Junio
de 2008
Phase relations of Fe-Si alloy up to core conditions: Implications
for the Earth inner core
Authors: Hidetoshi Asanuma, Eiji Ohtani et al
Link: Click here
Abstract
X-ray diffraction experiments were conducted
to 257 GPa and high temperature in situ on an iron-silicon alloy containing 3.4
wt% silicon, a candidate for the Earth's inner core forming material. The | results
revealed that fcc and hcp phases coexist up to 104 GPa. A single hcp phase is
stable at higher pressures at least up to 3600 K at 242 GPa and to 2400 K at 257
GPa. Dissolution of silicon in the liquid outer core following reaction with the
silicate mantle during core formation strongly suggests the existence of silicon
in the solid inner core. Our results revealed that the iron-3.4 wt% silicon alloy
in the inner core is likely to possess an hcp structure, which can explain the
inner core anisotropy observed in seismology. |
Junio
de 2008
Non-Fickian dispersion in porous media: 1. Multiscale measurements
using single-well injection withdrawal tracer tests
Authors: P. Gouze,
T. Le Borgneet al
Link: Click here
Abstract
We present a set of single-well injection
withdrawal tracer tests in a paleoreef porous reservoir displaying important small-scale
heterogeneity. An improved dual-packer probe was designed to perform dirac-like
tracer injection and accurate downhole automatic measurements of the tracer concentration
during the recovery phase. By flushing the tracer, at constant flow rate, for
increasing time duration, we can probe distinctly different reservoir volumes
and test the multiscale predictability of the (non-Fickian) dispersion models.
First we describe the characteristics, from microscale to meter scale, of the
reservoir rock. | Second, the
specificity of the tracer test setup and the results obtained using two different
tracers and measurement methods (salinity-conductivity and fluorescent dye-optical
measurement, respectively) are presented. All the tracer tests display strongly
tailed breakthrough curves (BTC) consistent with diffusion in immobile regions.
Conductivity results, measured over 3 orders of magnitude only, could have been
easily interpreted by the conventional mobile-immobile (MIM) diffusive mass transfer
model of asymptotic log-log slope of -2. However, the fluorescent dye sensor,
which allows exploring much lower concentration values, shows that a change in
the log-log slope occurs at larger time with an asymptotic value of -1.5, corresponding
to the double-porosity model. These results suggest that the conventional, one-slope
MIM transfer rate model is too simplistic to account for the real multiscale heterogeneity
of the diffusion-dominant fraction of the reservoir. |
Junio
de 2008
Geometrical and Taylor dispersion in a fracture with random obstacles:
An experimental study with fluids of different rheologies
Authors: A.
Boschan, I. Ippolito et al
Link: Click here
Abstract
The miscible displacement of a Newtonian
or shear-thinning fluid by another one of same rheological properties has been
studied optically in a flat transparent model fracture with a random distribution
of identical cylindrical obstacles on one of the walls. At the local scale, the
concentration variation on individual pixels satisfies a Gaussian convection-dispersion
relation with local transit time (x, y) and dispersivity l d (x, y). The variation | of
l d with the Péclet number Pe shows that it results from a combination
of geometrical and Taylor dispersion, respectively dominant at low and high Pe
values. Using shear-thinning solutions instead of a Newtonian fluid enhances the
velocity contrasts (and therefore geometrical dispersion) and reduces Taylor dispersion.
At the global scale, the front geometry is studied from the isoconcentration lines
c = 0.5 (equivalent to lines of constant (x, y) value): beyond a transition travel
time, their width in the direction parallel to the flow reaches a constant limit
varying linearly with Log(Pe) with a slope increasing with the shear-thinning
character of the fluid. These characteristics are compared to previous observations
on other model fractures with a self-affine roughness displaying channelization
effects. |
Junio
de 2008
A new polar magnetic index of geomagnetic activity
Authors:
Wladislaw Lyatsky and George V. Khazanov
Link: Click here
Abstract
We developed a new polar magnetic (PM)
index of geomagnetic activity which, similarly to the existing polar cap index,
was computed from magnetic field data from near-pole geomagnetic observatories.
However, we used a different method for its calculation, which provided the high
correlation of this index with both solar wind data and many events in geospace
environment. This improves significantly the reliability of forecasting geomagnetic
disturbances and such key parameters as cross-polar-cap voltage and Joule heating
in high-latitude ionosphere, which play an important role in the development of
global |
geomagnetic, ionospheric, and thermospheric disturbances. In this paper, we examined
PM index in the Northern Hemisphere only. We tested the PM index for 10-year period.
The correlation between PM index and upstream solar wind data for all these years
is very high (the squared correlation coefficient R 2 ˜ 0.74 which corresponds
to the linear correlation coefficients R ˜ 0.86). The PM index also shows
the high correlation with the cross-polar-cap voltage and hemispheric Joule heating
(the squared correlation coefficient R 2 between the actual and predicted values
of these parameters reaches ~0.81 which corresponds to the linear correlation
coefficients R ˜ 0.9), which results in significant increasing the prediction
reliability of these parameters. Thus, the polar magnetic (PM) index of geomagnetic
activity provides a significant increase in the forecasting reliability of geomagnetic
disturbances and related events in geospace environment, and it may be used as
an important input parameter in modeling ionospheric, magnetospheric, and thermospheric
processes. |
Junio
de 2008
Modeling the arrival at Earth of the interplanetary shock following
the 12 May 1997 solar event using HAFv2 and 3-D MHD HHMS models
Authors:
Z. K. Smith, T. R. Detman et al
Link: Click here
Abstract
The 12 May 1997 solar event is used
to demonstrate the improvements that have been made in recent years in the accuracy
of predicting, in the operational environment, the time of arrival at Earth of
solar-caused interplanetary shocks. This event provides a simple test case because
it was both well isolated in time from other events and well documented, with
the observation of a large flare (in optical and X-ray), a large halo CME, metric
type-II radio burst and near-Earth solar wind and interplanetary magnetic field
data. The | Hakamada-Akasofu-Fry
model (version 2: HAFv2) near-real-time prediction gave an error of 31 h. Because
the accuracy of predicted Sun-to-Earth transit times of such shocks depends both
on the model and our ability to determine the proper inputs from available observations,
we reexamined the predictions made for the 12 May event using inputs that could
have been available in near-real-time, both with HAFv2 and a second, ensemble-partnered
solar wind prediction model, the 3-D MHD Hybrid Heliospheric Model System (HHMS).
