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Septiembre de 2006
Sulfur's impact on core evolution and magnetic field generation
on Ganymede
Authors: Steven A. Hauck II ,
Jonathan M. Aurnou et al
Link: Click here
Abstract
Analysis of the melting relationships of potential core forming
materials in Ganymede indicate that fluid motions, a requirement
for a dynamo origin for the satellite's magnetic field, may
be driven, in part, either by iron (Fe) "snow" forming
below the core-mantle boundary or solid iron sulfide (FeS)
floating upward from the deep core. Eutectic melting temperatures
and eutectic sulfur contents in the binary Fe-FeS system decrease
with increasing pressure within the interval of core pressures
on Ganymede (<14 GPa). Comparison of melting temperatures
to adiabatic temperature
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gradients in the core
suggests that solid iron is thermodynamically stable at shallow
levels for bulk core compositions more iron-rich than eutectic
(i.e., <21 wt % S). Calculations based on high-pressure
solid-liquid phase relationships in the Fe-FeS system indicate
that iron snow or floatation of solid iron sulfide, depending
on whether the core composition is more or less iron-rich
than eutectic, is an inevitable consequence of cooling Ganymede's
core. These results are robust over a wide range of plausible
three-layer internal structures and thermal evolution scenarios.
For precipitation regimes that include Fe-snow, we present
scaling arguments that give typical Rossby and magnetic Reynolds
numbers consistent with dynamo action occurring in Ganymede's
core. Furthermore, by applying recently derived scaling relationships
relating magnetic field strength to buoyancy flux, we obtain
estimates of surface magnetic field strength comparable with
observed values.
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Septiembre de 2006
Single-crystal elasticity and sound velocities of (Mg0.94Fe0.06)O
ferropericlase to 20 GPa
Authors: Jennifer M. Jackson
, Stanislav V. Sinogeikin et al
Link: Click here
Abstract
The single-crystal elastic properties of high-spin (Mg0.94Fe0.06)O
ferropericlase were measured by Brillouin spectroscopy on
a sample compressed to 20 GPa with diamond anvil cells using
methanol-ethanol-water as a pressure-transmitting medium.
At room pressure, the adiabatic bulk (K 0S) and shear (µ0S)
moduli are K 0S = 163 ± 3 GPa and µ0S = 121 ±
2 GPa, in excellent agreement with ultrasonic results from
the same bulk sample (Jacobsen et al., 2002). A fit to all
our high-pressure Brillouin data using a third-order finite-strain
equation of state yields the following pressure derivatives
of the adiabatic bulk and shear moduli: K'0S = 3.9 ±
0.2 and µ'0S
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= 2.1 ± 0.1. Within the uncertainties, we find that K
0S and K'0S of (Mg0.94Fe0.06)O are unchanged from MgO. However,
µ0S and µ'0S of (Mg0.94Fe0.06)O are reduced by 8%
and 11%, respectively. The aggregate compressional (V P) and
shear (V S) wave velocities are reduced by 4% and 6%, respectively,
as compared to MgO. The pressure dependence of the single-crystal
elastic moduli and aggregate sound velocities is linear within
the investigated pressure range. The elastic anisotropy of (Mg0.94Fe0.06)O
is about 10% greater than that of MgO at ambient conditions.
At the highest pressure obtained here, the elastic anisotropy
of (Mg0.94Fe0.06)O is close to zero. On the basis of our measurements
and earlier ultrasonic measurements, we find that the pressure
derivatives of shear moduli obtained at room pressure for low
iron concentrations (<20 mol% FeO) of high-spin ferropericlase
are inconsistent with those inferred from the lower mantle PREM
model. |
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Septiembre de 2006
Critical point theory of earthquakes: Observation of correlated
and cooperative behavior on earthquake fault systems
Authors: Chien-chih Chen, John
B. Rundle et al
Link: Click here
Abstract
The critical point theory for earthquakes was originally proposed
to explain the scaling relations observed in earthquakes,
including the Gutenberg-Richter frequency- magnitude relation
and the Omori's law for aftershocks. In this model, main shocks,
their foreshocks and aftershocks
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are
all associated with the formation of a correlated, cooperative
spatial region with high stress. Until now, only indirect evidence
of the existence of these correlated regions has been reported.
Here in this paper we present observations and analyses that
allow us to directly map the high stress, spatially correlated
regions preceding four major earthquakes, i.e. the 1992 Landers
(California), 1995 Kobe (Japan), 1999 Chi-Chi (Taiwan) and 1999
Hector Mine (California) earthquakes. We therefore conclude
that the locations and extent of large main shocks and their
immediate aftershocks can be determined from seismicity data
taken prior to the main shocks, and provide additional evidence
in support of the critical point theory for earthquakes. |
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Agosto de 2006
Hydrogeological insights at Stromboli volcano (Italy) from
geoelectrical, temperature, and CO2 soil degassing investigations
Authors:A. Finizola , A. Revil
et al
Link: Click here
Abstract
Finding the geometry of aquifers in an active volcano is important
for evaluating the hazards associated with phreato-magmatic
phenomena and incidentally to address the problem of water
supply. A combination of electrical resistivity tomography
(ERT), self-potential, CO2, and temperature measurements provides
insights about the location and pattern of ground water
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flow
at Stromboli volcano. The measurements were conducted along
a NE-SW profile across the island from Scari to Ginostra, crossing
the summit (Pizzo) area. ERT data (electrode spacing 20 m, depth
of penetration of ~200 m) shows the shallow architecture through
the distribution of the resistivities. The hydrothermal system
is characterized by low values of the resistivity (<50 O
m) while the surrounding rocks are resistive (>2000 O m)
except on the North-East flank of the volcano where a cold aquifer
is detected at a depth of ~80 m (resistivity in the range 70-300
O m). CO2 and temperature measurements corroborate the delineation
of the hydrothermal body in the summit part of the volcano while
a negative self-potential anomaly underlines the position of
the cold aquifer. |
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Agosto de 2006
Low Earth orbit satellite navigation errors and vertical
total electron content in single-frequency GPS tracking
Authors: Miquel Garcia-Fernàndez
and Oliver Montenbruck
Link: Click here
Abstract
In the context of space applications, the GPS system is presently
a well-established and accepted tracking system. To meet the
basic navigation requirements, most satellites in a low Earth
orbit are equipped with single-frequency GPS receivers that
measure the coarse acquisition code as well as the L1 phase.
