|
Agosto de 2007
Surface wave tomography of the western United States from
ambient seismic noise: Rayleigh wave group velocity maps
Authors: M. P. Moschetti, M. H. Ritzwoller et al
Link: Click Here
Abstract
We have applied ambient noise surface wave tomography to data
that have emerged continuously from the EarthScope USArray Transportable
Array (TA) between October 2004 and January 2007. Estimated
Green's functions result by cross-correlating noise records
between every station-pair in the network. The 340 stations
yield a total of more than 55,000 interstation paths. Within
the 5- to 50-s period band, we
|
measure
the dispersion characteristics of Rayleigh waves using frequency-time
analysis. High-resolution group velocity maps at 8-, 16-, 24-,
30-, and 40-s periods are presented for the western United States.
The footprint of the TA encloses a region with a resolution of
about the average interstation spacing (~70 km). Velocity anomalies
in the group velocity maps correlate well with the dominant geological
features of the western United States. Coherent velocity anomalies
are associated with the Sierra Nevada, Peninsular, and Cascade
Ranges, Great Valley, Salton Trough, and Columbia basins, the
Columbia River flood basalts, the Snake River Plain and Yellowstone,
and mantle wedge features associated with the subducting Juan
de Fuca plate. |
|
Agosto de 2007
Towards a universal rule on the recurrence interval scaling
of repeating earthquakes?
Authors: Kate Huihsuan Chen, Robert M. Nadeau et al
Link: Click Here
Abstract
A recent analysis of recurrence properties of small repeating
earthquakes on a creeping oblique thrust fault in eastern Taiwan
reveals a weak variation in recurrence interval (T r ) with
seismic moment (M o ). Compared to the scaling of T r with M
o from repeating earthquake data near
|
Parkfield in California,
the repeating data from eastern Taiwan has recurrence intervals
that are 2 times shorter. Also in northeastern Japan, T r of repeating
quakes are ~4 times shorter than those expected from the Parkfield
scaling law. When adjusted to account for differences in the geodetically
derived slip rates for the three fault zones, however, the T r
-M o scaling is remarkably consistent among the three regions.
It suggests that the tectonic loading rate is likely the most
important factor that controls the repeat time. It also suggests
that there seems to exist a universal rule on recurrence interval
scaling of repeating earthquakes in diverse tectonic settings. |
|
Agosto de 2007
Joint inversion of teleseismic receiver functions and magnetotelluric
data using a genetic algorithm: Are seismic velocities and electrical
conductivities compatible?
Authors: M. Moorkamp, A. G. Jones et al
Link: Click Here
Abstract
Joint inversion of different kinds of geophysical data has the
potential to improve model resolution, under the assumption
that the different observations are sensitive to the same subsurface
features. Here, we examine the compatibility of P-wave teleseismic
receiver
|
functions
and long-period magnetotelluric (MT) observations, using joint
inversion, to infer one-dimensional lithospheric structure. We
apply a genetic algorithm to invert teleseismic and MT data from
the Slave craton; a region where previous independent analyses
of these data have indicated correlated layering of the lithosphere.
Examination of model resolution and parameter trade-off suggests
that the main features of this area, the Moho, Central Slave Mantle
Conductor and the Lithosphere-Asthenosphere boundary, are sensed
to varying degrees by both methods. Thus, joint inversion of these
two complementary data sets can be used to construct improved
models of the lithosphere. Further studies will be needed to assess
whether the approach can be applied globally. |
|
15 de
Agosto de 2007
Terremoto en Perú
Según el USGS, correspondió a
dos sismos consecutivo acaecidos el día 15 de agosto
frente a las costas de Pisco (18:40:56 hora local de
Perú). El primer sismo comenzó
a las 23:40:56 UTC con una magnitud Mw de 8.0, mientras que
|
el segundo comenzó a las 23:41:20 UTC con una magnitud
Mw de 7.5.
Hipocentros
- Del primer sismo: 13.36º S, 76.52º W, Prof = 30.2
Km
- Del segundo sismo: 12.23º S, 76.65º W, Prof = 178.6
Km
Energía total liberada: 33 mil veces la energía
liberada por la bomba atómica de Hiroshima.
Réplicas (hora local de Perú)
- 19:01: 5.6 Mw
- 19:21: 5.8 Mw
- 20:01: 5.8 Mw
- 20:31: 5.4 Mw
El evento principal incluyó un tsunami con una altura
máxima en la localidad de Paracas, donde las olas alcanzaron
los 5 m.
|
|
Agosto de 2007
Tsunami early warning using GPS-Shield arrays
Authors: Stephan V. Sobolev, Andrey Y. Babeyko et al
Link: Click Here
Abstract
The 2004 catastrophic Indian Ocean tsunami has strongly emphasized
the need for reliable tsunami early warning systems. Another
giant tsunamigenic earthquake may occur west of Sumatra, close
to the large city of Padang. We demonstrate that the presence
of islands between the trench and the Sumatran coast makes earthquake-induced
tsunamis especially sensitive to slip distribution on the rupture
plane as wave heights at Padang may differ by more than a factor
of 5 for earthquakes having the same seismic moment (magnitude)
and rupture zone geometry but different slip distribution. Hence
reliable prediction of tsunami wave heights for Padang cannot
be provided using traditional,
|
earthquake-magnitude-based
methods. We show, however, that such a prediction can be issued
within 10 minutes of an earthquake by incorporating special types
of near-field GPS arrays ("GPS-Shield"). These arrays
measure both vertical and horizontal displacements and can resolve
higher order features of the slip distribution on the fault than
the seismic moment if placed above the rupture zone or are less
than 100 km away of the rupture zone. Stations in the arrays are
located as close as possible to the trench and are aligned perpendicular
to the trench, i.e., parallel to the expected gradient of surface
coseismic displacement. In the case of Sumatra and Java, the GPS-Shield
arrays should be placed at Mentawai Islands, located between the
trench and Sumatra and directly at the Sumatra and Java western
coasts. We demonstrate that the "GPS-Shield" can also
be applied to northern Chile, where giant earthquakes may also
occur in the near future. Moreover, this concept may be applied
globally to many other tsunamigenic active margins where the land
is located above or close to seismogenic zones. |
|
Agosto de 2007
Decay of aftershock activity for Japanese earthquakes
Authors: K. Z. Nanjo, B. Enescu et al
Link: Click Here
Abstract
Aftershock decay is often correlated with the modified Omori's
law: dN/dt = t -1(1 + t/c)-p , where dN/dt is the occurrence
rate of aftershocks with magnitudes greater than a lower cutoff
m, t is time since a mainshock, t and c are characteristic times,
and p is an exponent. Extending this approach, we derive two
possibilities: (1) c is a constant independent of m and t scales
with m and (2) c scales with m and t is a constant independent
of m. These two are tested by using aftershock sequences of
four relatively recent and
|
large
earthquakes in Japan. We first determine for each sequence the
threshold magnitude above which all aftershocks are completely
recorded and use only events above this magnitude. Then, visual
inspection of the decay curves and statistical analysis shows
that the second possibility is the better approximation for our
sequences. This means that the power law decay of smaller aftershocks
starts after larger times from the mainshock. We find a close
association of our second result with a solution obtained for
a damage mechanics model of aftershock decay. The time delays
associated with aftershocks, according to the second possibility,
can be understood as the times needed to nucleate microcracks
(aftershocks). Our result supports the idea that the c value is
a real consequence of aftershock dynamics associated with damage
evolution. |
|
Agosto de 2007
Turbulent structure in Earth's fluid core inferred from time
series of geomagnetic dipole moment
Authors: Ataru Sakuraba and Yozo Hamano
Link: Click Here
Abstract
The physical grounds are discussed for assessing turbulent structures
hidden inside the Earth's liquid core by using a time series
of the geomagnetic dipole moment obtained from historical and
paleomagnetic data. We propose the idea that the time-averaged
wavenumber
|
spectra of electric
current density at the core-mantle boundary (CMB) and also velocity
near the CMB have relation to the frequency spectrum of the dipole
moment. We performed computer simulations of a magnetohydrodynamic
spherical dynamo to verify this idea. We show that the frequency
spectrum of the dipole moment in the simulation is similar to
that inferred by paleomagnetic observations. The simulation results
indicate that the underlying kinetic energy spectrum is proportional
to m -5/3 in a high wavenumber range, where m is the azimuthal
wavenumber. We speculate that a similar turbulent energy spectrum
may exist in the Earth's core with a peak near m = 5. |
|
Agosto de 2007
Thermal demagnetization of Martian upper crust by magma intrusion
Authors: Yoshiko Ogawa and Michael Manga
Link: Click Here
Abstract
The absent or weak magnetic field above large Martian volcanoes
may provide constraints on their formation and the carrier of
magnetization. We consider the ability of magma intrusions to
thermally demagnetize the shallow crust beneath
|
volcanoes
through heat conduction and hydrothermal circulation. If magnetization
is dominated by magnetite, the volume of crust that is thermally
demagnetized is similar to the volume of crust emplaced since
the dynamo field disappeared. If magnetization is dominated by
pyrrhotite, the volume of crust that is demagnetized is typically
more than twice the volume of magma intruded. Hydrothermal circulation
contributes negligible additional thermal demagnetization, beyond
that from heat conduction alone, for permeabilities less than
O(10-15) m2. |
|
Agosto de 2007
On the fractal nature of the magnetic field energy density
in the solar wind
Authors: B. Hnat, S. C. Chapman et al
Link: Click Here
Abstract
The solar wind exhibits scaling typical of intermittent turbulence
in the statistics of in situ fluctuations in both the magnetic
and velocity fields. Intriguingly, quantities not directly accessed
by theories of ideal, incompressible, MHD
|
turbulence, such as
magnetic energy density, B 2, nevertheless show evidence of simple
fractal (self-affine) statistical scaling. We apply a novel statistical
technique which is a sensitive discriminator of fractality to
the B 2 timeseries from WIND and ACE. We show that robust fractal
behaviour occurs at solar maximum and determine the scaling exponents.
The probability density function (PDF) of fluctuations at solar
maximum and minimum are distinct. Power law tails are seen at
maximum, and the PDF is reminiscent of a Lévy flight. |
|
Agosto
de 2007
Primer descubrimiento de impacto
meteorítico en el núcleo de hielo
profundo de la Antártica (Domo C
- Epica)
Autores: Biancamaria Narcisi, Jean
Robert Petit et al
Link: Clic Aquí
Abstract
Dos distintos estratos de polvo en el núcleo de hielo
del Domo C - Epica (75°06'S, 123°21'E, Plateau Oriental
de la Antártica) han permitido detectar dos eventos meteoríticos
individuales. Las partículas que forman parte de esos
estratos han sido analizados con microsonda electrónica
y muestran una peculiar textura y rasgos mineralógicos
y geoquímicos similares a los observados en escombros
extraterrestres encontrados en sedimentos profundos y capas
polares. Estimaciones preliminares del ingreso de material cósmico
en los estratos estudiados - obtenidos a partir de mediciones
realizadas con el Contador Coulter - indican un flujo de 4
a 5 órdenes de magnitud superior al flujo micrometeorítico
normal observado anualmente en el hielo y nieve de la Antártica
Oriental. Los eventos cósmicos fueron fechados a través
de modelos glacialógicos en 434 ± 6 y 481 ±
6 ka respectivamente y fueron ubicados como "Eventos de
Medio-Brunhes". Este es el primer reporte de horizontes
cósmicos bien fechados en núcleos de hielo profundos
de la Antártica. Esta investigación mejora significativamente
los registros extraterrestres de la Antártica y abre
la posibilidad de nuevas perspectivas de correlación
entre registros climáticos de la región polar
austral.
|
August - 2007
First discovery of meteoritic events in deep Antarctic (EPICA-Dome
C) ice cores
Authors: Biancamaria Narcisi, Jean Robert Petit et al
Link: Click Here
Abstract
Two distinct dust layers in the EPICA-Dome C ice core (75°06'S,
123°21'E, East Antarctic Plateau) have been shown to relate
to individual meteoritic events. Particles forming these layers,
investigated by electron microprobe, show peculiar textural,
mineralogical and geochemical features and closely resemble
extraterrestrial debris in deep-sea sediments and polar caps.
