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Junio de 2010
Determining leaf area index and leafy tree roughness using
terrestrial laser scanning
Authors: A. S. Antonarakis, K. S. Richards
et al
Link: Click here
Abstract
Vegetation roughness, and more specifically forest roughness,
is a necessary component in better defining flood dynamics
both in the sense of changes in river catchment characteristics
and the dynamics of forest changes and management. Extracting
roughness parameters from riparian forests can be a complicated
process involving different components for different required
scales and flow depths. For flow depths that enter a forest
canopy, roughness at both the woody branch and foliage level
is necessary. This study attempts to extract roughness for
this leafy component using a relatively new remote sensing
technique in the form of terrestrial laser scanning. Terrestrial
laser
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scanning is used in
this study due to its ability to obtain millions of points within
relatively small forest stands. This form of lidar can be used
to determine the gaps present in foliaged canopies in order
to determine the leaf area index. The leaf area index can then
be directly input into resistance equations to determine the
flow resistance at different flow depths. Leaf area indices
created using ground scanning are compared in this study to
indices calculated using simple regression equations. The dominant
riparian forests investigated in this study are planted and
natural poplar forests over a lowland section of the Garonne
River in Southern France. Final foliage roughness values were
added to woody branch roughness from a previous study, resulting
in total planted riparian forest roughness values of around
Manning's n = 0.170-0.195 and around n = 0.245-330 for in-canopy
flow of 6 and 8 m, respectively, and around n = 0.590 and around
n = 0.750 for a natural forest stand at the same flow depths.
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Junio de 2010
Collapse of the Maya: Could deforestation have contributed?
Authors: Robert J. Oglesby, Thomas L. Sever
et al
Link: Click here
Abstract
The collapse of the Maya civilization during the ninth century
A.D. is a major conundrum in the history of mankind. This
civilization reached a spectacular peak but then almost completely
collapsed in the space of a few decades. While numerous explanations
have been put forth to explain this collapse, in recent years,
drought has gained favor. This is because water resources
were a key for the Maya, especially to ensure their survival
during the lengthy dry season that occurs
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where
they lived. Natural drought is a known, recurring feature of
this region, as evidenced by observational data, reconstructions
of past times, and global climate model output. Results from
simulations with a regional climate model demonstrate that deforestation
by the Maya also likely induced warmer, drier, drought-like
conditions. It is therefore hypothesized that the drought conditions
devastating the Maya resulted from a combination of natural
variability and human activities. Neither the natural drought
or the human-induced effects alone were sufficient to cause
the collapse, but the combination created a situation the Maya
could not recover from. These results may have sobering implications
for the present and future state of climate and water resources
in Mesoamerica as ongoing massive deforestation is again occurring.
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Junio de 2010
Bridging great earthquake doublets through silent slip:
On- and off-fault aftershocks of the 2006 Kuril Island subduction
earthquake toggled by a slow slip on the outer rise normal
fault of the 2007 great earthquake
Authors: Yosihiko Ogata, Shinji Toda et
al
Link: Click here
Abstract
The November 2006 great thrust earthquake of moment magnitude
(Mw) 8.3 on the plate boundary in the Kuril (Chishima) Islands
triggered as many vigorous normal-faulting aftershocks in
the outer rise region seaward of the trench as on-fault aftershocks.
Here we show that the aftershocks for a 40 day period preceding
the January 2007 Mw = 8.1 event in the outer rise were less
frequently observed than the rate expected by both the Omori-Utsu
formula and the epidemic-type aftershock sequence (ETAS) model
while at
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the same time were
more frequent along the plate boundary. We discuss whether such
simultaneous quiescence and activation in the pervasive 2006
aftershocks might have been initiated by the onset of aseismic
slip on the 2007 rupture plane. The stable fault sliding in
an elastic half-space demonstrates progressive stress shadowing
in the outer rise and stress loading along the plate boundary.
Furthermore, the rate- and state-dependent friction of Dieterich
quantitatively simulates temporal changes in seismic behavior.
The same trends of the anomalous activities last in both zones
after the 2007 rupture, which suggests that the slips continuously
take place in the similar region on the fault throughout the
period before and after the rupture. The Kuril case implies
that temporal seismic quiescence in an aftershock sequence is
useful in evaluating the possibility of the subsequent triggered
event, thus suggesting itself as a potential earthquake-forecasting
model together with pervasive earthquake-clustering models.
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Junio de 2010
Detailed spatial changes in the stress field of the 1984
western Nagano earthquake region
Authors: Yohei Yukutake, Yoshihisa Iio
et al
Link: Click here
Abstract
We estimate the stress field after the 1984 western Nagano
earthquake from numerous focal mechanisms. To precisely determine
the focal mechanisms, we analyze the earthquakes that occurred
around the eastern part of the main shock fault, where station
coverage is fairly good. Most of the earthquakes that occurred
in this part, except the main shock fault, are reverse fault
types. In contrast, earthquakes that occurred near
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the
main shock fault have T axes distributed in a belt (strike slip,
oblique slip, and reverse fault types occurred equally). Using
the stress inversion method to quantitatively estimate the stress
field of these earthquakes revealed that the s2 axis near the
main shock fault is close to the vertical direction, whereas
it is close to the horizontal direction in other regions. At
the western edge of the study area, we observe that the s1 axis
rotates toward the NS direction and that the stress ratio (s1
- s2)/(s1 - s3) is low. We estimate that the magnitude of shmin
decreases near the main shock fault. Spatial variation of the
stress field around the eastern part of the main shock fault
is not generated by static stress changes caused by the main
shock. Local stress anomalies might have occurred around the
main shock fault before the main shock. |
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Junio de 2010
Recurrence rates of large explosive volcanic eruptions
Authors: N. I. Deligne, S. G. Coles et
al
Link: Click here
Abstract
A global database of large explosive volcanic eruptions has
been compiled for the Holocene and analyzed using extreme
value theory to estimate magnitude-frequency relationships.
The database consists of explosive eruptions with magnitude
(M) greater than or equal to 4. Two models are applied to
the data, one assuming no underreporting of eruptions and
the other taking underreporting into consideration. Results
from the latter indicate that the level of underreporting
is high and fairly constant from the start of the
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Holocene until about
1 A.D. and then decreases dramatically toward the present. Results
indicate there is only a ~20% probability that an explosive
eruption of M = 6 occurring prior to 1 A.D. is recorded. Analysis
of the data set in the time periods 1750 A.D. and 1900 A.D.
to present (assuming no underreporting) suggests that that these
periods are likely to be too short to give reliable estimates
of return periods for explosive eruptions with M > 6. Analysis
of the Holocene data set with corrections for underreporting
bias provide robust magnitude-frequency relationships up to
M = 7. Extrapolation of the model to greater magnitudes (M >
8) gives results inconsistent with geological data, predicting
eruption size upper limits much smaller than known eruptions
such as the Fish Canyon Tuff. We interpret this result as the
consequence of different mechanisms operating for explosive
eruptions with M > 7. |
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Junio de 2010
Toward an explanation for the present and past locations
of the poles
Authors: B. Steinberger and T. H. Torsvik
Link: Click here
Abstract
Earth's orientation relative to its spin axis is determined
by its nonhydrostatic inertia tensor. We show here that the
present-day nonhydrostatic inertia tensor can be modeled by
combining contributions due to large low shear velocity provinces
(LLSVPs) in the lowermost mantle and due to subduction. With
the first contribution only, the spin axis would be at ~67°N,
96°E (north Siberia). The distribution of recent subduction,
with largest amounts in the northwest Pacific
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(beneath
East Asia) and the southeast Pacific (beneath South America),
adds a secondary contribution which moves the spin axis toward
the observed poles. We use plate reconstructions to infer subduction
and inertia tensor through time, assuming that the LLSVP contribution
has remained constant. Motion of the pole toward Greenland since
~50 Ma is attributed to increased subduction beneath East Asia
and South America and a decrease beneath North America since
then. Motion of the pole toward Siberia before that is attributed
to large amounts of subduction beneath North America between
~120 and 50 Ma and decreasing amounts of subduction in East
Asia after 60-70 Ma. Greater stability of the spin axis since
~100 Ma can be attributed to a decrease in the amount of subduction
in polar latitudes and an increase in equatorial latitudes.
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Junio de 2010
Prediction of background levels for the Wind WAVES instrument
and implications for the galactic background radiation
Authors: D. S. Hillan, Iver H. Cairns et
al
Link: Click here
Abstract
We investigate and predict the observed background levels
for the TNR, RAD1, and RAD2 receivers when connected to the
X, Y, and Z antennas of the WAVES instrument on the spacecraft
Wind. The receivers are connected to either a single antenna,
in "SEP" mode, or a combination of antennas, in
"SUM" mode. With the TNR receiver in SEP (X) mode,
the predicted backgrounds agree to within 20% when modeled
using a two component model for the quasi-thermal plasma noise
(QTN). Calibrating the RAD1 in SEP (X) mode observations against
TNR
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allows us to calculate
the relative receiver gain GR1 = 1.43 ± 0.18. Using the
RAD1 data in SUM (X+Z) mode, the ratio of antenna gains is found
to be R = 6.5, in agreement with preflight measurements. Observed
differences between the SEP (X) and SUM (X+Z) modes are explained
for the first time, and the predicted levels of QTN and galactic
background are found to agree to within 20%. RAD2 is also calibrated
against RAD1 and TNR, yielding a total gain GR2Gy = 2.5 ±
0.3. Differences between the predicted and observed galactic
background spectra are used to estimate the effective antenna
lengths for the X and Y antennas, which are found to be between
the physical monopole antenna length L and the Hansen (1981)
prediction of L*sqr(2/3). The analyses are consistent with the
Novaco and Brown (1978) galactic background model, which decreases
much faster than that of Cane (1979). Our model background spectrum
is useful for theory-data comparisons of type II and III bursts.
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Junio de 2010
Tsunami waveform inversion including dispersive waves:
the 2004 earthquake off Kii Peninsula, Japan
Authors: Tatsuhiko Saito, Kenji Satake
et al
Link: Click here
Abstract
Long waves are often assumed to model tsunamis, but the wavelength
of the initial water height distribution produced by a large
submarine earthquake, particularly in the direction perpendicular
to the fault strike, is sometimes not much greater than the
water depth. The resulting tsunami may have a dispersive character
that cannot be simulated based on a conventional long-wave
approximation. The 2004 earthquake off Kii Peninsula (M 7.4)
on the southern coast of Japan indeed produced a dispersive
tsunami that was recorded at two stations located off Shikoku.