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However,
the resulting kinematic navigation solutions exhibit systematic
position errors caused by elevation-dependent ionospheric path
delays. In this study a simple analytical model is established,
which quantitatively relates the position error to the vertical
electron content and the mapping function. This model substantiates
the empirical evidence of a mean radial offset that increases
in proportion to the total electron content above the satellite.
It is furthermore shown that the ratio between this offset and
the vertical ionospheric path delay depends on the applied elevation
mask angle. Representative ratios of 3-5 are obtained for the
mapping function of the Lear ionosphere model and elevation
cutoff angles of 10°, 5°, and 0°. This analytical
result has further been confirmed by signal simulator tests
as well as flight data of the CHAMP satellite. |
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Agosto de 2006
Global gravity, bathymetry, and the distribution of submarine
volcanism through space and time
Authors: A. B. Watts , D. T.
Sandwell et al
Link: Click here
Abstract
The seafloor is characterized by numerous seamounts and oceanic
islands which are mainly volcanic in origin. Relatively few
of these features (<~0.1%), however, have been dated, and
so little is known about their tectonic setting. One parameter
that is sensitive to whether a seamount formed on, near, or
far from a mid-ocean ridge is the elastic thickness, T e ,
which is a proxy for the long-term strength of the lithosphere.
Most previous studies are based on using the bathymetry to
calculate the gravity anomaly for different values of T e
and then comparing the calculated and observed gravity anomaly.
The problem with such an approach is that bathymetry data
are usually limited to single-beam echo sounder data acquired
along a ship track and these data are too sparse to define
seamount shape. We therefore use the satellite-derived gravity
anomaly to predict the bathymetry for different values of
T e . By comparing the predicted bathymetry to actual shipboard
soundings in the vicinity of each locality in the Wessel global
seamount database, we have obtained 9758 T e estimates from
a wide range of submarine
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volcanic features in
the Pacific, Indian, and Atlantic oceans. Comparisons where
there are previous estimates show that bathymetric prediction
is a robust way to estimate T e and its upper and lower bounds.
T e at sites where there is both a sample and crustal age
show considerable scatter, however, and there is no simple
relationship between T e and age. Nevertheless, we are able
to tentatively assign a tectonic setting to each T e estimate.
The most striking results are in the Pacific Ocean where a
broad swath of "on-ridge" volcanism extends from
the Foundation seamounts and Ducie Island/Easter Island ridge
in the southeast, across the equator, to the Shatsky and Hess
rises in the northwest. Interspersed among the on-ridge volcanism
are "flank ridge" and "off-ridge" features.
The Indian and Atlantic oceans also show a mix of tectonic
settings. Off-ridge volcanism dominates in the eastern North
Atlantic and northeast Indian oceans, while flank ridge volcanism
dominates the northeastern Indian and western south Atlantic
oceans. We have been unable to assign the flank ridge and
off-ridge estimates an age, but the on-ridge estimates generally
reflect, we believe, the age of the underlying oceanic crust.
We estimate the volume of on-ridge volcanism to be ~1.1 ×
106 km3 which implies a mean seamount addition rate of ~0.007
km3 yr-1. Rates appear to have varied through geological time,
reaching their peak during the Late/Early Cretaceous and then
declining to the present-day.
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Agosto de 2006
Day-to-day variability in equatorial spread F: Is there
some physics missing?
Author: Roland T. Tsunoda
Link: Click here
Abstract
Attempts continue to be made, without notable success, to
identify the source of day-to-day variability in occurrence
of equatorial spread F (ESF). Most seek to uncover a controlling
factor in one of the parameters that describe the linear
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growth
rate of the Rayleigh-Taylor (RT) instability. It is possible,
however, that some physics is still missing in that description
of ESF generation. Consideration is given here to an F-region
response to a large-scale polarization electric field that is
generated by a sporadic-E layer instability (Cosgrove and Tsunoda,
2002) and mapped to the bottomside of the F 2 layer. Results
indicate that the large-scale wave structure, the most reliable
precursor for ESF, may be initiated by this process at the base
of the F 2 layer, where plasma drift is westward. |
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Agosto
de 2006
Tsunami travel time prediction using neural networks
Authors: Rahul Barman , B. Prasad
Kumar et al
Link: Click here
Abstract
The present work reports the development of a nonlinear technique
based on artificial neural network (ANN) for prediction of
tsunami travel time in the Indian Ocean. The expected times
of arrival
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(ETA)
computation involved 250 representative coastal stations encompassing
35 countries. A travel time model is developed using ANN approach.
The ANN model uses non-linear regression where a Multi-layer
Perceptron (MLP) is used to tackle the non-linearity in the
computed ETA. The back-propagation feed forward type network
is used for training the system using the resilient back-propagation
algorithm. The model demonstrates a high degree of correlation,
proving its robustness in development of a real-time tsunami
warning system for Indian Ocean. |
Agosto
de 2006
Magnitude estimation using the first three seconds P-wave
amplitude in earthquake early warning
Authors: Yih-Min Wu and Li Zhao
Link: Click here
Abstract
Pd is the peak amplitude of displacement in the first three
seconds after the arrival of the P wave. We investigated the
attenuation of Pd with the hypocentral distance R in southern
California as a
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function
of magnitude M, and obtained the following relationship: log
(P d ) = -3.463 + 0.729 × M - 1.374 × log (R) ±
0.305. Given an earthquake location determined by the P-wave
arrival times at stations close to the epicenter, this relationship
can be used to define a so-called "Pd magnitude" of
earthquakes. Our result shows that for earthquakes in southern
California the Pd magnitudes agree with the catalog magnitudes
with a standard deviation of 0.18 for events less than magnitude
6.5. Therefore, Pd is a robust measurement for estimating the
magnitudes of earthquakes and has practical application in earthquake
early warning systems. |
Agosto
de 2006
Distinguishing ionospheric models using Schumann resonance
spectra
Authors: Earle R. Williams , Vadim
C. Mushtak et al
Link: Click here
Abstract
A number of models for Schumann resonance (SR) behavior in
uniform approximations of the real Earth-ionosphere cavity
now populate the literature. These models are treated in terms
of variously formulated propagation parameters: as the complex
eigenvalue of the propagation problem, as the complex incident
angle's sine, as the phase velocity and attenuation rate,
or as a
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pair
of complex characteristic altitudes. This study compares a priori
theoretical propagation parameters with the corresponding quantities
recovered from calculated Schumann resonance spectra by means
of Lorentzian spectral fits. To estimate the ultimate accuracy
of the recovery procedure, the influence of source-receiver
separation is excluded by assuming a globally uniform distribution
of lightning sources. The comparisons show a practically acceptable
agreement, within several percent, agreement between recovered
and a priori parameters for all models studied. When judged
against real Schumann resonance observations, these results
shed light on problems with certain models. More importantly,
the results reaffirm the ability of procedures based on SR observations
to resolve global features of the ionosphere's state and structure.
