidal oscillations in time, with
the Fourier amplitude and phase spectra describing the contribution
of each sinusoid to the total signal. By extension, three-component
signals can be thought of as superpositions of sinusoids oscillating
in the x-, y-, and z-directions, which, when considered one
frequency at a time, trace out elliptical motion in three-space.
Thus the total three-component signal can be thought of as
a superposition of ellipses. The information contained in
the Fourier spectra of the x-, y-, and z-components of the
signal can then be re-
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expressed
as Fourier spectra of the elements of these ellipses, namely:
the lengths of their semi-major and semi-minor axes, the strike
and dip of each ellipse plane, the pitch of the major axis,
and the phase of the particle motion at each frequency. The
same type of reasoning can be used with windowed Fourier transforms
(such as the S transform), to give time-varying spectra of the
elliptical elements. These can be used to design signal-adaptive
polarization filters that reject signal components with specific
polarization properties. Filters of this type are not restricted
to reducing the whole amplitude of any particular ellipse; for
example, the 'linear' part of the ellipse can be retained while
the 'circular' part is rejected. This paper describes the mathematics
behind this technique, and presents three examples: an earthquake
seismogram that is first separated into linear and circular
parts, and is later filtered specifically to remove the Rayleigh
wave; and two shot gathers, to which similar Rayleigh-wave filters
have been applied on a trace-by-trace basis. |
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Mayo de 2006
Computation of Large Anisotropic Seismic Heterogeneities
(CLASH)
Authors: Éric Beucler and Jean-Paul Montagner
Link: www.blackwell-synergy.com/doi/abs/10.1111/j.1365-246X.2005.02813.x
Abstract
A general tomographic technique is designed in order (i) to
operate in anisotropic media; (ii) to account for the uneven
seismic sampling and (iii) to handle massive data sets in
a reasonable computing time. One modus operandi to compute
a 3-D body wave velocity model relies on surface wave phase
velocity measurements. An intermediate step, shared by other
approaches, consists in translating, for each period of a
given mode branch, the phase velocities integrated along ray
paths into local velocity perturbations. To this end, we develop
a method, which accounts for the azimuthal anisotropy in its
comprehensive form. The weakly non-linear forward problem
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allows
to use a conjugate gradient optimization. The Earth's surface
is regularly discretized and the partial derivatives are assigned
to the individual grid points. Possible lack of lateral resolution,
due to the inescapable uneven ray path coverage, is taken into
account through the a priori covariances on parameters with
laterally variable correlation lengths. This method allows to
efficiently separate the 2psi and the 4psi anisotropic effects
from the isotropic perturbations. Fundamental mode and overtone
phase velocity maps, derived with real Rayleigh wave data sets,
are presented and compared with previous maps. The isotropic
models concur well with the results of Trampert & Woodhouse.
Large 4psi heterogeneities are located in the tectonically active
regions and over the continental lithospheres such as North
America, Antarctica or Australia. At various periods, a significant
4psi signature is correlated with the Hawaii hotspot track.
Finally, concurring with the conclusions of Trampert & Woodhouse,
our phase velocity maps show that Rayleigh wave data sets do
need both 2psi and 4psi anisotropic terms. |
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Abril de 2006
Equivalent transmissivity of heterogeneous leaky aquifers
for steady state radial flow
Authors: Nadim K. Copty, Murat Savas Sarioglu and Angelos
N. Findikakis.
Link: www.agu.org/pubs/crossref/2006/
2005WR004673.shtml
Abstract
The purpose of this study is to estimate the equivalent transmissivity
for two-dimensional steady state radial flow toward a fully
penetrating well in a heterogeneous leaky aquifer. The log
transmissivity of the pumped aquifer is modeled as a multivariate
random spatial function with stationary mean and exponential
semivariogram, while the aquitard conductance is assumed to
be spatially uniform. An approximate expression for
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the
upscaled or block transmissivity, defined as the transmissivity
of an equivalent homogeneous leaky aquifer system with the same
pumping, is proposed. The proposed relation expresses the upscaled
transmissivity as a weighted average of the log transforms of
the point transmissivity values. Monte Carlo simulations are
used to numerically validate the proposed upscaling expression
for wide ranges of aquifer parameters. The relation of the weighing
function to the statistical parameters of the transmissivity
field is discussed. The first and second moments of the upscaled
transmissivity are also estimated as a function of the problem
parameters. The expected value of the equivalent transmissivity
is shown to increase from the geometric mean to the arithmetic
mean as the aquitard conductance increases from zero to infinity.