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For
the foreshock (M 7.1), on the contrary, a dominant dispersive
tsunami was not recognized at these stations. Because dispersive
waves show strong directional dependence with respect to the
fault strike, the above difference indicates that the strikes
of the main shock and the foreshock were different. We conducted
a tsunami waveform inversion analysis based on the dispersive
tsunami equations to estimate the initial water height distribution
of the main shock. The estimated initial water height distribution
overlapped with the aftershock region, suggesting that the fault
strike was perpendicular to the trough axis, and the total displaced
water volume was 1.7-2.0 × 109 m3. When we used the conventional
long-wave approximation, the estimated initial water height
distribution extended considerably from the aftershock area,
because artificial sources were needed outside the aftershock
area to reproduce the observed dispersive waves. |
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Junio de 2010
Early Mars hydrology: Meridiani playa deposits and the
sedimentary record of Arabia Terra
Authors: Jeffrey C. Andrews-Hanna, Maria
T. Zuber et al
Link: Click here
Abstract
The Meridiani Planum region of Mars has been identified as
a region of past aqueous activity by a combination of orbital
and in situ observations that revealed evidence for sulfate-rich
dirty evaporites formed in a playa setting. We investigate
the hydrology and sedimentary record of this area using global
and regional hydrological models in which groundwater flow
is driven by a combination of precipitation, evaporation,
and the surface topography. Groundwater evaporation results
in evaporite precipitation and cementation of aeolian sediments,
allowing the accumulation of deposits of substantial thickness,
which then affect the
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subsequent patterns
of groundwater flow. Hydrological activity is initially predicted
to be isolated to the deepest craters and depressions, primarily
within the Arabia Terra region surrounding Meridiani. As these
depressions fill with sediments, the groundwater upwelling spreads
laterally across broad regions of Arabia Terra, including Meridiani
Planum, as well as regional topographic lows such as the northern
lowlands and large impact basins. The model predictions are
borne out by observations of large intracrater deposits, inverted
valley networks, finely layered deposits, spectral evidence
for hydrated sulfates, and pedestal craters that preserve the
remnants of a much larger deposit that once covered much of
Arabia Terra. The results suggest that the inferred playa at
Meridiani was part of a regionally extensive zone of groundwater
upwelling. This hydrological cycle requires that conditions
in the late Noachian to early Hesperian must have been conducive
to the existence of liquid water at the surface throughout much
of the low latitudes of Mars. |
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Junio de 2010
Relationship between ocean velocity and motionally induced
electrical signals: 1. In the presence of horizontal velocity
gradients
Author: Zoltan B. Szuts
Link: Click here
Abstract
Motionally induced electric fields and electric currents in
the ocean depend to first order solely on the vertical dimension.
We investigate the significance of two-dimensional (2-D) perturbations
that arise in the presence of horizontal velocity gradients.
The full electric response is calculated for two schematic
geometries that contain horizontal velocity gradients, have
a two-layer ocean with a layer of sediment beneath, and are
described by four nondimensional parameters. When considered
over the realistic ranges of oceanic aspect ratio
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(the
ratio of water depth to the width of velocity), sediment thickness,
and sediment conductivity, velocity errors arising from 2-D
perturbations are found to be less than a few percent of the
dominant one-dimensional (1-D) signal. All errors depend on
the aspect ratio to the power of 1.9 (1) for signals induced
by the vertical (horizontal) component of the Earth's magnetic
field. Depth-uniform velocity errors are proportional to the
1-D sediment conductance ratio, whereas depth-varying velocity
errors are independent of sediment thickness or conductivity.
Errors are weakly (proportionally) dependent on the jet depth
for signals induced by the vertical (horizontal) component of
the magnetic field. Two-dimensional perturbations decay away
from the forcing region with a half width of 0.2-1 times the
1-D effective water depth. This study extends the first-order
theory to the maximum expected aspect ratios for oceanic flow
and finds small perturbations with simple dependencies on the
nondimensional parameters. |
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Mayo de 2010
A mechanism for spatial and temporal earthquake clustering
Authors: E. A. Jagla and A. B. Kolton
Link: Click here
Abstract
The Gutenberg-Richter law states that the size-frequency distribution
of earthquakes follows a power law. This trend is usually
justified using spring-block models, where slips with the
appropriate statistics of sizes have been numerically observed.
However, prominent spatial and temporal clustering features
of earthquakes, as those implied by the Omori law of aftershocks,
are not accounted for by this kind of model unless
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they
are complemented with ad hoc assumptions, such as stress recovery
laws after slip events, or the phenomenological rate-and-state
equations to describe friction. We show that when a mechanism
of structural relaxation is incorporated into a spring-block
model, realistic earthquake patterns following the Gutenberg-Richter
and Omori laws are obtained. Moreover, features well known from
laboratory friction experiments, such as velocity weakening
and increase of static friction with contact time, appear as
a consequence of the relaxational mechanism as well, without
making any a priori assumptions on the velocity dependence of
the friction force in the model. In this way, our model shows
that a single physical mechanism may be a unifying concept behind
the Gutenberg-Richter and Omori laws and the rate-and-state
equations of rock friction. |
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Mayo de 2010
Hierarchical asperity model for multiscale characteristic
earthquakes: A numerical study for the off-Kamaishi earthquake
sequence in the NE Japan subduction zone
Authors: Takane Hori and Shin'ichi Miyazaki
Link: Click here
Abstract
We develop a numerical model to simulate the occurrence of
multiscale characteristic earthquakes in the off-Kamaishi
area of the northeast Japan subduction zone. A notable feature
of these earthquake sequences is that microearthquakes occur
during the later half of interseismic period between two successive
M ~ 4.9 characteristic earthquakes, which we call
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moderate earthquakes.
We introduce a microasperity, the rupture area of the microearthquakes,
with a significantly smaller nucleation size than the size of
a hyper-asperity, the rupture area for moderate earthquakes.
In contrast, the nucleation size in the rest of the hyper-asperity,
which we call a conditional asperity, is set to be comparable
to the spatial size of the hyper-asperity. Such a hierarchical
asperity model successfully reproduces the observed earthquake
sequences. The simulation results show that both microearthquakes
and moderate earthquakes nucleate at the microasperity. The
stress level in the conditional asperity determines whether
a rupture terminates just outside the microasperity or propagates
out into the conditional asperity. |
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Mayo de 2010
Three-dimensional electrical resistivity image of magma
beneath an active continental rift, Taupo Volcanic Zone, New
Zealand
Authors: Wiebke Heise and T. Grant Caldwell
et al
Link: Click here
Abstract
Magmatic activity in regions of continental extension may
result in huge (>400 km3) explosive eruptions of viscous,
gas-rich silicic-magma. Geochemical and geological data suggest
that the large volumes of magma erupted are produced by extracting
interstitial liquid from a
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long-lived
'mush zone' (a mixture of solid crystals and liquid melt) that
accumulates in liquid-dominated lenses at the top of a much
thicker region of lower melt-fraction mush. Such lenses will
be highly electrically conductive compared with normal mid-crustal
rocks. Here we use results of 220 magnetotelluric (MT) soundings
to construct a 3-D electrical resistivity image of the northern
(silicic) part of New Zealand's Taupo Volcanic Zone, a young
continental rift associated with very high heat flow and intense
silicic volcanism. The electrical resistivity image shows a
plume-like structure of high conductivity, interpreted to be
a zone of interconnected melt, rising from depths >35 km
beneath the axis of extension. |
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Mayo de 2010
Dynamics of the ascent and eruption of water containing
dissolved CO2 on Mars
Authors: Alistair Simon Bargery and Lionel
Wilson
Link: Click here
Abstract
Volcanic activity on Mars can interact with the crust, providing
CO2 to the hydrosphere and fracturing the cryosphere. We examine
both heat transfer between magma and cryosphere ice and CO2
transfer between magma and aquifer water. Over the short time
scales required to fracture the cryosphere, plausible amounts
of CO2 transferred from magma are readily dissolved at the
base of an aquifer under typical pressure and temperature
conditions. We examine the physical mechanism by which, in
some circumstances, subsurface water containing dissolved
CO2 is released
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through fractures
to the surface and emerges as a liquid fountain. All of the
bulk densities of mixtures of water and released CO2, for the
range of surface pressures found on Mars, are significantly
greater than the atmosphere density. Using a series of models
we find a relationship between the dissolved CO2 fraction in
an aquifer and the height of a fountain that would form at the
surface above a crustal fracture connecting the aquifer to the
surface. Water eruption velocities range from ~54 to ~163 m
s-1, leading to water fountain heights of at least a few hundred
meters up to a few thousand meters with large CO2 content. Such
fountains could be responsible for both the erosion observed
at the source of Athabasca Vallis and the series of arcuate
ridges surrounding the eastern end of the source graben of Mangala
Vallis. Any mixtures of water and carbon dioxide that supplied
the outflow channels in the Hesperian or Amazonian may have
erupted as such fountains. |
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Mayo de 2010
Thermochemical pressurization of faults during coseismic
slip
Authors: N. Brantut, A. Schubnel et al
Link: Click here
Abstract
During earthquakes, frictional heating on the fault plane
induces a temperature rise and thus a pore pressure rise,
which is known as thermal pressurization (TP). Coseismic mineral
dehydrations may occur because of this temperature increase
and are included within the TP framework. Dehydrations are
modeled as a source term for pore pressure because of the
total volume change and as a sink term for temperature because
they are endothermic. The reaction occurs within the slipping
zone when a threshold temperature Ts is reached. Dehydration
reaction kinetic is modeled using a first-order reaction rate.
Using energy and fluid mass conservation, we derive analytically
the equations of evolution of pore pressure, temperature,
and reaction extent in the undrained, adiabatic case using
a constant reaction rate. We investigate the values of the
kinetic rate constant required to
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produce
a significant effect, which are much higher than laboratory
data reported in the literature on clay, serpentine, and phyllosilicate
dehydration. We show, however, that such high values can be
reached if the temperature dependency of the rate constant is
taken into account. Next, we include fluid and heat transport
and use an Arrhenius law to calculate the rate constant as a
function of temperature. The subsequent set of differential
equations is then solved numerically. The main effect of dehydration
reactions is an increase of pore pressure and a stabilization
of the temperature during slip. We explore a wide range of parameters
in order to determine in which cases dehydration can be considered
as a nonnegligible process. For high-permeability rocks (>10-18
m2) and when the amount of water that can be released is of
the order of 10%, dehydration is an important mechanism as it
delays the onset of melting, which would normally occur even
within the TP framework. If the onset temperature is low compared
to the initial temperature T0 (Ts - T0 150°C), overpressure
can occur. If the reactions are highly endothermic and if their
kinetic is fast enough, frictional melting would not occur unless
the dehydration reactions are completed within the slipping
zone. |
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Mayo de 2010
Impact of the receiver fault distribution on aftershock
activity
Authors: S. Hainzl, G. Zöller et al
Link: Click here
Abstract
Aftershock models are usually based either on purely empirical
relations ignoring the physical mechanism or on deterministic
calculations of stress changes on a predefined receiver fault
orientation. Here we investigate the effect of considering
more realistic fault systems in models based on static Coulomb
stress changes. For that purpose, we perform earthquake simulations
with elastic half-space stress interactions, rate-and-state
dependent frictional
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earthquake nucleation,
and extended ruptures with heterogeneous (fractal) slip distributions.
We find that the consideration of earthquake nucleation on multiple
receiver fault orientations does not influence the shape of
the temporal Omori-type aftershock decay, but changes significantly
the predicted spatial patterns and the total number of triggered
events. So-called stress shadows with decreased activity almost
vanish, and activation decays continuously with increasing distance
from the main shock rupture. The total aftershock productivity,
which is shown to be almost independent of the assumed background
rate, increases significantly if multiple receiver fault planes
exist. The application to the 1992 M7.3 Landers, California,
aftershock sequence indicates a good agreement with the locations
and the total productivity of the observed directly triggered
aftershocks. |
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Mayo de 2010
Modeling and forecasting climate variables using a physical-statistical
approach
Authors: Edward P. Campbell and Mark J.