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Agosto
de 2006
The apparently isotropic Australian upper mantle
Authors: Maggy Heintz , Brian L.
N. Kennett et al
Link: Click here
Abstract
We investigate shear wave splitting measurements performed
on two years of data recorded at stations deployed in the
TASMAL experiment, a network of 20 broadband seismological
stations designed to record data on each side of the controversial
Tasman Line in Australia. Whereas a subset of measurements
previously performed on one year of data exhibited a curvilinear
pattern similar to that of the Tasman
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Line, suggesting anisotropy frozen in the lithosphere, considering
the whole data set drastically changes the situation: apparent
isotropy in the Australian upper mantle is observed at numerous
stations. This apparent isotropy together with the EW or NS
orientations of the polarization plane of the fast S wave (phi
) observed at some stations is consistent with a two-layer anisotropic
system underneath the Australian continent, with a perpendicular
orientation of in each layer. From the latest tomographic results,
the transition between the Precambrian western and Phanerozoic
eastern Australia appears to define blocks of various thickness.
Unlike the situation across the TESZ in Europe, these blocks
do not seem to be correlated with a different behavior in terms
of seismic anisotropy. |
Agosto
de 2006
Laboratory measurements of P- and S-wave velocities in polycrystalline
plagioclase and gabbronorite up to 700 °C and 1 GPa: Implications
for the low velocity anomaly in the lower crust
Authors: Yoshio Kono , Masahiro
Ishikawa et al
Link: Click here
Abstract
P- (Vp) and S-wave (Vs) velocities of polycrystalline plagioclase
(An51±1) and gabbronorite including plagioclase (An49±2)
(62.3vol.%) were measured up to 700°C at 0.6, 0.8, and
1.0GPa during heating and cooling. Both
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polycrystalline
plagioclase and gabbronorite show reversible and discontinuous
change in temperature derivative of Vp and Vs, dVp/dT and dVs/dT,
respectively, at about 400°C. The dVp/dT and dVs/dT of the
polycrystalline plagioclase changes from -0.8 × 10-4 to
-3.4 × 10-4 km s-1°C-1 and from -1.1 × 10-4
to -3.3 × 10-4 km s-1°C-1, respectively. The dVp/dT
and dVs/dT of the gabbronorite varies from -1.0 × 10-4
to -3.4 × 10-4 km s-1°C-1 and from -0.4 × 10-4
to -3.5 × 10-4 km s-1°C-1, respectively. The reversible
and discontinuous changes in dVp/dT and dVs/dT are attributed
to a phase transformation of plagioclase. The present data suggest
that the sharp decrease in Vp and Vs would cause low velocity
anomaly under dry and subsolidus melt-absent conditions in the
mid-to-lower crust of relatively high heat flow regions. |
Agosto
de 2006
Long-range triggered earthquakes that continue after the
wave train passes
Author: Emily E. Brodsky
Link: Click here
Abstract
Large earthquakes can trigger distant earthquakes in geothermal
areas. Some triggered earthquakes happen while the surface
waves pass through a site, but others occur hours or even
days later. Does this prolonged seismicity require a special
mechanism to store the stress from the seismic waves that
differs from ordinary aftershock mechanisms? These questions
have driven studies of long-range triggering since the phenomenon's
discovery. Here I attempt to answer the questions by examining
the statistics
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of triggered sequences. Two separate observations are consistent
with the prolonged sequences being simply local aftershocks
of earthquakes triggered early in the wave train. First, the
sequences obey Omori's Law over both short (1 hour) and longer
(5 day) time intervals. Secondly, the number of observed triggered
earthquakes in the first hour after the wave train can be predicted
from the number of earthquakes triggered during the wave train.
Even the very vigorous 10-day triggering at Long Valley from
the 1992 Landers M w 7.3 earthquakes can be interpreted as the
aftershocks of either a local M ˜ 4.1 earthquake or an
equivalent combination of several smaller mainshocks. Therefore,
long-range triggering does not need to include a mechanism to
produce sustained stresses other than the process that generates
aftershocks of the earthquakes that occur while the wave train
is passing. |
Agosto
de 2006
Improved modeling of Lunar Prospector neutron spectrometer
data: Implications for hydrogen deposits at the lunar poles
Authors: David J. Lawrence , W.
C. Feldman et al
Link: Click here
Abstract
New models have been computed for the Lunar Prospector (LP)
thermal and epithermal neutron counting rates using the particle
transport code MCNPX. This work improves upon previous studies
by using one code to model the neutron production, transport,
and detection processes, and by examining the sensitivity
of epithermal neutrons to elements other than hydrogen. Our
modeling results for standard anhydrous lunar
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soils
show that when hydrogen is not included in a soil, epithermal
neutrons are most sensitive to variations in the abundances
of Fe, Gd, and Sm, which is consistent with measured epithermal
neutron data. We use our current modeling results, in conjunction
with known mineral compositions of lunar soils and other lunar
global data sets to conclude that the best explanation for a
decrease in the counting rate of epithermal neutrons near both
lunar poles is the presence of hydrogen. We have further concluded
that the average hydrogen abundance near both lunar poles is
100-150 ppm and is likely buried by 10 ± 5 cm of dry
lunar soil, a result that is consistent with previous studies.