Preliminary estimates of cosmic debris input at the studied
layers, obtained from Coulter Counter measurements, are 4-5
orders of magnitude greater than the yearly micrometeorite
flux in East Antarctic snow and ice. The cosmic events are accurately
dated through glaciological models at 434 ± 6 and 481
± 6 ka, respectively and are located in the core climatic
stratigraphy near the "Mid-Brunhes Event". This is
the first report of well-dated cosmic horizons in deep Antarctic
ice cores. It significantly improves the extraterrestrial record
of Antarctica and opens new correlation perspectives between
long climatic records of the South polar region.
|
|
Agosto de 2007
Impact of El Niño-Southern Oscillation on European
climate
Authors: S. Brönnimann
Link: Click Here
Abstract
El Niño-Southern Oscillation (ENSO) is arguably the most
important global climate pattern. While the effects in the Pacific-North
American sector and the tropical regions are relatively well
understood, the impacts on the circulation in the North Atlantic-European
sector are discussed
|
more controversially.
Studies from the past 10 years demonstrate that ENSO does affect
European climate. However, some of the effects undergo a seasonal
modulation or are nonlinear. The signal can be modified by other
factors and might be nonstationary on multidecadal scales, contributing
to a large interevent variability. Here I review observational
and model-based evidence for ENSO's effect on European climate
and discuss possible mechanisms, also including troposphere-stratosphere
coupling. The paper ends with a schematic depiction of the effects
and a discussion of their relevance with respect to our scientific
understanding of the climate system and of their relevance for
seasonal climate forecasts. |
|
Agosto de 2007
Streaming potentials of granular media: Influence of the
Dukhin and Reynolds numbers
Authors: A. Bolève, A. Crespy et al
Link: Click Here
Abstract
Laboratory experiments are performed to understand the controlling
parameters of the electrical field associated with the seepage
of water through a porous material. We use seven glass bead
packs with varying mean grain size in an effort to obtain a
standard material for the investigation of these electrical
potentials. The mean grain size of these samples is in the range
56-3000 µm. We use pure NaCl electrolytes with conductivity
in the range 10-4 to 10-1 S m-1 at 25°C. The flow conditions
cover viscous and inertial laminar flow conditions but not turbulent
flow. In the relationship between the streaming potential coupling
coefficient and the grain size, three distinct domains are defined
by the values of two dimensionless numbers, the Dukhin and the
|
Reynolds numbers. The Dukhin number represents the ratio between
the surface conductivity of the grains (due to conduction in the
electrical double layer coating the surface of the grains) and
the pore water electrical conductivity. At high Dukhin numbers
( 1) and low Reynolds numbers ( 1), the magnitude of the streaming
potential coupling coefficient decreases with the increase of
the Dukhin number and depends on the mean grain diameter (and
therefore permeability) of the medium. At low Dukhin and Reynolds
numbers ( 1), the streaming potential coupling coefficient becomes
independent of the microstructure and is given by the well-known
Helmholtz-Smoluchowski equation widely used in the literature.
At high Reynolds numbers, the magnitude of the streaming potential
coupling coefficient decreases with the increase of the Reynolds
number in agreement with a new model developed in this paper.
A numerical application is made illustrating the relation between
the self-potential signal and the intensity of seepage through
a leakage in an embankment. |
|
Agosto de 2007
Frequency sensitive moment tensor inversion for light to
moderate magnitude earthquakes in eastern Africa
Authors: A. Barth, F. Wenzel et al
Link: Click Here
Abstract
We provide a procedure for the routine determination of moment
tensors from earthquakes with magnitudes as low as M W 4.4 using
data recorded by only a few permanent seismic stations at regional
to teleseismic distances. Waveforms are inverted for
|
automatically determined
frequency pass-bands that depend on source-receiver locations
as well as the earthquake magnitude. Inversion results are stable
against small variations in the frequency band and provide low
data variances, i.e., a good fit between observed and modelled
waveform traces. The total frequency band used for our procedure
ranges from 10 mHz to 29 mHz (periods of 35 s to 100 s). This
enables us to determine focal mechanisms for earthquakes that
were not derived previously by routine procedures of CMT or other
agencies. As a case study, we determine focal mechanism solutions
of 38 light to moderate magnitude earthquakes in eastern Africa
between 1995 and 2002. |
|
Agosto de 2007
Comparison of Fourier and wavelet techniques in the determination
of geomagnetic field line resonances
Authors: A. Boudouridis and E. Zesta
Link: Click Here
Abstract
Fourier transforms have traditionally been used in the past
for time-frequency analysis of numerous physical problems. Recently,
a new time-frequency analysis technique using temporally confined
base functions, called wavelets, has been applied to many problems.
The advantage of the wavelets over the conventional sinusoidal
functions is their time localization property,
|
providing information not only about the frequency
or scale size of the features present but also about their location
in the time series. Here we compare the performance of the two
techniques on the time-frequency analysis of ground magnetometer
data and especially their ability to pick up the field line
resonance (FLR) frequency of a resonating magnetic field line
using automated FLR determination techniques. We find that the
automated techniques work better with the Fourier transforms
giving less variable FLR frequency. The high temporal resolution
of the wavelet transforms can be useful for detailed analyses
of short time periods, but it becomes a disadvantage when it
comes to automated techniques for the entire dayside magnetosphere,
yielding a highly variable FLR frequency.
|
|
Agosto de 2007
Wavelet-based multifractal analysis of field scale variability
in soil water retention
Authors: Takele B. Zeleke and Bing C. Si
Link: Click Here
Abstract
Better understanding of spatial variability of soil hydraulic
parameters and their relationships to other soil properties
is essential to scale-up measured hydraulic parameters and to
improve the predictive capacity of pedotransfer functions. The
objective of this study was to characterize scaling properties
and the persistency of water retention parameters and soil physical
properties. Soil texture, bulk density, organic carbon content,
and the parameters of the van Genuchten water
|
retention function were determined on 128 soil cores from a
384-m transect with a sandy loam soil, located at Smeaton, SK,
Canada. The wavelet transform modulus maxima, or WTMM, technique
was used in the multifractal analysis. Results indicate that
the fitted water retention parameters had higher small-scale
variability and lower persistency than the measured soil physical
properties. Of the three distinct scaling ranges identified,
the middle region (8–128 m) had a multifractal-type scaling.
The generalized Hurst exponent indicated that the measured soil
properties were more persistent than the fitted soil hydraulic
parameters. The relationships observed here imply that soil
physical properties are better predictors of water retention
values at larger spatial scales than at smaller scales.
|
|
Agosto
de 2007
Convection under a lid of finite conductivity: Heat flux
scaling and application to continents
Authors: C. Grigné, S. Labrosse et al
Link: Click Here
Abstract
A scaling law for the heat flux out of a convective fluid covered
totally or partially by a finitely conducting lid is proposed.
This scaling is constructed in order to quantify the heat transfer
out of the Earth's mantle, taking into account the effect of
the dichotomy between oceans and continents, which imposes heterogeneous
thermal boundary conditions at the surface of the mantle. The
effect of these heterogeneous boundary conditions is studied
here using simple two-dimensional models, with the mantle
|
represented
by an isoviscous fluid heated from below and continents represented
by nondeformable lids of finite thermal conductivity set above
the surface of the model. We use free-slip boundary conditions
under the oceanic and continental zones in order to study in an
isolated way the possible thermal effect of continents, independently
of all mechanical effect. A systematic study of the heat transfer
as a function of the Rayleigh number of the fluid, of the width
of the lid, and of its thermal properties is carried out. We show
that estimates of continental lithosphere thickness imply a strong
insulating effect from continents on mantle heat loss, at least
locally. The heat flux below continents was low in the past and
of the order of the present one if the continental thickness has
remained broadly constant over the Earth's history. |
|
Agosto de 2007
Effects of Mount Pinatubo volcanic eruption on the hydrological
cycle as an analog of geoengineering
Authors: Kevin E. Trenberth and Aiguo Dai
Link: Click Here
Abstract
The problem of global warming arises from the buildup of greenhouse
gases such as carbon dioxide from burning of fossil fuels and
other human activities that change the composition of the atmosphere
and alter outgoing longwave radiation (OLR). One geoengineering
solution being proposed is to reduce the incoming
|
sunshine by emulating
a volcanic eruption. In between the incoming solar radiation and
the OLR is the entire weather and climate system and the hydrological
cycle. The precipitation and streamflow records from 1950 to 2004
are examined for the effects of volcanic eruptions from El Chichón
in March 1982 and Pinatubo in June 1991, taking into account changes
from El Niño-Southern Oscillation. Following the eruption
of Mount Pinatubo in June 1991 there was a substantial decrease
in precipitation over land and a record decrease in runoff and
river discharge into the ocean from October 1991-September 1992.
The results suggest that major adverse effects, including drought,
could arise from geoengineering solutions. |
|
Agosto de 2007
Bayesian multivariate linear regression with application
to change point models in hydrometeorological variables
Authors: O. Seidou , J. J. Asselin et al
Link: Click Here
Abstract
Multivariate linear regression is one of the most popular modeling
tools in hydrology and climate sciences for explaining the link
between key variables. Piecewise linear regression is not always
appropriate since the relationship may experiment sudden changes
due to climatic, environmental, or anthropogenic perturbations.
To address this issue, a practical and general approach to the
Bayesian analysis of the multivariate regression model is presented.
The approach allows simultaneous single change point detection
in a multivariate sample and can account for missing data in
the response variables and/or in the explicative variables.
It also improves on recently published change point detection
methodologies by allowing a more
|
flexible
and thus more realistic prior specification for the existence
of a change and the date of change as well as for the regression
parameters. The estimation of all unknown parameters is achieved
by Monte Carlo Markov chain simulations. It is shown that the
developed approach is able to reproduce the results of Rasmussen
(2001) as well as those of Perreault et al. (2000a, 2000b). Furthermore,
two of the examples provided in the paper show that the proposed
methodology can readily be applied to some problems that cannot
be addressed by any of the above-mentioned approaches because
of limiting model structure and/or restrictive prior assumptions.
The first of these examples deals with single change point detection
in the multivariate linear relationship between mean basin-scale
precipitation at different periods of the year and the summer-autumn
flood peaks of the Broadback River located in northern Quebec,
Canada. The second one addresses the problem of missing data estimation
with uncertainty assessment in multisite streamflow records with
a possible simultaneous shift in mean streamflow values that occurred
at an unknown date. |
|
Julio de 2007
Global mantle flow and the development of seismic anisotropy:
Differences between the oceanic and continental upper mantle
Authors: Clinton P. Conrad, Mark D. Behn et al
Link: Click Here
Abstract
Viscous shear in the asthenosphere accommodates relative motion
between Earth's surface plates and underlying mantle, generating
lattice-preferred orientation (LPO) in olivine aggregates and
a seismically anisotropic fabric. Because this fabric develops
with the evolving mantle flow field, observations of seismic
anisotropy can constrain asthenospheric flow patterns if the
contribution of fossil lithospheric anisotropy is small. We
use global viscous mantle flow models to characterize the relationship
between asthenospheric deformation and LPO and compare the predicted
pattern of anisotropy to a global compilation of observed
|
shear wave splitting
measurements. For strain axis (ISA, the LPO after infinite deformation)
faster than the ISA changes along flow lines. Thus we expect the
ISA to approximate LPO throughout most of the asthenosphere, greatly
simplifying LPO predictions because strain integration along flow
lines is unnecessary. Approximating LPO with the ISA and assuming
A-type fabric (olivine a axis parallel to ISA), we find that mantle
flow driven by both plate motions and mantle density heterogeneity
successfully predicts oceanic anisotropy (average misfit 13°).