We use updated methods of estimating the initial shock velocity (from Smith et
al., 2005b) and examine the influence of the background solar wind. This reexamination
reduced the error from the original 31 h to less than 4 h. Achieving near-real-time
accuracies to this level would greatly enhance operations of technologies such
as electrical power grids, satellite systems, and polar flight communications.
|
Junio
de 2008
Equation of state of NaMgF3 postperovskite: Implication for the
seismic velocity changes in the D region
Authors: Justin Hustoft, Krystle
Catalli et al
Link: Click here
Abstract
Recent first-principles calculations
have proposed that bulk sound speed (V F) decreases at the perovskite (Pv) postperovskite
(PPv) transition. Yet some measurements suggest an increase in V F at the PPv
transition in (Mg,Fe)SiO3 and NaMgF3. | Our
new measurements on NaMgF3 up to 50 GPa under quasi-hydrostatic stress conditions
reveal that the V F of PPv remains lower than that of Pv near the PPv transition,
consistent with a first-principles study on NaMgF3. Combined with similar observations
in (Mg,Fe)SiO3, MgGeO3, and CaIrO3, our result on NaMgF3 suggests that the V F
decrease is a robust trend in the PPv transition and is controlled more by crystal
structure than by chemical composition. Our finding also strengthens the proposal
that the elastic properties of PPv can explain the small P-wave increase across
the D discontinuity and the anticorrelation between the V F and S-wave velocity
anomalies in the D region. |
Junio
de 2008
The structure of the base of the outer core inferred from seismic
waves diffracted around the inner core
Authors: Zuihong Zou, Keith D.
Koper et al
Link: Click here
Abstract
We systematically searched for seismograms
of waves diffracted around the inner core (PKP Cdiff ) from all the temporary
seismic arrays with data currently available at the IRIS DMC, as well as some
permanent regional seismic arrays including F-NET in Japan and GRF in Germany,
to assemble the largest high-quality PKP Cdiff database ever created. PKP Cdiff
waves preferentially sample the base of the outer core and so contain important
clues about Earth structure in this region. We measured PKP DF -PKP Cdiff differential
traveltimes and PKP Cdiff /PKP DF amplitude | ratios
in the distance range of 154°-160° and modeled the observations using
grid searches and full wave theory synthetic seismograms. The optimum model found
by fitting the differential traveltimes has relatively low velocity at the base
of the outer core as in AK135, which is consistent with many previous traveltime
studies. However, the optimum model found by fitting the amplitude ratios (PKP
Cdiff /PKP DF ) does not exhibit this feature, and instead is closer to PREM.
The discrepancy may be explained by two likely causes. One is that small-scale
topography or roughness on the ICB tends to scatter energy away from PKP Cdiff
waves by generating trailing coda waves. The other is that there exists a thin
layer with relatively low Q at the base of the outer core. This might be expected
if there are suspended solid particles at the base of the outer core, as proposed
decades ago. Both mechanisms could generate smaller PKP Cdiff amplitudes without
significantly affecting PKP Cdiff traveltimes. |
Junio
de 2008
Probabilistic seismic hazard in the San Francisco Bay area based
on a simplified viscoelastic cycle model of fault interactions
Authors:
Fred F. Pollitz and David P. Schwartz et al
Link: Click here
Abstract
We construct a viscoelastic cycle model
of plate boundary deformation that includes the effect of time-dependent interseismic
strain accumulation, coseismic strain release, and viscoelastic relaxation of
the substrate beneath the seismogenic crust. For a given fault system, time-averaged
stress changes at any point (not on a fault) are constrained to zero; that is,
kinematic consistency is enforced for the fault system. The dates of last rupture,
mean recurrence times, and the slip distributions of the (assumed) repeating ruptures
are key inputs into the viscoelastic cycle model. This simple formulation allows
construction of stress evolution at all points in the plate boundary zone for
purposes of probabilistic seismic hazard analysis (PSHA). Stress |
evolution is combined with a Coulomb failure stress threshold at representative
points on the fault segments to estimate the times of their respective future
ruptures. In our PSHA we consider uncertainties in a four-dimensional parameter
space: the rupture peridocities, slip distributions, time of last earthquake (for
prehistoric ruptures) and Coulomb failure stress thresholds. We apply this methodology
to the San Francisco Bay region using a recently determined fault chronology of
area faults. Assuming single-segment rupture scenarios, we find that future rupture
probabilities of area faults in the coming decades are the highest for the southern
Hayward, Rodgers Creek, and northern Calaveras faults. This conclusion is qualitatively
similar to that of Working Group on California Earthquake Probabilities, but the
probabilities derived here are significantly higher. Given that fault rupture
probabilities are highly model-dependent, no single model should be used to assess
to time-dependent rupture probabilities. We suggest that several models, including
the present one, be used in a comprehensive PSHA methodology, as was done by Working
Group on California Earthquake Probabilities. |
Mayo
de 2008
A new global model for P wave speed variations in Earth's mantle
Authors: Chang Li, Robert D. van der Hilst et al
Link: Click here
Abstract
We document our tomographic method and
present a new global model of three-dimensional (3-D) variations in mantle P wave
velocity. The model is parameterized by means of rectangular cells in latitude,
longitude, and radius, the size of which adapts to sampling density by short-period
(1 Hz) data. The largest single data source is ISC/NEIC data reprocessed by Engdahl
and coworkers, from which we use routinely picked, short-period P, Pg, Pn, pP,
and pwP data (for earthquakes during the period 1964~2007). To improve the resolution
in the lowermost and uppermost mantle, we use differential times of core phases
(PKP AB - PKP DF , PKP AB - PKP BC , P diff - PKP DF ) and surface-reflected waves
(PP-P). The low-frequency differential times (P diff , |
PP) are measured by waveform cross correlation. Approximate 3-D finite frequency
kernels are used to integrate the long-period data (P diff , PP) and short-period
(P, pP, PKP) data. This global data set is augmented with data from regional catalogs
and temporary seismic arrays. A crust correction is implemented to mitigate crustal
smearing into the upper mantle. We invert the data for 3-D variations in P wave
speed and effects of hypocenter mislocation subject to norm and gradient regularization.