The localized hydrogen abundance for small (<20 km) areas
of permanently shaded regions remains highly uncertain and could
range from 200 ppm H up to 40 wt% H2O in some isolated regions.
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Agosto
de 2006
Modeling the isotopic composition of Antarctic snow using
backward trajectories: Simulation of snow pit records
Authors: M. M. Helsen , R. S. W.
van de Wal et al
Link: Click here
Abstract
The quantitative interpretation of isotope records (d18O,
dD, and d excess) in ice cores can benefit from a comparison
of observed meteorology with associated isotope variability.
For this reason we studied four isotope records from snow
pits in western Dronning Maud Land (DML), Antarctica, covering
the period 1998-2001. Timing and magnitude of snowfall events
on these locations were monitored using sonic height rangers.
For the distinguished snowfall events we evaluated the isotopic
composition of the moisture during transport by combining
backward trajectory calculations with isotopic modeling, using
a Rayleigh-type distillation model (MCIM). The initial isotope
ratio of the moisture was determined from
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monthly mean isotope fields from a general circulation model
(ECHAM4). The trajectory analysis showed that the southern Atlantic
Ocean is the major moisture source for precipitation in DML.
Modeling results along the trajectories revealed that most of
the isotopic depletion occurred during the last day of the transport.
Finally, a diffusion model was applied to describe the diffusion
in the firn layer such that the modeled isotopes could be compared
with the observed isotope records. The resulting modeled isotope
profiles were mostly in good agreement with the observed seasonal
variability in the snow. However, at low temperatures (especially
on the Antarctic interior), magnitude of the total distillation
was underestimated. Regarding the d excess parameter, our results
show a large influence of advection height on the final value
of d excess in precipitation. This in turn points to the importance
of the vertical structure of d excess over the oceanic source
region, which obscures the classical interpretation of this
parameter in terms of temperature and relative humidity in the
moisture source region. |
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Agosto de 2006
Monitoring of an infiltration experiment using the self-potential
method
Authors: B. Suski , A. Revil
et al
Link: Click here
Abstract
An infiltration test was performed from a ditch with the purpose
of monitoring the evolution of the piezometric levels using
self-potential measurements made at the ground surface. We
used a set of 18 piezometers and a network of 41 nonpolarizable
(Pb/PbCl2) electrodes. The variations of the self-potential
signals are linearly correlated to the piezometric level changes
with an apparent voltage coupling coefficient of -5.5
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± 0.9 mV m-1. We measured, independently of this infiltration
test, the three material properties entering the macroscopic
field equations. They are the resistivity distribution of the
soil, its mean hydraulic conductivity, and its intrinsic streaming
potential coupling coefficient (-5.8 ± 1.1 mV m-1). Then,
we modeled numerically the infiltration test and the associated
self-potential signals using a two-dimensional finite difference
code. The numerical model reproduces fairly well the observed
results. This investigation demonstrates the effectiveness of
the self-potential method in field conditions to monitor small
variations (<0.60 m) of the water table. It offers for the
first time a test of the electrokinetic theory in the field
with independent evaluation of the material properties entering
the field equations. |
Agosto
de 2006
Initiation propagation and termination of elastodynamic ruptures
associated with segmentation of faults and shaking hazard
Author: Bruce E. Shaw
Link: Click here
Abstract
Using a model of a complex fault system, we examine the initiation,
propagation, and termination of ruptures and their relationship
to fault geometry and shaking hazard. We find concentrations
of epicenters near fault step overs and ends; concentrations
of terminations near
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fault
ends; and persistent propagation directivity effects. Taking
advantage of long sequences of dynamic events, we directly measure
shaking hazards, such as peak ground acceleration exceedance
probabilities, without need for additional assumptions. This
provides a new tool for exploring shaking hazard from a physics-based
perspective, its dependence on various physical parameters,
and its correlation with other geological and seismological
observables. Using this capability, we find some significant
aspects of the shaking hazard can be anticipated by measures
of the epicenters. In particular, asymmetries in the relative
peak ground motion hazard along the faults appear well correlated
with asymmetries in epicentral locations. |
Agosto
de 2006
High-precision isotopic characterization of USGS reference
materials by TIMS and MC-ICP-MS
Authors: Dominique Weis , Bruno
Kieffer et al
Link: Click here
Abstract
The Pacific Centre for Isotopic and Geochemical Research (PCIGR)
at the University of British Columbia has undertaken a systematic
analysis of the isotopic (Sr, Nd, and Pb) compositions and
concentrations of a broad compositional range of U.S. Geological
Survey (USGS) reference materials, including basalt (BCR-1,
2; BHVO-1, 2), andesite (AGV-1, 2), rhyolite (RGM-1, 2), syenite
(STM-1, 2), granodiorite (GSP-2), and granite (G-2, 3). USGS
rock reference materials are geochemically well characterized,
but there is neither a systematic methodology nor a database
for radiogenic isotopic compositions, even for the widely
used BCR-1. This investigation represents the first comprehensive,
systematic analysis of the isotopic composition and concentration
of USGS reference materials and provides an important database
for the isotopic community. In addition, the range of equipment
at the PCIGR, including a Nu Instruments Plasma MC-ICP-MS,
a
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Thermo
Finnigan Triton TIMS, and a Thermo Finnigan Element2 HR-ICP-MS,
permits an assessment and comparison of the precision and accuracy
of isotopic analyses determined by both the TIMS and MC-ICP-MS
methods (e.g., Nd isotopic compositions). For each of the reference
materials, 5 to 10 complete replicate analyses provide coherent
isotopic results, all with external precision below 30 ppm (2
SD) for Sr and Nd isotopic compositions (27 and 24 ppm for TIMS
and MC-ICP-MS, respectively). Our results also show that the
first- and second-generation USGS reference materials have homogeneous
Sr and Nd isotopic compositions. Nd isotopic compositions by
MC-ICP-MS and TIMS agree to within 15 ppm for all reference
materials. Interlaboratory MC-ICP-MS comparisons show excellent
agreement for Pb isotopic compositions; however, the reproducibility
is not as good as for Sr and Nd. A careful, sequential leaching
experiment of three first- and second-generation reference materials
(BCR, BHVO, AGV) indicates that the heterogeneity in Pb isotopic
compositions, and concentrations, could be directly related
to contamination by the steel (mortar/pestle) used to process
the materials. Contamination also accounts for the high concentrations
of certain other trace elements (e.g., Li, Mo, Cd, Sn, Sb, W)
in various USGS reference materials. |
Agosto
de 2006
Magnetohydrodynamic simulation of the interaction between
interplanetary strong shock and magnetic cloud and its consequent
geoeffectiveness
Authors: Ming Xiong , Huinan Zheng
et al
Link: Click here
Abstract
Numerical studies have been performed to interpret the observed
"shock overtaking magnetic cloud (MC)" event by
a 2.5 dimensional magnetohydrodynamic (MHD) model in the heliospheric
meridional plane. Results of an individual MC simulation show
that the MC travels with a constant bulk flow speed. The MC
is injected with a very strong inherent magnetic field over
that in the ambient flow and expands rapidly in size initially.