Continental anisotropy is less well fit (average misfit 41°),
but lateral variations in lithospheric thickness improve the fit
in some continental areas. This suggests that asthenospheric anisotropy
contributes to shear wave splitting for both continents and oceans
but is overlain by a stronger layer of lithospheric anisotropy
for continents. The contribution of the oceanic lithosphere is
likely smaller because it is thinner, younger, and less deformed
than its continental counterpart. |
|
Julio de 2007
A correlated fracture network: Modeling and percolation properties
Authors: Mohsen Masihi and Peter R. King
Link: Click Here
Abstract
We present a model of fractures based on the idea that the elastic
free energy due to the fracture density follows a Boltzmann
distribution. The resulting expression for the spatial correlation
in the displacement of fractures is used as an objective function
in a simulated annealing algorithm to generate realizations
of correlated
|
fracture
networks. This approach determines the appropriate statistical
distribution for the fractures (e.g., length distribution) rather
than imposing them as is done conventionally. The model consists
of two families of parallel fractures which are perpendicular
under isotropic conditions. There also exists a positive correlation
between the position of fractures and their lengths; that is,
large fractures have their neighbors located at greater distance
than small fractures. Finally, we use the realizations of correlated
fracture networks in the basic methodology of the percolation
approach and investigate the model percolation properties. In
particular, the scaling exponents of the connectivity are found
to be different from the conventional, uncorrelated values. |
|
Julio de 2007
Global 30-240 keV proton precipitation in the 17-18 April
2002 geomagnetic storms: 3. Impact on the ionosphere and thermosphere
Authors: Xiaohua Fang, Aaron J. Ridley et al
Link: Click Here
Abstract
NOAA Polar Orbiting Environmental Satellites 30-240 keV proton
precipitation measurements in the 17-18 April 2002 geomagnetic
storms are used with a Monte Carlo ion transport model to obtain
ionization and heating rates that are subsequently fed into
the Global Ionosphere Thermosphere Model to investigate the
proton impact on the ionosphere and thermosphere. Simulation
results show that after the addition of proton precipitation
in a moderate geomagnetic storm (specifically, the one during
mid-April 2002), there are places at low altitudes (100-120
km) on the nightside
|
undergoing significant
increases in electron and nitric oxide (NO) densities. The enhancement
can be as large as several factors or even by an order of magnitude.
Moreover, the temporal profiles of the enhancement in ionospheric
electron densities demonstrate a direct correlation with proton
precipitation imposed on the topside boundary, and there is no
integral effect. This is in contrast with a continuous buildup
process illustrated in the time variation of the thermospheric
NO density enhancement because NO at these altitudes has a long
lifetime. In addition, by including high-energy precipitating
protons in a global ionosphere thermosphere coupled model, significant
changes take place in the ion convection (locally around ±20%)
and in the neutral winds (locally around ±40%). This study
represents the first attempt to understand the global influence
of proton precipitation on the ionosphere and thermosphere using
in situ observational data. |
|
Julio de 2007
Wavelet-based multifractal analysis of field scale variability
in soil water retention
Authors: Takele B. Zeleke and Bing C. Si
Link: Click Here
Abstract
Better understanding of spatial variability of soil hydraulic
parameters and their relationships to other soil properties
is essential to scale-up measured hydraulic parameters and to
improve the predictive capacity of pedotransfer functions. The
objective of this study was to characterize scaling properties
and the persistency of water retention parameters and soil physical
properties. Soil texture, bulk density, organic carbon content,
|
and
the parameters of the van Genuchten water retention function were
determined on 128 soil cores from a 384-m transect with a sandy
loam soil, located at Smeaton, SK, Canada. The wavelet transform
modulus maxima, or WTMM, technique was used in the multifractal
analysis. Results indicate that the fitted water retention parameters
had higher small-scale variability and lower persistency than
the measured soil physical properties. Of the three distinct scaling
ranges identified, the middle region (8-128 m) had a multifractal-type
scaling. The generalized Hurst exponent indicated that the measured
soil properties were more persistent than the fitted soil hydraulic
parameters. The relationships observed here imply that soil physical
properties are better predictors of water retention values at
larger spatial scales than at smaller scales. |
|
Julio de 2007
Radiative transfer mixing models of meteoritic assemblages
Authors: Samuel J. Lawrence and Paul G. Lucey
Link: Click Here
Abstract
A spectral mixing model for the expected conditions on asteroid
surfaces based on Hapke's radiative transfer theory for intimate
mixtures is presented. This model calculates the visible/near-infrared
reflectance spectrum of an intimate mixture incorporating the
modal mineralogy, the mineral chemistry, the particle size,
and the degree of space weathering as factors, and includes
an improved treatment for the spectral effects of coarse-grained
Fe,Ni-metal. This model can reproduce the spectra of geologically
plausible mixtures of silicate components and meteorites with
known mineral
|
abundances, chemistry,
and particle sizes. We compare the model spectral effects of the
coarse-grained Fe,Ni-metal naturally present on asteroid surfaces
to the spectral effects of submicroscopic iron produced by lunar-style
space weathering. The model spectral effects of large asteroid
surface concentrations of coarse-grained Fe,Ni-metal are similar
to the spectral effects of SMFe in some cases. We model the composition
of asteroid 4 Vesta and find it to be consistent with a minimally
space-weathered eucritic composition. Finally, we predict the
spectral properties of some example of ordinary chondrite parent
bodies. When modeled with particle sizes matching the optically
dominant particle size of the lunar surface and minimal abundances
of SMFe, these model ordinary chondrite parent bodies plot outside
the S(IV) asteroid field on a Band area ratio versus Band I center
plot. We find that a combination of larger particle sizes and
increased SMFe abundances is the most effective way to move these
models into the S(IV) field. |
|
Julio de 2007
Recovering tracer test input functions from fluid electrical
conductivity logging in fractured porous rocks
Authors: S. A. Mathias, A. P. Butler et al
Link: Click Here
Abstract
A radially convergent tracer test was carried out in an unconfined
Chalk aquifer of Berkshire, United Kingdom. Fluorescent tracers
were injected into two boreholes lying 32 m (PL10A) and 54 m
(PL10B) from the abstraction hole. The tracers were also mixed
with an NaCl solution so that vertical distributions of tracer
within the injection
|
wells
could be monitored using fluid electrical conductivity (FEC) logging.
The breakthrough curve (BTC) from PL10A was unimodal and had a
first arrival time of 14 min. The BTC from PL10B exhibited two
distinct peaks and a first arrival time of just 4 min. The tracer
test input functions were derived by numerically modeling the
observed FEC logs of the injection wells. These were then convoluted
with a conventional, Fickian matrix diffusion dual-porosity model.
The results suggested that the multiple peaks were due to the
way in which the tracers left the injection wells and migrated
into the aquifer. FEC log inversion proved to be an effective
method for predicting borehole flow data obtained by flowmeters
and recovering tracer test input functions for radially convergent
tracer tests. |
|
Julio de 2007
Stochastic uncertainty analysis for solute transport in randomly
heterogeneous media using a Karhunen-Loève-based moment
equation approach
Authors: Gaisheng Liu , Zhiming Lu et al
Link: Click Here
Abstract
A new approach has been developed for solving solute transport
problems in randomly heterogeneous media using the Karhunen-Loève-based
moment equation (KLME) technique proposed by Zhang and Lu (2004).
The KLME approach combines the Karhunen-Loève decomposition
of the underlying random conductivity field and the perturbative
and polynomial expansions of dependent variables including the
hydraulic head, flow velocity, dispersion coefficient, and solute
concentration. The equations obtained in this approach are sequential,
and their structure is formulated in the same form as the original
governing equations such that any existing simulator, such as
Modular
|
Three-Dimensional Multispecies
Transport Model for Simulation of Advection, Dispersion, and Chemical
Reactions of Contaminants in Groundwater Systems (MT3DMS), can
be directly applied as the solver. Through a series of two-dimensional
examples, the validity of the KLME approach is evaluated against
the classical Monte Carlo simulations. Results indicate that under
the flow and transport conditions examined in this work, the KLME
approach provides an accurate representation of the mean concentration.
For the concentration variance, the accuracy of the KLME approach
is good when the conductivity variance is 0.5. As the conductivity
variance increases up to 1.0, the mismatch on the concentration
variance becomes large, although the mean concentration can still
be accurately reproduced by the KLME approach. Our results also
indicate that when the conductivity variance is relatively large,
neglecting the effects of the cross terms between velocity fluctuations
and local dispersivities, as done in some previous studies, can
produce noticeable errors, and a rigorous treatment of the dispersion
terms becomes more appropriate. |
|
Julio de 2007
Fractal parameters of individual soot particles determined
using electron tomography: Implications for optical properties
Authors: Kouji Adachi , Serena H. Chung et al
Link: Click Here
Abstract
The morphologies of soot particles are both complex and important.
They influence soot atmospheric lifetimes, global distributions,
and climate impacts. Particles can have complex geometries with
overlapping projecting parts and pores that are difficult to
infer from the conventional techniques used to study them. We
used electron tomography with a transmission electron microscope
(TEM) to determine three-dimensional (3D) properties such as
fractal dimension (D f), radius of gyration (R g), volume (V),
surface area (A s), and structural coefficient (k
|
a)
for individual soot particles from the ambient air of an Asian
dust (AD) episode and from a U.S. traffic source. The respective
median values of D f are 2.4 and 2.2, of R g are 274 and 251 nm,
of A s/V are 9.2 and 13.7 × 107 m-1, and of k a are 0.67
and 0.71. The corresponding parameters, when calculated from 2D
projections such as TEM images, are considerably less precise
and commonly erroneous. Unlike other methods that have been used
to derive fractal parameters, our method is applicable to particles
of any D f. Using the 3D data, we estimate that mass-normalized
scattering cross sections of our AD and traffic soot particles
are respectively about 15 and 30 times greater than those of unaggregated
spheres, which is the shape assumed in global models to estimate
radiative forcing. Accurate 3D information can be used to compute
more precise optical properties, which are important for estimating
direct radiative forcing and improving our understanding of the
climate impact of soot. |
|
Julio de 2007
Structure and mechanical properties of faults in the North
Anatolian Fault system from InSAR observations of coseismic
deformation due to the 1999 Izmit (Turkey) earthquake
Authors: Yariv Hamiel and Yuri Fialko
Link: Click Here
Abstract
We study the structure and mechanical properties of faults in
the North Anatolian Fault system by observing near-fault deformation
induced by the 1999 M w 7.4 Izmit earthquake (Turkey). We use
interferometric Synthetic Aperture Radar (InSAR) and Global
Positioning System observations to analyze the coseismic surface
deformation in the near field of the Izmit rupture. The overall
observed coseismic deformation is consistent with deformation
predicted by a dislocation model assuming a uniform elastic
crust. Previous InSAR studies revealed small-scale changes in
the radar range across the nearby faults of the North Anatolian
fault system (in particular, the Mudurnu Valley and Iznik faults)
(e.g., Wright et al., 2001). We demonstrate that these anomalous
range changes are consistent with an elastic response
|
of compliant fault zones
to the stress perturbation induced by the Izmit earthquake. We
examine the spatial variations and mechanical properties of fault
zones around the Mudurnu Valley and Iznik faults using three-dimensional
finite element models. In these models, we include compliant fault
zones having various geometries and elastic properties and apply
stress changes deduced from a kinematic slip model of the Izmit
earthquake. The best fitting models suggest that the inferred
fault zones have a characteristic width of a few kilometers, depth
in excess of 10 km, and reductions in the effective shear modulus
of about a factor of 3 compared to the surrounding rocks. The
characteristic width of the best fitting fault zone models is
consistent with field observations along the North Anatolian Fault
system (Ambraseys, 1970). Our results are also in agreement with
InSAR observations of small-scale deformation on faults in the
Eastern California Shear Zone in response to the 1992 Landers
and 1999 Hector Mine earthquakes (Fialko et al., 2002; Fialko,
2004). The inferred compliant fault zones likely represent intense
damage and may be quite commonly associated with large crustal
faults. |
|
Julio de 2007
Capillary pressure as a unique function of electric permittivity
and water saturation
Authors: Willem-Jan Plug, Evert Slob et al
Link: Click Here
Abstract
The relation between capillary pressure (P c ) and interfacial
area has been investigated by measuring P c and the electric
permittivity at 100 kHz simultaneously as function of the water
saturation, (S w ). Drainage and imbibition experiments have
been conducted for sand-distilled water-gas (CO2/N2) systems.