Spatial resolution is ~100 km in the best sampled upper mantle regions. Our model,
which is available online and which will be updated periodically, reveals in unprecedented
detail the rich variation in style of subduction of lithospheric slabs into the
mantle. The images confirm the structural complexity of downwellings in the transition
zone discussed in previous papers and show with more clarity the structure of
slab fragments stagnant in the transition zone beneath east Asia. They also reveal
low wave speed beneath major hot spots, such as Iceland, Afar, and Hawaii, but
details of these structures are not well resolved by the data used. |
Mayo
de 2008
Doppler spreading of internal gravity waves by an inertia-wave
packet
Authors: J. C. Vanderhoff, K. K. Nomura et al
Link: Click
here
Abstract
Present Doppler-spreading models for
the high wave number end of atmospheric and oceanic internal wave spectra either
neglect the time dependence of the background long-wave shear | entirely,
or ignore time-dependent effects in their parameterization of dissipation. Through
ray tracing and numerical simulations the Doppler spreading of an idealized interaction
between a short internal-wave packet by an inertia-wave packet is examined. The
results are sufficient to show that time dependence in the long-wave shear can
make a significant difference to short-wave behavior, and will need to be taken
into account in future modeling efforts. |
Mayo
de 2008
High-pressure polymorphism of Fe2P and its implications for meteorites
and Earth's core
Authors: Przemyslaw Dera, Barbara Lavina et al
Link: Click here
Abstract
Minerals with composition (Fe,Ni)2P,
are rare, though important accessory phases in iron and chondritic meteorites.
The occurrence of these minerals in meteorites is believed to originate either
from the equilibrium condensation of protoplanetary materials in solar nebulae
or from the later accretion and condensation processes | in
the cores of parent bodies. Fe-Ni phosphides are considered a possible candidate
for a minor phase present in the Earth's core, and at least partially responsible
for the observed density deficit with respect to pure iron. We report results
of high-pressure high-temperature X-ray diffraction experiments with synthetic
barringerite (Fe2P) up to 40 GPa and 1400 K. A new phase transition to the Co2Si-type
structure has been found at 8.0 GPa, upon heating. The high-pressure phase can
be metastably quenched to ambient conditions at room temperature, and then, if
heated again, transforms back to barringerite, providing an important constraint
on the thermodynamic history of meteorite. |
Mayo
de 2008
The 26 May 2006 magnitude 6.4 Yogyakarta earthquake south of Mt.
Merapi volcano: Did lahar deposits amplify ground shaking and thus lead to the
disaster?
Authors: T. R. Walter, R. Wang et al
Link: Click here
Abstract
Indonesia is repeatedly unsettled by
severe volcano- and earthquake-related disasters, which are geologically coupled
to the 5-7 cm/a tectonic convergence of the Australian plate beneath the Sunda
Plate. On Saturday, 26 May 2006, the southern coast of central Java was struck
by an earthquake at 2254 UTC in the Sultanate Yogyakarta. Although the magnitude
reached only M w = 6.4, it left more than 6,000 fatalities and up to 1,000,000
homeless. The main disaster area was south of Mt. Merapi Volcano, located within
a |
narrow topographic and structural depression along the Opak River. The earthquake
disaster area within the depression is underlain by thick volcaniclastic deposits
commonly derived in the form of lahars from Mt. Merapi Volcano, which had a major
influence leading to the disaster. In order to more precisely understand this
earthquake and its consequences, a 3-month aftershock measurement campaign was
performed from May to August 2006. We here present the first location results,
which suggest that the Yogyakarta earthquake occurred at 10-20 km distance east
of the disaster area, outside of the topographic depression. Using simple model
calculations taking material heterogeneity into account we illustrate how soft
volcaniclastic deposits may locally amplify ground shaking at distance. As the
high degree of observed damage may have been augmented by the seismic response
of the volcaniclastic Mt. Merapi deposits, this work implies that the volcano
had an indirect effect on the level of earthquake destruction. |
Mayo
de 2008
A study of the 2006 and 2007 earthquake sequence of Pisco, Peru,
with InSAR and teleseismic data
Authors: M. E. Pritchard and E. J. Fielding
Link:
Click here
Abstract
We combine interferometric synthetic
aperture radar (InSAR) and teleseismic body waves to study the largest earthquake
(M w 8.1) in a sequence of events on the subduction megathrust near Pisco, Peru.
Our analysis includes some of the first InSAR | data
from the ALOS satellite and wide swath data from the Envisat satellite. The teleseismic
data indicate the slip maximum occurred 60-90 seconds after the mainshock started.
The InSAR data constrain the main slip patch to be about 70 km from the hypocenter,
suggesting an extremely low rupture velocity (<1.5 km/s) or long slip rise
time. No large earthquake has occurred in the 2007 rupture area since at least
1746 and possibly 1687, suggesting significant aseismic deformation in the area.
