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Julio de 2006
Explosion energy of the 2004 eruption of the Asama Volcano,
central Japan, inferred from ionospheric disturbances
Authors: Kosuke Heki
Link: Click here
Abstract
The Japanese dense array of Global Positioning System recorded
ionospheric disturbances as
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changes in Total Electron
Content ~ 12 minutes after the September 1 2004 eruption of
the Asama Volcano, Central Japan. The disturbance had a period
of one and a quarter minutes and propagated as fast as ~ 1.1
km/s, suggesting its origin as the acoustic wave generated
by the explosion. By comparing the disturbance amplitudes
with those by a surface mine blast with a known energy, the
overall Asama explosion energy is inferred to be about 2 ×
1014 J.
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Julio
de 2006
Structure of the San Andreas Fault at SAFOD from P-wave tomography
and fault-guided wave mapping
Authors: Peter Malin , Eylon Shalev et al
Link: Click here
Abstract
Fault-guided waves reveal a low-velocity fault segment a few
hundred meters southwest of the main strand of the San Andreas
Fault (SAF) system. In 2004, the San Andreas Fault Observatory
at Depth (SAFOD) Main Hole was
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drilled
2.5 km underground and 0.7 km west of the SAF surface trace.
A 3-component, 4.5-Hz seismograph was installed near the bottom
of this hole. This instrument recorded fault zone guided (Fg)
waves originating from earthquakes along the main SAF ~2 km
north and 3 km south of the SAFOD site. This ~5 km length corresponds
to a distinctive low-velocity structure imaged in 2003 using
microearthquakes recorded on the Pilot Hole array. Because this
structure transmits Fg-waves from the main fault, it is probably
connected to the main SAF and is most likely a major, unmapped
fault. |
Julio
de 2006
Quantitative network model predictions of saturation behind
infiltration fronts and comparison with experiments
Author: David A. DiCarlo
Link: Click here
Abstract
Infiltrations with certain boundary and initial conditions
show an anomalous behavior where the water saturation and
pressure profile are inverted in the vertical direction. This
occurs when the water at the infiltration front oversaturates
the porous medium, causing drainage behind the front, and
is most likely due to the front being sharp at the pore scale.
Here we attempt to quantitatively model water flow at infiltration
fronts by using a physically based network model that
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includes
viscous effects. The network model includes pore and throat
elements of different shapes and sizes, and a connection topology
based on geologic media. Viscous effects are added quasi-statically
through calculating the water pressure in each element for a
particular applied flux. For media with a small initial saturation,
the infiltration flux range for which saturation overshoot occurs
is predicted by the network model. For initially dry media,
the model predictions scale correctly with media size but require
an independent parameter to predict the absolute transition
flux. The predicted magnitude of the overshoot does not match
particularly well with the measured magnitude for coarse media,
although this may be the result of certain simplified assumptions
in the model. The results suggest that pore-scale modeling can
make certain quantitative predictions of saturation overshoot.
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Julio
de 2006
Bayesian neural network for rainfall-runoff modeling
Authors: Mohammad Sajjad Khan and Paulin Coulibaly
Link: Click here
Abstract
In this paper, a Bayesian learning approach is introduced
to train a multilayer feed-forward network for daily river
flow and reservoir inflow simulation in a cold region river
basin in Canada. In Bayesian approach, uncertainty about the
relationship between inputs and outputs is initially taken
care of by an assumed prior distribution of parameters (weights
and biases). This prior distribution is updated to posterior
distribution using a likelihood function following Bayes'
theorem while data are observed. This posterior distribution
is called the objective function of a network in the Bayesian
learning approach. The objective function is maximized using
a suitable optimization technique. Once the network is trained,
the predictive distribution of the network outputs is obtained
by integrating over the posterior distribution of weights.
In this study, Gaussian prior distribution and a Gaussian
noise model are used in defining posterior distribution.
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The
network has been optimized using a scaled conjugate gradient
technique. Posterior distribution of weights is approximated
to Gaussian during prediction. Prediction performance of the
Bayesian neural network (BNN) is compared with the results obtained
from a standard artificial neural network (ANN) model and a
widely used conceptual rainfall-runoff model, namely, HBV-96.