The main
|
capillary
cycles and the scanning curves show hysteresis with the drainage
curves displaying higher values than the imbibition curves. The
100 kHz permittivity data also show hysteresis between drainage
and imbibition. Furthermore non-monotonic behavior is observed,
which is analogous to the interfacial area characteristics obtained
from network and micro-pore models. The permittivity behavior
is attributed to polarization of the gas-water and water-solid
interfaces. The permittivity hysteresis is provoked by the different
phase distributions and geometries. Our results show that P c
is a unique function of the permittivity and S w , and therefore
this work provides clear evidence that the permittivity is a measure
for the interfacial area. |
|
Julio de 2007
Magnetic hysteresis parameters and Day plot analysis to characterize
diagenetic alteration in gas hydrate-bearing sediments
Authors: S. A. Klapp, H. Klein et al
Link: Click Here
Abstract
Due to experimental difficulties grain size distributions of
gas hydrate crystallites are largely unknown in natural samples.
For the first time, we were able to determine grain size distributions
of six natural gas hydrates for samples retrieved from the Gulf
of Mexico and from Hydrate Ridge offshore Oregon from varying
depths. High-energy
|
synchrotron radiation
provides high photon fluxes as well as high penetration depth
and thus allows for investigation of bulk sediment samples. The
gas hydrate crystallites appear to be (log-) normally distributed
in the natural samples and to be of roughly globular shape. The
mean grain sizes are in the range from 300-600 µm with a
tendency for bigger grains to occur in greater depth, possibly
indicating a difference in the formation age. Laboratory produced
methane hydrate, starting from ice and aged for 3 weeks, shows
half a log-normal curve with a mean value of ~40 µm. This
one order-of-magnitude smaller grain sizes suggests that care
must be taken when transposing grain-size sensitive (petro-)physical
data from laboratory-made gas hydrates to natural settings. |
|
Julio de 2007
The instantaneous rate dependence in low temperature laboratory
rock friction and rock deformation experiments
Authors: N. M. Beeler, T. E. Tullis et al
Link: Click Here
Abstract
Earthquake occurrence probabilities that account for stress
transfer and time-dependent failure depend on the product of
the effective normal stress and a lab-derived dimensionless
coefficient a. This coefficient describes the instantaneous
dependence of fault strength on deformation rate, and determines
the duration of precursory slip. Although an instantaneous rate
dependence is observed for fracture, friction, crack growth,
and low temperature plasticity in laboratory experiments, the
physical origin of this effect during earthquake faulting is
obscure. We examine this rate dependence in laboratory experiments
on different rock types using a normalization scheme modified
from one
|
proposed
by Tullis and Weeks [1987]. We compare the instantaneous rate
dependence in rock friction with rate dependence measurements
from higher temperature dislocation glide experiments. The same
normalization scheme is used to compare rate dependence in friction
to rock fracture and to low-temperature crack growth tests. For
particular weak phyllosilicate minerals, the instantaneous friction
rate dependence is consistent with dislocation glide. In intact
rock failure tests, for each rock type considered, the instantaneous
rate dependence is the same size as for friction, suggesting a
common physical origin. During subcritical crack growth in strong
quartzofeldspathic and carbonate rock where glide is not possible,
the instantaneous rate dependence measured during failure or creep
tests at high stress has long been thought to be due to crack
growth; however, direct comparison between crack growth and friction
tests shows poor agreement. The crack growth rate dependence appears
to be higher than the rate dependence of friction and fracture
by a factor of two to three for all rock types considered. |
|
Julio de 2007
Permeability evolution in quartz fault gouges under hydrothermal
conditions
Authors: Silvio B. Giger, Eric Tenthorey et al
Link: Click Here
Abstract
The permeability (k) of fine-grained quartz aggregates were
measured in situ during hot pressing (HPing) experiments to
explore the evolution of fluid transport properties of fault
zones during the interseismic period. Experiments were conducted
at temperatures of 150°C and between 700 and 850°C,
with confining and pore water pressures of 250 and 150 MPa,
respectively. Significant permeability reduction was observed
between 700 and 850°C, with permeability reduction rates
(r = (1/t) ln (k to /k t )), ranging from approximately 6 ×
10-5 s-1 at 700°C to a maximum of approximately 7.4 ×
10-4 s-1 at 850°C. Permeability decreased exponentially
with time, and the permeability reduction rate increased with
|
increasing temperature,
increasing differential stress, and decreasing grain size. Analysis
of the permeability-porosity relationships indicates that permeability
in the simulated gouge at high temperature shuts off at a critical
porosity of 0.045 ± 0.004. The presence of microstructures,
such as grain interpenetration, grain shape truncation, arrays
of fluid inclusions, and development of quartz overgrowths on
grains, indicate that k reduction was controlled by dissolution-precipitation
creep processes. Extrapolation of the permeability reduction rates,
measured in this study, to temperatures typical of the continental
seismogenic regime highlights the strongly time-dependent nature
of permeability in natural fault wear products at depths of nucleation
of major earthquakes. Within the recurrence time of large earthquakes,
quartz-rich fault zones in the fluid-active midcrustal to lower
continental crustal regimes can evolve from high-permeability
conduits to low-permeability seals. Episodic changes in the fluid
transport properties of faults during the interseismic period
are likely to impact on the pore pressure evolution of fault wear
products. |
|
Julio
de 2007
Pronóstico de Tsunamis con foco en la zona de Perú-Chile
mediante la utilización de un algoritmo adaptativo inverso
Autores: Alejandro Sánchez y Kwok Fai Cheung
Link: Click Aquí
Abstract
El método inverso permite realizar pronósticos
de tsunamis mediante regresión de datos de nivel del
mar a campo cercano v/s un conjunto de mareogramas calculados
por unidad de deslizamiento de subfallas predefinidas. El presente
paper describe un mejoramiento realizado al método inverso,
basado en la resolución del tiempo ocupado en el movimiento
de la subfalla durante el proceso de inversión. Esto
proporciona adicionales grados de libertad en el análisis
de regresión y contabilización de la propagación
de la ruptura derivada de un terremoto tsunamigénico.
Se aplicó el método a la zona de subducción
ubicada entre Perú y Chile, donde los parámetros
asociados al foco sísmico son deducidos a partir de estudios
previos históricos y sismotectónicos. Un riguroso
análisis del tsunami de Antofagasta de 1995 demuestra
el incremento de la confiabilidad del algoritmo adaptativo inverso
y su aplicación para pronósticos a campo cercano
y campo lejano.
|
July 2007
Tsunami forecast using an adaptive inverse algorithm for
the Peru-Chile source region
Authors: Alejandro Sánchez and Kwok Fai Cheung
Link: Click Here
Abstract
The inverse method provides tsunami forecasts through regression
of near-field water-level data against a set of mareograms computed
for unit slip of predetermined subfaults. The present paper
describes an improvement to this inverse method by resolving
the timing of subfault movement in the inversion. This provides
additional degrees of freedom in the regression analysis and
accounts for rupture propagation and rise time of tsunamigenic
earthquakes. The method is implemented for the Peru-Chile subduction
zone, in which the seismic source parameters are deduced from
previous studies of seismotectonics and historical earthquakes.
A hindcast analysis of the 1995 Antofagasta-Chile tsunami demonstrates
the capability of the adaptive inverse algorithm in improving
the near-field inversion results and consequently the far-field
forecast.
|
|
Junio de 2007
Regional tectonics of the Coso geothermal area along the
intracontinental plate boundary in central eastern California:
Three-dimensional Vp and Vp/Vs models, spatial-temporal seismicity
patterns, and seismogenic deformation
Authors: Egill Hauksson and Jeffrey Unruh
Link: Click Here
Abstract
We synthesize the tectonics of the southern Walker Lane belt
and Coso Range in central eastern California using regional
earthquake data. First, we invert for three-dimensional models
of the V p and V p /V s structure of the upper and middle crust.
Using these models, we also determine three-dimensional V s
and Poisson's ratio models. The changes in seismic velocities
across the region are small, except for low velocities in sedimentary
basins and a ~2-km positive elevation of the basement velocities
(V p > 6 km/s) beneath the southern Sierra Nevada. Localized
low-V p and low-V s zones beneath the central Coso Range image
a geothermal reservoir at 0- to 3-km depth, as well as distinct
low-velocity anomalies in the depth range of ~8 to ~12 km. Because
the V p /V s has average crustal values within this broader
zone, we interpret the anomaly to indicate a zone of few percent
geothermal brines extending from 8- to 12-km depth. In addition,
an embedded highly localized poorly resolved zone (possibly
as small as 1 km3) of slightly above average V p /V s and higher
|
Poisson's ratio is a
tentative suggestion of a small volume percent of magma present
at depth of ~10 km. Second, we relocated the seismicity in the
region using absolute traveltimes and differential traveltimes
determined from waveform cross correlation. The relocated seismicity
forms several spatially clustered lineaments along the southeast
side of the Sierra Nevada and in the Indian Wells Valley and vicinity
of the Coso geothermal field, which coincide with mapped late
Quaternary faults in the region. The base of seismicity shallows
from a regional depth of about ~11 to ~5 km beneath the central
Coso Range, which we interpret as evidence for shallowing of the
brittle-ductile transition zone beneath the geothermal field.
In addition to abundant background seismicity, two large earthquake
swarms, located 5 to 8 km to the west of Coso, occurred in April
to May 1992 and May to June 2001. Two dual main shock-aftershock
sequences also occurred as follows: the 1994 sequence near Ridgecrest
and the later Coso earthquake sequence from late 1996 to early
1998, with the pairs of main shocks spaced 47 days and 16 months
apart, respectively. Kinematic analysis of the focal mechanisms
indicates that the crustal stress loading process varies across
the region. The low-V p anomaly, abundant seismicity, and crustal
thinning provide quantitative evidence for the Coso region being
an extensional releasing step over between two northwest-striking
dextral faults: The Little Lake and Airport Lake fault zones to
the south, and the Owens Valley fault to the north. |
|
Junio de 2007
Magnetic hysteresis parameters and Day plot analysis to characterize
diagenetic alteration in gas hydrate-bearing sediments
Authors: Randolph J. Enkin, Judith Baker et al
Link: Click Here
Abstract
The J meter coercivity spectrometer is a machine capable of
rapid and simple measurement of magnetic hysteresis, isothermal
remanence acquisition and magnetic viscosity of rocks and sediments.
The J meter was used to study a suite of samples collected from
strata in the gas hydrate-bearing JAPEX/JNOC/GSC Mallik 5L-38
well (69.5°N, 134.6°W) in the Mackenzie Delta of the
northwestern Canadian Arctic. The Day plot of magnetic hysteresis
ratios for these samples is exotic in that the points do not
plot along a
|
hyperbola
as is usually observed. Rather, they plot as a scatter which is
shown to contour into vertical slices using coercivity field (HC)
or saturation magnetization (JS), and horizontal slices using
the relative quantity of superparamagnetism (JSPM/JS). Optical
microscopy reveals that the magnetic minerals are detrital magnetite
and authigenic greigite. Greigite is dominant in sands which in
situ had >70% gas hydrate saturation and in silts in which
gas hydrate growth was blocked by insufficient porosity. We infer
that the silts were the accumulation sites for solutes which had
been excluded from the pore waters in neighboring coarser-grained
sediments during the course of gas hydrate formation. Consequently,
we conclude that magnetic properties are related to gas hydrate-related
processes, and as such, may have potential as a method of remote
sensing for gas hydrate deposits. |
|
Junio de 2007
Contemporary stress orientations in the Andean retroarc between
34°S and 39°S from borehole breakout analysis
Authors: C. Guzmán, E. Cristallini et al
Link: Click Here
Abstract
In this paper we present the results of the analysis of borehole
breakouts from 115 wells drilled within Neuquén Basin
in the Andean retroarc between 34° and 39°S (Argentina).