Palmer
Link: Click here
Abstract
In climatology it is common for studies to use either process
models derived from physical principles or empirical models,
which are rarely combined in any formal way. In part, this
is because it is difficult to develop process models for climate
variables such as monthly or seasonal rainfall that may be
thought of as outputs from
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complex
physical processes. Models for these so-called climate outputs
therefore typically use empirical methods, often incorporating
modeled data as predictors. Our application is concerned with
using simplified models of the El Niño-Southern Oscillation
to drive forecasts of climate outputs such as monthly rainfall
in southeast Australia. We develop a method to couple an empirical
model with a process model in a sequential formulation familiar
in data assimilation. This allows us to model climate outputs
directly, and it offers potential for building new seasonal
forecasting approaches drawing on the strengths of both empirical
and physical modeling. It is also easy to update the model as
more data become available. |
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Mayo de 2010
Deep roots of upper plate faults and earthquake generation
illuminated by volcanism
Authors: Ryosuke Ando and Satoshi Okuyama
Link: Click here
Abstract
For many years, the generation mechanism of disastrous intraplate
earthquakes in the upper plate of a subduction zone have remained
unclear largely because the roots of upper plate intraplate
faults have particularly quiet inter-seismic nature and limited
visibility. Here we propose that the 2008 Mw 6.9 Iwate-Miyagi
Nairiku, Japan, Earthquake, occurred on a dipping fault in
a
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volcanic region, contains
previously-unreported co-seismic evidence of a ductile shear
zone (DSZ) present on the down-dip fault extension beneath the
seismogenic layer, and DSZ plays a dominant role in the seismogenesis.
We found the evidence in the spatial pattern of the co-seismic
ground displacement, which was well captured by synthetic aperture
radar, reflecting geothermal anomalies there. A dynamic forward
model including the inter-seismic deformation of DSZ naturally
reproduced this displacement pattern without kinematic inversions
and consistently explained other independent observations. This
finding should make breakthroughs in observational and theoretical
studies of earthquake generation. |
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Abril de 2010
Inner core-mantle gravitational locking and the super-rotation
of the inner core
Authors: Mathieu Dumberry and Jon Mound
Link: Click here
Abstract
Seismological observations suggest that the Earth's solid
inner core has been rotating faster than the mantle over the
past several decades, consistent with the results of some
numerical geodynamo models. However, the hemispherical anisotropy
structure of the inner core, also seismically observed, may
require the inner core to remain at a relatively fixed longitudinal
alignment with respect to the mantle, perhaps due to gravitational
locking between them. Both of these seismic observations may
be compatible if the differential rotation of the inner core
is oscillatory in nature, with no mean offset over geologically
long timescales. In this work, we
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investigate the possible rates of rotation
of an oscillating inner core and the dynamics of coupling
within the core-mantle system from an angular momentum perspective.
We find that gravitational coupling between the inner core
and mantle acts to prevent differential rotation, although
the period at which 'locking' occurs differs depending on
which of the inner core or mantle is driving the motion. We
also show that for an internally generated torque, a long
period (longer than 100 yr) oscillation of the inner core
with a rate equal to 0.25° yr-1, on the high end of the
rates inferred from seismic observations, is possible. However,
the mantle oscillations entrained by gravitational coupling
in such a scenario are only marginally compatible with the
observed changes in length of day. We show that, in order
to explain the seismically inferred rotation rates, either
the gravitational coupling must be lower than previous estimates,
or the electromagnetic coupling at the core-mantle boundary
must be stronger than typical estimates.
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Abril de 2010
Core field acceleration pulse as a common cause of the
2003 and 2007 geomagnetic jerks
Authors:A. Chulliat, E. Thébault
et al
Link: Click here
Abstract
Laboratory experiments have established that many of the materials
comprising the Earth are strongly anisotropic in terms of
seismic-wave speeds. Observations of azimuthal and radial
anisotropy in the upper mantle are attributed to the lattice-preferred
orientation of olivine caused by the shear strains associated
with deformation, and provide some of the most direct evidence
for deformation and flow within the Earth's interior. Although
observations of crustal radial anisotropy would improve our
understanding of crustal deformation and flow patterns resulting
from tectonic processes, large-scale observations have been
limited to regions of particularly thick crust. Here we show
that observations from ambient noise tomography in the western
United States reveal strong deep (middle to lower)-crustal
radial anisotropy that is confined mainly to the geological
provinces that have undergone significant extension during
the Cenozoic Era (since ~65 Myr ago). The coincidence of crustal
radial anisotropy
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with the extensional provinces of the western United States
suggests that the radial anisotropy results from the lattice-preferred
orientation of anisotropic crustal minerals caused by extensional
deformation. These observations also provide support for the
hypothesis that the deep crust within these regions has undergone
widespread and relatively uniform strain in response to crustal
thinning and extension.
Competing interests statement
The authors declare no competing financial interests.
Online Methods
Inversion of surface-wave dispersion measurements for a three-dimensional
S-wave speed model proceeds in two steps: (1) the inversion
of surface-wave dispersion measurements from the interstation
empirical Green's functions by ambient noise tomography (ANT)
and from earthquake data by multiple-plane-wave earthquake
tomography (MPWT), to produce dispersion maps; and (2) inversion
of the dispersion maps for the three-dimensional S-wave speed
model. Inversions I, II and III differ only in the amplitudes
of radial anisotropy allowed in the deep (middle to lower)
crust and in the uppermost mantle.
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Abril de 2010
Modelling sound propagation in the Southern Ocean to estimate
the acoustic impact of seismic research surveys on marine
mammals
Authors:Monika Breitzke and Thomas Bohlen
Link: Click here
Abstract
Modelling sound propagation in the ocean is an essential tool
to assess the potential risk of air-gun shots on marine mammals.
Based on a 2.5-D finite-difference code a full waveform modelling
approach is presented, which determines both sound exposure
levels of single shots and cumulative sound exposure levels
of multiple shots fired along a seismic line. Band-limited
point source approximations of compact air-gun clusters deployed
by R/V Polarstern in polar regions are used as sound sources.
Marine mammals are simulated as static receivers. Applications
to deep and shallow water models
|
including
constant and depth-dependent sound velocity profiles of the
Southern Ocean show dipole-like directivities in case of single
shots and tubular cumulative sound exposure level fields beneath
the seismic line in case of multiple shots. Compared to a semi-infinite
model an incorporation of seafloor reflections enhances the
seismically induced noise levels close to the sea surface. Refraction
due to sound velocity gradients and sound channelling in near-surface
ducts are evident, but affect only low to moderate levels. Hence,
exposure zone radii derived for different hearing thresholds
are almost independent of the sound velocity structure. With
decreasing thresholds radii increase according to a spherical
20 log10 r law in case of single shots and according to a cylindrical
10 log10 r law in case of multiple shots. A doubling of the
shot interval diminishes the cumulative sound exposure levels
by -3 dB and halves the radii. The ocean bottom properties only
slightly affect the radii in shallow waters, if the normal incidence
reflection coefficient exceeds 0.2. |
|
Abril de 2010
Seismoelectromagnetic waves radiated by a double couple
source in a saturated porous medium
Authors: Yongxin Gao and Hengshan Hu
Link: Click here
Abstract
Studied in this paper are the properties of seismoelectromagnetic
waves radiated by a double couple in a saturated porous medium
arising from the electrokinetic effect. First, using the Pride's
equations, we derive the Green's function of the magnetic
field due to a single point force as a complement of previous
authors' works, in which only the Green's functions of the
solid displacement, the relative fluid-solid displacement
and the electric field were expressed. Furthermore, we extend
these Green's functions to cater for the moment tensor sources.
Then we derive the Green's functions of the solid displacement,
the electric and magnetic fields in the frequency-space domain
excited by a double couple source, which is frequently used
in earthquake seismology. To visualize these fields, the radiation
patterns are calculated and displayed. The results illustrate
that the radiation pattern of the electric far field for the
longitudinal (or transverse) wave is the same in shape as
that of the far field of the P (or S) wave in
|
elastodynamics. For
a transverse wave, the electric and magnetic far fields share
the same radiation patterns in shape, while the electric and
magnetic near fields do not. For each of the four body waves,
the far, intermediate and near fields are compared at different
receiver-to-source distances, respectively. The electromagnetic
(EM) wave has a much longer near-field-dominating distance than
the seismic waves. We calculate the waveforms in the time-space
domain by numerically Fourier transforming the Green's functions
into the time domain. In order to validate these Green's functions
and the waveforms, we calculate the waveforms again by another
method. The main idea of the method is regarding the source
as a displacement-stress-EM discontinuity vector. The result
shows that the waveforms from those two methods are in excellent
agreement. In the waveforms, there are the electric fields accompanying
both the P and S waves, as well as the magnetic field accompanying
the S wave. We testify that the S wave generally has a weaker
capacity than the P wave in inducing an electric field. In the
waveforms, there is also an independently propagating EM wave,
which has a much higher speed than the seismic waves, and reaches
the observation point immediately after the source launched.
By comparing the waveforms at different receiving locations,
we find that waveforms differ at different observation orientations. |
|
Abril de 2010
Detecting seasonal variations in seismic velocities within
Los Angeles basin from correlations of ambient seismic noise
Authors: Ueli Meier , Nikolai M. Shapiro
et al
Link: Click here
Abstract
We analyse 3 yr of continuous seismic records from broad-band
stations of the Caltech Regional Seismic Network (CI) in vicinity
of the Los Angeles basin. Using correlations of ambient seismic
noise, relative velocity variations in the order of 0.1 per
cent can be measured between all interstation pairs. It is
the first time that such an extensive study between 861 interstation
pairs over such a large area has been carried out. We perform
these measurements using the 'stretching' technique, assuming
that one of the two waveforms is merely a stretched version
of the other. Obviously this assumption is always violated
and the two waveforms are generally decorrelated due to temporal
changes in the Earth
|
crust, due to different sources or simply because the cross-correlations
are not fully converged. We investigate the stability of these
measurements by repeating each measurement over various time-windows
of equal length. On average between all interstation pairs in
the Los Angeles basin a seasonal signal in the relative velocity
variation is observed, with peaks and troughs during winter
and summer time, respectively. Generally the observed signal
decreases with increasing interstation distance and relative
traveltime perturbations can only be measured up to an interstation
distance of 60 km. Furthermore, the traveltime perturbations
do not depend on azimuth of station pairs, suggesting that they
are not related to seasonal variations of the noise sources.
Performing a simple regionalization by laterally averaging measurements
over a subset of stations we found the sedimentary basin showing
the most consistent signal and conclude that the observed seasonality
might be induced either by changes in the ground-water aquifer
or thermo-elastic strain variations that persist down to a depth
of 15-22 km. |
|
Abril de 2010
A finite-difference algorithm for full waveform teleseismic
tomography
Authors:S. Roecker , B. Baker and J. McLaughlin
Link: Click here
Abstract
We adapt a 2-D spectral domain finite difference waveform
tomography algorithm previously used in active source seismological
imaging to the case of a plane wave propagating through a
2.5-D viscoelastic medium in order to recover P and S wave
speed variations from body waves recorded at teleseismic distances.