Consequently, the diameter of the MC increases in an asymptotic
speed while its angular width contracts gradually. Meanwhile,
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simulations of MC-shock interaction are also presented, in which
both a typical MC and a strong fast shock emerge from the inner
boundary and propagate along the heliospheric equator, separated
by an appropriate interval. The results show that the shock
first catches up with the preceding MC, then penetrates through
the MC, and finally merges with the MC-driven shock into a stronger
compound shock. The morphologies of shock front in interplanetary
space and MC body behave as a central concave and a smooth arc,
respectively. The compression and rotation of the magnetic field
serve as an efficient mechanism to cause a large geomagnetic
storm. The MC is highly compressed by the overtaking shock.
Contrarily, the transport time of the incidental shock influenced
by the MC depends on the interval between their commencements.
Maximum geoeffectiveness results from when the shock enters
the core of preceding MC, which is also substantiated to some
extent by a corresponding simplified analytic model. Quantified
by the Dst index, the specific result is that the geoeffectiveness
of an individual MC is largely enhanced with 80% increment in
maximum by an incidental shock. |
Agosto
de 2006
How probability weighting affects participation in water
markets
Authors: Ram Ranjan and Jason F.
Shogren.
Link: Click here
Abstract
The behavioral tendency to overestimate probabilities of loss
can affect a farmer's participation in water markets. We examine
this issue with a theoretical model of a nonexpected utility
maximizing farmer who places subjective weights on the actual
probabilities of loss of water rights due to market transactions.
The farmer bargains over sharing of surpluses with the buyer
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of
water. The farmer then incorporates the bargaining outcome in
his intertemporal expected benefit maximization problem that
accounts for the possible loss of water rights due to its sale
out of agriculture. Three key results emerge. First, subjective
weighting of probabilities leads to discounting of resources
when farmers overestimate probabilities of loss. Second, if
farmers have idiosyncratic time preferences, total water supply
in the market would depend on the level of heterogeneity in
the population. Third, considering the case of two farmers,
we find that the farmer with lower endowments bears the burden
of risk reduction, whereas the one with higher endowments sells
more water for profits. As the level of risk increases, however,
the relative difference in risk sharing declines. |
Agosto
de 2006
Mitigating atmospheric noise for InSAR using a high resolution
weather model
Authors: J. Foster, B. Brooks et
al
Link: Click here
Abstract
A high resolution weather model is used to predict atmospheric
delays for the acquisition times of synthetic aperture radar
images over Hawaii.
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Refraction
of the radar by water vapor in the atmosphere in Hawaii leads
to apparent ground-motions with wavelengths and magnitudes similar
to the actual ground motions generated by tectonic and volcanic
processes. We examine the potential for a weather model to help
characterize the atmospheric component in InSAR scenes and find
that in the best cases it models the observed delays well, reducing
the variance at wavelengths of 30 km and greater by ~60%, while
even in the worst cases it provides an independent means of
quantifying the expected variance in the image due to the atmosphere. |
Agosto
de 2006
The Mw 7.8, 2001 Kunlunshan earthquake: Extreme rupture speed
variability and effect of fault geometry
Authors: D. P. Robinson , C. Brough
et al
Link: Click here
Abstract
By analyzing body wave seismograms, we show that the rupture
speed on the Main Kunlun Fault during the M w 7.8 2001 Kunlunshan,
Tibet, earthquake was highly variable and the rupture process
consisted of three stages. In the first stage, the rupture
accelerated from rest to an average speed of 3.3 km/s over
a distance of 120 km. The rupture then propagated for another
150 km at an apparent rupture speed exceeding the P
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wave speed. In the final stage, the earthquake fault bifurcates,
and the rupture front slowed down. The region of highest rupture
velocity is found to coincide with the region of highest fault
slip, has the longest slip duration, and is where off-fault
ground cracking is observed in the field. Stress drops are found
to be higher in regions of higher rupture speeds. The greatest
concentration of aftershocks is located near the fault bifurcation
zone and hence coincides with the region of highest fault slip,
highest stress drop and highest rupture velocity. The fault
width is no more than 10 km in most places and is about 20 km
in the region of highest slip. This narrow fault width is attributed
to the fact that crust below this depth is sufficiently warm
not to permit brittle failure to occur. The remarkable similarity
of this earthquake with the 1906 California earthquake, in spite
of occurring in very different tectonic regimes, is discussed.
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Agosto
de 2006
Evidence in support of the climate change-Atlantic hurricane
hypothesis
Author: James B. Elsner
Link: Click here
Abstract
The power of Atlantic tropical cyclones is rising rather dramatically
and the increase is correlated with an increase in the late
summer/early fall sea surface temperature over the North Atlantic.