The first-order present-day stress orientation in the Andean
retroarc is expected to be mainly controlled by the plate boundary
forces (azimuth 80°) and the topographic forces (E-W). The
obtained maximum horizontal stress (SHmax)
|
hasa preferred trend
with a resultant direction of azimuth 88.7° and a 95% confidence
interval of 13.3° consistent with the expected trend. The
horizontal stress trajectory map achieved for this region shows
that the SHmax along the study area is not completely uniform.
To the north of Colorado River, the SHmax shows an ESE tendency
interpreted as significant influenced by the topographic forces.
To the south of Colorado River, SHmax has an ENE trend similar
to the expected based on plate boundary forces. To the southeast
of the region, a NE direction was found, probably showing a basement
structural control in the stress field geometry. The stress orientations
obtained for the whole region show that plate boundary forces,
drag basal, and topographic forces are strongly controlling the
stress direction distribution. |
|
Junio de 2007
Source analysis of the February 12th 2007, Mw 6.0 Horseshoe
earthquake: Implications for the 1755 Lisbon earthquake
Authors: Daniel Stich, Flor de Lis Mancilla et al
Link: Click Here
Abstract
While very large earthquakes are generally confined to subduction
zones, the SW Iberian margin -setting of the famous Mw 8.5-8.7,
1755 Lisbon tsunami earthquake- may be an exception to this
rule. Evidence for active subduction is not conclusive here,
but instead plate convergence in old oceanic lithosphere with
large brittle layer thickness can account for the occurrence
of great
|
earthquakes
along moderate-length faults. We estimate the source parameters
of the February 12th 2007, Horseshoe earthquake. Regional moment
tensor inversion yields an Mw 6.0, reverse to strike-slip faulting
source in the upper mantle. Modelling teleseismic, surface-reflected
body waves (pP, pwP, sP) indicates a source depth of 40 km beneath
the seafloor. Analysing apparent source time functions allows
identifying the preferred fault plane (strike N245°E/ dip
55°/ rake 50°), and estimating rupture area (53 km2) and
average slip (0.27 m). Scaling the source characteristics to the
size of the 1755 earthquake suggests a fault length of 230-315
km, being compatible with the length of mapped faults in the area.
|
|
Junio de 2007
Magma intrusion and deformation predictions: Sensitivities
to the Mogi assumptions
Author: Timothy Masterlark
Link: Click Here
Abstract
Mogi's (1958) magma intrusion model is widely used to predict
observed deformation of active volcanoes. The model simulates
a small spherical expansion source (SES) embedded in a homogeneous,
isotropic, Poisson-solid half-space (HIPSHS). This study computes
surface displacement due to SESs at depth using a combination
of analytical and finite element models (FEMs), for which the
HIPSHS assumptions are not required. Interferometric synthetic
aperture radar (InSAR) data suggest that Okmok volcano, Alaska,
subsided more than a meter owing to lava extrusion during its
1997 eruption. Inverse methods, which use an HIPSHS model, precisely
locate an SES at a depth of 3100
|
m beneath the center
of the caldera. A series of alternative model configurations relax
the combined suite of HIPSHS assumptions and sequentially isolate
the effects of each assumption. Forward modeling predictions are
relatively insensitive to topographic effects and layered elastic
properties, somewhat sensitive to anisotropic elastic properties
and Poisson's ratio, and very sensitive to the presence of weak
materials within a caldera. Inverse methods, combined with analytical
and FEM-generated impulse response functions, isolate the influence
of each HIPSHS assumption on SES depth and pressure estimations.
Results are particularly sensitive to a model configuration simulating
a weak caldera, for which the estimated SES depth (4500 m) is
significantly deeper than the estimate for the HIPSHS model. For
deformation data having high signal-to-noise ratios, such as the
co-eruption InSAR data for Okmok volcano, both forward and inverse
deformation prediction errors attributed to the Mogi (1958) assumptions
can greatly exceed observation uncertainties. |
|
Junio de 2007
Generation of VLF waves in the ionosphere with coherent dual-site
excitation
Authors: A. Y. Wong, E. Wei et al
Link: Click Here
Abstract
High Power Auroral Stimulation (HIPAS) Observatory and High
Frequency Active Auroral
|
Research
Program (HAARP) are two radiating facilities in the Arctic region
separated by a distance of the order of VLF wavelengths. The current-carrying
plasma in the E region of the ionosphere above each facility can
be modulated to radiate VLF waves via HF heating. Experiments
demonstrated that VLF waves can be coherently excited by these
two facilities through constructive interference that is sensitive
to the phase difference between these two sources. |
|
Junio de 2007
A global index earthquake approach to probabilistic assessment
of extremes
Authors: Eric M. Thompson, Laurie G. Baise et al
Link: Click Here
Abstract
This paper applies recent innovations in flood frequency analysis
to regional series of earthquake magnitudes in the global Centroid
Moment Tensor (CMT) catalog from 1976 to 2005. Probability plot
correlation coefficient hypothesis tests and L-moment goodness-of-fit
evaluations reveal that the Gumbel (GUM) distribution provides
a good approximation to the probability distribution function
(pdf) of series of annual maximum (AM) earthquake magnitudes.
Homogeneity tests based on the theory of L-moments further reveal
that broad regions of the
|
globe are homogeneous
in the sense that the AM observations of earthquake magnitudes
are well approximated by a GUM pdf with fixed upper moments. The
homogeneity of global earthquake data across broad tectonic environments
enables us to pool data into a regional pdf of earthquake magnitudes,
termed an index earthquake distribution. Research in hydrology
has shown that frequency analysis based on pooling of data using
an analogous index-flood method are much more accurate than frequency
analysis based on site or region specific data. The index earthquake
distribution is a dimensionless GUM distribution with fixed scale
parameter so only the mean earthquake magnitude must be estimated
for a region to define the frequency distribution of large earthquakes.
We show how the degree of spatial homogeneity of earthquake magnitudes
across broad tectonic environments can be exploited to yield improved
estimates of the risk posed by extreme earthquake magnitudes.
|
|
Junio de 2007
Effect of the incorporation of FeAlO3 into MgSiO3 perovskite
on the post-perovskite transition
Authors: Daisuke Nishio-Hamane, Kiyoshi Fujino et al
Link: Click Here
Abstract
Effect of the incorporation of FeAlO3 into MgSiO3 perovskite
on post-perovskite transition was investigated in Mg0.85Fe0.15Al0.15Si0.85O3
on the basis of high pressure and temperature in-situ X-ray
diffraction experiments using a laser
|
heated
diamond anvil cell. Results demonstrate that single perovskite
is stable up to 143 GPa and 2500 K and perovskite and post-perovskite
coexist at 157-162 GPa and 1600-2500 K for the pressure scales
by Tsuchiya (2003). Post-perovskite formed as single phase at
176-178 GPa and 1600-2600 K. The post-perovskite transition pressure
in Mg0.85Fe0.15Al0.15Si0.85O3 was much higher than that in MgSiO3.
The present experimental study indicates that the incorporation
of FeAlO3 component expands the stability region of perovskite
toward high pressure. The FeAlO3 concentration may strongly influence
the thickness of the D" layer at the lowermost of the lower
mantle. |
|
Junio de 2007
Interferogram formation in the presence of complex and large
deformation
Authors: Sang-Ho Yun, Howard Zebker et al
Link: Click Here
Abstract
Sierra Negra volcano in Isabela island, Galápagos, erupted
from October 22 to October 30 in 2005. During the 8 days of
eruption, the center of Sierra Negra's caldera subsided about
5.4 meters. Three hours prior to the onset of the eruption,
an earthquake (Mw 5.4) occurred, near the caldera. Because of
the large and complex phase gradient due to the huge subsidence
and
|
the earthquake, it is
difficult to form an interferogram inside the caldera that spans
the eruption. The deformation is so large and spatially variable
that the approximations used in existing InSAR software (ROI,
ROI_PAC, DORIS, GAMMA) cannot properly coregister SAR image pairs
spanning the eruption. We have developed here a two-step algorithm
that can form intra-caldera interferograms from these data. The
first step involves a "rubber-sheeting" SAR image coregistration.
In the second step we use range offset estimates to mitigate the
steep phase gradient. Using this new algorithm, we retrieve an
interferogram with the best coverage to date inside the caldera
of Sierra Negra. |
|
Junio de 2007
Risks due to X-ray flares during astronaut extravehicular
activity
Authors: David S. Smith and John M. Scalo
Link: Click Here
Abstract
Solar hard X-ray flares can expose astronauts on lunar and deep
space extravehicular activities (EVAs) to dangerous acute biological
doses. We combine calculations of radiative transfer through
shielding materials with subsequent transfer through tissue
to show that hazardous doses, taken as =0.1 Gy, should occur
with a probability of about 10% per 100 hours of accumulated
EVA
|
inside
the current spacesuit. The rapid onset and short duration of X-ray
flares and the lack of viable precursor events require strategies
for quick retreat, in contrast to solar proton events, which usually
take hours to deliver significant fluence and can often be anticipated
by flares or other light speed precursors. Our results contrast
with the view that only particle radiation poses dangers for human
space exploration. Heavy-element shields provide the most efficient
protection from X-ray flares, since X rays produce no significant
secondary radiation. We calculate doses due to X-ray flares behind
aluminum shields and estimate the required shield masses to accompany
EVA rovers. |
|
Junio de 2007
SPADE: A rock-crushing and sample-handling system developed
for Mars missions
Authors: Candice J. Hansen and David A. Paige et al
Link: Click Here
Abstract
A novel system has been developed to access and analyze the
interior of rocks on Mars by crushing rocks. A miniature rock
crusher has been prototyped along with a method for
|
distribution of the
fines and fragments produced by the rock crusher to inspection
and analysis instruments. The science goals and advantages of
this approach are substantial with respect to understanding the
geologic and climate history of Mars via the investigation of
its mineralogy and petrology. The system is described in detail,
and results of various performance metrics are reported. Engineering
considerations, constraints on functionality, fault tolerance,
and its previously-planned deployment on the 2009 Mars Science
Laboratory mission are described. |
|
Junio de 2007
Antarctic atmospheric temperature trend patterns from satellite
observations
Authors: Celeste M. Johanson and Qiang Fu
Link: Click Here
Abstract
Tropospheric temperatures in the Antarctic are retrieved by
linearly combining satellite-borne Microwave Sounding Unit (MSU)
channels 2 and 4 observations. We show good agreement between
satellite-inferred temperature trends and radiosonde observations.
It is illustrated that the Antarctic troposphere has cooled
in the summer and fall seasons since 1979, in agreement with
|
Thompson
and Solomon (2002). It is shown that significant tropospheric
warming prevails during Antarctic winters and springs, but we
also find significant winter cooling over half of East Antarctica.
We find the largest winter tropospheric warming of about 0.6 K/decade
for 1979-2005 between 120°W and 180°W. Homogeneous winter
tropospheric warming over Antarctica from the ERA-40 reanalysis
is not supported by the MSU observations. While MSU stratospheric
temperatures exhibit the expected large cooling during the spring
and summer seasons, we also find large stratospheric warming over
half the southern hemisphere high latitudes in the winter and
spring seasons. |
|
Junio de 2007
Electrical resistivity tomography monitoring of permafrost
in solid rock walls
Authors: Michael Krautblatter and Christian Hauck
Link: Click Here
Abstract
This article describes the first attempt to conduct electrical
resistivity tomography (ERT) in solid permafrost-affected rock
faces. Electrode design, instrument settings, and processing
routines capable of measuring under relevant conditions were
developed. Four transects, with NW, NE, east (E) and south (S)
aspects, were installed in solid rock faces between Matter Valley
and Turtmann Valley, Switzerland, at 3070-3150 m above sea level.