A transferable efficacy that permits recovery of arbitrarily
heterogenous models on moderately sized computers provides
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the primary motivation
for choosing this algorithm. Synthetic waveforms can be generated
either by specifying an analytic solution for a background plane
wave in a 1-D model and solving for the source distribution
that would produce it, or by solving for a scattered field excited
by a plane wave source and then adding the background wavefield
to it. Because the former approach typically involves a concentration
of sources at the free surface, the latter tends to be more
stable numerically. We adapt a gradient approach to solve the
inverse problem to maintain tractability; calculating the gradient
does not require much more computational effort than does the
forward problem. The waveform tomography algorithm can be applied
in a straightforward way to perform receiver function migration
and traveltime inversion. |
|
Abril de 2010
Case history of combined airborne time-domain electromagnetics
and power-line field survey in Chibougamau, Canada
Authors: Marc A. Vallée, Richard
S. Smith et al
Link: Click here
Abstract
Exploration for volcanogenic massive sulfides requires good
geologic understanding. Geologic knowledge often is limited
by a lack of outcrops. This is especially true in Canada under
residual glacial covers. Geologic information must therefore
be complemented by information obtained using means such as
geophysical and geochemical observations. Electromagnetic
(EM) methods extend lithological understanding to depths beyond
the overburden. Massive sulfides are highly conductive and,
depending on their
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depth
and volume, may be detected easily by airborne EM surveys. They
are more often equant than graphitic sediments, which typically
have longer strike length. Current EMtechniques that identify
massive sulfides operate in the frequency or time domain, the
latter being more common. Additional information can be provided
by using power-line fields as a source of EM signals when the
powerlines are appropriately located in the area of interest.
We have worked in an active exploration area near Chibougamau,
Canada, known for a large occurrence of massive sulfide deposits.
The geology is a sequence of volcanic formations with felsic
and mafic intrusions. Our magnetic technique responded well
to mafic rocks. An airborne time-domain EM survey mapped localized
and intrasedimentary conductors in that area. We learned in
our study that power-line EM fields can be used to map large-extent
conductive formations and narrow geologic faults. |
|
Abril de 2010
Analytic solution of the gravity anomaly of irregular 2D
masses with density contrast varying as a 2D polynomial function
Author: Xiaobing Zhou
Link: Click here
Abstract
The analytic solution of the gravity anomaly caused by a 2D
irregular mass body with the density contrast varying as a
polynomial function in the horizontal and vertical directions
is extrapolated from a historical version in which the analytic
solution for the gravity anomaly was given only at the origin
of the coordinate system to any point for the density function
in terms of variables relative to that origin. To calculate
the gravity anomaly at stations that are not at origins, a
coordinate transformation is performed, in which case the
polynomial density contrast function must also be expressed
in the transformed coordinates, or a transformed solution
must be
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obtained. These analytic
solutions can be obtained at any station using (1) a solution
transformation method, in which the density function and boundary
of a mass body are kept intact, or (2) a coordinate transformation
method, in which polynomial coefficient and boundary of a mass
body are transformed accordingly. The issue of singularity and
instability of the analytic methods has been related to case
studies. Caution should be exercised in modeling or interpreting
the gravity survey data using the analytic methods for large
target-distance-to-target-size ratios outside the range of numerical
stability. Compared with other published methods, the analytic
solution results agree very well with other numerical or seminumerical
methods, indicating the solution is correct and can be applied
for any gravity anomaly calculation caused by an irregular 2D
mass body with the density-contrast approximated as a polynomial
function of horizontal position and/or vertical position when
the observation is within the range of numerical stability. |
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Abril de 2010
The wedge model revisited: A physical modeling experiment
Link: Click here
Abstract
To assess seismic amplitude effects commonly associated with
the classic wedge model in geophysics, we built a scaled physical
model of a simple high-velocity wedge immersed in water. In
addition to demonstrating the well-known tuning effect related
to thin beds, a 2D marine zero-offset seismic survey over
the physical model shows a surprising number of high-amplitude
dipping events corresponding to elastic multimodes, multiples,
and mixed-mode reflections having
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nonreciprocal
raypaths. These events cause additional complexities in the
amplitudes of the top-wedge and base-wedge reflections that
are not observed in simple acoustic seismic responses of the
wedge model. Finite-difference, acoustic, exploding-reflector
numerical model data, calculated using the same wedge geometry
and velocity model, assisted in the identification of these
events. It was found that the amplitudes of mixed-mode multiples
in data recorded over high-velocity rocks with a wedge-like
geometry might be significant. We also discovered that there
is a maximum number of zero-offset pure-mode multiples within
the wedge for a given wedge taper angle. Conventional P-wave
migration of the physical model data confirmed that the multimode
reflections degrade the quality of the migrated image. |
|
Abril 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. |
|
Abril 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
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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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Abril de 2010
Geophysical experiments for the pre-reclamation assessment
of industrial and municipal waste landfills
Authors:R Balia and B Littarru
Link: Click here
Abstract
Two examples of combined application of geophysical techniques
for the pre-reclamation study of old waste landfills in Sardinia,
Italy, are illustrated. The first one concerned a mine tailings
basin and the second one a municipal solid waste landfill;
both disposal sites date back to the 1970-80s. The gravity,
shallow reflection, resistivity and induced polarization methods
were employed
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in different combinations
at the two sites, and in both cases useful information on the
landfill's geometry has been obtained. The gravity method proved
effective for locating the boundaries of the landfill and the
shallow reflection seismic technique proved effective for the
precise imaging of the landfill's bottom; conversely the electrical
techniques, though widely employed for studying waste landfills,
provided mainly qualitative and debatable results. The overall
effectiveness of the surveys has been highly improved through
the combined use of different techniques, whose individual responses,
being strongly dependent on their specific basic physical characteristic
and the complexity of the situation to be studied, did not show
the same effectiveness at the two places. |
|
Abril de 2010
Core field acceleration pulse as a common cause of the
2003 and 2007 geomagnetic jerks
Authors:A. Chulliat, E. Thébault
et al
Link: Click here
Abstract
Using observatory data, we report the detection of a geomagnetic
jerk in 2007, which we relate to a jump in the second derivative
of the geomagnetic field previously noted in satellite
data. Although not of worldwide extent, this jerk is very
intense in the South Atlantic region. Using the
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CHAOS-2
model, we show that both this jerk and the previous 2003 jerk
are caused by a single core field acceleration pulse reaching
its maximum power near 2006.0. This pulse seems to be simultaneously
occurring in several regions of the core surface where it corresponds
to dominant n = 5 and 6 spherical harmonic modes. Geometrical
attenuation explains why the 2003 and 2007 jerks are local and
not fully synchronized at the Earth's surface. Our results suggest
that this core field acceleration pulse is the relevant phenomenon
to be investigated from the point of view of core dynamics,
rather than the jerks themselves. |
|
Abril de 2010
Thermal structure of the subduction zone in western Japan
derived from seismic attenuation data
Authors: Andri Dian Nugraha, Jim Mori et
al
Link: Click here
Abstract
We use a well determined three-dimensional attenuation model
to infer the temperature distribution of the subduction zone
in western Japan. The tomographic attenuation model was
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derived from about
19,000 t* determinations for P- waves recorded at 128 stations.
The attenuation model with a previously determined velocity
model was used to infer the temperature distribution of the
subduction zone from depths of 15 to 80 km, using the high temperature
background method. We can clearly see the cooler slab (400°C
to 700°C) that is being subducted under the hotter mantle
material (425°C to 850°C). We also show that the deep
low-frequency earthquakes are occurring in regions just above
the slab with temperatures of about 500°C to 600°C.
|
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Abril de 2010
Seafloor topography, ocean infragravity waves, and background
Love and Rayleigh waves
Authors: Yoshio Fukao, Kiwamu Nishida et
al
Link: Click here
Abstract
We propose that background Love and Rayleigh waves in a frequency
range 5-20 mHz are generated primarily by ocean infragravity
waves in the same frequency range by a linear coupling process
with seafloor topography. Wavelengths of infragravity waves
in this frequency range are on the order of 10 to 40 km in
the deep ocean. The seafloor topography with wavelengths of
this order is dominated by abyssal hills, which are the most
widespread physiographic forms on Earth, covering as much
as 85% of the Pacific floor.
|
Interaction
of infragravity waves in the deep ocean with these hills generates
a random distribution of point-like tangential forces on the
seafloor which may be large enough to excite Love and Rayleigh
waves simultaneously. We quantify this idea by using the known
statistical property of hills distribution in the Pacific and
by noting that heights of abyssal hills are an order of magnitude
smaller than depths of the deep ocean, so that the topography-related
phase velocity change can be neglected. The model is reasonably
consistent with the Love to Rayleigh wave amplitude ratio reported
at 10-20 mHz and the observed background Rayleigh wave spectrum
with a characteristic plateau around 8 mHz. Contribution of
topographic coupling in shallow, coastal seas is not included
in our simple model but should be important, especially at frequencies
above 20 mHz. |
|
Abril de 2010
Depth of the Moho discontinuity beneath the Japanese islands
estimated by traveltime analysis
Author: Akio Katsumata
Link: Click here
Abstract
The crustal structure beneath the Japanese islands, including
depth distributions of the Conrad and Moho discontinuities,
was estimated using a tomographic inversion of regional body
wave arrival times. Depth distributions of the bottom of the
surface layer, the Conrad, and the Moho were modeled with
two-dimensional B spline functions, while velocity distributions
in layers were expressed by three-dimensional B spline functions.
The depth of the discontinuities
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and the velocity in
the layers were estimated simultaneously by the least squares
method. The velocity structure was sequentially estimated from
shallower parts to deeper parts to avoid correlation between
them. This sequential analysis provided improved depth resolution.
The deepest region of the Moho discontinuity was located in
central Honshu, reaching about 40 km. The Moho discontinuity
was generally deep in the central part of the islands, whereas
it was relatively shallow in the Kanto, southwestern Chubu,
and Chugoku districts in Honshu and in northern Kyushu. Some
of the shallow Moho regions would be related to graben formation
due to tensile tectonic stress since the Miocene. The results
were compared with those of seismic refraction surveys and receiver
function analyses, and it was found that the obtained model
was consistent with many of these studies. |
|
Abril de 2010
Dynamics of soap bubble bursting and its implications to
volcano acoustics
Authors: V. Vidal, M. Ripepe et al
Link: Click here
Abstract
In order to assess the physical mechanisms at stake when giant
gas bubbles burst at the top of a magma conduit, laboratory
experiments have been performed. An overpressurized gas cavity
is
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bursts. The acoustic
signal produced by the bursting is investigated. The key result
is that the amplitude and energy of the acoustic signal strongly
depend on the film rupture time. As the rupture time is uncontrolled
in the experiments and in the field, the measurement of the
acoustic excess pressure in the atmosphere, alone, cannot provide
any information on the overpressure inside the bubble before
explosion. This could explain the low energy partitioning between
infrasound, seismic and explosive dynamics often observed on
volcanoes. |
|
Marzo de 2010
Earth's dynamo limit of predictability
Authors: G. Hulot, F. Lhuillier et al
Link: Click here
Abstract
Earth's magnetic field is currently decreasing, reducing the
protection it offers against charged particles coming from
space and increasing space weather hazards within the near-Earth
environment. Modeling the future evolution of the
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field
is thus of considerable interest. But how far in the future
this can conceivably be done is still an open question. Here
we report on the first systematic investigation of the limit
of predictability of fully consistent 3D numerical dynamo simulations,
and suggest that the Earth's dynamo is likely unpredictable
beyond a century, making decade timescale forecasts of the main
magnetic field conceivable, but rendering longer-term predictions,
such as the timing of the next reversal, totally unpredictable.
|
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Marzo de 2010
Northern Rocky Mountain streamflow records: Global warming
trends, human impacts or natural variability?