The slip deficit apparently cannot be filled with rapid after-slip. In addition,
the area where the Nazca Ridge is subducting appears to be either a seismic gap
or a persistent area of aseismic slip. |
Mayo
de 2008
Primary oil migration through buoyancy-driven multiple fracture
propagation: Oil velocity and flux
Authors: Z.-H. Jin and S. E. Johnson
Link: Click here
Abstract
We present a fracture-mechanics-based
formulation to investigate primary oil migration through the propagation of an
array of periodic, parallel fractures in a sedimentary rock with elevated pore
fluid pressure. The rock is | assumed
to be a linearly elastic medium. The fracture propagation and hence oil migration
velocity are determined using a fracture mechanics criterion together with the
lubrication theory of fluid mechanics. We find that fracture interactions have
profound effects on the primary oil migration behavior. For a given fracture length,
the mass flux of oil migration decreases dramatically with an increase in fracture
density. The reduced oil flux is due to the decreased fracture propagation velocity
as well as the narrowed fracture opening that result from the fracture interactions.
|
Mayo de
2008
A self-replication model for long channelized lava flows on the Mars
plains
Authors: S. M. Baloga and L. S. Glaze et al
Link: Click here
Abstract
A model is presented for channelized
lava flows emplaced by a self-replicating, levee-building process over long distances
on the plains of Mars. Such flows may exhibit morphologic evidence of stagnation,
overspills, and upstream breakouts. However, these processes do not inhibit the
formation and persistence of a prominent central channel that can often be traced
for more than 100 km. The two central assumptions of the self-replication model
are (1) the flow advances at the average upstream velocity of the molten core
and (2) the fraction of the lava that travels faster than | the
average upstream velocity forms stationary margins in the advancing distal zone
to preserve the self-replication process. For an exemplary 300 km long flow north
of Pavonis Mons, the model indicates that ~8 m of crust must have formed during
emplacement, as determined from the channel and levee dimensions. When combined
with independent thermal dynamic estimates for the crustal growth rate, relatively
narrow constraints are obtained for the flow rate (2250 m3 s-1), emplacement duration
(600 d), and the lava viscosity of the molten interior (106 Pa s). Minor, transient
overspills and breakouts increase the emplacement time by only a factor of 2.
The primary difference between the prodigious channelized Martian flows and their
smaller terrestrial counterparts is that high volumetric flow rates must have
persisted for many hundreds of days on Mars, in contrast to a few hours or days
on Earth. |
|
Mayo
de 2008
Computer simulation of the role of groundwater seepage in forming
Martian valley networks
Authors: Wei Luo and Alan D. Howard
Link:
Click here
Abstract
The role of groundwater in forming Martian
valley networks is simulated in a computer model as seepage erosion by contributing
to surface runoff and as seepage weathering by causing accelerated weathering
of bedrock, which makes its subsequent erosion and removal easier. Simulation
results show that seepage erosion cannot mobilize large grain size sediment and
is marginally effective at generating integrated valley networks with realistic
rates of aquifer recharge. On the other hand, seepage weathering may play a major
role in forming Martian valley networks. Seepage weathering combined with fluvial
runoff | creates stubby deep
canyons with abrupt headwalls that are similar in morphology to terrestrial and
Martian valley systems attributed to erosion by groundwater. Depending on the
relative contribution of groundwater weathering to surface runoff erosion, a continuum
of valley network morphology can be generated. Eolian modification masks the original
differences in fluvial landforms, making different scenarios visually more similar.
Martian valley networks may have developed through a range of combinations of
runoff erosion and seepage weathering, which can complicate the interpretation
of the processes based on final landform morphology. Unequivocal identification
of seepage involvement of valley incision on Mars may not be possible without
knowledge of subsurface properties (hydraulic conductivity, layering, degree of
cementation, etc.) and the grain sizes of sediment transported through the valley
systems. |
Mayo
de 2008
Current systems in the Jovian magnetosphere
Authors: Tadanori
Moriguchi, Aoi Nakamizo et al
Link: Click here
Abstract
The plasma dynamics in the Jovian magnetosphere
is characterized by rapid rotation of the planet. In this paper, a new magnetohydrodynamic
(MHD) simulation scheme is developed to precisely calculate the current systems
in the Jovian magnetosphere and to relate them with the plasma dynamics. A field-aligned
current (FAC) pattern that is expected for the ionosphere to drive the corotation
is reproduced in the present simulation result with current away from the ionosphere
at lower latitudes and current toward the ionosphere at higher latitudes. In the
magnetospheric region, a thin equatorial current |
sheet with eastward current flowing around Jupiter (ring current) dominates in
the middle magnetosphere. By tracing current lines, it is found that the upward
FAC on the low-latitude side in the ionosphere is first connected to the ring
current and then to the Chapman-Ferraro current through the plasma sheet current.
Downward FAC in the high-latitude ionosphere is spirally connected to the tail
lobe. From the distribution of J·E, the places of dynamo and energy consumption
region are investigated to understand the energy balance associated with the corotation
by magnetosphere-ionosphere (M-I) current system. From these analyses, the FAC,
ring current, the Chapman-Ferraro current and plasma distribution in the Jovian
magnetosphere are understood as a self-consistently organized structure through
corotation and confinement. In addition, some interesting field-line structures
and plasmoid formation are found by tracing the magnetic field line. |
Mayo
de 2008
Effects of supershear rupture speed on the high-frequency content
of S waves investigated using spontaneous dynamic rupture models and isochrone
theory
Authors: A. Bizzarri, P. Spudich et al
Link: Click here
Abstract
In this paper we achieve three goals:
(1) We demonstrate that crack tips governed by friction laws, including slip weakening,
rate- and state-dependent laws, and thermal pressurization of pore fluids, propagating
at supershear speed have slip velocity functions with reduced high-frequency content
compared to crack tips traveling at subshear speeds. This is demonstrated using
a fully dynamic, spontaneous, three-dimensional earthquake model, in which we
calculate fault slip velocity at nine points (locations) distributed along a quarter
circle on the fault where the rupture is traveling at supershear speed in the
in-plane direction and subshear speed in the antiplane | direction.
This holds for a fault governed by the linear slip-weakening constitutive equation,
by slip weakening with thermal pressurization of pore fluid, and by rate- and
state-dependent laws with thermal pressurization. The same is also true even assuming
a highly heterogeneous initial shear stress field on the fault. (2) Using isochrone
theory, we derive a general expression for the spectral characteristics and geometric
spreading of two pulses arising from supershear rupture, the well-known Mach wave,
and a second lesser known pulse caused by rupture acceleration. (3) We demonstrate
that the Mach cone amplification of high frequencies overwhelms the de-amplification
of high-frequency content in the slip velocity functions in supershear ruptures.