DC resistivity in the transects was measured repeatedly during
the summer and compared by applying a time-lapse inversion routine.
Resistivity values were calibrated using observed rock surface
conditions of thawed,
|
damp rocks (1-8 kO
m), deeply frozen rocks (18-80 kO m), and the transition from
damp thawed to frozen rocks (8-18 kO m). Mean surface layer resistivities
of transects respond to air temperatures below 0°C with a
rapid increase by a factor of 1.4 to 2.9 from values of 12-15
kO m to values of 22-31 kO m. Rock layers at depths of 2-6 m display
a general trend of resistivity decrease in summer, corresponding
to a persistent thawing process. Their response to anomalously
cool August temperatures occurs with a time lag of 2 to 4 weeks.
Only transects E, NE and NW display persistent, high-resistivity
permafrost bodies (>50 kO m) mostly at depths of 6-10 m. The
maximum thaw depth of a continuous thawing front above permafrost
is 6 m. However, the ERT results emphasize the role of heat transfer
by deep-reaching cleft water systems. Thus permafrost occurs in
lenses rather than layers. ERT provides rapid detection of ice
and water distribution in permafrost-affected bedrock. |
|
Junio de 2007
Dynamics of Strombolian ash plumes from thermal video: Motion,
morphology, and air entrainment
Author: Matthew R. Patrick
Link: Click Here
Abstract
Imaging volcanic plumes is essential to provide an observational
basis for understanding and modeling plume dynamics. During
June-July 2004, ~150 Strombolian ash plumes were imaged at Stromboli
volcano, Italy, with a forward looking infrared radiometer (FLIR)
thermal video camera (30 Hz). Of these, 25-80 plumes were suited
for different levels of quantitative analyses. In this study
some simple analyses are applied to constrain basic parameters
for the dynamics of Strombolian plumes during their initial
ascent (~130 m). Plume rise rates covered both gas thrust (>15
m s-1) and buoyant regimes (<15 m s-1), which in turn controlled
lateral spreading rates
|
and
air entrainment rates. The half angle of lateral spreading of
the plume front averaged 7.3(±1.6)° for gas thrust
regimes and 13.5(±1.6)° for buoyant regimes, equating
to mean air entrainment coefficients of 0.06-0.12 for gas thrust
regimes and 0.22(±0.03) for buoyant regimes. These factors
were also linked to plume morphologies, which included jets, starting
plumes and thermals. A "rooted thermal" form was observed
and presumed as an intermediary between starting plumes and discrete
thermals. Plume rise could be approximated by a power law dependence
with time. Rooted thermals spread and entrained air at rates approaching
those of a discrete thermal but rose at a rate similar to that
of a starting plume. Phenomena including helical motion and sedimentation
were visible in the FLIR imagery. These results demonstrate that
emergent plume behavior is progressive and highly transient. Furthermore,
this study offers empirical reinforcement that entrainment dynamics
are intrinsically different in (1) gas thrust versus buoyantly
driven regimes and (2) plume fronts versus steady plumes. |
|
Junio de 2007
Anomalous resistivity by fluctuation in the lower-hybrid
frequency range
Authors: Peter H. Yoon and Anthony T. Y. Lui et al
Link: Click Here
Abstract
The present paper discusses anomalous resistivity that arises
from electrostatic fluctuation in the lower-hybrid frequency
range. The implicit
|
physical situation is the vicinity of the reconnection
onset region. By employing locally uniform approximation, the
unmagnetized ion response and fully magnetized electron linear
response are computed analytically. On the basis of the response
functions, anomalous transport coefficients including the anomalous
resistivity are constructed. An order-of-magnitude analysis
reveals that the anomalous resistivity is much higher in magnitude
than the classical resistivity. Dependence of the anomalous
resistivity on plasma beta and the cross-field drift speed is
also discussed.
|
|
Junio de 2007
Subcritical creep compaction of quartz sand at diagenetic
conditions: Effects of water and grain size
Authors: F. M. Chester, J. S. Chester et al
Link: Click Here
Abstract
Compactional creep was measured for aggregates of quartz sand
that were subjected to an effective pressure of 34.5 MPa and
temperature of 150°C for times up to 6 months. The effects
of grain size and water on creep were determined by loading
quartz aggregates of 255 ± 60, 130 ±18, and 35
± 12 µm grain size under nominally dry, water vapor,
static liquid water, and flowing water conditions. All aggregates
displayed transient, decelerating creep; volume strain rates
|
as
low as 2 × 10-10 s-1 were achieved. Volume strain rate increases
systematically with decrease in grain size and increase in exposure
to water at comparable strain or time. Despite the fact that the
effective pressure applied was far less than the critical pressures
for short-term cataclastic compaction of the quartz aggregates,
grain-scale microstructures indicate that the underlying mechanism
of creep is crack growth. Creep rates are explained by subcritical
crack growth, as governed by water-silicate reactions at crack
tips, controlled by access of water at dry and vapor conditions,
and solute chemistry under static and flowing liquid water conditions.
Extrapolation of the experimental results to natural compaction
of quartz sand during burial and diagenesis over times of tens
of millions of years indicates that porosity loss through subcritical
cracking and grain rearrangement of medium-grained, porous, wet
quartz sands can reach ~10%. |
|
Junio de 2007
A fractal approach to the recession of spring hydrographs
Authors: S. Hergarten, S. Birk et al
Link: Click Here
Abstract
We present an analytical theory for the recession of spring
hydrographs after precipitation events, focusing on short time
scales. While other
|
approaches mostly predict
an exponential decay over long times, we obtain a power-law decrease
of discharge at short times. Similar to established well functions,
the power-law exponent is related to the spatial dimension. In
extension to these models, our approach relates the exponent to
the dimension of a highly permeable conduit system. This dimension
may take non-integer values, and we provide a geometric realization
for fracture systems with fractal dimensions between two and three.
|
|
Junio de 2007
Extreme runup from the 17 July 2006 Java tsunami
Authors: Hermann M. Fritz, Widjo Kongko et al
Link: Click Here
Abstract
The 17 July 2006 magnitude Mw 7.8 earthquake off the south coast
of western Java, Indonesia, generated a tsunami that effected
over 300 km of coastline and killed more than 600 people, with
locally focused runup heights exceeding 20 m. This slow earthquake
was hardly felt on Java, and wind waves breaking masked any
preceding
|
withdrawal
of the water from the shoreline, making this tsunami difficult
to detect before impact. An International Tsunami Survey Team
was deployed within one week and the investigation covered more
than 600 km of coastline. Measured tsunami heights and run-up
distributions were uniform at 5 to 7 m along 200 km of coast;
however there was a pronounced peak on the south coast of Nusa
Kambangan, where the tsunami impact carved a sharp trimline in
a forest at elevations up to 21 m and 1 km inland. Local flow
depth exceeded 8 m along the elevated coastal plain between the
beach and the hill slope. We infer that the focused tsunami and
runup heights on the island suggest a possible local submarine
slump or mass movement. |
|
Junio de 2007
An analysis of the scale heights in the lower topside ionosphere
based on the Arecibo incoherent scatter radar measurements
Authors: Libo Liu, Huijun Le et al
Link: Click Here
Abstract
We statistically analyze the ionospheric scale heights in the
lower topside ionosphere based on the electron density (N e)
and temperature profiles observed from the incoherent scatter
radar (ISR) at Arecibo (293.2°E, 18.3°N), Puerto Rico.
In this study, a database containing the Arecibo ISR observations
from 1966 to 2002 has been used in order to investigate the
diurnal and seasonal variations and solar activity dependences
of the
|
vertical scale height
(VSH), which is deduced from the electron concentration profiles
defined as the value of -dh/d(ln(N e)), and the effective scale
height (H m), which is defined as the scale height in the Chapman-a
function to approximate the N e profiles. As a measure of the
slope of the height profiles of the topside electron density,
the derived VSH and H m show marked diurnal and seasonal variations
and solar activity dependences. Their features are discussed in
terms of thermal structures in the lower topside ionosphere. We
also investigate the quantitative relationships between H m, VSH,
and plasma scale height (H p) over Arecibo. The similarities and
differences in these scale heights are discussed. Results suggest
that both the contributions from topside temperature structure
and diffusion processes can also greatly control VSH and H m through
changing the profile shape. |
|
Junio
de 2007
Un Estudio
numérico de las resonancias de Schumann en Marte mediante
la utilización del método FDTD
Autores: A. Soriano, E.
A. Navarro et al
Link: Click Aquí
Abstract
Es posible que las ondas electromagnéticas que naturalmente
se forman cerca de la superficie de Marte debido a descargas
electróstaticas originadas en tormentas de arena ("demonios
de arena") o por actividad geológica, queden atrapadas
en una cavidad resonante formada entre la superficie y la ionósfera
inferior, del mismo modo como ocurre en la Tierra, amplificando
de este modo las resonancias de Schumann.
En el paper se aplica la técnica FDTD (Diferencias Finitas
en el Dominio del Tiempo) para modelar la atmósfera de
Marte con el objetivo de determinar las frecuencias resonantes
de Schumann y la aparición de campos electromagnéticos
naturales en el rango ELF (Frecuencias Extremadamente Bajas).
Se proporciona una herramienta matemática para analizar
la conductividad eléctrica basal de la atmósfera
marciana, mediante la obtención de las frecuencias resonantes
de Schumann y su dependencia de diversas fuentes de ionización
junto con la actividad solar.
También se estudia el efecto que tiene el potenciamiento
local de la conductividad basal a gran altitud sobre las resonancias
de Schumann.
A partir de los resultados se elabora un criterio para fijar
un límite superior para el modelo numérico.
|
Junio de 2007
A numerical study of the Schumann resonances in Mars with
the FDTD method
Authors: A. Soriano, E. A. Navarro et al
Link: Click Here
Abstract
Natural electromagnetic waves generated near the surface by
electrostatic discharges in dust storms (dust devils) or by
geological activity could be trapped in the resonant cavity
formed by the surface and lower ionosphere of Mars, as it occurs
on Earth giving rise to Schumann resonances. The finite difference
time-domain technique (FDTD) is applied to model the atmosphere
of Mars in order to determine Schumann resonant frequencies,
and natural electromagnetic fields at the extremely low frequency
range (ELF). A numerical tool is provided to analyze the electrical
conductivity profile of the Martian atmosphere, with the aim
of obtaining Schumann resonance frequencies and their dependence
on various ionization sources and solar activity. The effect
of local enhancement of conductivity profile at several altitudes
is also studied to understand its consequences on Schumann resonances.
A criterion to fix the upper limit of the numerical model is
established from these results.
|
|
Mayo de 2007
Numerical simulations of coronal hole-associated neutral
solar wind as expected at the Solar Orbiter position
Authors: R. D'Amicis, S. Orsini et al
Link: Click Here
Abstract
Neutral hydrogen is indicative of the behavior of the main solar
wind component formed by protons out to at least 5 R . In fact,
beyond this distance, the characteristic time for charge exchange
between hydrogen atoms and protons becomes larger than the coronal
expansion timescale, causing the neutrals to decouple from the
charged solar wind. The mean free path of the neutral component
rapidly increases with the radial distance so that neutrals
generated at heliocentric distances =24 R fly unperturbed and
eventually are detected by Solar Orbiter (perihelion at approximately
48 R ), since their mean free path
|
is long enough to let
neutrals reach the neutral solar wind detector. However, the computation
of the differential flux shows that the bulk of the flux detected
at the Solar Orbiter vantage point mainly comes from about 9 R
. Neutrals retain information on the three-dimensional distribution
of hydrogen at the level where they are generated as the proton
velocity distribution is frozen within the generated neutrals
and transferred up to the Solar Orbiter position. In the present
study, we report our preliminary results from our simulation of
the neutral solar wind distribution as predicted at the Solar
Orbiter position and considering the evolution of a coronal hole-emerging
solar wind whose major parameters are estimated by the Solar and
Heliospheric Observatory (SOHO) Ultraviolet Coronagraph Spectrometer
(UVCS) experiment. The synergy between corona remote sensing and
in situ neutral particle observations will enable us to infer
the degree of anisotropy, if any, in the neutral and charged coronal
hydrogen close to the Sun. |
|
Mayo de 2007
Water vapor diffusion in Mars subsurface environments
Authors: Troy L. Hudson, Oded Aharonson et al
Link: Click Here
Abstract
The diffusion coefficient of water vapor in unconsolidated porous
media is measured for various soil simulants at Mars-like pressures
and subzero temperatures. An experimental chamber which simultaneously
reproduces a low-pressure, low-temperature, and low-humidity
environment is used to monitor water flux from an ice source
through a porous diffusion barrier. Experiments are performed
on four types of simulants: 40-70 µm glass beads, sintered
glass filter disks, 1-3 µm dust (both loose and packed),
and JSC Mars-
|
1.