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Abril de 2006
Solar activity reconstructed over the last 7000 years:
The influence of geomagnetic field changes.
Authors: I. G. Usoskin, S. K. Solanki and M. Korte.
Link: www.agu.org/pubs/crossref/2006/
2006GL025921.shtml
Abstract
The long-term solar activity, as manifested by sunspot number,
has been recently reconstructed on multi-millennium time scales
by S. K. Solanki et al. (2004) from the measured concentration
of 14C in tree rings. The exact level of the reconstructed
solar activity depends, however, on independently evaluated
data of the geomagnetic dipole strength variations. Recently,
a new series
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of
the palaeomagnetic dipole moment reconstruction for the last
7000 years has been presented by M. Korte and C. G. Constable
(2005a) on the basis of a thorough analysis of global samples.
The new palaeomagnetic series yields a systematically lower
dipole moment in the past, compared to the earlier geomagnetic
reconstructions. We have revised the earlier sunspot activity
reconstruction since 5000 BC, using the new geomagnetic data
series, and found that it is roughly consistent with the previous
results during most of the period, although the revised sunspot
number values are in general higher. Nonetheless, it is confirmed
with the new palaeomagnetic series that the Sun spends only
2-3% of the time in a state of high activity, similar to the
modern episode. This strengthens the conclusion that the modern
high activity level is very unusual during the last 7000 years.
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Abril de 2006
Seismology: Breaking new ground (Nature)
Author: Naomi Lubick
Link: www.nature.com/nature/journal
/v440/n7086/full/440864a.html
Abstract
In 1906, a great earthquake destroyed San Francisco, and galvanized
US seismologists. Naomi Lubick looks back at the event that
changed the country's geological scene.
The Great Earthquake and subsequent fire that destroyed San
Francisco in 1906 began at 5:12 a.m. on 18 April. More than
3,000 people are thought to have died following the magnitude-7.9
tremor. The metropolis of San Francisco, built on gold-rush
fortunes, was almost utterly destroyed in three days of fire,
and officials spent years playing down the possibility of
another 'big one'. Yet the earthquake also jump-started seismology
in the United States, inspiring it to catch up with
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countries
such as Britain, Japan and Germany. The US scientific community
had already encountered several major earthquakes. Three tremors
of magnitude 8 or more racked the New Madrid region in the US
Midwest in 1811 and 1812. And the city of Charleston, South
Carolina, was seriously damaged during an 1886 earthquake.
But the 1906 earthquake happened in the right time and place
to act as a catalyst for science. Chance brought together several
ingredients: the right people, the right technology, key ideas
in need of testing - and a huge earthquake delivering the data.
"It took that large an event to make seismology a national
priority," says Jack Boatwright, a seismologist at the
US Geological Survey (USGS) in Menlo Park, California.
Around the world, the discipline of seismology began to coalesce
in the late nineteenth century, as non-specialists interested
in earthquakes began to work together. In India ... |
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Abril de 2006
Distinct regional differences in crustal thickness along
the axis of the Mariana Trough, inferred from gravity anomalies
Authors: Kazuya Kitada , Nobukazu Seama, Toshitsugu Yamazaki,
Yoshifumi Nogi, Kiyoshi Suyehiro
Link: www.agu.org/pubs/crossref/
2006/2005GC001119.shtml
Abstract
We have compiled extensive gravity and bathymetry data for
the whole Mariana Trough, which were collected during several
Japanese scientific cruises over the last few years. This
study aims to clarify the lateral distribution of the local
differences in geochemical signatures, which have been observed
locally in the Mariana Trough. Shipboard free-air gravity
anomaly data from eight Japan Agency for Marine-Earth Science
and Technology (JAMSTEC) cruises were compiled with those
crossover errors of 2.85 mgal. Mantle Bouguer anomalies (MBA)
were calculated by subtracting the predictable gravity signal
due to the seawater/crust and crust/mantle density boundaries.