The BNN model outperformed the conceptual model and slightly
outperformed the standard ANN model in simulating mean, peak,
and low river flows and reservoir inflows. The significant contribution
of the Bayesian method over the conventional ANN approach, among
others, is the uncertainty estimation of the outputs in the
form of confidence intervals which are particularly needed in
practical water resources applications. Prediction confidence
limits (or intervals) indicate the extent to which one can rely
on predictions for decision making. It is shown that the BNN
can provide reliable streamflow and reservoir inflow forecasts
without a loss in model prediction accuracy as compared to standard
ANN and conceptual model HBV. Another significant advantage
of BNN approach is that the overfitting and underfitting problems
are automatically taken care of by the Bayesian learning algorithm,
which conversely remain serious problems with conventional ANN
learning algorithm. |
Julio
de 2006
Climate change scenarios for seasonal precipitation in South
America from IPCC-AR4 models
Authors: Carolina Vera , Gabriel Silvestri et al
Link: Click here
Abstract
A subset of climate simulations of the 20th century from the
IPCC-AR4 is analyzed to assess the ability of these models
to reproduce the observed climatological seasonal precipitation
in South America during the period 1970-1999. Changes of the
model climatology in a climate change
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scenario
(SRESA1b) for the period 2070-2099 are also discussed. Results
show that models are able to reproduce the main features of
the precipitation seasonal cycle over South America, although
the precipitation in the SACZ region and the precipitation maximum
over southeastern South America observed during the cold season
are not well- represented. There is a general consensus among
models that the precipitation changes projected are mainly:
i) an increase of summer precipitation over southeastern subtropical
South America; ii) a reduction of winter precipitation over
most of the continent; and iii) reduction of precipitation in
all seasons along the southern Andes. |
Julio
de 2006
Simulation study of the one-dimensional Burridge-Knopoff
model of earthquakes
Authors: Takahiro Mori and Hikaru Kawamura
Link: Click here
Abstract
Spatiotemporal correlations of the one-dimensional spring-block
(Burridge-Knopoff) model of earthquakes are extensively studied
by means of numerical computer simulations. Particular attention
is paid to clarifying how the statistical properties of earthquakes
depend on the frictional and elastic properties of earthquake
faults. It is found that as the velocity-weakening tendency
of the friction force gets weaker, the
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system
tends to be more critical, while as the velocity-weakening tendency
gets stronger, the system tends to be more off critical with
enhanced features of a characteristic earthquake. The model
exhibits several eminent precursory phenomena prior to the large
event in its spatiotemporal correlations. Preceding the main
shock, the frequency of smaller events is gradually enhanced,
whereas just before the main shock, it is suppressed in a close
vicinity of the epicenter of the upcoming event (the Mogi doughnut).
The timescale of the onset of the doughnut-like quiescence depends
on the extent of the frictional instability. Under certain conditions,
preceding the main shock, the apparent B value of the magnitude
distribution increases significantly. The existence of such
distinct precursory phenomena may open a way to the prediction
of the time and the position of the upcoming large event. |
Julio
de 2006
Upper mantle velocity-temperature conversion and composition
determined from seismic refraction and heat flow
Authors: H. K. C. Perry , C. Jaupart et al
Link: Click here
Abstract
We compile upper mantle P n velocities from seismic refraction/wide-angle
reflection surveys in the southern Superior Province of the
Canadian Shield and compare them with temperatures at the
Moho deduced from heat flow data. Calculated Moho temperatures
and P n velocities correlate well, showing that in this area,
P n depends primarily on temperature. The obtained values
of delta V(P n )/delta T depend weakly on the assumed
value of Moho heat flow and are on the order of -6.0 ×
10-4 ± 10% km s-1 K-1, within the range of temperature
derivatives obtained in laboratory studies of ultramafic rocks.
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Comparison
between observed P n velocities and predicted values for several
mineralogical models at Moho temperatures allows constraints
on both the Moho heat flow and the shallow mantle composition.
For all Moho heat flows, undepleted (clinopyroxene-rich) mantle
compositions do not allow a good fit to the data. For depleted
mantle compositions, temperatures consistent with the observed
P n velocities correspond to values of Moho heat flow larger
than 12 mW m-2. For our preferred Moho heat flow of 15 mW m-2,
the best fit mantle composition is slightly less depleted than
models for average Archean subcontinental lithospheric mantle.
This may be due to rejuvenation by melt-related metasomatism
during the Keweenawan rifting event. The similarity in P n -
T conversion factors estimated from this empirical large-scale
geophysical study and those from laboratory data provides confidence
in the absolute temperature values deduced from heat flow measurements
and seismic studies. |
Julio
de 2006
On the chaotic nature of solar-terrestrial environment: Interplanetary
Alfvén intermittency
Authors: A. C.-L. Chian , Y. Kamide et al
Link: Click here
Abstract
We present an overview of observational and theoretical evidence
of chaos and intermittency in the solar-terrestrial environment
including solar dynamo, solar atmosphere, solar wind, and
terrestrial magnetosphere-ionosphere-atmosphere. The chaotic
nature of space
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plasmas
is studied by a nonlinear model of Alfvén waves described
by the low-dimensional limit of the derivative nonlinear Schrödinger
equation given by its stationary solutions in the frame moving
with the driver wave velocity. A periodic window of the bifurcation
diagram is constructed to identify two types of Alfvén
chaos related to type-I intermittency and crisis-induced intermittency.