Consequently, when earthquake ruptures travel at supershear speed, a net enhancement
of high-frequency radiation is expected, and the alleged "low" peak
accelerations observed for the 2002 Denali and other large earthquakes are probably
not caused by diminished high-frequency content in the slip velocity function,
as has been speculated. |
Mayo
de 2008
Geoelectrical inference of mass transfer parameters using temporal
moments
Authors: Frederick D. Day-Lewis, Kamini Singha et al
Link:
Click here
Abstract
We present an approach to infer mass
transfer parameters based on (1) an analytical model that relates the temporal
moments of mobile and bulk concentration and (2) a bicontinuum modification | to
Archie's law. Whereas conventional geochemical measurements preferentially sample
from the mobile domain, electrical resistivity tomography (ERT) is sensitive to
bulk electrical conductivity and, thus, electrolytic solute in both the mobile
and immobile domains. We demonstrate the new approach, in which temporal moments
of collocated mobile domain conductivity (i.e., conventional sampling) and ERT-estimated
bulk conductivity are used to calculate heterogeneous mass transfer rate and immobile
porosity fractions in a series of numerical column experiments. |
Mayo
de 2008
Rapid, precise, and high-sensitivity acquisition of paleomagnetic
and rock-magnetic data: Development of a low-noise automatic sample changing system
for superconducting rock magnetometers
Authors: Joseph L. Kirschvink,
Robert E. Kopp et al
Link: Click here
Abstract
Among Earth sciences, paleomagnetism
is particularly linked to the statistics of large sample sets as a matter of historical
development and logistical necessity. Because the geomagnetic field varies over
timescales relevant to sedimentary deposition and igneous intrusion, while the
fidelity of recorded magnetization is modulated by original | properties
of rock units and by alteration histories, "ideal" paleomagnetic results
measure remanent magnetizations of hundreds of samples at dozens of progressive
demagnetization levels, accompanied by tests of magnetic composition on representative
sister specimens. We present an inexpensive, open source system for automating
paleomagnetic and rock magnetic measurements. Using vacuum pick-and-place technology
and a quartz-glass sample holder, the system can in 1 h measure remanent magnetizations,
as weak as a few pAm2, of ~30 specimens in two vertical orientations with measurement
errors comparable to those of the best manual systems. The system reduces the
number of manual manipulations required per specimen approximately eightfold.
|
Mayo de
2008
ArcheoInt: An upgraded compilation of geomagnetic field intensity
data for the past ten millennia and its application to the recovery of the past
dipole moment
Authors: A. Genevey, Y. Gallet et al
Link: Click here
Abstract
This paper presents a compilation of
intensity data covering the past 10 millennia (ArcheoInt). This compilation, which
upgrades the one of Korte et al. (2005), contains 3648 data and incorporates additional
intensity and directional data sets. A large majority of these data (~87%) were
acquired on archeological artifacts, and the remaining ~13% correspond to data
obtained from volcanic products. The present compilation also includes important
metadata for evaluating the intensity data quality and providing a foundation
to guide improved selection criteria. We show that ~50% of the data set fulfill
reasonable reliability standards which take into account the anisotropic nature
of most studied objects (potsherds), the stability of the magnetization, and the
data dispersion. The temporal and geographical distributions of this sub-data
set are similar to those of the main data set, with ~72% of the data dated from
the past | three
millennia and ~76% obtained from western Eurasia. Approximately half of the selected
intensity data are associated with at least an inclination value. To constrain
the axial and full dipole evolution over the past three millennia requires that
we avoid any overrepresentation of the western Eurasian data. We introduce a first-order
regional weighting scheme based on the definition of eight widely distributed
regions of 30° width within which the selected data are numerous enough. The
regional curves of virtual axial dipole moments (VADM) and of mixed VADM-virtual
dipole moments (VDM) averaged over sliding windows of 200 years and 500 years
testify for strong contributions from either equatorial dipole or nondipole components.
The computation of global VADM and mixed VADM/VDM variation curves, assuming an
equal weight for each region, yields a dipole evolution marked by a distinct minimum
around 0 B.C./A.D. followed by a maximum around the third-fourth century A.D.
A second minimum is present around the eighth century A.D. This variation pattern
is compatible with the one deduced from earlier, more sophisticated analysis based
on the inversion of both intensity and directional data. In particular, there
is a good agreement among all VADMs and dipole moment estimates over the historical
period, which further strengthens the validity of our weighting scheme. |
Abril
de 2008
Aquifer structure identification using stochastic inversion
Authors: Dylan R. Harp, Zhenxue Dai et al
Link: Click here
Abstract
This study presents a stochastic inverse
method for aquifer structure identification using sparse geophysical and hydraulic
response data. The method is based on updating structure parameters from a transition
probability model to iteratively modify the aquifer structure and parameter | zonation.
The method is extended to the adaptive parameterization of facies hydraulic parameters
by including these parameters as optimization variables. The stochastic nature
of the statistical structure parameters leads to nonconvex objective functions.
A multi-method genetically adaptive evolutionary approach (AMALGAM-SO) was selected
to perform the inversion given its search capabilities. Results are obtained as
a probabilistic assessment of facies distribution based on indicator cokriging
simulation of the optimized structural parameters. The method is illustrated by
estimating the structure and facies hydraulic parameters of a synthetic example
with a transient hydraulic response. |
Abril
de 2008
MILAMIN: MATLAB-based finite element method solver for large problems
Authors: M. Dabrowski, M. Krotkiewski et al
Link: Click here
Abstract
The finite element method (FEM) combined
with unstructured meshes forms an elegant and versatile approach capable of dealing
with the complexities of problems in Earth science. Practical applications often
require high-resolution models that necessitate advanced computational strategies.