The crustal thickness variation along the spreading axis was
estimated from the MBA. Different features in crustal thickness,
its variation, and segment length for each segment, allow
us to identify four distinct regional differences in magmatic
activity along the spreading axis of the Mariana Trough. Segment
in
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region
A (to the north of 20°35'N) shows the largest sectional
dimensions of crust along the axis and it is probably affected
by an additional supply from island arc magma sources. A variety
of crustal thickness values and of along-axis crustal thickness
variations in region B (between 15°38'N and 20°35'N)
suggests two types of segments. One is similar to a slow spreading
ridge segment that has a plume-like mantle upwelling under the
spreading axis, and the other is a magma-starved segment. Region
C (between 14°22'N and 15°38'N) is a less magmatic region
(individual crustal thickness averages of 3.4-4.1 km). Region
D (to the south of 14°22'N) has higher individual crustal
thickness averages of 5.9-6.9 km, suggesting higher magmatic
activity with a sheet-like mantle upwelling under the spreading
axis. Different features in the MBA for off-axis areas suggest
that these four regions have existed since the Mariana Trough
started spreading. Moreover, comparison between our results
of crustal thickness and previous geochemical results indicates
that less magmatic spreading segments with thin crust, which
are locally distributed in both regions B and C, probably result
from mantle source depleted of water and incompatible elements.
This suggests that lateral compositional variation of water
and incompatible elements exists on a segment scale in the mantle
source beneath the spreading axis of the Mariana Trough. |
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Abril de 2006
Unusually quick development of a 4000 nT substorm during
the initial 10 min of the 29 October 2003 magnetic storm
Authors: M. Yamauchi , T. Iyemori, H. Frey, M. Henderson
Link: www.agu.org/pubs/crossref/
2006/2005JA011285.shtml
Abstract
Global geomagnetic field data, IMAGE FUV data, and many other
in situ observations are presented for the initial 10 min
of the magnetic storm starting 29 October 2003 at around 0610
UT. Within 1 min after sudden commencement (SC), two independent
strong westward ionospheric electrojets (>2000 nT) at the
inner magnetospheric region started simultaneously, one in
the evening-midnight sector and the other in the morning sector.
Both activities expanded and accompanied auroral expansion.
The locations (inner magnetosphere), morphologies (expansion),
and intensities (>2000 nT) of both
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activities fall into substorm expansive phases. Having such
simultaneous independent 2000 nT level expansions makes this
event unique. The interplanetary magnetic field condition before
the SC was not favorable in causing an AL < -2000 nT activity.
A timing analysis indicates that these strong westward electrojets
were most likely triggered by the interplanetary shock, with
the triggering location not farther than the geosynchronous
distance. They are also probably maintained by the direct energy
pumping from the solar wind because cross-tail current derived
from the closely located GOES-10 and Polar did not decrease
very much during this period. A local but even stronger geomagnetic
(nearly 4000 nT) and auroral activity started only 6 min after
the start of SC at postmidnight where and when the above two
expanding activities met each other, although the relation between
the onset of 4000 nT activity and the preceding expansions is
not clear. The suddenness of this third activity (3000 nT change
within 2 min) is another unique feature. |
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Abril de 2006
A least-squares variance analysis method for shape and
depth estimation from gravity data
Authors: E M Abdelrahman et al
Link: www.iop.org/EJ/abstract/1742-2140/3/2/005
Abstract
We have developed a simple method to estimate the shape (shape
factor) and the depth of a buried structure simultaneously
from modified first moving average residual anomalies (second
moving average residuals) obtained from gravity data using
filters of successively greater window
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lengths. The method is based on computing the variance of the
depths determined from all second moving average residual anomaly
profiles using the least-squares method for each shape factor.