We show that an Alfvén chaotic attractor is composed
of chaotic saddles and unstable periodic orbits and explain
the links between these unstable structures and Alfvén
intermittency. The role of interplanetary Alfvén intermittency
in the solar wind driving of intense geomagnetic activities
is discussed. |
Julio
de 2006
Direct gas injection into saturated glass beads: Transition
from incoherent to coherent gas flow pattern
Authors: Helmut Geistlinger , Gunnar Krauss et al
Link: Click here
Abstract
The transition from incoherent to coherent buoyancy-driven
gas flow is investigated in two-dimensional tanks filled with
glass beads using a high-resolution optical-gravimetrical
setup.
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Both a grain-size (d k )- and flow rate (Q)-dependent transition
are observed in the gas flow pattern. Standard quasistatic criteria
do not explain the experimental results, since they do not take
into account the competition between stabilizing friction forces
and destabilizing capillary and gravitational forces. Conceptualizing
the steady state tortuous gas flow as core-annulus flow and
applying Hagen-Poiseuille flow for a straight capillary, we
propose a flow rate and grain-size-dependent stability criterion
that accounts for the experimental results and is used to classify
the experiments in a d k -Q diagram. |
Julio
de 2006
Theory of borehole stability when drilling through salt formations
Authors: José M. Carcione et al
Link: Click here
Abstract
We present a mathematical analysis of borehole stability when
drilling through rock salt. First, we consider an elastic
transversely isotropic medium and find the optimal mud weight
as a function of the vertical overburden and horizontal tectonic
stresses. Then, the Zener and Maxwell mechanical models are
used to model the effects of transient and steady-state creep
flow, respectively, in isotropic media. Under certain conditions
such as the absence of dilatational
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anelasticity,
the Burger model can be used to describe the steady-state flow,
including transient creep effects. The type of creep is regulated
by critical octahedral-stress values that depend on temperature
and pressure. A typical drilling results in conditions of plane
strain, whose solution is given by Kirsch's equations. In this
case, the borehole is subject to minimum and maximum horizontal
stresses, which differ from the vertical stress. The analysis
provides expressions for the shape of the borehole-cross section,
the borehole-wall closure time, and the optimal mud weight to
avoid wall collapse or expansion. It is shown that an anisotropic
state of tectonic stress may require mud pressures exceeding
the overburden stress and that the calculation should consider
the joint optimization of the shape and area of the borehole
cross section. |
Julio
de 2006
Ultrasonic borehole velocity imaging
Authors: Kenneth W. Winkler and Ralph D'Angelo
Link: Click here
Abstract
We describe a new technique for making high-resolution velocity
images of rocks using refracted ultrasonic waves. The use
of refracted waves makes this technique potentially suitable
for imaging borehole walls. In the laboratory, we use a single-transmitter,
two-receiver, first-arrival method for making velocity measurements,
with a
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spatial
resolution on the order of 1 cm. Our acoustic pulses are centered
near 200 kHz. Scans of a borehole wall reveal dipping thin layers
and fractures. When external stress is applied perpendicular
to the borehole, stress concentrations appear on our images
as axial bands of high and low velocities. Breakouts created
by high stress also can be imaged. On a planar sample, a velocity
image reveals shale laminations and carbonate stringers. For
field applications, this technique offers the potential for
imaging in both conductive and nonconductive muds and provides
images based on a physical property (velocity) that currently
is not used for fine-scale borehole imaging. |
Julio
de 2006
Prediction of vein connectivity using the percolation approach:
model test with field data
Authors: M Belayneh, M Masihi, S K Matthäi and P R King
Link: Click here
Abstract
Evaluating the uncertainty in fracture connectivity and its
effect on the flow behaviour of natural fracture networks
formed under in situ conditions is an extremely difficult
task. One widely used probabilistic approach is to use percolation
theory, which is well adapted to estimate the connectivity
and conductivity of geometrical objects near the percolation
threshold. In this paper, we apply scaling laws from percolation
theory to predict the connectivity of vein sets
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exposed
on the southern margin of the Bristol Channel Basin. Two vein
sets in a limestone bed interbedded with shales on the limb
of a rollover fold were analysed for length, spacing and aperture
distributions. Eight scan lines, low-level aerial photographs
and mosaics of photographs taken with a tripod were used. The
analysed veins formed contemporaneously with the rollover fold
during basin subsidence on the hanging wall of a listric normal
fault. The first vein set, V1, is fold axis-parallel (i.e. striking
~100°) and normal to bedding. The second vein set, V2, strikes
140° and crosscuts V1. We find a close agreement in connectivity
between our predictions using the percolation approach and the
field data. The implication is that reasonable predictions of
vein connectivity can be made from sparse data obtained from
boreholes or (limited) sporadic outcrop. |
Julio
de 2006
A method for determining gas-hydrate or free-gas saturation
of porous media from seismic measurements
Author: Matthias Zillmer
Link: Click here
Abstract
The occurrence of gas hydrate or free gas in a porous medium
changes the medium's elastic properties. Explicit formulas
for gas-hydrate or free-gas saturation of pore space on the
basis of the Frenkel-Gassmann equations describe the elastic
moduli and seismic velocities of a porous medium for low frequencies.