Authors: Jeannine-Marie St. Jacques, David
J. Sauchyn et al
Link: Click here
Abstract
The ~60 year Pacific Decadal Oscillation (PDO) is a major
factor controlling streamflow in the northern Rocky Mountains,
causing dryness during its positive phase, and wetness during
its negative phase. If the PDO's influence is not incorporated
into a trend analysis of streamflows, it can produce detected
declines that are actually
|
artifacts of this
low-frequency variability. Further difficulties arise from the
short length and discontinuity of most gauge records, human
impacts, and residual autocorrelation. We analyze southern Alberta
and environs instrumental streamflow data, using void-filled
datasets from unregulated and regulated gauges and naturalized
records, and Generalized Least Squares regression to explicitly
model the impacts of the PDO and other climate oscillations.
We conclude that streamflows are declining at most gauges due
to hydroclimatic changes (probably from global warming) and
severe human impacts, which are of the same order of magnitude
as the hydroclimate changes, if not greater. |
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Marzo de 2010
El empuje Occidental Andino, la Falla de
San Ramón y el riesgo sísmico en Santiago de
Chile.
Autores: Rolando Armijo, Rodrigo Rauld et
al
Link: Clic aquí
Abstract
La importancia de las estructuras del
borde oeste del flanco occidental de Los Andes, paralelo a
la zona de subducción, aparece actualmente minimizada.
Esto dificulta nuestra comprensión del Sistema Andes-Altiplano
y que es una de las más importantes cadenas montañosas
de la Tierra. Nosotros analizamos una sección clave
de la tectónica de Los Andes en la latitud 33.5°
S, donde la cadena presenta una etapa temprana de su evolución,
con el objetivo de resolver la arquitectura primaria de la
orogenia. Nos enfocamos en el sistema de propagación
de fallas activas en Los Andes, detrás de la Falla
de San Ramón , lo cual es crítico para evaluar
el riesgo sísmico de la ciudad de Santiago y crucial
para descifrar la estructura del Empuje Occidental Andino
(WAT). La Falla de San Ramón es una rampa de empuje
en la zona frontal de un sector de desprendimiento con un
deslizamiento promedio de ~ 0.4 mm/año. La existencia
de terrenos escarpados modernos a distintas escalas indica
la posibilidad de eventos sísmicos pasados de magnitudes
de hasta 7.4 Mw. El WAT desciende hacia el este de la Falla
de San Ramón, cruzando 12 Km de la cobertura de la
raíz andina y debajo del basamento anticlinal frontal
cordillerano, en un rango de ~ 5 Km de alto y sobre 700 Km
de longitud.
Nosotros proponemos un modelo tectónico de primer orden
de los Andes involucrando una embriónica subducción
intracontinental consistente con las observaciones geológicas
y geofísicas. La etapa primaria del movimiento hacia
el oeste, con predomino del WAT a 33.5° S, está
evolucionando dentro de una configuración de movimiento
doble. También hemos deducido un modelo de crecimiento
del WAT del Altiplano similar al que ocurre en la zona cordillerana
de los Himalayas Tibetanos. Nosotros sugerimos que la subducción
intracontinental del WAT es un sustituto mecánico de
una zona de colisión, lo que vuelve obsoleto el actual
paradigma sobre la orogenia andina.
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Marzo de 2010
The West Andean Thrust, the San Ramón Fault, and
the seismic hazard for Santiago, Chile
Authors: Rolando Armijo, Rodrigo Rauld
et al
Link: Click here
Abstract
The importance of west verging structures at the western flank
of the Andes, parallel to the subduction zone, appears currently
minimized. This hampers our understanding of the Andes-Altiplano,
one of the most significant mountain belts on Earth. We analyze
a key tectonic section of the Andes at latitude 33.5°S,
where the belt is in an early stage of its evolution, with
the aim of resolving the primary architecture of the orogen.
We focus on the active fault propagation-fold system in the
Andean cover behind the San Ramón Fault, which is critical
for the seismic hazard in the city of Santiago and crucial
to decipher the structure of the West Andean Thrust (WAT).
The San Ramón Fault is a thrust ramp at the front of
a basal detachment with average slip rate of ~0.4 mm/yr. Young
scarps at various scales imply plausible seismic events up
to Mw 7.4. The WAT steps down eastward from the San Ramón
Fault, crossing 12 km of Andean cover to root beneath the
Frontal Cordillera basement anticline, a range ~5 km high
and >700 km long. We propose a first-order tectonic model
of the Andes involving an embryonic intracontinental subduction
consistent with geological and geophysical observations. The
stage of primary westward vergence with dominance of the WAT
at 33.5°S is evolving into a doubly vergent configuration.
A growth model for the WAT-Altiplano similar to the Himalaya-Tibet
is deduced. We suggest that the intracontinental subduction
at the WAT is a mechanical substitute of a collision zone,
rendering the Andean orogeny paradigm obsolete.
|
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Marzo de 2010
Information theory-based approach for location of monitoring
water level gauges in polders
Authors: Leonardo Alfonso, Arnold Lobbrecht
et al
Link: Click here
Abstract
Data collection is a critical activity in the management of
water systems because it supports informed decision making.
Data are collected by means of monitoring networks in which
water level gauges are of particular interest because of their
implications for flood management. This paper introduces a
number of modifications to previously published methods that
use information theory to design hydrological monitoring networks
in order to make the methods applicable to the design of water
level
|
monitors for highly
controlled polder systems. The new contributions include the
use of a hydrodynamic model for entropy analysis, the introduction
of the quantization concept to filter out noisy time series,
and the use of total correlation to evaluate the performance
of three different pairwise dependence criteria. The resulting
approach, water level monitoring design in polders (WMP), is
applied to a polder in the Pijnacker region, Netherlands. Results
show that relatively few monitors are adequate to collect the
information of a polder area in spite of its large number of
target water levels. It is found, in addition, that the directional
information transfer DITYX is more effective in finding independent
monitors, whereas DITXY is better for locating sets of monitors
with high joint information content. WMP proves to be a suitable
and simple method as part of the design of monitoring networks
for polder systems. |
|
Marzo de 2010
Entropy theory for derivation of infiltration equations
Author: Vijay P. Singh
Link: Click here
Abstract
An entropy theory is formulated for modeling the potential
rate of infiltration in unsaturated soils. The theory is composed
of six parts: (1) Shannon entropy, (2) principle of maximum
entropy (POME), (3) specification of information on infiltration
in terms of constraints, (4) maximization of entropy in accordance
with POME, (5) derivation of the probability distribution
of infiltration, and (6) derivation of infiltration equations.
The theory is illustrated with the derivation of six infiltration
equations commonly used in hydrology, watershed management,
and agricultural irrigation, including Horton, Kostiakov,
Philip two-
|
term,
Green-Ampt, Overton, and Holtan equations, and the determination
of the least biased probability distributions of these infiltration
equations and their entropies. The theory leads to the expression
of parameters of the derived infiltration equations in terms
of measurable quantities (or information), called constraints,
and in this sense these equations are rendered nonparametric.
Furthermore, parameters of these infiltration equations can
be expressed in terms of three measurable quantities: initial
infiltration, steady infiltration, and soil moisture retention
capacity. Using parameters so obtained, infiltration rates are
computed using these six infiltration equations and are compared
with field experimental observations reported in the hydrologic
literature as well as the rates computed using parameters of
these equations obtained by calibration. It is found that infiltration
parameter values yielded by the entropy theory are good approximations.
|
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Marzo de 2010
Size-resolved chemical composition of aerosol emitted by
Erebus volcano, Antarctica
Authors:E. Ilyinskaya, C. Oppenheimer et
al
Link: Click here
Abstract
Persistent, open-vent degassing of Erebus volcano, Antarctica,
is a significant point source of gases and aerosol to the
austral polar troposphere. We report here on the chemical
composition and size distribution of the Erebus aerosol, focusing
on the water-soluble fraction. The aerosol was sampled at
the rim of the active crater using a cascade impactor, which
collected and sized particles in 14 size bins from >10
to 0.01 µm. The soluble fraction of the Erebus aerosol
is distinct from other volcanic sources in several respects.
It is dominated by chloride-bearing particles (over 30% of
total mass) and has an unusually high Cl-/SO42- molar ratio
of
|
3.5. Coarse particles
contribute little to the total mass of the soluble fraction.
Elevated concentrations of F-, Cl-, Br-, and SO42- are found
in a narrow particle size fraction of 0.1-0.25 µm. The
detection of particulate Br- reinforces our understanding of
the potential for quiescent volcanic emissions to deplete tropospheric
ozone. The small aerosol size reflects the low atmospheric temperature
and humidity, which inhibit particle growth. Halide-alkali metal
salts (Na, K)(Cl, F) appear to be the most abundant species
in the aerosol. The concentration of Pb is high compared to
other volcanoes; its exsolution may be promoted by the high
abundance of halogens in Erebus magma. Despite the previously
reported high NOx content in the plume, we did not detect significant
quantities of nitrate in the near-vent aerosol. Our findings
emphasize the potential regional significance of emissions from
Erebus for understanding the Antarctic atmospheric composition
and glaciochemical records. |
|
Marzo de 2010
Lava dome collapse detected using passive seismic interferometry
Author: B. J. Baptie
Link: Click here
Abstract
The collapse of the lava dome at the Soufrière Hills
Volcano on Montserrat in July 2003 is the largest recorded
in historical times. I use noise correlation Green's functions
to measure the changes in seismic properties that resulted
from this collapse. Continuous three component
|
seismic
data recorded at two pairs of stations were cross-correlated
to retrieve three-component Green's functions along two paths
that intersect the volcanic edifice before and after the dome
collapse. Particle motion analysis shows that the Green's functions
are dominated by Rayleigh waves and are consistent with the
expected Green's tensor for a vertical point force source at
one station recorded by a three-component receiver at the other.
Following the collapse, there is a clear decorrelation and phase
shift in the Green's functions corresponding to a change in
velocity of approximately 0.5% that can be interpreted in terms
of the unloading of the lava dome. |
|
Marzo de 2010
Normal and reverse faulting driven by the subduction zone
earthquake cycle in the northern Chilean fore arc
Authors: John P. Loveless, Richard W. Allmendinger
et al
Link: Click here
Abstract
Despite its location in a convergent tectonic setting, the
Coastal Cordillera of northern Chile between 21°S and
25°S is dominated by structures demonstrating extension
in the direction of plate convergence. In some locations,
however, normal faults have been reactivated as reverse faults,
complicating the interpretation of long-term strain. In order
to place these new observations in a tectonic context, we
model stress changes induced on these faults by the subduction
earthquake cycle. Our simulations predict that interseismic
locking on the plate boundary encourages normal slip on fore-arc
|
faults, which may
result from elastic rebound due to interplate earthquakes or
from seismic or aseismic motion that takes place within the
interseismic period. Conversely, stress generated by strong
subduction zone earthquakes, such as the 1995 Mw = 8.1 Antofagasta
event, provides a mechanism for the reverse reactivation we
document here. Upper plate fault slip in response to the low-magnitude
stress changes induced by the subduction earthquake cycle suggests
that the absolute level of stress on these faults is very low.