The minimum variance is used as a criterion for determining
the correct shape and depth of the buried structure. When the
correct shape factor is used, the variance of the depths is
always less than the variances computed using wrong shape factors.
The method is applied to synthetic data with and without random
errors, complex regional anomalies and interference from neighbouring
structures, and tested on a field example from the USA. |
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Abril de 2006
Exploration of a geothermal reservoir using geoelectrical
resistivity inversion: case study at Hammam Mousa, Sinai,
Egypt
Author: Gad El-Qady
Link: www.iop.org/EJ/abstract/1742-2140/3/2/002
Abstract
Geoelectrical resistivity is a pioneer geophysical technique
used in geothermal exploration. With the advent of computing
technology, it has become convenient to apply sophisticated
data analysis and inversion to geoelectrical resistivity field
data. In this work, a geoelectrical resistivity survey was
conducted in the Hammam Mousa area to explore the geothermal
resources and groundwater aquifer. The survey comprises 19
vertical electrical soundings (VES) using the well-known Schlumberger
array with AB/2 up to 1000 m. Interpretation of one-dimensional
(1D)
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inversion
gave a layered-earth resistivity model using a nonlinear least-squares
method. However, some resistivity sections of the 1D inversion
were not fully resolved for the complicated geologic structure.
Therefore, we carried out a two-dimensional (2D) inversion based
on the ABIC least-squares method for the same data set. The
general distribution of resistivity shows a very low value near
Hammam Mousa Hot Spring. The 2D resistivity cross section clearly
elaborates the subsurface structure in the spring area and it
elucidates and gives an explanation for the hot water source
in the area. It is concluded that the hydrothermal system in
the Hammam Mousa area is adequately delineated from the 1D and
2D inversions of vertical electric sounding data using a Schlumberger
electrode array. Accordingly, a proposal for geothermal drilling
in the study area is recommended. |
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Abril de 2006
Reconciling a geophysical model to data using a Markov
chain Monte Carlo algorithm: An application to the Yellow
Sea-Korean Peninsula region
Authors: Michael E. Pasyanos, Gregory A. Franz et al
Link: www.agu.org/pubs/crossref/2006
/2005JB003851.shtml
Abstract
In an effort to build seismic models that are the most consistent
with multiple data sets we have applied a new probabilistic
inverse technique. This method uses a Markov chain Monte Carlo
(MCMC) algorithm to sample models from a prior distribution
and test them against multiple data types to generate a posterior
distribution. While computationally expensive, this approach
has several advantages over deterministic models, notably
the seamless reconciliation of different data types that constrain
the model, the proper handling of both data and model uncertainties,
and the ability to easily incorporate a variety of prior information,
all in a straightforward, natural fashion. A real advantage
of the technique is
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that
it provides a more complete picture of the solution space. By
mapping out the posterior probability density function, we can
avoid simplistic assumptions about the model space and allow
alternative solutions to be identified, compared, and ranked.
Here we use this method to determine the crust and upper mantle
structure of the Yellow Sea and Korean Peninsula region. The
model is parameterized as a series of seven layers in a regular
latitude-longitude grid, each of which is characterized by thickness
and seismic parameters (Vp, Vs, and density). We use surface
wave dispersion and body wave traveltime data to drive the model.