A key assumption of the model is that either gas hydrate or
free gas is present in the pore space in addition to water.
Under this assumption, the method uses measured P- and S-wave
velocities and bulk density along with estimates of the moduli
and
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densities
of the solid and fluid phases present to determine whether gas
or hydrate is present. The method then determines the saturation
level of either the gas or the hydrate. I apply the method to
published velocity and density data from seismic studies at
the antarctic Shetland margin and at the Storegga slide, offshore
Norway, and to borehole log and core data from Ocean Drilling
Program (ODP) Leg 164 at Blake Ridge, offshore South Carolina.
A sensitivity analysis reveals that the standard deviations
of the gas-hydrate and free-gas saturations reach 30%-70% of
the saturations if the standard deviations of the P- and S-wave
velocities and of the bulk density are 50 m/s and 0.05 g/cm3,
respectively. I conclude that a reliable quantification of gas
hydrate and free gas can be achieved by seismic methods only
if the seismic velocities and bulk density of the medium are
determined with high accuracy from the measured data. |
Julio
de 2006
Ground-penetrating radar theory and application of thin-bed
offset-dependent reflectivity
Authors: John H. Bradford and Jacob C. Deeds
Link: Click here
Abstract
Offset-dependent reflectivity or amplitude-variation-with-offset
(AVO) analysis of ground-penetrating radar (GPR) data may
improve the resolution of subsurface dielectric permittivity
estimates. A horizontally stratified medium has a limiting
layer thickness below which thin-bed AVO analysis is necessary.
For a typical GPR signal,
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this
limit is approximately 0.75 of the characteristic wavelength
of the signal. Our approach to modeling the GPR thin-bed response
is a broadband, frequency-dependent computation that utilizes
an analytical solution to the three-interface reflectivity and
is easy to implement for either transverse electric (TE) or
transverse magnetic (TM) polarizations. The AVO curves for TE
and TM modes differ significantly. In some cases, constraining
the interpretation using both TE and TM data is critical. In
two field examples taken from contaminated-site characterization
data, we find quantitative thin-bed modeling agrees with the
GPR field data and available characterization data. |
Julio
de 2006
Induced-polarization detection and mapping of contaminant
plumes
Authors: John A. Sogade, Francesca Scira-Scappuzzo et al
Link: Click here
Abstract
Several laboratory and scaled model investigations suggest
that organic contaminants affect the surface electrical properties
of exposed soils/rocks and therefore produce measurable induced
polarization (IP) signatures. However, there is little field
evidence of an IP methodology for contaminant mapping. A 2D
time-domain IP method is developed for mapping the FS-12 contaminant
plume at the Massachusetts Military Reservation (MMR) located
in Cape Cod, Massachusetts. The FS-12 plume consists of approximately
265 m3 of fuel that erupted from a broken underground pipeline
in the early 1970s. Benzene and ethylene dibromide (EDB) are
the primary contaminants at FS-12, with concentrations exceeding
the allowed maximum
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concentration
levels (MCL), while other constituents of the plume did not
exceed their MCL. Therefore, the contaminants of interest are
benzene and EDB, partly because of their health risk and partly
because they present the highest concentrations (2400 and 1000
µg/L, respectively) among the plume constituents and are
therefore more likely to be related to the polarization source.
IP data were acquired along a survey line that partially transects
the plume extending over contaminated and uncontaminated zones
and were inverted to give 2D resistivity and chargeability plots
to 100 m depth and a horizontal extent of 400 m. By separately
inverting IP data derived from time windows located at short
and long decay times, a time-domain gross (spectral) chargeability
difference is produced. Both the chargeability and gross spectral
chargeability difference show good agreement with the known
location of the plume from monitoring wells, with the IP chargeability
section suggesting contaminant distribution detail that cannot
otherwise be inferred from the sparse borehole distribution. |
Julio
de 2006
Parallel 3-D simulation of seismic wave propagation in heterogeneous
anisotropic media: a grid method approach
Authors:Hongwei Gao and Jianfeng Zhang
Link: Click here
Abstract
This paper presents a parallel numerical technique for modelling
wave propagation in 3-D heterogeneous anisotropic media. The
scheme is developed by following a so-called 3-D grid method
of the elastic-isotropic case. The proposed parallel algorithm
needs small data exchanges between subdomains in contrast
to that developed based on other numerical techniques; therefore,
it is more suitable for a PC-Cluster. The algorithm is implemented
on a mesh of mixed tetrahedrons and parallelepipedons, thus
providing an accurate description of arbitrary 3-D surface
and interface topographies and an easy generation of a non-uniform,
unstructured mesh. The unstructured mesh means that the proposed
algorithm can reduce the memory
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requirement by flexibly assigning small grid spacing in regions
with low velocities and larger grid spacing in regions with
higher velocities. Like the 3-D grid method, the resulting anisotropic
scheme naturally satisfies the free-surface boundary conditions
of arbitrary surface topography. As a result, the near-surface
scattering effects can be more accurately modelled. The proposed
scheme can handle a general anisotropy without any interpolations.