A debate concerns the nature of these increases with some
studies attributing them to a natural
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climate
fluctuation, known as the Atlantic Multidecadal Oscillation
(AMO), and others suggesting climate change related to anthropogenic
increases in radiative forcing from greenhouse-gases. Here tests
for causality using the global mean near-surface air temperature
(GT) and Atlantic sea surface temperature (SST) records during
the Atlantic hurricane season are applied. Results show that
GT is useful in predicting Atlantic SST, but not the other way
around. Thus GT "causes" SST providing additional
evidence in support of the climate change hypothesis. Results
have serious implications for life and property throughout the
Caribbean, Mexico, and portions of the United States. |
Agosto
de 2006
Three years of Atmospheric Infrared Sounder radiometric calibration
validation using sea surface temperatures
Authors: H. H. Aumann , Steve Broberg
et al
Link: Click here
Abstract
This paper evaluates the absolute accuracy and stability of
the radiometric calibration of the Atmospheric Infrared Sounder
(AIRS) by analyzing the difference between the brightness
temperatures measured at 2616 cm-1 and those calculated at
the top of the atmosphere (TOA), using the Real-Time Global
Sea Surface Temperature (RTGSST) for cloud-free night tropical
oceans between ±30° latitude. The TOA correction
is based on radiative transfer. The analysis of the first
3 years of AIRS radiances verifies the absolute calibration
at 2616 cm-1 to better than 200 mK, with better than 16 mK/yr
stability. The AIRS radiometric calibration uses an internal
full aperture wedge blackbody with the
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National
Institute of Standards and Technology (NIST) traceable prelaunch
calibration coefficients. The calibration coefficients have
been unchanged since launch. The analysis uses very tight cloud
filtering, which selects about 7000 cloud-free tropical ocean
spectra per day, about 0.5% of the data. The absolute accuracy
and stability of the radiometry demonstrated at 2616 cm-1 are
direct consequences of the implementation of AIRS as a thermally
controlled, cooled grating-array spectrometer and meticulous
attention to details. Comparable radiometric performance is
inferred from the AIRS design for all 2378 channels. AIRS performance
sets the benchmark for what can be achieved with a state-of-the-art
hyperspectral radiometer from polar orbit and what is expected
from future hyperspectral sounders. AIRS was launched into a
705 km altitude polar orbit on NASA's Earth Observation System
(EOS) Aqua spacecraft on 4 May 2002. AIRS covers the 3.7-15.4
micron region of the thermal infrared spectrum with a spectral
resolution of V/delta V= 1200 and has returned 3.7 million spectra
of the upwelling radiance each day since the start of routine
data gathering in September 2002. |
Agosto
de 2006
Relationship between geochemical parameters and the occurrence
of Dehalococcoides DNA in contaminated aquifers
Authors: Xiaoxia Lu , John T. Wilson
et al
Link: Click here
Abstract
Strains of Dehalococcoides are the only microbes known that
can completely dechlorinate PCE, TCE, cis-DCE, and vinyl chloride
to ethylene. Either naturally occurring strains or bioaugmentation
cultures of Dehalococcoides are widely used for in situ bioremediation
of contaminated groundwater. Naturally occurring strains have
an important role in natural attenuation of PCE, TCE, cis-DCE,
and vinyl chloride in groundwater. This study evaluated the
relationship between selected biogeochemical parameters and
the presence of Dehalococcoides DNA in field-scale plumes.
A total of 81 monitoring wells were sampled from 15 groundwater
plumes at 10 locations across the United States (one sample
per monitoring well). The presence of Dehalococcoides DNA
was
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determined
with an assay based on the polymerase chain reaction (PCR) using
DNA primers targeting the 16S rRNA gene of Dehalococcoides.
The groundwater samples were also analyzed for concentrations
of O2, NO3 -1 plus NO2 -1 - N, CH4, H2, Fe (II), SO4 -2, TOC,
Cl-1, and benzene, toluene, ethylbenzene, and xylene (BTEX)
compounds and for alkalinity, ORP, electrical conductivity,
pH, and temperature. Dehalococcoides DNA was unequivocally detected
in 26 wells, most of which exhibited methanogenic conditions.
A two-sample Kolmogorov-Smirnov test was used to compare the
distribution of each parameter in water where Dehalococcoides
DNA was present to the distribution where Dehalococcoides DNA
was absent. The only parameters for which the distributions
were different at 95% confidence were NO3 -1 plus NO2 -1 - N,
CH4, and ORP. Using these three statistically significant geochemical
parameters as descriptors, a predictive model for the presence
of Dehalococcoides DNA was developed using logistic regression
with a binary response. Under conditions where data of direct
biochemical assay are not available a calculated probability
could be used to properly calibrate computer models of natural
attenuation. |
Agosto
de 2006
Using uplifted Holocene beach berms for paleoseismic analysis
on the Santa María Island, south-central Chile
Authors: B. Bookhagen , H. P. Echtler
et al
Link: Click here
Abstract
Major earthquakes (M > 8) have repeatedly ruptured the
Nazca-South America plate interface of south-central Chile
involving meter scale land-level changes. Earthquake recurrence
intervals, however, extending beyond limited historical records
are virtually unknown, but would provide
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crucial
data on the tectonic behavior of forearcs. We analyzed the spatiotemporal
pattern of Holocene earthquakes on Santa María Island
(SMI; 37°S), located 20 km off the Chilean coast and approximately
70 km east of the trench. SMI hosts a minimum of 21 uplifted
beach berms, of which a subset were dated to calculate a mean
uplift rate of 2.3 ± 0.2 m/ky and a tilting rate of 0.022
± 0.002 °/ky. The inferred recurrence interval of
strandline-forming earthquakes is ~180 years. Combining coseismic
uplift and aseismic subsidence during an earthquake cycle, the
net gain in strandline elevation in this environment is ~ 0.4
m per event. |
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Agosto de 2006
Modeling the geomagnetic effects caused by the solar eclipse
of 11 August 1999
Authors: J. J. Curto , B. Heilig
at al
Link: Click here
Abstract
The solar eclipse of 11 August 1999 was total along a belt
crossing Central Europe, where there
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is
a high density of magnetic observatories. We studied the transient
geomagnetic effects produced by the temporary cutoff of the
ionizing solar radiations during the eclipse. It is the first
case in which the phenomenon has been analysed in a multisite
context. For observatories along the umbra path, a model based
on the equivalent electric current schema is proposed. |
Agosto
de 2006
A nonparametric stochastic downscaling framework for daily
rainfall at multiple locations
Authors: R. Mehrotra and Ashish
Sharma
Link: Click here
Abstract
Use of General Circulation Models (GCMs) for climate change
impact assessment is often limited by their incapability at
representing local features and dynamics at spatial scales
finer than the in-built GCM grid scale. This has led to the
development of downscaling techniques for transfer of coarse
GCM simulated weather output to finer spatial resolutions.