We therefore developed "Million a Minute" (MILAMIN), an efficient MATLAB
implementation of FEM that is capable of setting up, solving, and postprocessing
two-dimensional problems with one million unknowns in one minute on a modern desktop
computer. MILAMIN allows the user to achieve numerical resolutions that are necessary
to resolve the heterogeneous nature of geological materials. In this paper we
provide the technical knowledge required to develop such models without the need
to buy a commercial FEM package, programming compiler-language code, or hiring
a computer specialist. It has been our special aim that all the components of
MILAMIN perform efficiently, individually and as a package. While some of the
components rely on readily | available
routines, we develop others from scratch and make sure that all of them work together
efficiently. One of the main technical focuses of this paper is the optimization
of the global matrix computations. The performance bottlenecks of the standard
FEM algorithm are analyzed. An alternative approach is developed that sustains
high performance for any system size. Applied optimizations eliminate Basic Linear
Algebra Subprograms (BLAS) drawbacks when multiplying small matrices, reduce operation
count and memory requirements when dealing with symmetric matrices, and increase
data transfer efficiency by maximizing cache reuse. Applying loop interchange
allows us to use BLAS on large matrices. In order to avoid unnecessary data transfers
between RAM and CPU cache we introduce loop blocking. The optimization techniques
are useful in many areas as demonstrated with our MILAMIN applications for thermal
and incompressible flow (Stokes) problems. We use these to provide performance
comparisons to other open source as well as commercial packages and find that
MILAMIN is among the best performing solutions, in terms of both speed and memory
usage. The corresponding MATLAB source code for the entire MILAMIN, including
input generation, FEM solver, and postprocessing, is available from the authors
( http://www.milamin.org) and can be downloaded as auxiliary material. |
Abril
de 2008
GEOMAGIA50: An archeointensity database with PHP and MySQL
Authors: K. Korhonen, F. Donadini et al
Link: Click here
Abstract
The GEOMAGIA50 database stores 3798
archeomagnetic and paleomagnetic intensity determinations dated to the past 50,000
years. It | also
stores details of the measurement setup for each determination, which are used
for ranking the data according to prescribed reliability criteria. The ranking
system aims to alleviate the data reliability problem inherent in this kind of
data. GEOMAGIA50 is based on two popular open source technologies. The MySQL database
management system is used for storing the data, whereas the functionality and
user interface are provided by server-side PHP scripts. This technical brief gives
a detailed description of GEOMAGIA50 from a technical viewpoint. |
Abril
de 2008
Meteorites on Mars observed with the Mars Exploration Rovers
Authors: C. Schröder, D. S. Rodionov et al
Link: Click here
Abstract
Reduced weathering rates due to the
lack of liquid water and significantly greater typical surface ages should result
in a higher density of meteorites on the surface of Mars compared to Earth. Several
meteorites were identified among the rocks investigated during Opportunity's traverse
across the sandy Meridiani plains. Heat Shield Rock is a IAB iron meteorite and
has been officially recognized as "Meridiani Planum." Barberton is | olivine-rich
and contains metallic Fe in the form of kamacite, suggesting a meteoritic origin.
It is chemically most consistent with a mesosiderite silicate clast. Santa Catarina
is a brecciated rock with a chemical and mineralogical composition similar to
Barberton. Barberton, Santa Catarina, and cobbles adjacent to Santa Catarina may
be part of a strewn field. Spirit observed two probable iron meteorites from its
Winter Haven location in the Columbia Hills in Gusev Crater. Chondrites have not
been identified to date, which may be a result of their lower strengths and probability
to survive impact at current atmospheric pressures. Impact craters directly associated
with Heat Shield Rock, Barberton, or Santa Catarina have not been observed, but
such craters could have been erased by eolian-driven erosion. |
Abril
de 2008
Coseismic displacement and tectonic implication of 1951 Longitudinal
Valley earthquake sequence, eastern Taiwan
Authors: Yuan-Hsi Lee, Guin-Ting
Chen et al
Link: Click here
Abstract
The Longitudinal Valley Fault (LVF)
in eastern Taiwan is an extremely active fault with 3-4 cm of displacements consumed
each year along its length. The fault forms the suture zone between the Philippine
Sea and Eurasian plates as a result of an oblique arc continental collision. From
22 October to 5 December 1951, four earthquakes (Ms > 7) shook the LVF. We
used triangulation (from 1917 to 1921 to 1976-1978) and interseismic GPS (from
1990 to 1995) data to estimate coseismic |
displacements of the 1951 earthquake sequences. Coseismic displacement progressively
decreases from north to south and the azimuth changes from north to NE, then to
a NW direction. According to the inverted faulting mechanism, the Longitudinal
Valley fault can be separated into three segments. Both the northern and central
segments have a high dip angle to the east, but the southern segment is of listric
fault geometry. The northern segment exhibits dominantly left lateral strike-slip
faulting with reverse component, while the middle exhibits thrusting dominantly,
and the southern segment exhibits thrusting with left-lateral motion associated
with a smaller coseismic displacement. In addition, this three-segment deformation
model can explain the pattern of recent crustal deformation along the LVF and
Coastal Range. |
Abril
de 2008
San Andreas Fault damage at SAFOD viewed with fault-guided waves
Authors: Yong-Gang Li and Peter E. Malin
Link: Click here
Abstract
Highly damaged rocks within the San
Andreas fault zone at Parkfield form a low-velocity waveguide for seismic waves,
giving rise to fault-guided waves. Prominent fault-guided waves have been observed
at the San Andreas Fault Observatory at Depth (SAFOD) site, including a surface
array across the fault zone and a borehole seismograph |
placed in the SAFOD well at a depth of ~2.7 km below ground. The resulting observations
are modeled here using 3-D finite-difference methods. To fit the amplitude, frequency,
and travel-time characteristics of the data, the models require a downward tapering,