In this paper, the transversely isotropic medium with a tilted
symmetry axis, as typically caused by a system of parallel cracks
or fine layers, is discussed in detail. A paraxial absorbing
boundary condition in a 3-D general anisotropic case is also
proposed. Comparisons with analytical solutions demonstrate
the accuracy of the parallel algorithm. Computed 3-D radiation
patterns illustrate shear-wave splitting, as predicted by the
theory. We show the generality and flexibility of the algorithm
by modelling wave propagation in an anisotropic half-space with
a hemispherical crater on the surface and in mixed isotropic/anisotropic
models with horizontal and inclined interfaces. |
Julio
de 2006
The effect of energy feedbacks on continental strength
Authors: Klaus Regenauer-Lieb et al
Link: Click
here
Abstract
The classical strength profile of continents1, 2 is derived
from a quasi-static view of their rheological response to
stress-one that does not consider dynamic interactions between
brittle and ductile layers. Such interactions result in complexities
of failure in the brittle-ductile transition and the need
to couple energy to understand strain localization. Here we
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investigate
continental deformation by solving the fully coupled energy,
momentum and continuum equations. We show that this approach
produces unexpected feedback processes, leading to a significantly
weaker dynamic strength evolution. In our model, stress localization
focused on the brittle-ductile transition leads to the spontaneous
development of mid-crustal detachment faults immediately above
the strongest crustal layer. We also find that an additional
decoupling layer forms between the lower crust and mantle. Our
results explain the development of decoupling layers that are
observed to accommodate hundreds of kilometres of horizontal
motions during continental deformation. |
|
Julio de 2006
S-transform view of geomagnetically induced currents during
geomagnetic superstorms
Authors: Antti Pulkkinen and Ryuho Kataoka
Link: www.agu.org/pubs/crossref/2006/
2006GL025822.shtml
Abstract
A novel time-frequency analysis method (S-transform) capable
of handling noisy non-stationary signals is applied to study
the properties of geomagnetically induced current (GIC) fluctuations
in the Finnish natural gas pipeline. New local time- and storm
phase-dependent S-transform spectral properties of
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auroral
region GIC fluctuations during geomagnetic superstorms are reported.
More specifically, the S-transform spectra have two distinct
regions containing the most of the spectral power that persisted
from storm to storm: main phase-related wide-band fluctuations
driven possibly by a substorm-type ionospheric activity centered
around the local midnight and recovery phase-related narrow-band
fluctuations associated with Pc5 range geomagnetic pulsations
in the local morning region. Based on this observed "stability",
a new S-transform-based statistical approach using, for example,
an ensemble of different S-transform responses for known storms
is proposed for GIC prediction. |
|
Julio de 2006
Thermal equation of state of Fe3S and implications for
sulfur in Earth's core
Authors: Christopher T. Seagle, Andrew J. Campbellet, Dion
L. Heinz, Guoyin Shen and Vitali B. Prakapenka
Link: www.agu.org/pubs/crossref/2006/
2005JB004091.shtml
Abstract
Iron (Fe) and coexisting Fe3S were studied simultaneously
using synchrotron X-ray diffraction and a laser-heated diamond
anvil cell (DAC). The thermal equation of state (EOS) of Fe3S
was investigated up to pressures of 80 GPa and temperatures
of 2500 K. Fitting a third-order Birch-Murnaghan EOS to the
room temperature data yielded bulk modulus K0 = 156(7) GPa
(values in parentheses are standard deviation) and pressure
derivative K'0 = 3.8(3) calibrated against
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NaCl in the B2 structure. The room temperature data were also
calibrated against the EOS of hcp-Fe for comparison and aid
in the determination of the thermal pressure contribution of
Fe3S. This fit yielded bulk modulus K0 = 113(9) GPa and pressure
derivative K'0 = 5.2(6). The thermal pressure contribution of
Fe3S was assumed to be of the form Delta P thermal = aK Delta
T, where aK is constant. The best fit to the data
yielded aK = 0.011(2) GPa K-1. Iron and Fe3S
coexisted in the high-pressure, high-temperature experiments,
and a density relationship between Fe and Fe3S was found to
be linear and independent of temperature. Extrapolation of the
data to the core-mantle boundary (CMB), using an assumed temperature
of 3500 K at the CMB, a 2% volume change associated with melting,
and applying a small adjustment to account for the nickel content
of the core indicates that 14.7(11) wt % sulfur is adequate
to resolve the density deficit of the outer core. |
|
Junio de 2006
Tectonic discrimination diagrams revisited
Author: Pieter Vermeesch
Link: www.agu.org/pubs/crossref/2006/
2005GC001092.shtml
Abstract
The decision boundaries of most tectonic discrimination diagrams
are drawn by eye. Discriminant analysis is a statistically
more rigorous way to determine the tectonic affinity of oceanic
basalts based on their bulk-rock chemistry. This method was
applied to a database of 756 oceanic basalts of known tectonic
affinity (ocean island, mid-ocean ridge, or island arc). For
each of these training data, up to 45 major, minor, and trace
elements were measured. Discriminant analysis assumes multivariate
normality. If the same covariance structure is shared by all
the classes (i.e., tectonic affinities), the decision boundaries
are linear, hence the term linear discriminant analysis (LDA).