Furthermore, seismic hazard analysis for northern Chile requires
consideration of not only the plate boundary earthquake cycle
but also the cycle on fore-arc faults that may or may not coincide
with the interplate pattern. Though the relationships between
permanent strain and deformation calculated using elastic models
remain unclear, the compatibility of modeled stress fields with
the distribution of fore-arc faulting suggests that interseismic
strain accumulation and coseismic deformation on the subduction
megathrust both play significant roles in shaping structural
behavior in the upper plate. |
|
Marzo de 2010
Compatibility of induction methods for mantle soundings
Authors: J. Vozar and V. Y. Semenov
Link: Click here
Abstract
Formulations that form the basis of experimental impedances
in induction soundings result from the impedance boundary
conditions or from the simplified theoretical models. The
formulations are essentially different for the magnetotelluric
and magnetovariation sounding methods. In order to increase
reliability of mantle investigations, studies of the mantle's
electrical properties are often carried out by the joint inversion
of impedances obtained by both sounding methods. A forward
modeling approach is used to verify the accuracy of merging
the long-period impedances obtained by the magnetotelluric
and
|
magnetovariation
methods. The spherical modeling of the responses above 2-D and
3-D mantle inhomogeneities has shown that the different induction
methods can give mutually inconsistent results and the combination
of their responses can be problematic in practice. For this
reason much attention is given to the generalized horizontal
spatial gradient sounding method which results in impedance
functions that in space and frequency domains closely resemble
the magnetotelluric impedances. In this study some interesting
properties of the induction arrows above a spherical inhomogeneity,
excited by an inhomogeneous external field, are estimated for
long periods. A final comprehensive model, assuming a shell
of realistic conductance at the Earth's surface, is evidence
that the generalized horizontal spatial gradient method is promising
for the study of mantle inhomogeneities and can be reliably
used in combination with the magnetotelluric method in a specific
way. |
|
Marzo de 2010
A hidden Markov model for earthquake declustering
Author: Zhengxiao Wu
Link: Click here
Abstract
The hidden Markov model (HMM) and related algorithms provide
a powerful framework for statistical inference on partially
observed stochastic processes. HMMs have been successfully
implemented in many disciplines,
|
though
not as widely applied as they should be in earthquake modeling.
In this article, a simple HMM earthquake occurrence model is
proposed. Its performance in declustering is compared with the
epidemic-type aftershock sequence model, using a data set of
the central and western regions of Japan. The earthquake clusters
and the single earthquakes separated using our model show some
interesting geophysical differences. In particular, the log-linear
Gutenberg-Richter frequency-magnitude law (G-R law) for the
earthquake clusters is significantly different from that for
the single earthquakes. |
|
Marzo de 2010
Nonlinear VLF effects in the topside ionosphere
Authors: E. V. Mishin, M. J. Starks et
al
Link: Click here
Abstract
The Demeter satellite observed intense broadband lower- and
upper-hybrid electrostatic
|
waves
and plasma perturbations associated with high-power whistler-mode
signals from the very-low frequency (VLF) transmitter NWC. This
paper shows that the Demeter observations can be explained by
nonlinear interactions driven by VLF pump waves, thereby suggesting
that nonlinear effects are responsible for energy losses of
high-power VLF signals in the ionosphere. |
|
Marzo de 2010
Markov-switching model for nonstationary runoff conditioned
on El Niño information
Authors:E. Gelati, H. Madsen et al
Link: Click here
Abstract
We define a Markov-modulated autoregressive model with exogenous
input (MARX) to generate runoff scenarios using climatic information.
Runoff parameterization is assumed to be conditioned on a
hidden climate state following a Markov chain, where state
transition probabilities are functions of the climatic input.
MARX allows stochastic modeling of nonstationary runoff, as
runoff
|
anomalies
are described by a mixture of autoregressive models with exogenous
input, each one corresponding to a climate state. We apply MARX
to inflow time series of the Daule Peripa reservoir (Ecuador).
El Niño-Southern Oscillation (ENSO) information is used
to condition runoff parameterization. Among the investigated
ENSO indexes, the NINO 1+2 sea surface temperature anomalies
and the trans-Niño index perform best as predictors.
In the perspective of reservoir optimization at various time
scales, MARX produces realistic long-term scenarios and short-term
forecasts, especially when intense El Niño events occur.
Low predictive ability is found for negative runoff anomalies,
as no climatic index correlating properly with negative inflow
anomalies has yet been identified. |
|
Marzo de 2010
Rheologic and structural controls on the deformation of
Okmok volcano, Alaska: FEMs, InSAR, and ambient noise tomography
Authors: Timothy Masterlark, Matthew Haney
et al
Link: Click here
Abstract
Interferometric synthetic aperture radar (InSAR) data indicate
that the caldera of Okmok volcano, Alaska, subsided more than
a meter during its eruption in 1997. The large deformation
suggests a relatively shallow magma reservoir beneath Okmok.
Seismic tomography using ambient ocean noise reveals two low-velocity
zones (LVZs). The shallow LVZ corresponds to a region of weak,
fluid-saturated materials within the caldera and extends from
the caldera surface to a depth of 2 km. The deep LVZ clearly
indicates the presence of the magma reservoir beneath Okmok
that is significantly deeper (>4 km depth)
|
compared
to previous geodetic-based estimates (3 km depth). The deep
LVZ associated with the magma reservoir suggests magma remains
in a molten state between eruptions. We construct finite element
models (FEMs) to simulate deformation caused by mass extraction
from a magma reservoir that is surrounded by a viscoelastic
rind of country rock embedded in an elastic domain that is partitioned
to account for the weak caldera materials observed with tomography.
This configuration allows us to reduce the estimated magma reservoir
depressurization to within lithostatic constraints, while simultaneously
maintaining the magnitude of deformation required to predict
the InSAR data. More precisely, the InSAR data are best predicted
by an FEM simulating a rind viscosity of 7.5 × 1016 Pa
s and a mass flux of -4.2 × 109 kg/d from the magma reservoir.
The shallow weak layer within the caldera provides a coeruption
stress regime and neutral buoyancy horizon that support lateral
magma propagation from the central magma reservoir to extrusion
near the rim of the caldera. |
|
Marzo de 2010
Retrieving electric resistivity data from self-potential
measurements by cross-correlation
Authors: Evert Slob, Roel Snieder et al
Link: Click here
Abstract
We show that the two-point cross-correlation of self-potential
field recordings is equal to the electric resistivity between
the two points. This holds under the condition that spatially
and temporally uncorrelated noise sources exist
|
throughout
the volume. These sources should have a known amplitude spectrum
and their correlated strengths should be proportional to the
dissipative medium property function. Natural fluctuations,
such as thermal noise, may occur that satisfy the necessary
conditions. When these fluctuations are random deviations from
a state of thermal equilibrium, the fluctuation-dissipation
theorem can be used to describe these sources. Other types of
sources may exist, such as the ones creating the self-potential
field through coupling with fluctuations in pressure, temperature
and chemical potential gradients. |
|
Marzo de 2010
Correlation of earthquake source parameters inferred from
dynamic rupture simulations
Authors: Jan Schmedes, Ralph J. Archuleta
et al
Link: Click here
Abstract
We analyzed 315 dynamic strike-slip rupture models computed
up to 5.0 Hz to get a quantitative understanding of the correlation
and amplitude distributions of parameters describing the earthquake
source, such as slip and rupture velocity. To account for
the epistemic uncertainty of the problem, we constructed a
database of dynamic ruptures computed by ourselves and other
authors. This database contains ruptures computed using different
models of initial stress, peak stress, and critical slip-weakening
distance. Using the set of computed ruptures, we constructed
probability density functions (pdfs) for
|
the amplitude distributions of the source parameters and for
the correlation between the source parameters. We tried to extract
parameter pairs that showed a small variability in the spatial
correlation given the large epistemic uncertainty in the input.
We only analyzed the areas on the fault with subshear propagation
speed. The principal findings are as follows: (1) Final slip
amplitude does not show correlation with the local rupture velocity.
(2) Final slip amplitude correlates well with risetime. (3)
Rupture velocity correlates well with peak slip rate and the
duration of the impulsive part of the slip rate function. (4)
The pdf of rupture velocity, risetime, and peak slip rate depends
on the distance from the nucleation zone. (5) Fracture energy
is not the single controlling factor for the rupture velocity;
the slope of the linear slip-weakening curve has a significant
effect on the rupture velocity. (6) The crack length (length
that is slipping at a given time) decreases with the distance
from the nucleation zone. |
|
Marzo de 2010
Estimation of aquifer lower layer hydraulic conductivity
values through base flow hydrograph rising limb analysis
Authors: Valentijn R. N. Pauwels and Peter
A. Troch
Link: Click here
Abstract
The estimation of catchment-averaged aquifer hydraulic conductivity
values is usually performed through a base flow recession
analysis. Relationships between the first time derivatives
of the base flow and the base flow values themselves, derived
for small and large values of time, are used for this purpose.
However, in the derivation of the short-time equations, an
initially fully saturated aquifer without recharge with sudden
drawdown is assumed, which occurs very rarely in reality.
It is demonstrated that this approach leads to a nonnegligible
error in the
|
parameter
estimates. A new relationship is derived, valid for the rising
limb of a base flow hydrograph, succeeding a long rainless period.
Application of this equation leads to accurate estimates of
the aquifer lower layer saturated hydraulic conductivity. Further,
it has been shown analytically that, if base flow is modeled
using the linearized Boussinesq equation, the base flow depends
on the effective aquifer depth and the ratio of the saturated
hydraulic conductivity to the drainable porosity, not on these
three parameters separately. The results of the new short-time
expression are consistent with this finding, as opposed to the
use of a traditional base flow recession analysis. When base
flow is modeled using the nonlinear Boussinesq equation, the
new expression can be used, without a second equation for large
values of time, to estimate the aquifer lower layer hydraulic
conductivity. Overall, the results in this paper suggest that
the new methodology outperforms a traditional recession analysis
for the estimation of catchment-averaged aquifer hydraulic conductivities.
|
|
Marzo de 2010
Imaging and characterization of solute transport during
two tracer tests in a shallow aquifer using electrical resistivity
tomography and multilevel groundwater samplers
Authors: Kerstin Müller, Jan Vanderborght,
et al
Link: Click here
Abstract
The relevance of aquifer heterogeneity for flow and transport
is recognized broadly; however, its characterization is hampered
by the inaccessibility of the subsurface. Time-lapse electrical
resistivity tomography (ERT) offers the possibility of imaging
noninvasively subsurface transport. We present results of
two tracer tests that were carried out successively in a shallow
aquifer at the Krauthausen test site (Germany). The breakthroughs
of an electrically conductive and a resistive tracer were
monitored with ERT and local multilevel groundwater samplers
(MLS) along two cross sections perpendicular to the mean flow
|
direction.
Sinking of the conductive salt tracer due to density effects
was observed with ERT. We applied a stream tube model to characterize
the spatially variable transport. ERT-derived stream tube parameters
showed similar patterns for the two tracer experiments, reflecting
the effect of aquifer heterogeneity on transport. MLS data did
not show similar spatial patterns, which indicates that these
measurements may be prone to subtle changes of the flow field
in the small sampling volume and mixing within screened wells.