We find that when properly tuned (i.e., the Markov chains have
had adequate time to fully sample the model space and the inversion
has converged), the technique behaves as expected. The posterior
model reflects the prior information at the edge of the model
where there is little or no data to constrain adjustments, but
the range of acceptable models is significantly reduced in data-rich
regions, producing values of sediment thickness, crustal thickness,
and upper mantle velocities consistent with expectations based
on knowledge of the regional tectonic setting. |
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Abril de 2006
Unsolved problems in the lowermost mantle
Authors: Kei Hirose, Shun-ichiro Karato et al
Link: www.agu.org/pubs/crossref/2006/
2006GL025691.shtml
Abstract
Many characteristics of D" layer may be attributed to
the recently discovered MgSiO3 post-perovskite phase without
chemical heterogeneities. They include a sharp discontinuity
at the top of D", regional variation in seismic anisotropy,
and a steep Clapeyron slope. However, some features remain
unexplained. The seismically inferred
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velocity jump is too large
in comparison to first principles calculations, and the sharpness
of the discontinuity may require a chemical boundary. Chemical
heterogeneity may play an important role in addition to the
phase transformation from perovskite to post-perovskite. Phase
transformation and chemical heterogeneity and the attendant
changes in physical properties, such as rheology and thermal
conductivity, are likely to play competing roles in defining
the dynamical stability of the D" layer. Revealing the
relative roles between phase transition and chemical anomalies
is an outstanding challenge in the study of the role of D"
in thermal-chemical evolution of the Earth. |
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Marzo de 2006
Monte Carlo modeling of toroidal ion distributions and
ion temperatures at high altitudes equatorward of the cusp:
Effect of finite gyroradius
Authors: I. A. Barghouthi and M. A. Atout
Link: www.agu.org/pubs/crossref/2006
/2005JA011437.shtml
Abstract
We report that the effect of finite gyroradius is responsible
for production of the H+ and O+ toroids at high altitudes
equatorward of the cusp that are observed by TIDE and TIMAS
ion instruments on board the polar spacecraft. The energization
of charged particles, owing to interaction with electromagnetic
turbulence, has an important influence on the plasma outflow
in space. The effect of wave-particle interactions (WPI) on
H+ and O+ outflow at high altitudes equatorward of the cusp
was investigated by using Monte Carlo method. The Monte Carlo
model includes the effect of WPI, gravity, polarization electrostatic
field, and the divergence of the geomagnetic field within
the simulation tube (1.2-10 Earth radii, R E ). As the ions
drift upward along the geomagnetic field lines, they
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interact
with the electromagnetic turbulence and consequently get heated
in the direction perpendicular to the geomagnetic field. The
mirror force converts some of the gained ion energy in the perpendicular
direction into parallel kinetic energy. These effects combine
to form an ion-conic velocity distribution. However, as the
ions are heated and move to higher altitudes, the ion gyroradius
? i may become comparable to the perpendicular wavelength of
the electromagnetic turbulence lambda . As the ratio
Rho/lambda becomes >1, then the heating rate turns
to be self-limited and the ion velocity distribution displays
toroidal features. A comparison has been made between the Monte
Carlo calculations obtained in this study and observations of
H+ and O+ ion velocity distributions and temperatures. The comparison
showed a remarkably close agreement in the corresponding results
for the ion velocity distribution and its temperature. As a
result of the comparison, we were able to predict the characteristic
value of the perpendicular wavelength of the electromagnetic
turbulence lambda at high altitudes equatorward of the
cusp. To our knowledge, this represents the first successful
comparison of observed toroids with a theoretical model. |
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Marzo de 2006
Probabilistic seismic hazard estimation in low-seismicity
regions considering non-Poissonian seismic occurrence
Authors: Céline Beauval, Sebastian Hainzl and Frank
Scherbaum
Link: http://www.blackwell-synergy.com/doi/abs/10.1111/j.1365-246X.2006.02863.x
Abstract
In low-seismicity regions, such as France or Germany, the
estimation of probabilistic seismic hazard must cope with
the difficult identification of active faults and with the
low amount of seismic data available. Since the probabilistic
hazard
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method was initiated, most
studies assume a Poissonian occurrence of earthquakes. Here
we propose a method that enables the inclusion of time and space
dependences between earthquakes into the probabilistic estimation
of hazard. Combining the seismicity model Epidemic Type Aftershocks-Sequence
(ETAS) with a Monte Carlo technique, aftershocks are naturally
accounted for in the hazard determination. The method is applied
to the Pyrenees region in Southern France. The impact on hazard
of declustering and of the usual assumption that earthquakes
occur according to a Poisson process is quantified, showing
that aftershocks contribute on average less than 5 per cent
to the probabilistic hazard, with an upper bound around 18 per
cent. |
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Marzo de 2006
Simulation of active tectonic processes for a convecting
mantle with moving continents
Authors: Valeriy Trubitsyn, Mikhail Kaban, Walter Mooney,
Christoph Reigber and Peter Schwintzer
Link: http://www.blackwell-synergy.com/doi/abs/10.1111/j.1365-246X.2006.02832.x
Abstract
Numerical models are presented that simulate several active
tectonic processes. These models include a continent that
is thermally and mechanically coupled with viscous mantle
flow. The assumption of rigid continents allows use of solid
body equations to describe the continents' motion and to calculate
their velocities. The starting point is a quasi-steady state
model of mantle convection with temperature/pressure-dependent
viscosity. After placing a continent on top of the mantle,
the convection pattern changes.