In contrast with this, quadratic discriminant analysis (QDA)
allows the classes to have different covariance structures.
To solve the statistical problems associated with the constant-sum
constraint of geochemical data, the training
|
data must be transformed to log-ratio space before performing
a discriminant analysis. The results can be mapped back to the
compositional data space using the inverse log-ratio transformation.
An exhaustive exploration of 14,190 possible ternary discrimination
diagrams yields the Ti-Si-Sr system as the best linear discrimination
diagram and the Na-Nb-Sr system as the best quadratic discrimination
diagram. The best linear and quadratic discrimination diagrams
using only immobile elements are Ti-V-Sc and Ti-V-Sm, respectively.
As little as 5% of the training data are misclassified by these
discrimination diagrams. Testing them on a second database of
182 samples that were not part of the training data yields a
more reliable estimate of future performance. Although QDA misclassifies
fewer training data than LDA, the opposite is generally true
for the test data. Therefore LDA is a cruder but more robust
classifier than QDA. Another advantage of LDA is that it provides
a powerful way to reduce the dimensionality of the multivariate
geochemical data in a similar way to principal component analysis.
This procedure yields a small number of "discriminant functions,"
which are linear combinations of the original variables that
maximize the between-class variance relative to the within-class
variance. |
|
Junio de 2006
Multifractality and rainfall extremes: A review
Authors: Daniele Veneziano, Andreas Langousis et al
Link: http://www.agu.org/pubs/crossref/2006/
2005WR004716.shtml
Abstract
The multifractal representation of rainfall and its use to
predict rainfall extremes have advanced significantly in recent
years. This paper summarizes this body of work and points
at some open questions. The need for a coherent overview comes
in part from the use of different terminology, notation, and
analysis methods in the literature and in part from the fact
that results are dispersed and not always readily available.
|
Two
important trends have marked the use of multifractals for rainfall
and its extremes. One is the recent shift of focus from asymptotic
scaling properties (mainly for the intensity-duration-frequency
curves and the areal reduction factor) to the exact extreme
distribution under nonasymptotic conditions. This shift has
made the results more relevant to hydrologic applications. The
second trend is a more sparing use of multifractality in modeling,
reflecting the limits of scale invariance in space-time rainfall.
This trend has produced models that are more consistent with
observed rainfall characteristics, again making the results
more suitable for application. Finally, we show that rainfall
extremes can be analyzed using rather rough models, provided
the parameters are fitted to an appropriate range of large-deviation
statistics. |
|
Junio de 2006
Numerical simulations of frictional melting: Small dependence
of shear stress drop on viscosity parameters
Authors: S. Sirono, K. Satomi et al
Link: www.agu.org/pubs/crossref/2006/
2005JB003858.shtml
Abstract
Frictional melting during coseismic slipping strongly affects
dynamic sliding. An idealized numerical simulation of shear
stress drop due to lubrication of fault surfaces by frictional
melting is performed to investigate dynamical adjustment of
melt thickness and its dependence on material properties.
We numerically solved one-dimensional heat conduction equations
coupled with hydrodynamic equations of motion of the melt
layer. We adopt a simple Arrhenius-type temperature dependence
of melt viscosity with a parametric factor eta sub 0.