30-40-m wide fault-core embedded in a 100-200-m wide jacket. Compared with the
intact wall rocks, the core velocities are reduced by ~40% and jacket velocities
by ~25%. Based on the depths of earthquakes generating guided waves, we estimate
that the low-velocity waveguide along the fault at SAFOD extends at least to depths
of ~7 km, more than twice the depth reported in pervious studies. |
Abril
de 2008
Topography growth drives stress rotations in the central Andes:
Observations and models
Authors: Oliver Heidbach, Giampiero Iaffaldano
et al
Link: Click here
Abstract
Recent numerical models that couple
global mantel circulation with lithosphere dynamics show that growth of the central
Andes controls the 30% reduction of convergence velocity between the Nazca and
South America plates observed over the | past
10 Ma. The increase of gravitational potential energy due to topographic growth
is also a major control on the stress pattern. Here we use numerical models which
reproduce the Nazca/South America convergence history to predict the change of
stress pattern in the central Andes for the past 10 Ma. Comparison of the modeled
stress orientations at present-day with the observed ones results in ±23.9°
mean deviation. Based on this good agreement we attempt to predict paleostress
orientations 10 Ma ago. Interestingly, the modeled stress orientations 3.2 Ma
ago are very similar to the present-day orientations. From this result we infer
that stress rotations occurred between 10 and 3.2 Ma ago, when topography was
considerably lower. |
Abril
de 2008
Enhanced GPS inversion technique applied to the 2004 Sumatra earthquake
and tsunami
Authors: Andreas Hoechner, Andrey Y. Babeyko et al
Link:
Click here
Abstract
Since the devastating earthquake and
tsunami in 2004 offshore Sumatra, many source models have been put forward. Recent
studies clearly show that modern GPS-processing could achieve high resolving power
for slip in near real time, which is crucial for determining tsunami initial conditions, | provided
accurate GPS-processing and inversion. Here, we propose an inversion technique
with improved representation of the subduction zone geometry and physically justified
boundary conditions. We show that the discrepancy between the inversion of near-
and far field GPS data for the 2004 event, which is often explained by postseismic
slip, can be eliminated by using our inversion method and IASP91 earth model.
Inverted source models, including versions with splay faulting, are shown to be
consistent with satellite altimetry data of offshore tsunami wave height, suggesting
that displacement at the splay fault might have been present but was likely a
second order process. |
Abril
de 2008
Geophysical mapping of ground ice using a combination of capacitive
coupled resistivity and ground-penetrating radar, Northwest Territories, Canada
Authors: Gregory P. De Pascale, Wayne H. Pollard et al
Link: Click here
Abstract
The nature and distribution of ground
ice are two of the most unpredictable geological variables in near-surface deposits
characterized by continuous permafrost. Subsurface information about ground ice
distribution and structure can be obtained either by invasive and environmentally
destructive techniques like drilling and excavation or by noninvasive low-impact
geophysical methods. In this study, coordinated measurements by two complementary
geophysical tools, capacitive-coupled resistivity (CCR) and ground-penetrating
radar (GPR) were used to map ground ice in a variety of locations in the Mackenzie
Delta region | of the western
Canadian Arctic. Both CCR and GPR systems are highly portable (especially on snow
covered surfaces) and very effective in collecting data under winter conditions
when cold ground temperatures ensure that nearly all liquid water is frozen and
signal penetration is enhanced. CCR and GPR readily detect stratigraphic differences
including the contacts between massive ice deposits and enclosing sediments. GPR
is widely used in permafrost research, but CCR has been used in only a few studies.
This is the first study to combine results from both systems by collecting complementary
data sets along coincident transects. We demonstrate that when combined, these
data increase the quality and interpretation of subsurface information beyond
what could be determined by either of the instruments alone. The complementary
nature of these two geophysical tools facilitated the detection and mapping of
massive ground ice, ice-rich sediments, ice wedges, thermokarst, and basic stratigraphic
relationships. This study breaks new ground by documenting the benefits of using
these techniques together in permafrost investigations. |
Abril
de 2008
Antarctic ice mass balance estimates from GRACE: Tidal aliasing
effects
Authors: Philip Moore and Matt A. King et al
Link: Click
here
Abstract
Geophysical interpretation of GRACE
gravity fields has provided estimates of Antarctic ice mass change. Such analyses
rely on proper consideration of ocean tidal effects through the models CSR4 and
FES2004. In general, mis-modeling of tidal constituents with aliasing period less
than 30 day will not have significant impact on ice mass change. However, for
constituents, such as K1, K2, and S2, the aliasing period is sufficiently large
to potentially compromise long-term variability studies. Here we quantify tidal
aliasing over Antarctica by simulating GRACE signatures |
due to differences between CSR4 and FES2004, and the best available circum-Antarctic
model, TPXO6.2. The S2 simulations are in close agreement with the observed S2
signal from GRACE. Simulations of ice mass change show that over 2002-2006 long-term
K1 and K2 aliasing is equivalent to a rate error of 4.5 ± 1.3 km3/a of
ice with CSR4, but only 0.2 ± 0.2 km3/a with FES2004. After spatial averaging
and destriping, K1 plus K2 mis-modeling in CSR4 (FES2004) introduce point-wise
errors up to 5 (2) mm/a in equivalent water height over a 3.5 year period. With
observed mass change equivalent to less than 30 mm/a of water height over much
of Antarctica, the simulations show tidal aliasing uncertainty at the 2-3 mm/a
level for August 2002-January 2006, or ~10% of the signal. With GRACE Release
04, the revised estimate (April 2002-January 2006) of published ice volume decrease
is 164 ± 80 km3/a of ice, although this value depends very much on the
GIA model and GRACE analysis approach. |
Abril
de 2008
Un modelo Bayesiano de selección aplicado a las Redes Neuronales
Artificiales utilizado para el modelamiento de los recursos de agua.