A theoretical framework is presented that applies to environments
that are not necessarily isothermal or isobaric. For most of our
samples, we find diffusion coefficients in the range of 2.8 to
5.4 cm2 s-1 at 600 Pascal and 260 K. This range becomes 1.9-4.7
cm2 s-1 when extrapolated to a Mars-like temperature of 200 K.
Our preferred value for JSC Mars-1 at 600 Pa and 200 K is 3.7
± 0.5 cm2 s-1. The tortuosities of the glass beads is about
1.8. Packed dust displays a lower mean diffusion coefficient of
0.38 ± 0.26 cm2 s-1, which can be attributed to transition
to the Knudsen regime where molecular collisions with the pore
walls dominate. Values for the diffusion coefficient and the variation
of the diffusion coefficient with pressure are well matched by
existing models. The survival of shallow subsurface ice on Mars
and the providence of diffusion barriers are considered in light
of these measurements. |
|
Mayo de 2007
Morphology, chemistry, and spectral properties of Hawaiian
rock coatings and implications for Mars
Authors: Michelle E. Minitti, Catherine M. Weitz et al
Link: Click Here
Abstract
We studied coatings on five glass-rich basalts from the Big
Island of Hawaii. The coatings are characterized by complex
morphologies and their thicknesses range from 3 to 80 µm.
Coating chemistries are predominantly hydrated silica with minor
amounts of Fe-, Ti-, and S-bearing materials. Visible, near
infrared, and thermal infrared spectra of the coatings demonstrate
that coatings as thin as 3 µm mask the spectral character
of the substrate basalt. The Fe-, Ti-, and S-bearing components
control the VNIR coating
|
spectra. The hydrated
silica component of the coatings dominates the thermal infrared
coating spectra. The coating chemistries are consistent with leaching
and/or dissolution of basalt glass or tephra in aqueous, acidic,
and oxidizing conditions and subsequent precipitation of insoluble
phases. Similar alteration conditions are thought to have occurred
on Mars, making formation of such coatings on Martian lithologies
feasible. Given the capabilities of various Mars missions, if
coatings like those of this study were present on Martian lithologies,
their chemical and/or spectral signatures would be detectable.
Chemical and spectral data from thin (=10 µm), Fe- and Ti-bearing
coatings are consistent with phases capable of explaining the
high-SiO2 component of Type 2 surfaces previously identified in
the Thermal Emission Spectrometer data set and are potentially
consistent with spectral data from the Mars Exploration Rovers.
|
|
Mayo de 2007
Power variations of Schumann resonances related to El Niño
and La Niña phenomena
Authors: Heng Yang and Victor P. Pasko
Link: Click Here
Abstract
A three dimensional Finite Difference Time Domain (FDTD) model
of the Earth-ionosphere cavity with the realistic conductivity
profile is employed to study the intensity variations of Schumann
resonances (SR) associated with the El Niño and La Niña
phenomena. Comparison of the results derived from our FDTD model
and the
|
previous
studies by other authors on related subjects shows that the intensity
of the Schumann resonances varies with the spatial shifts of the
thunderstorm regions under El Niño and La Niña conditions.
Due to the different spatial field distributions of SR electrical
and magnetic components in the Earth-ionosphere cavity, the different
power variation patterns are clearly observed in the electrical
and magnetic components with the motion of the thunderstorm center
in our FDTD results. A new method is proposed to detect the shifts
of the thunderstorm regions related to the El Niño and
La Niña phenomena using the combination of both electrical
and magnetic components of Schumann resonances at a single station.
|
|
Mayo de 2007
Geomagnetic field modeling from satellite attitude control
magnetometer measurements
Authors: Jeong Woo Kim, Jong Sun Hwang et al
Link: Click Here
Abstract
To demonstrate the utility of satellite attitude control magnetometer
measurements for mapping main field variations, we analyzed
the three-axis magnetometer (TAM) measurements that provide
attitude control for the KOMPSAT-1 satellite. Initial processing
involved transforming the TAM's magnetic measurements from the
Earth-Centered Inertial coordinates (ECI) to the Earth-Centered
Earth-Fixed coordinates (ECEF) and then to spherical coordinates.
The magnetic field of the satellite body produces symmetric
|
signatures in the ascending
and descending orbital measurements and thus can be readily removed.
Spectral correlation filtering of the orbital observations helped
to eliminate the dynamic external field and solar activity noise
components. The ascending and descending data were then spectrally
reconstructed to estimate the total magnetic field with minimum
track line noise. Correlation coefficients of 0.97 and 0.96 mark
the correlation of the KOMPSAT-1 total geomagnetic intensity map
with the Ørsted and IGRF2000 core magnetic field models,
respectively. Power spectra from Gauss coefficients of KOMPSAT-1
model showed closeness with the models from Ørsted data
and IGRF2000 model. The spherical harmonic coefficients calculated
from the KOMPSAT-1 model by conjugate gradient inversion are strongly
coherent with the Ørsted and IGRF2000 coefficients through
degree 9. |
|
Mayo de 2007
Influences of urban fabric on pyroclastic density currents
at Pompeii (Italy): 1. Flow direction and deposition
Authors: L. Gurioli, E. Zanella et al
Link: Click Here
Abstract
To assess ways in which the products of explosive eruptions
interact with human settlements, we performed volcanological
and rock magnetic analyses on the deposits of the A.D. 79 eruption
at the Pompeii excavations (Italy). During this eruption the
Roman town of Pompeii was covered by 2.5 m of fallout pumice
and then partially destroyed by pyroclastic density currents
(PDCs). Anisotropy of magnetic susceptibility measurements performed
on the fine matrix of the deposits allowed the quantification
of the variations in flow direction and emplacement mechanisms
of the parental PDCs that entered
|
the town. These results, integrated with volcanological
field investigations, revealed that the presence of buildings,
still protruding through the fallout deposits, strongly affected
the distribution and accumulation of the erupted products. All
of the PDCs that entered the town, even the most dilute ones,
were density stratified currents in which interaction with the
urban fabric occurred in the lower part of the current. The
degree of interaction varied mainly as a function of obstacle
height and density stratification within the current. For examples,
the lower part of the EU4pf current left deposits up to 3 m
thick and was able to interact with 2- to 4-m-high obstacles.
However, a decrease in thickness and grain size of the deposits
across the town indicates that even though the upper portion
of the current was able to decouple from the lower portion,
enabling it to flow over the town, it was not able to fully
restore the sediment supply to the lower portion in order to
maintain the deposition observed upon entry into the town.
|
|
Mayo de 2007
Improving the precision of high-rate GPS
Authors: Kristine M. Larson, Andria Bilich et al
Link: Click Here
Abstract
In order to improve the accuracy of high-rate (1 Hz) displacements
for geophysical applications such as seismology it is important
to reduce systematic errors at seismic frequencies. One such
GPS error source that overlaps with seismic frequencies and
is not currently modeled is multipath. This study investigates
the frequencies and repetition of multipath in high-rate GPS
time series in order to maximize the effectiveness of
|
techniques relying upon
the geometric repeatability of GPS satellite orbits. The implementation
of the aspect repeat time adjustment (ARTA) method described here
uses GPS position time series to estimate time-varying and site-dependent
shifts. As demonstrated for high-rate GPS sites in southern California
this technique significantly reduces positioning noise at periods
from 20 to 1000 s. For a 12-hour time series, ARTA methods improve
the standard deviation of the north component from 8.2 to 5.1
mm and the east component from 6.3 to 4.0 mm. After applying ARTA
corrections, common mode errors are removed by stacking. This
method further improves the standard deviations to 3.0 and 2.6
mm for the north and east components, respectively. |
|
Mayo de 2007
Finite-element modeling of subglacial cavities and related
friction law
Authors: O. Gagliardini, D. Cohen et al
Link: Click Here
Abstract
Sliding velocity and basal drag are strongly influenced by changes
in subglacial water pressure or subglacial water storage associated
with opening and closing of water cavities in the lee of bedrock
obstacles. To better understand this influence, finite-element
simulations of ice flowing past bedrock obstacles with cavity
formation are carried out for different synthetic periodic bedrock
shapes. In the numerical model, the cavity roof is treated as
an unknown free
|
surface
and is part of the solution. As an improvement over earlier studies,
the cases of nonlinear ice rheology and infinite bedrock slopes
are treated. Our results show that the relationship between basal
drag and sliding velocity, the friction law, can be easily extended
from linear to nonlinear ice rheology and is bounded even for
bedrocks with locally infinite slopes. Combining our results with
earlier works by others, a phenomenological friction law is proposed
that includes three independent parameters that depend only on
the bedrock geometry. This formulation yields an upper bound of
the basal drag for finite sliding velocity and a decrease in the
basal drag at low effective pressure or high velocity. This law
should dramatically alter results of models of temperate glaciers
and should also have important repercussions on models of glacier
surges and fast glacier flows. |
|
Mayo
de 2007
Thermal technique for controlling hydraulic fractures
Authors: Ruiting Wu, Leonid N. Germanovich et al
Link: Click Here
Abstract
A simple technique is introduced to control hydraulic fractures
in laboratory samples. This is achieved by heating or cooling
samples prior to
|
injecting the fracturing
liquid, so that the induced thermal stresses govern the fracture
orientation. We developed a simple theoretical model to parameterize
experiments in laboratory settings and for materials that are
different from ours. We also illustrate the utility of the technique
with two examples: (1) mixed mode I+III hydraulic fracture propagation
and (2) visualization of the fluid flow in the created fracture.
In some cases, a similar technique may also be applicable for
controlling the orientation of hydraulic fractures in field conditions.
|
|
Mayo
de 2007
Precipitation of southwestern Canada: Wavelet, scaling, multifractal
analysis, and teleconnection to climate anomalies
Authors: Thian Yew Gan, Adam Kenea Gobena et al
Link: Click Here
Abstract
Using wavelets, statistically significant interannual and interdecadal
oscillations that occurred haphazardly have been detected in
southwestern (SW) Canadian seasonal precipitation anomalies.
At interannual scales, station precipitation anomalies show
unstable relations with large-scale climate anomalies such as
the El Niño-Southern Oscillation (ENSO), Pacific Decadal
Oscillation (PDO), Pacific/North America (PNA), East Pacific
(EP) and West Pacific (WP) patterns, and the Central North Pacific
(CNP) index. Not all significant precipitation activities could
be matched by similar activities in one or more climate anomalies
considered. Inconsistent wavelet coherence and phase difference
between the leading principal components (PC) of regional precipitation
anomalies and climate indices as well as weak Pearson's correlations
between band-passed precipitation PCs and climate indices for
the 2-3 year and 3-8 year scales provide supporting evidence
for unstable
|
precipitation climate relationships at the interannual scale.
On the other hand, interdecadal precipitation variability is mainly
associated with low-frequency variability in CNP, PDO and ENSO.