Between 50% and 10% of the tracer was recovered in the ERT-derived
breakthrough curves. Compared with transport simulations in
a homogeneous aquifer, ERT-derived time-integrated changes in
electrical conductivity were locally larger but focused in a
smaller area. MLS data indicated that in this area, ERT did
not underestimate the tracer recovery. The relatively low tracer
recovery was attributed to undetected tracer breakthrough in
regions with low ERT sensitivity and in regions where the length
of the tracer plume and the electrical conductivity contrast
were small. |
|
Marzo de 2010
An elastic plate model for interseismic deformation in
subduction zones
Authors: Ravi V. S. Kanda and Mark Simons
Link: Click here
Abstract
Geodetic observations of interseismic surface deformation
in the vicinity of subduction zones are frequently interpreted
using simple kinematic elastic dislocation models (EDM). In
this theoretical study, we develop a kinematic EDM that simulates
plate subduction over the interseismic period (the elastic
subducting plate model (ESPM)) having only 2 more degrees
of freedom than the well-established back slip model (BSM):
an elastic plate thickness and the fraction of flexural stresses
due to bending at the trench that are released continuously.
Unlike the BSM, in which steady state deformation in both
plates is assumed to be negligible, the ESPM includes deformation
in the subducting and overriding plates (owing to plate thickness),
while still preserving the correct sense of convergence velocity
between the subducting and overriding
|
plates, as well as zero net steady state vertical offset between
the two plates when integrated over many seismic cycles. The
ESPM links elastic plate flexure processes to interseismic deformation
and helps clarify under what conditions the BSM is appropriate
for fitting interseismic geodetic data at convergent margins.
We show that the ESPM is identical to the BSM in the limiting
case of zero plate thickness, thereby providing an alternative
motivation for the BSM. The ESPM also provides a consistent
convention for applying the BSM to any megathrust interface
geometry. Even in the case of nonnegligible plate thickness,
the deformation field predicted by the ESPM reduces to that
of the BSM if stresses related to plate flexure at the trench
are released either continuously and completely at shallow depths
during the interseismic period or deep in the subduction zone
(below ~100 km). However, if at least a portion of these stresses
are not continuously released in the shallow portion of the
subduction zone (via seismic or aseismic events), then the predicted
surface velocities of these two models can differ significantly
at horizontal distances from the trench equivalent to a few
times the effective interseismic locking depth. |
|
Marzo de 2010
Tandem afterslip on connected fault planes following the
2008 Nima-Gaize (Tibet) earthquake
Authors:I. Ryder, R. Bürgmann et al
Link: Click here
Abstract
On 9 January 2008 a M 6.4 normal-faulting earthquake occurred
in central Tibet, near the border of Nima and Gaize counties
and just north of the Bangong-Nujiang suture zone. A week
later, a M 5.9 aftershock occurred a few kilometers to the
northwest of the main shock. Here we consider interferometric
synthetic aperture radar (InSAR) data from the Japanese ALOS
and European Envisat satellites, covering both the coseismic
phase and 9 months of postseismic deformation. The coseismic
interferograms clearly show surface deformation resulting
from both main shock and aftershock ruptures, and data inversions
using elastic dislocation models
|
suggest
that two northwest dipping faults form a synthetic system, with
the more steeply dipping aftershock plane meeting the main shock
plane at depth. Postseismic interferograms show first-order
similarities with their coseismic counterparts, indicating that
afterslip occurred on both main shock and aftershock rupture
surfaces during the months following the earthquakes. The afterslip
occurred at comparable depths to the coseismic slip, but the
amount of slip was about an order of magnitude smaller. A slip
template method is used to obtain moment release estimates at
different postseismic time intervals and hence document the
time dependence of the postseismic transient. The exponential
decay time of the afterslip is 34 days, and the moment release
due to the afterslip was about 10% of the coseismic moment.
Models of viscoelastic stress relaxation in a Maxwell half-space
place a strong lower bound on midcrust to lower crustal viscosity
of 3 × 1017 Pa s. Postseismic data covering a longer time
span have the potential to improve this constraint. |
|
Marzo de 2010
A dynamo model for axisymmetrizing Saturn's magnetic field
Author: S. Stanley
Link: Click here
Abstract
Magnetic field measurements demonstrate that Saturn's internally
generated magnetic field has an extremely small dipole tilt.
The nearly-perfect axisymmetry of Saturn's dipole is troubling
because of Cowling's theorem which states that an axisymmetric
magnetic field cannot be maintained by a dynamo. A possible
mechanism
|
to
axisymmetrize the observed field involves differential rotation
in a stably-stratified electrically conducting layer surrounding
the dynamo. Here we use numerical dynamo models to study the
axisymmetrizing effects of stably stratified layers surrounding
the dynamo. We find that a thin stably-stratified layer which
undergoes differential rotation due to thermal winds as a result
of pole to equator temperature differences can produce a more
axisymmetrized field. Surprisingly, we find that the direction
of the zonal flows and their equatorial symmetry is a crucial
factor for magnetic field axisymmetry since some zonal flows
act to destabilize the dynamo producing non-axisymmetric fields.
|
|
Marzo de 2010
Inverting geodetic time series with a principal component
analysis-based inversion method
Authors: A. P. Kositsky and J.-P. Avouac
Link: Click here
Abstract
The Global Positioning System (GPS) system now makes it possible
to monitor deformation of the Earth's surface along plate
boundaries with unprecedented accuracy. In theory, the spatiotemporal
evolution of slip on the plate boundary at depth, associated
with either seismic or aseismic slip, can be inferred from
these measurements through some inversion procedure based
on the theory of dislocations in an elastic half-space. We
describe and test a principal component analysis-based inversion
method (PCAIM), an inversion strategy that relies on principal
component analysis of the surface displacement time series.
We prove that the fault
|
slip
history can be recovered from the inversion of each principal
component. Because PCAIM does not require externally imposed
temporal filtering, it can deal with any kind of time variation
of fault slip. We test the approach by applying the technique
to synthetic geodetic time series to show that a complicated
slip history combining coseismic, postseismic, and nonstationary
interseismic slip can be retrieved from this approach. PCAIM
produces slip models comparable to those obtained from standard
inversion techniques with less computational complexity. We
also compare an afterslip model derived from the PCAIM inversion
of postseismic displacements following the 2005 8.6 Nias earthquake
with another solution obtained from the extended network inversion
filter (ENIF). We introduce several extensions of the algorithm
to allow statistically rigorous integration of multiple data
sources (e.g., both GPS and interferometric synthetic aperture
radar time series) over multiple timescales. PCAIM can be generalized
to any linear inversion algorithm. |
|
Febrero de 2010
Acoustic properties of gas hydrate-bearing consolidated
sediments and experimental testing of elastic velocity models
Authors: Gao W. Hu, Yu G. Ye et al
Link: Click here
Abstract
Although elastic velocities (Vp, Vs) can be used to assess
the distribution and concentration of marine gas hydrates
in situ and several existing models relate hydrate saturation
to acoustic velocity, the accuracy of these models is uncertain
because of the difficulty in determining hydrate saturations
and velocities of intact hydrate-bearing sediments. In this
paper, the acoustic properties of gas hydrate-bearing consolidated
sediments were investigated experimentally. Hydrate saturation
(Sh) and acoustic velocities were measured in one system by
time domain
|
reflectometry
and ultrasonic methods, respectively, during gas hydrate formation
and subsequent dissociation in a water-saturated artificial
core. Acoustic velocities change little at low hydrate saturations
(0% to ~10%), whereas they increase rapidly when hydrate saturation
is between 10% and 30%. We verified two commonly used models,
i.e., the weighted equation (WE) and the Biot-Gassmann theory
modified by Lee (BGTL). In the 0% to 40% hydrate saturation
range, the WE model is consistent with the measured Vp data,
while a combination of the WE and the Vp/Vs ratio in the BGTL
predicts Vs corresponding to the observed data. As hydrate saturation
is more than 30%, however, the BGTL is more suitable for predicting
both Vp and Vs. This suggests that gas hydrate may be treated
as a component within a matrix of consolidated sediments when
hydrate saturation exceeds 30%. However, when Sh is less than
30%, the hydrate locates in the pore fluid or partly adheres
to the sediment frame. |
|
Febrero de 2010
Schumann resonances excitation due to positive and negative
cloud-to-ground lightning
Authors: V. V. Surkov and M. Hayakawa
Link: Click here
Abstract
In this paper, we have applied a statistical approach to the
problem of Schumann resonances excitation due to both negative
and positive cloud-to-ground lightning (-CG and +CG,
|
respectively).
Correlation matrices and a power spectrum of natural electromagnetic
variations resulted from the global lightning activity are estimated.
On the basis of these estimates we have shown that the global
+CG lightning activity can make a significant contribution to
the low-frequency portion of power spectrum and to the Schumann
resonances. Despite their infrequent occurrence compared to
the negative flashes, the global +CG lightning can be of primary
importance in generating the power spectrum since their charge
moment and continuing current are, on average, larger than those
of the -CG lightning. |
|
Febrero de 2010
Kinematics and source zone properties of the 2004 Sumatra-Andaman
earthquake and tsunami: Nonlinear joint inversion of tide
gauge, satellite altimetry, and GPS data
Authors: S. Lorito, A. Piatanesi et al
Link: Click here
Abstract
We (re)analyzed the source of the 26 December 2004 Sumatra-Andaman
earthquake and tsunami through a nonlinear joint inversion
of an inhomogeneous data set made up of tide gauges, satellite
altimetry, and far-field GPS recordings. The purpose is twofold:
(1) the retrieval of the main kinematics rupture parameters
(slip, rake, and rupture velocity) and (2) the inference of
the rigidity of the source zone. We independently estimate
the slip from tsunami data and the seismic moment from geodetic
data to derive the rigidity. Our results confirm that the
source of the
|
2004
Sumatra-Andaman earthquake has a complex geometry, constituted
by three main slip patches, with slip peaking at ~30 m in the
southern part of the source. The rake direction rotates counterclockwise
at the northern part of the source, according to the direction
of convergence along the trench. The rupture velocity is higher
in the deeper than in the shallower part of the source, consistent
with the expected increase of rigidity with depth. It is also
lower in the northern part, consistent with known variations
of the incoming plate properties and shear velocity. Our model
features a rigidity (20-30 GPa) that is lower than the preliminary
reference Earth model (PREM) average for the seismogenic volume.
The source rigidity is one of the factors controlling the tsunami
genesis: for a given seismic moment, the lower the rigidity,
the higher the induced seafloor displacement. The general consistence
between our source model and previous studies supports the effectiveness
of our approach to the joint inversion of geodetic and tsunami
data for the rigidity estimation. |
|
Febrero de 2010
Modelo para la fuente del terremoto de Pisco, Perú,
de magnitud Mw 8.0: Implicaciones para las características
sismogénicas de los grandes terremotos de subducción
Autores: A. Sladen, H. Tavera et al
Link: Click aquí
Abstract
Nosotros utilizamos un Radar de Apertura Sintética
Interferométrica, ondas de cuerpos telesísmicos,
formas de ondas de tsunamis registradas por tsunámetros,
observaciones de campo de ascenso de las costas, subsidencia
y preludios con el objetivo de desarrollar y testear un modelo
refinado de la historia espaciotem-poral del deslizamiento
ocurrido durante el terremoto Mw 8.0 ocurrido en Pisco el
día 15 de agosto de 2007. La solución preferida
muestra dos distintos sectores de gran deslizamiento. Uno
de estos sectores se ubica cerca del epicentro mientras que
el mayor experimentó una ruptura 60 Km más al
sur, en la latitud de la Península de Paracas. El deslizamiento
en el segundo sector comenzó 60 s después del
deslizamiento iniciado en el primer sector. Nosotros observamos
una importante anticorrelación entre la distribución
de deslizamiento cosísmica y la distribución
posterior al shock determinada a partir de la red sísmica
peruana. El modelo de fuente propuesto es compatible con las
medidas regionales del runup y abre los registros de tsunami.