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The mantle
flow subsequently passes through several stages, eventually
resembling the mantle structure under present-day continents:
(a) Extension tectonics and marginal basins form on boundary
of a continent approaching to subduction zone, roll back of
subduction takes place in front of moving continent; (b) The
continent reaches the subduction zone, the extension regime
at the continental edge is replaced by strong compression. The
roll back of the subduction zone
still continues after closure of the marginal basin and the
continent moves towards the upwelling. As a result the ocean
becomes non-symmetric and (c) The continent overrides the upwelling
and subduction in its classical form stops. The third stage
appears only in the upper mantle model with localized upwellings
overrides the upwelling and subduction in its classical form
stops. The third stage appears only in the upper mantle model
with localized upwellings. |
01-mar-06
Detectan hidratos de gas de fácil explotación
en costa chilena (Noticias Ambientales ACHS)
Auspiciosos resultados preliminares tuvo la expedición
científica de la comisión encargada de explorar
la presencia de hidratos de gas en la costa chilena. Un estudio
realizado durante 20 días a bordo del buque oceanográfico
Vidal Gormaz de la Armada, ubicó vastos yacimientos |
de hidratos de gas (forma
sólida) y gas libre, cuyo estado en forma de burbujas
facilita su potencial explotación como fuente energética.
Ya se había establecido la existencia de yacimientos
de gas en el océano, pero la exploración entre
el 3 y 22 de febrero permitió un análisis más
acotado del sector entre la desembocadura de los ríos
Maule y Bio Bío. Los resultados de la exploración
estarán listos en forma parcial a fines de año.
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Diciembre 2005
Layer stripping of azimuthal anisotropy from P-wave reflection
moveout in orthogonal survey lines
Authors: Shangxu Wang et al 2006 J. Geophys. Eng. 3 1-11
doi:10.1088/1742-2132/3/1/001
Link: http://www.iop.org/EJ/abstract/1742-2140/3/1/001
Abstract
This paper presents a layer-stripping procedure to determine
interval measurements of fracture parameters in multi-layered
fractured media with vertically varying strike directions.
The procedure is based on the P-wave travel time difference
between two orthogonal seismic survey lines, and this difference
is referred to as the P-wave
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Azimuthal
Moveout Response (AMR). The interval AMR of a fracture target
for a fixed offset is a function of cos(x) with respect to the
line azimuth and the fracture-strike azimuth i. Consequently
two pairs of orthogonal survey lines can be used to determine
the local fracture strike i if the interval AMR of the target
is known. In the case of a weakly fractured overburden underlain
by a fractured target, layer stripping can be achieved through
the alignment of the top-target event by performing NMO correction
separately for all survey lines. The interval AMR of the target
layer may then be calculated from the residual moveout the bottom-target
event, if any. In the general case, a ray-tracing procedure,
similar to that used in AVO analysis, is required to perform
effective layer stripping. Full-wave modelling is used to verify
and illustrate these procedures. |