Composite analysis of winter precipitation shows that ENSO, PDO,
PNA and WP offer better separation of positive and negative precipitation
anomalies than EP and CNP. However, the effect of ENSO is found
to be stronger than the others. Precipitation power spectrum plots
mostly reveal two linear decay regions of different slopes separated
by a breakpoint located approximately at 20 to 30 days, while
empirical probability plots reveal power law behavior and hyperbolic
intermittency in these data, whose correlation dimensions (D 2)
are between 8 and 9. Different multifractal behaviors are observed
among stations because the amount of different rainfall generating
mechanisms vary from station to station, as reflected by the haphazard
nature of oscillations detected in most precipitation data. Although
the leading PCs of winter regional precipitation show modest correlations
at zero- to three-season lead times with ENSO and PDO indices,
the high D2 values and absence of consistent interannual precipitation
activities suggest that prediction of SW Canadian seasonal precipitation
by teleconnection with climate indices is likely limited. Adding
other predictor fields such as sea surface temperature and/or
sea level pressure may be useful. |
|
Mayo de 2007
Meteoric smoke concentration in the Vostok ice core estimated
from superparamagnetic relaxation and some consequences for
estimates of Earth accretion rate
Authors: Luca Lanci, Dennis V. Kent et al
Link: Click Here
Abstract
We measured the magnetization of glacial and interglacial ice
from the Vostok core to estimate the meteoric smoke concentration
in Antarctic ice.
|
We have found that,
within the uncertainty of the method, the smoke concentration
in ice in Antarctica is equivalent to that previously measured
in Greenland ice. The virtually identical smoke concentrations
despite the different ice accumulation rates in Greenland and
Antarctica suggest that wet deposition is the main deposition
mechanism for such ultra-small particles. Given the typical scavenging
ratios for atmospheric aerosols, this would imply that previous
estimates of accretion rate based on dry deposition are likely
to be appreciably overestimated. |
|
Mayo de 2007
Role of topography in isotherm perturbation: Apatite (U-Th)/He
and fission track results from the Malta tunnel, Tauern Window,
Austria
Authors: Jurgen P. T. Foeken, Cristina Persanoet al
Link: Click Here
Abstract
Apatite (U-Th)/He and fission track age determinations of samples
from a 20-km-long, near-horizontal tunnel in the Hochalm-Ankogel
Dome (eastern Tauern Window, Austria) are presented in order
to determine the role of (paleo)-topography in perturbing isotherms
in the shallow crust. Apatite fission track ages (26-8 Ma) show
no systematic correlation with distance along the tunnel or
elevation. Two age components in the fission track data indicate
cooling through ~120°C at approximately 20 Ma and ~80°C
at approximately 6 Ma. Surface and
|
tunnel
(U-Th)/He ages (17-9 Ma and 13-5 Ma, respectively) are consistently
younger than the equivalent fission track ages. (U-Th)/He ages
vary systematically along the tunnel with older ages at the northern
and southern tunnel portals and younger ages in the central section.
Geological factors (faulting, lithology) appear to have had little
effect on this age distribution. The (U-Th)/He age pattern is
inconsistent with rock cooling underneath the present-day Hochalm-Ankogel
Dome topography. The age minimum is interpreted to coincide with
a paleotopographic maximum in the Hochalm-Ankogel Dome and suggests
that the 40-60°C isotherms were warped sufficiently to affect
apatite (U-Th)/He ages. The tunnel apatite (U-Th)/He ages match
synthetic He ages derived from a two-dimensional numerical model
of landscape evolution. Integrating the thermal model results
with the data allows the relief development to be constrained
and we calculate that the present-day Hochalm-Ankogel Dome topography
was formed at 7 to 10 Ma. |
|
Mayo de 2007
Parkfield earthquakes: Characteristic or complementary?
Authors: Susana Custódio and Ralph J. Archuleta
Link: Click Here
Abstract
We model the two most recent M w ~ 6 Parkfield, California,
earthquakes, which occurred in 1966 and 2004, from a nonlinear
global inversion of near-fault strong motion seismograms. Our
rupture models are characterized by spatially variable slip
amplitude and rake, rupture velocity, and risetime. The rupture
models indicate that the two earthquakes generated slip in regions
of the fault that are not identical, as earlier suggested. Given
the sparse seismic data set available for
|
the 1966 earthquake,
we conduct a series of tests to verify our results: (1) we perform
synthetic tests in order to study the resolution of the 1966 seismic
data set; (2) we perform an inversion of the 2004 earthquake using
a data set equivalent to the 1966 earthquake; and (3) we model
the 1966 data set under the a priori assumption that it was similar
to the 2004 earthquake. All of the tests, as well as independent
observations, indicate that slip during the 1966 and 2004 Parkfield
earthquakes occurred in different regions of the fault. This result
implies that regions of a fault that are frictionally locked may
remain locked even during a main shock (moderate-size earthquake).
In this scenario, large earthquakes occur when all the locked
regions of a fault are "synchronized" and ready to slip
at the same time. |
|
Mayo de 2007
Scrubbing process and chemical equilibria controlling the
composition of light hydrocarbons in natural gas discharges:
An example from the geothermal fields of El Salvador
Authors: F. Tassi, O. Vaselli et al
Link: Click Here
Abstract
The compositional features of fluids from both fumarolic discharges
and productive geothermal wells of Ahuachapan-Chipilapa, Berlin-Chinameca,
and San Vicente geothermal systems (El Salvador) are described
and discussed in order to investigate the complex geochemical
interactions involving geothermal fluids within the shallowest
part of the hydrothermal circulation pathways. Our results highlight
that secondary processes are able to strongly affect and modify
the chemical characteristics of geothermal gases once they discharge
to the surface as natural manifestations, mainly in relation
to the chemical-physical properties of each gas species. The
effects of both gas dissolution in shallow aquifers
|
and
gas-water-rock chemical interactions on gas discharge composition
make it difficult to get a correct evaluation of the thermodynamic
conditions that characterize the geothermal reservoirs by applying
the common geoindicators based on the chemical equilibria of the
H2O-CO2-H2-CH4-CO system. Differently, the composition of the
C1-C2-C3 alkanes and the C3 and C4 alkane-alkene pair, established
within the geothermal reservoirs under the control of chemical
reactions, remains stable in samples collected from discharging
gas vents. These results suggest that the relative abundances
of hydrocarbons characterized by similar structure and molecular
size seem to be mainly regulated by the diffusion velocity of
gases through the liquid-dominated system. Therefore the chemical
features of the light organic gas fraction of naturally discharging
fluids can be successfully utilized for the evaluation of geothermal
reservoir temperatures and redox conditions, providing useful
indications in terms of geothermal exploration and exploitation.
On this basis, the distribution, speciation, and relative abundances
of light hydrocarbons can also be considered highly promising
in geochemical monitoring of active volcanic systems. |
|
Mayo de 2007
Geometry and P and S velocity structure of the "African
Anomaly"
Authors: Yi Wang and Lianxing Wen
Link: Click Here
Abstract
We constrain the geometry and P and S velocity structure of
a low-velocity anomaly in the lower mantle beneath southern
Africa (we term it the "African Anomaly") on the basis
of forward traveltime and waveform modeling of seismic data
sampling a great arc across the anomaly from the East Pacific
Rise to the Japan Sea. Our collected data set consists of direct
S, direct P, Sdiff, ScS, PcP, SKS, and SKKS phases recorded
by three temporary broadband PASSCAL seismic arrays deployed
in Africa between 1994 and 2002, the Tanzania seismic array
(1994-1995), the Kaapvaal seismic array (1997-1999), and the
Ethiopia/Kenya seismic array (2000-2002) for earthquakes occurring
in the East Pacific Rise, Drake Passage, South Sandwich islands,
Iran, Hindu Kush, Xinjiang, and the Japan Sea. The seismic data
provide excellent sampling of the African Anomaly in the lower
mantle along the
|
specific great arc.
In order to accurately account for the contributions from the
African Anomaly, we relocate all the events using a global seismic
shear velocity tomographic model and seismic data recorded by
the Global Seismographic Network and correct for the contributions
from the seismic heterogeneities outside the African Anomaly.
The seismic observations suggest that the African Anomaly locally
extends 1300 km above the core-mantle boundary beneath southern
Africa (around -25°N, 27°E) and exhibits a "bell-like"
geometry with both the southwestern and the northeastern flanks
dipping toward its center with the lateral dimension of the anomaly
increasing with depth. The base is about 4000 km wide extending
broadly in both the southwestward and the northeastward directions.
The seismic data can best be explained by a shear velocity structure
with average velocity decreases of -5% in the base and -2% to
-3% in the mid-lower mantle above the base, and a compressional
velocity structure with a uniform S to P velocity perturbation
ratio of 3:1 for the entire African Anomaly. These geometric and
velocity features suggest that the mid-lower mantle portion of
the African Anomaly is an integral component of the very low velocity
province and the African Anomaly is compositionally distinct and
geologically stable. |
|
Mayo de 2007
Seismic velocity reductions caused by the 2003 Tokachi-Oki
earthquake
Authors: Justin L. Rubinstein, Naoki Uchida et al
Link: Click Here
Abstract
We use four repeating earthquake sequences located near Hokkaido
to identify velocity changes caused by the M w 8.0 2003 Tokachi-Oki
earthquake. Using a moving window cross-correlation technique,
we identify delays in the arrival time of seismic waves that
accumulate linearly with time into the seismogram. This behavior
is indicative of multiple scattering within
|
a
medium where the seismic velocity has been reduced. For all of
our earthquake-receiver geometries, we find evidence of significant
velocity reductions close to the receiver. The correlation of
the size of the velocity reductions with both strong shaking and
site characteristics suggests that these velocity reductions are
caused by damage to near-surface materials created by nonlinear
strong ground motion. For earthquake/receiver geometries where
the seismic waves cross the Tokachi-Oki rupture zone, we identify
particularly large reductions in velocity as a result of the earthquake.
For these geometries, we believe that the rupture zone of the
Tokachi-Oki earthquake or the shallow crust above it represents
a second region where seismic velocities are reduced as a result
of the main shock. |
|
Mayo de 2007
From steep-slope volcano to flat caldera floor
Authors: Stéphanie Barde-Cabusson and Olivier Merle
Link: Click Here
Abstract
Most laboratory experiments of caldera collapse have dealt with
reservoir emptying below a flat-lying overburden without an
overlying analogue volcanic edifice on top. The overload and
the role of topography are then neglected so that the final
|
flat
floor within the caldera is directly linked to the initial one.
In addition, caldera subsidence is commonly attributed to the
collapse of the top of a magma chamber linked to eruptions delivering
large amounts of volcanic products. Analogue experiments show
that the deformation of a weak clay-rich core resulting from the
hydrothermal alteration in a volcanic edifice can, in certain
conditions, reproduce the structures of a caldera. In particular,
it is a way to explain the flat floor of a caldera when resurfacing
resulting from new eruptions or destructive processes seems unlikely.
|
|
Mayo de
2007
Outer rise stress changes related to the subduction of the
Juan Fernandez Ridge, central Chile
Authors: V. Clouard , J. Campos et al
Link: Click Here
Abstract
Although outer rise seismicity is less common than interplate
seismicity in subduction zones, a significant level of seismicity
has occurred between the Nazca trench and Juan Fernandez Ridge,
in central Chile, during the past 20 years. We first study the
9 April 2001 (Mw = 7.0) event and determine its focal mechanism,
depth, and source time function by body-waveform inversion from
teleseismic broadband data. The results indicate tensional faulting
in the upper part of the mechanical lithosphere. Its strike
(41°) is similar
|
to
those observed in events down dip of the slab at about 100 km
depth, which could indicate that these earthquakes occur in preexisting
structures formed at the trench. Compressive outer rise events
have also occurred during the 1980s in front of the rupture zone
of the 1985 Mw 7.8 Valparaíso Earthquake. To understand
their relation with the state of stress of the lithosphere, we
construct yield stress envelopes of the oceanic lithosphere, including
static and dynamic stresses. Dynamic stresses are due either to
slab pull, ridge push, resistive, and drag forces. We explain
the sequence of compressive and tensional events by the accumulation
of stress prior to 1985 when the subduction is assumed to be locked
and after by the unlockage of the subduction by the Valparaíso
interplate event. The yield stress envelope analysis enables us
to quantify the accumulation of compressive forces before 1985
and the tensional force after. |