After the initial transient stage, the evolution of the melt
layer
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asymptotes
to the late stage, where the melt thickness increases as root
square (t) and the generated shear stress drops as 1/root
square (t). An analytic self-similar solution of the temperature
profile for the late stage is obtained. The balance between
the viscous heat generation and conductive heat loss characterizes
the late stage. For small eta sub 0, the melt thickness
is proportional to eta sub 0, whereas it will saturate
for a larger eta sub 0. This is because the strong temperature
dependence of viscosity enables an automatic adjustment of viscosity,
which drops through the temperature increase due to the viscous
heating even if a large eta sub 0 is chosen. Numerical
results and the thickness data of natural pseudotachylyte layers
were compared. A maximum ratio X/ root square (D) (X
is thickness, D is sliding distance) exists above which no solution
is found. The effective thermal conductivity of the melt layer
should be large, probably due to the sliding surface roughness.
Comparison with laboratory experimental results showed that
the normal stress applied to the sample is an important parameter
for stress drop. |
|
Junio de 2006
Earthquake recurrence as a record breaking process
Authors: Jörn Davidsen, Peter Grassberger et al
Link:
www.agu.org/pubs/crossref/2006/
2006GL026122.shtml
Abstract
Extending the central concept of recurrence times for a point
process to recurrent events in space-time allows us to characterize
seismicity as a record breaking process using only
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spatiotemporal
relations among events. Linking record breaking events with
edges between nodes in a graph generates a complex dynamical
network isolated from any length, time or magnitude scales set
by the observer. For Southern California, the network of recurrences
reveals new statistical features of seismicity with robust scaling
laws. The rupture length and its scaling with magnitude emerges
as a generic measure for distance between recurrent events.
Further, the relative separations for subsequent records in
space (or time) form a hierarchy with unexpected scaling properties. |
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Junio de 2006
On the geomagnetic effects of solar wind interplanetary
magnetic structures
Authors: E. Echer, W. D. Gonzalez et al
Link:
www.agu.org/pubs/crossref/2006/
2005SW000200.shtml
Abstract
We present in this work a statistical study of the geoeffectiveness
of the solar wind magnetic interplanetary structures over
the entire observational period (1964–2003). The structures
studied were magnetic clouds (MCs, 170 events), corotating
interaction regions (CIRs, 727 events) and interplanetary
shocks (830 events). The geoeffectiveness was assessed in
terms of the geomagnetic index Kp, AE, and Dst peak values
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within
2 days after the interplanetary structure had passed near Earth's
orbit. Frequency distributions were obtained that give the probability
of every interplanetary structure being followed by intense,
moderate, weak, or quiet (none) magnetic activity levels. The
knowledge of probability distribution is important in schemes
to forecast space weather conditions after the detection, by
in situ solar wind observations, of an interplanetary structure
approaching Earth. We observed that magnetic clouds are more
efficient than shocks or CIRs in producing all the geomagnetic
activity disturbances; CIRs are themselves more geoeffective
as measured by the AE activity. We have confirmed that compound
structures (shocks plus MCs) are more geoeffective in every
type of magnetospheric activity than isolated structures. |
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Junio de 2006
Global models of surface wave
attenuation
Authors: Colleen A. Dalton and Göran Ekström
Link: http://www.agu.org/pubs/crossref/2006/
2005JB003997.shtml
Abstract
A large data set of fundamental mode Rayleigh wave amplitudes
is analyzed to derive global models of surface wave attenuation
(1/Q). The data set consists of measurements of Rayleigh wave
amplitude anomalies in the period range 50-250 s for 347 earthquakes
observed at 179 seismic stations. The amplitude anomalies
are considered to depend on four factors: intrinsic attenuation
along the ray path, elastic focusing effects along the ray
path, a source factor accounting for uncertainties in the
strength of excitation, and a receiver factor accounting for
uncertainties in the response at the station. The amplitude
data are inverted simultaneously for
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global
maps of attenuation expanded in spherical harmonics up to degree
12, global maps of phase velocity expanded to degree 20, and
source and receiver correction factors. All four variable types
are shown to be important in explaining the amplitude anomalies.
A data set of phase delay measurements provides additional constraints
on velocity structure. The maps of attenuation obtained by simultaneous
inversion for elastic and anelastic models contain important
features that are not robustly imaged when the effect of focusing
on wave amplitude is ignored. These include high attenuation
along western North America and along the East Pacific Rise
and other ridge systems and low attenuation associated with
stable continental interiors. The global attenuation maps exhibit
a strong correlation with phase velocity maps corrected for
the effect of the crust, particularly for periods <200 s.