This paper presents a nonparametric stochastic spatial downscaling
framework for multisite daily rainfall occurrence and amount.
At site rainfall occurrences are downscaled using a nonparametric
nonhomogeneous hidden Markov model (NNHMM) that represents
spatial dependence across the rainfall occurrence field using
a dynamic weather state indicative of the centroid and average
wetness fraction of the rainfall
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occurrence
field. The rainfall amounts on the wet days are downscaled using
a nonparametric kernel density approach that accommodates variations
in the rainfall downscaling model at individual locations. Spatial
dependence in the rainfall amounts is simulated by driving each
of the single-site amounts model with spatially correlated random
numbers. The proposed framework is applied for downscaling of
rainfall at a network of 30 rain gauge stations around Sydney
in Australia, and its performance is evaluated. The analyses
of the results show that the logic of providing separate treatments
for rainfall occurrence and amounts at individual locations
imparts considerable accuracy in the representation of characteristics
of interest in hydrologic studies. These characteristics include
representation of rainfall spell patterns, spatial distribution
of the rainfall occurrence and amount fields, representation
of low and high rainfall extremes at individual stations and
across the field, as well as common indicators of water balance
and variability that are of importance in a catchment scale
water balance simulation. |
Agosto
de 2006
Configuration of the Indian Moho beneath the NW Himalaya
and Ladakh
Authors: S. S. Rai, K. Priestley
et al
Link: Click here
Abstract
Teleseismic receiver function analysis of seismograms recorded
on a ~700 km long profile of 17 broadband seismographs traversing
the NW Himalaya shows a progressive northward deepening of
the Indian Moho from ~40 km
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beneath
Delhi south of the Himalayan foredeep to ~75 km beneath Taksha
at the Karakoram Fault. Similar studies by Wittlinger et al.
(2004) to the north of the Karakoram Fault show that the Moho
continues to deepen to ~90 km beneath western Tibet before shallowing
substantially to 50-60 km at the Altyn Tagh Fault. The continuity
of the Indian Moho imaged in the receiver functions reported
here, along with those of Wittlinger et al. (2004), suggest
that in this part of the Himalayan orogen the Indian plate may
penetrate as far as the Bangong Suture, and possibly as far
north as the Altyn Tagh. |
Agosto
de 2006
A new approach to directly determine the secular variation
from magnetic satellite observations
Authors: M. Mandea and N. Olsen
Link: Click here
Abstract
Observatory monthly means provide an excellent opportunity
to study the temporal changes of the geomagnetic field at
a given location. Unfortunately, determination of the global
pattern of changes using observatory data is hampered by their
uneven distribution. Satellite data provide excellent global
coverage, but the spacecraft
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movement
makes direct comparisons of satellite and observatory data difficult.
To investigate short-period secular variation in observatory
and satellite data, we developed an approach to extract satellite
monthly means for "virtual observatories" at 400 km
altitude, using CHAMP magnetic measurements. Comparison of these
virtual observatory monthly means with the corresponding ground
values shows a remarkably well-correlated signal at time-scales
of months to years, which is beyond the temporal resolution
limit of recent global models. Here, we describe this newly
developed approach, its validation, and discuss how it can be
used to understand short-period changes of the recent geomagnetic
field. |
Agosto
de 2006
Rapid estimation of first-order rupture characteristics for
large earthquakes using surface waves: 2004 Sumatra-Andaman
earthquake
Authors: Charles J. Ammon , Aaron
A. Velasco et al
Link: Click here
Abstract
Broadband surface waves from large earthquakes can be rapidly
processed to estimate seismic moment, faulting duration, and
general slip distribution, supplying important information
for hazard response efforts, including tsunami warnings. Deconvolution
of surface-wave Green's
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functions removes propagation effects, yielding azimuthally
varying effective source time functions. Even a single fault-perpendicular
time function can provide robust first-order estimates of the
temporal history of seismic radiation and associated slip distribution.
Simplified two-dimensional finite-fault modeling using a few
surface-wave time functions can reliably characterize the smooth
component of the overall rupture process. These procedures are
demonstrated using 13 Rayleigh wave observations for the 26
December 2004 Sumatra-Andaman earthquake, yielding satisfactory
agreement with models based on more complete seismic data sets.
Depending on source and station locations, stable slip estimates
can be obtained within 15-60 minutes of the onset of a large
earthquake. |
Agosto
de 2006
Seismic velocity and attenuation structures in the top 400
km of the Earth's inner core along equatorial paths
Authors: Wen-che Yu , Lianxing
Wen et al
Link: Click here
Abstract
We study seismic velocity and attenuation structures in the
top 400 km of the Earth's inner core based on modeling of
differential traveltimes, amplitude ratios, and waveforms
of the PKiKP-PKIKP phases observed at the epicentral distance
range of 120°-141° and the PKPbc-PKIKP phases observed
at the distance range of 146°-160° along equatorial
paths. Our data are selected from the seismograms recorded
in the Global Seismographic Network from 1990 to 2001 and
many regional seismic networks. The observed PKiKP-PKIKP and
PKPbc-PKIKP phases exhibit distinctive "east-west"
hemispheric patterns: (1) At the distance ranges of 131°-141°
and 146°-151°, PKIKP phases arrive about 0.3 s earlier
than the theoretical arrivals based on the Preliminary Reference
Earth Model (PREM) for the PKIKP phases sampling the "eastern
hemisphere" (40°E-180°E) of the inner core and
about 0.4 s later for those sampling the "western hemisphere"
(180°W-40°E). At the distance range of 151°-160°,
PKIKP phases arrive about 0.7 s earlier than the predicted
arrivals based on PREM for those sampling the eastern hemisphere
and about 0.1 s later for those sampling the western hemisphere.