A partir de los set de datos posteriores nosotros identificamos
el error de tiempo de 12 minutos del sistema de pronóstico
de tsunamis como proveniente de la deslocalización
de la fuente causada por el uso de sólo un tsunámetro
ubicado en un acimut no óptimo. La comparación
de nuestro modelo de fuente con las observaciones del tsunami
validan que la ruptura no se extendió a la fosa y confirma
que el evento de Pisco no es un terremoto- tsunami a pesar
de la aparentemente baja velocidad de ruptura (<1.5 km/s).
Nosotros apoyamos la interpretación de que el terremoto
consistió de dos sub-eventos, cada uno con una velocidad
de ruptura convencional (2-4 Km/s). El retraso entre los dos
subeventos podría reflejar el tiempo de nucleación
del segundo shock o, alternativamen-te, el tiempo requerido
para que el deslizamiento posterior incrementara el nivel
de tensión en la segunda aspereza al nivel necesario
de superar el roce estático. El modelo de fuente predice
deslizamiento de ascenso a poca distancia de la costa y subsidencia
en el terreno con la línea de pivoteo siguiendo muy
de cerca la línea de la costa. Este patrón es
consistente con nuestra observación de desplazamiento
vertical muy pequeño a lo largo de la orilla y que
comprobamos cuando visitamos el área epicentral en
los días posteriores al evento. A nuestro entender,
este terremoto representa uno de los mejores ejemplos que
vinculan la geomorfología de la línea de costa
y el patrón de deformación superficial inducido
por grandes rupturas interplacas.
|
Febrero de 2010
Source model of the 2007 Mw 8.0 Pisco, Peru earthquake:
Implications for seismogenic behavior of subduction megathrusts
Authors: A. Sladen, H. Tavera et al
Link: Click here
Abstract
We use Interferometric Synthetic Aperture Radar, teleseismic
body waves, tsunami waveforms recorded by tsunameters, field
observations of coastal uplift, subsidence, and runup to develop
and test a refined model of the spatiotemporal history of
slip during the Mw 8.0 Pisco earthquake of 15 August 2007.
Our preferred solution shows two distinct patches of high
slip. One patch is located near the epicenter while another
larger patch ruptured 60 km further south, at the latitude
of the Paracas peninsula. Slip on the second patch started
60 s after slip initiated on the first patch. We observed
a remarkable anticorrelation between the coseismic slip distribution
and the aftershock distribution determined from the Peruvian
seismic network. The proposed source model is compatible with
regional runup measurements and open ocean tsunami records.
From the latter data set, we identified the 12 min timing
error of the tsunami forecast system as being due to a mislocation
of the source, caused by the use of only one tsunameter located
in a nonoptimal azimuth. The comparison of our source model
with the tsunami observations validate that the rupture did
not extend to the trench and confirms that the Pisco event
is not a tsunami earthquake despite its low apparent rupture
velocity (<1.5 km/s). We favor the interpretation that
the earthquake consists of two subevents, each with a conventional
rupture velocity (2-4 km/s). The delay between the two subevents
might reflect the time for the second shock to nucleate or,
alternatively, the time it took for afterslip to increase
the stress level on the second asperity to a level necessary
for static triggering. The source model predicts uplift offshore
and subsidence on land with the pivot line following closely
the coastline. This pattern is consistent with our observation
of very small vertical displacement along the shoreline when
we visited the epicentral area in the days following the event.
This earthquake represents, to our knowledge, one of the best
examples of a link between the geomorphology of the coastline
and the pattern of surface deformation induced by large interplate
ruptures.
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Febrero de 2010
Satellite-based measurements of surface deformation reveal
fluid flow associated with the geological storage of carbon
dioxide
Authors: D. W. Vasco, A. Rucci et al
Link: Click here
Abstract
Interferometric Synthetic Aperture Radar (InSAR) data, gathered
over the In Salah CO2 storage project in Algeria, provide
an early indication that satellite-based geodetic methods
can be effective in monitoring the geological storage of carbon
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dioxide.
An injected mass of 3 million tons of carbon dioxide from one
of the first large-scale carbon sequestration efforts, produces
a measurable surface displacement of approximately 5 mm/year.
Using geophysical inverse techniques, we are able to infer flow
within the reservoir layer and within a seismically detected
fracture/fault zone intersecting the reservoir. We find that,
if we use the best available elastic Earth model, the fluid
flow need only occur in the vicinity of the reservoir layer.
However, flow associated with the injection of the carbon dioxide
does appear to extend several kilometers laterally within the
reservoir, following the fracture/fault zone. |
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Febrero de 2010
Fabric induced weakness of tectonic faults
Authors: André Niemeijer, Chris
Marone et al
Link: Click here
Abstract
Mature fault zones appear to be weaker than predicted by both
theory and experiment. One explanation involves the presence
of weak minerals, such as talc. However, talc is only a minor
constituent of most fault zones and thus the question arises:
what proportion of a weak
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mineral
is needed to satisfy weak fault models? Existing studies of
fault gouges indicate that >30% of the weak phase is necessary
to weaken faults - a proportion not supported by observations.
Here we demonstrate that weakening of fault gouges can be accomplished
by as little as 4 wt% talc, provided the talc forms a critically-aligned,
through-going layer. Observations of foliated fault rocks in
mature, large-offset faults suggest they are produced as a consequence
of ongoing fault displacement and thus our observations may
provide a common explanation for weakness of mature faults.
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Febrero de 2010
Reducing the effects of noise on atmospheric imaging radars
using multilag correlation
Authors: K. D. Le, R. D. Palmer et al
Link: Click here
Abstract
Atmospheric imaging radars offer the capability to scrutinize
structures within the illuminated volume at high temporal
and spatial resolutions. The retrieval of the mean signal
power using an imaging radar is obtained by subtracting the
noise power from the covariance function at lag zero. The
results obtained at low signal-to-noise ratio (SNR)
are problematic when the noise power is unsuccessfully estimated,
and are difficult to interpret when adaptive weights are used
because of the temporally varying noise power. In
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this
paper, a processing technique that improves the retrieval of
the mean signal power by exploiting the temporal correlation
difference between the desired signal and system noise is presented.
Simulations of its performance are presented for the special
case of a Gaussian received spectrum for variations in SNR,
normalized spectrum width, and number of time series samples.
The technique is also applied to real data collected with the
Turbulent Eddy Profiler, a vertically pointing phased array
radar developed at the University of Massachusetts, between
1435 and 1457 UTC 15 June 2003. Even though the performance
of this technique in terms of its variance and bias depends
on the SNR, spectrum width, and number of time series samples,
results from both simulations and real data are promising as
an enhanced mean signal power in the low SNR regions is obtained.
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Febrero de 2010
Clay clast aggregates in gouges: New textural evidence
for seismic faulting
Authors: Sébastien Boutareaud, Anne-Marie
Boullier et al
Link: Click here
Abstract
Spherical aggregates named clay-clast aggregates (CCAs) have
been reported from recent investigations on retrieved clay-bearing
fault gouges from shallow depth seismogenic faults and rotary
shear experiments conducted on clay-bearing gouge at seismic
slip rates. The formation of CCAs appears to be related to
the shearing of a smectite-rich granular material that expands
and becomes fluidized. We have conducted additional high-velocity
rotary shear experiments and low-velocity double-shear
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experiments.
We demonstrate that a critical temperature depending on dynamic
pressure- temperature conditions is needed for the formation
of CCAs. This temperature corresponds to the phase transition
of pore water from liquid to vapor or to critical, which induced
gouge pore fluid expansion and therefore a thermal pressurization
of the fault. A detailed examination by energy dispersive X-ray
spectrometry (EDX-SEM) element mapping, SEM, and transmission
electron microscopy (TEM) shows strong similar characteristics
of experimental and natural CCAs with a concentric well-organized
fabric of the cortex and reveals that their development may
result from the combination of electrostatic and capillary forces
in a critical reactive medium during the dynamic slip weakening.
Accordingly, the occurrence of CCAs in natural clay-rich fault
gouges constitutes new unequivocal textural evidence for shallow
depth thermal pressurization and consequently for past seismic
faulting. |
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Febrero de 2010
Seismic cycle stress change in western Taiwan over the
last 270 years
Authors: M. Mouyen, R. Cattin et al
Link: Click here
Abstract
The island of Taiwan is affected by intense seismic activity,
which includes large events as the disastrous 1999 Chi-Chi
earthquake. To improve seismic hazard assessment in this area,
we estimate the effect of both interseismic loading and major
events since 1736 on the state of
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stress
of major active faults. We focus our approach on western Taiwan,
which is the most densely populated part of Taiwan. We pay a
specific attention to faults geometry and to both interseismic
and coseismic slip distributions. Our results suggest that both
earthquakes and interseismic loading before 1999 increase the
Coulomb stress in the north-western part of the Chelungpu fault,
a region which experienced the highest coseismic slip during
the Chi-Chi earthquake. More importantly our results reveal
a Coulomb stress increase in the southern part of the Changhua
thrust fault, below a densely populated area. |
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Febrero de 2010
Impact of fracture development on the effective permeability
of porous rocks as determined by 2-D discrete fracture growth
modeling
Authors :A. Paluszny and S. K. Matthai
Link: Click here
Abstract
Fracture networks exert a strong influence on fluid flow in
the subsurface. We present a 2-D linear elastic finite element
model that generates fracture patterns in incremental iterative
steps. A subcritical failure criterion is applied to simulate
quasi-static multiple crack propagation. We study their impact
on fluid flow as a function of fracture density. Fractures
are represented by closed polygons. Geomechanical apertures
are a by-product of growth and depend on the current stress
state. Fracture arrest, closure, and coalescence are handled
by a geometric kernel. Traction and cohesion along fracture
walls are not taken into account. All patterns are generated
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assuming
plane strain and applying displacement tensile boundary conditions.
We assume randomly distributed flaw positions with a uniform
probability distribution and Gaussian-distributed flaw lengths.
A piecewise fracture permeability is derived from the parallel
plate law. We measure the effective permeability, keff, and
fracture-matrix flux ratio, qf/qm across the percolation threshold.
Before the percolation threshold we observe an increase in keff
of up to two orders of magnitude. Models with fixed apertures
overpredict keff by up to six orders of magnitude, as they disregard
variations in the aperture distribution due to fracture interaction.
After percolation our model predicts steady linear increase
in effective permeability. The qf/qm ratio better captures the
initial increase in hydraulic conductivity of the system as
opposed to the keff measurements. Results corroborate that fracture
percolation and stress-dependent aperture distribution due to
mechanical interactions control the evolution of the keff of
the system. Results depend on the number of initial flaws used
for fracture set growth. |