The correlation suggests that the variability in both Q and
velocity in the shallow upper mantle is primarily thermal in
origin. |
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Junio
de 2006
Are the Earth and the Moon compositionally alike? Inferences
on lunar composition and implications for lunar origin and
evolution from geophysical modeling
Authors: A. Khan , J. Maclennan et al
Link: http://www.agu.org/pubs/crossref/2006/
2005JE002608.shtml
Abstract
The main objective of the present study is to discuss in detail
the results obtained from an inversion of the Apollo lunar
seismic data set, lunar mass, and moment of inertia. We inverted
directly for lunar chemical composition and temperature using
the model system CaO-FeO-MgO-Al2O3-SiO2. Using Gibbs free
energy minimization, stable mineral phases at the temperatures
and pressures of interest, their modes and physical properties
are calculated. We determine the compositional range of the
oxide elements, thermal state, Mg#, mineralogy and physical
structure of the lunar interior, as well as
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constraining core size and density. The results indicate a lunar
mantle mineralogy that is dominated by olivine and orthopyroxene
(~80 vol%), with the remainder being composed of clinopyroxene
and an aluminous phase (plagioclase, spinel, and garnet present
in the depth ranges 0-150 km, 150-200 km, and >200 km, respectively).
This model is broadly consistent with constraints on mantle
mineralogy derived from the experimental and observational study
of the phase relationships and trace element compositions of
lunar mare basalts and picritic glasses. In particular, by melting
a typical model mantle composition using the pMELTS algorithm,
we found that a range of batch melts generated from these models
have features in common with low Ti mare basalts and picritic
glasses. Our results also indicate a bulk lunar composition
and Mg# different to that of the Earth's upper mantle, represented
by the pyrolite composition. This difference is reflected in
a lower bulk lunar Mg# (~0.83). Results also indicate a small
iron-like core with a radius around 340 km. |
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Mayo de 2006
Ignorance is bliss: Or seven reasons not to use uncertainty
analysis.
Authors:F. Pappenberger and K. J. Beven.
Link: www.agu.org/pubs/crossref/2006/
2005WR004820.shtml
Abstract
Uncertainty analysis of models has received increasing attention
over the last two decades in water resources research. However,
a significant part of the community is still reluctant to
embrace the estimation of uncertainty in hydrological and
hydraulic modeling. In this paper, we summarize and explore
seven common arguments: uncertainty analysis is not necessary
given physically realistic models; uncertainty analysis
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cannot be used in hydrological and hydraulic
hypothesis testing; uncertainty (probability) distributions
cannot be understood by policy makers and the public; uncertainty
analysis cannot be incorporated into the decision-making process;
uncertainty analysis is too subjective; uncertainty analysis
is too difficult to perform; uncertainty does not really matter
in making the final decision. We will argue that none of
the arguments against uncertainty analysis rehearsed are,
in the end, tenable. Moreover, we suggest that one reason
why the application of uncertainty analysis is not normal
and expected part of modeling practice is that mature guidance
on methods and applications does not exist. The paper
concludes with suggesting that a Code of Practice is needed
as a way of formalizing such guidance.
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Mayo de 2006
Observations of gas flow in porous media using a light
transmission technique.
Authors: Laila Parker, Rockie Yarwood et al.
Link:
www.agu.org/pubs/crossref/2006/
2005WR004080.shtml
Abstract
A novel technique for quantitative nondestructive study of
two-dimensional disposition of gas phase in unsaturated porous
media is presented. Carbon dioxide was pumped through a backlit
1 cm thick chamber packed with translucent sand, which was
variably saturated with water containing the pH indicator
dye methyl red. As the carbon dioxide dissolved in the pore
water, lowering the pH and changing the dye color, a
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CCD camera captured images of the resultant changes in transmitted
light. These digital image files were then processed using a
series of calibrated steps to relate light intensity to dye
attenuation, dye attenuation to solution pH, and solution pH
to aqueous and gaseous carbon dioxide concentration. The final
product was a series of false-color images showing the development
of the gaseous carbon dioxide plume. Limitations were found
that will require further development, including more attention
to calibration of dye concentration versus observed transmission
and investigation of dye solubility across the range of pH values
employed. With refinement, this technique may prove to be a
useful tool in studying the complexities of gas phase transport
in variably saturated porous media. |
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Mayo de 2006
Genetic algorithms for the design of looped irrigation
water distribution networks
Authors: Juan Reca and Juan Martínez.
Link: www.agu.org/pubs/crossref/2006/
2005WR004383.shtml
Abstract
A new computer model called Genetic Algorithm Pipe Network
Optimization Model (GENOME) has been developed with the aim
of optimizing the design of new looped irrigation water distribution
networks. The model is based on a genetic algorithm method,
although relevant modifications and improvements have been
implemented to adapt the model to this specific problem. It
makes
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use
of the robust network solver EPANET. The model has been tested
and validated by applying it to the least cost optimization
of several benchmark networks reported in the literature. The
results obtained with GENOME have been compared with those found
in previous works, obtaining the same results as the best published
in the literature to date. Once the model was validated, the
optimization of a real complex irrigation network has been carried
out to evaluate the potential of the genetic algorithm for the
optimal design of large-scale networks. Although satisfactory
results have been obtained, some adjustments would be desirable
to improve the performance of genetic algorithms when the complexity
of the network requires it. |