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(2)
Amplitude ratios of the PKIKP/PKiKP phases at the distance range
of 131°-141° and of the PKIKP/PKPbc phases at the distance
range of 146°-151° are, in general, smaller for the
PKIKP phases sampling the eastern hemisphere than for those
sampling the western hemisphere. At distances greater than 151°,
the PKIKP/PKPbc amplitude ratios become indistinguishable for
the two hemispheres. These observations can be best explained
by two different types of seismic velocity and attenuation models
along equatorial paths, one for each hemisphere, in the top
400 km of the inner core. For the eastern hemisphere, the velocity
structure has a velocity increase of 0.748 km/s across the inner
core boundary (ICB), a small velocity gradient of 0.0042 (km/s)/100
km in the top 235 km, followed by a steeper velocity gradient
of 0.1 (km/s)/100 km extending from 235 km to 375 km, and a
velocity gradient of 0.01 (km/s)/100 km in the deeper portion
of the inner core; the attenuation structure has an average
Q value of 300 in the top 300 km and an average Q value of 600
in the deeper portion of the inner core. For the western hemisphere,
the velocity structure has a velocity increase of 0.645 km/s
across the ICB and a velocity gradient of 0.049 (km/s)/100 km
in the top 375 km; the attenuation structure has an average
Q value of 600 in the top 375 km of the inner core. Our results
suggest that the inner core hemispheric variations in velocity
extend deeper than 375 km below the ICB and the top 235 km of
the inner core in the eastern hemisphere is anomalous compared
to the rest of the inner core in having a small velocity gradient,
high velocity, and high attenuation. |
Agosto
de 2006
Characterization of permeability anisotropy using wavelet
analysis
Authors:Roseanna M. Neupauer ,
Kaye L. Powell et al
Link: Click here
Abstract
Wavelet analysis is an image analysis technique that can extract
local information at multiple scales. Because of this capability,
wavelet analysis can be used to identify dominant scales in
statistically heterogeneous and anisotropic random fields.
We develop and test a wavelet
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analysis method for identifying dominant scales and orientations
in permeability fields and for identifying boundaries between
regions with different dominant orientations. We evaluate three
different wavelets (fully anisotropic Morlet wavelet, Mexican
hat wavelet, and Cauchy wavelet) and show that the Morlet wavelet
is the most effective of these three wavelets in identifying
dominant orientations. We also investigate the use of several
different quantitative wavelet measures in identifying dominant
scales and orientations in permeability fields. The technique
is demonstrated using both a synthetic data set with known characteristics
and a laboratory-collected permeability data set from Massillon
sandstone |
Agosto
de 2006
Application of Hopfield neural network for extracting Doppler
spectrum from ocean echo
Authors: Renzhuo Gui , Zijie Yang
et al
Link: Click here
Abstract
This paper proposes the method of a Hopfield-type neural network
(HNN) for extracting Doppler spectrum from ocean echo. First,
it introduces the basic principle of HNN for optimized processing.
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Second,
expanding the principle of utilizing autoregression (AR) to
estimate frequency spectrum, we point out how to apply HNN in
spectrum estimation. Last, the three methods are utilized to
process actual data, that is, the conventional fast Fourier
transform method, modern spectrum estimation-AR method, and
the spectrum estimation method based on HNN. The results obtained
by the three methods prove that the spectrum estimation method
based on HNN is feasible for extracting the Doppler spectrum
from ocean echo. |
Julio
de 2006
P and S velocity structure of the upper mantle beneath the
Transantarctic Mountains, East Antarctic craton, and Ross Sea
from travel time tomography
Authors: Timothy Watson , Andrew Nyblade et al
Link: Click here
Abstract
P and S wave travel times from teleseismic earthquakes recorded
by the Transantarctic Mountains Seismic Experiment (TAMSEIS)
have been used to tomographically image upper mantle structure
beneath portions of the Transantarctic Mountains (TAM), the
East Antarctic (EA) craton, and the West An tarctic rift system
(WARS) in the vicinity of Ross Island, Antarctica. The TAM
form a major tectonic boundary that divides the stable EA
craton and the tectonically active WARS. Relative arrival
times were determined using a multichannel cross-correlation
technique on teleseismic P and S phases from earthquakes with
mb = 5.5. 3934 P waves were used from 322 events, and 2244
S waves were used from 168 events. Relative travel time residuals
were inverted for upper mantle structure using VanDecar's
method. The P wave
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tomography
model reveals a low-velocity anomaly in the upper mantle of
approximately dVp = -1 to -1.5% in the vicinity of Ross Island
extending laterally 50 to 100 km beneath the TAM from the coast,
placing the contact between regions of fast and slow velocities
well inland from the coast beneath the TAM. The magnitude of
the low-velocity anomaly in the P wave model appears to diminish
beneath the TAM to the north and south of Ross Island. The depth
extent of the low-velocity anomaly is not well constrained,
but it probably is confined to depths above ~200 km. The S wave
model, within resolution limits, is consistent with the P wave
model. The low-velocity anomaly within the upper mantle can
be attributed to a 200-300 K thermal anomaly, consistent with
estimates obtained from seismic attenuation measurements. The
presence of a thermal anomaly of this magnitude supports models
invoking a thermal buoyancy contribution to flexurally driven
TAM uplift, at least in the Ross Island region of the TAM. Because
the magnitude of the anomaly to the north and south of Ross
Island may diminish, the thermal contribution to the uplift
of the TAM could be variable along strike, with the largest
contribution in the Ross Island region. The tomography results
reveal faster than average velocities beneath East Antarctica,
as expected for cratonic upper mantle. |
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