Autores:
Greer B. Kingston, Holger R. Maier et al
Link: Click Aquí Abstract
Las
Redes Neuronales Artificiales (RNAs) han probado ser extremadamente valiosas como
herramientas en el campo de la ingeniera de los recursos de agua. Sin embargo,
una de las más difíciles tareas asociadas a las RNAs corresponde
a determinar el nivel óptimo de complejidad requerido para modelar el problema,
dado que no existe un método formal de selección. Este paper presenta
un Modelo de Selección Bayesiano (MSB) que permite un objetivo acercamiento
para comparar modelos de diversas complejidades con el objetivo de seleccionar
la arquitectura neuronal óptima. Nuestra solución utiliza Cadenas
de Markov y el Método de Monte Carlo con posteriores simulaciones para
ponderar la evidencia a favor de diversos modelos. En este estudio se utilizan
tres conocidos métodos para comparar la idoneidad de incorporar la RNA
adecuada dentro del marco MSB. Sin embargo, se sabe que puede ser particularmente
difícil ponderar de un modo confiable la evidencia que presentan distintos
modelos de RNAs. El acercamiento MSB incorpora un procedimiento para ponderar
la evidencia mediante la inspección de distribuciones marginales posteriores
de los pesos sinápticos que van desde la capa oculta a la capa de salida,
lo cual indica de un modo directo cualquier posible redundancia presente en los
nodos de la capa oculta. La disponibilidad de esta estimación es una de
las mayores ventajas de nuestro acercamiento respecto de los métodos convencionales
de selección, los cuales no proporcionan la posibilidad de testeo (en su
lugar dejan todo en las manos del criterio de selección subjetivo del modelador).
Las ventajas de un acercamiento totalmente Bayesiano son demostradas en dos casos
sintéticos y uno real sobre recursos de agua.
|
Abril de 2008
Bayesian model selection applied to artificial
neural networks used for water resources modeling
Authors: Greer B. Kingston,
Holger R. Maier et al
Link: Click here
Abstract
Artificial neural networks (ANNs) have
proven to be extremely valuable tools in the field of water resources engineering.
However, one of the most difficult tasks in developing an ANN is determining the
optimum level of complexity required to model a given problem, as there is no
formal systematic model selection method. This paper presents a Bayesian model
selection (BMS) method for ANNs that provides an objective approach for comparing
models of varying complexity in order to select the most appropriate ANN structure.
The approach uses Markov Chain Monte Carlo posterior simulations to estimate the
evidence in favor of competing models and, in this study, three known methods
for doing this are compared in terms of their suitability for being incorporated
into the proposed BMS framework for ANNs. However, it is acknowledged that it
can be particularly difficult to accurately estimate the evidence of ANN models.
Therefore, the proposed BMS approach for ANNs incorporates a further check of
the evidence results by inspecting the marginal posterior distributions of the
hidden-to-output layer weights, which unambiguously indicate any redundancies
in the hidden layer nodes. The fact that this check is available is one of the
greatest advantages of the proposed approach over conventional model selection
methods, which do not provide such a test and instead rely on the modeler's subjective
choice of selection criterion. The advantages of a total Bayesian approach to
ANN development, including training and model selection, are demonstrated on two
synthetic and one real world water resources case study. |
Abril
de 2008
Demonstrating fractal scaling of baseflow residence time distributions
using a fully-coupled groundwater and land surface model
Authors: Stefan
J. Kollet and Reed M. Maxwell et al
Link: Click here
Abstract
The influence of the vadose zone, land
surface processes, and macrodispersion on the shape and scaling behavior of residence
time distributions of baseflow is studied using a fully coupled watershed model
in conjunction with a | Lagrangian,
particle-tracking approach. Numerical experiments are used to simulate groundwater
flow paths from recharge locations along the hillslope to the streambed. These
experiments are designed to isolate the influences of topography, vadose zone/land
surface processes, and macrodispersion on subsurface transport of tagged parcels
of water. The results of these simulations agree with previous observations that
such distributions exhibit a power law form and fractal behavior, which can be
identified from plots of the residence time distribution and the power spectra.
It is shown that vadose zone/land surface processes significantly affect both
the residence time distributions and their spectra. |
Marzo
de 2008
Non-ideal liquidus curve in the Fe-S system and Mercury's snowing
core
Authors: Bin Chen, Jie Li et al
Link: Click here
Abstract
We conducted multi-anvil experiments
to investigate the melting behavior of the iron-sulfur system at moderate pressures.
Our data reveal a positive departure from ideal solution behavior at 14 GPa, as
indicated by the presence of two | inflection
points on the liquidus curve of iron-rich compositions. In contrast, the shape
of the liquidus curve at 10 GPa is consistent with nearly ideal mixing between
end-member components. Combined with existing data at lower pressures and above
20 GPa, our results suggest a negative liquidus temperature gradient under conditions
found at shallow depths in Mercury's core. At the present time, the core is most
likely precipitating solid iron in the form of snow, at a single depth or in two
distinct zones. Formation and segregation of iron snow would alter the thermal
and chemical state of the core and influence the origin and surface expression
of the planet's magnetic field. |
Marzo
de 2008
On the estimation of seismic intensity in earthquake early warning
systems
Authors: Shunroku Yamamoto, Paul Rydelek et al
Link: Click
here
Abstract
The infamous Tokai Earthquake, which
by some accounts is overdue, is expected to be a magnitude = 8 event that will
cause unprecedented damage in regions of Japan. To mitigate hazards from large
earthquakes in Japan, an earthquake early warning (EEW) system was developed that
is able to determine hypocentral locations from P-arrival | data
within a few seconds and then transmit this information before the onset of large
ground motions from the later arrival of S-waves. We introduced a new source parameter
for seismic intensity magnitude that can be estimated from the real-time P-wave
data during the early stages of fault rupture for most earthquakes M = 6.5. The
use of this parameter results in a significant improvement in the uncertainty
in the estimated seismic intensity compared to estimates derived from earthquake
magnitude. A pre-established relation between the P- and S-wave seismic intensity
therefore enables an EEW system to issue a rapid and reasonably reliable prediction
of the amount of ground shaking that may be expected from the damaging S-waves.
|
Marzo
de 2008
Ambient seismic noise tomography and structure of eastern
North America
Authors: Chuntao Liang & Charles A. Langston
Link: Click here
Abstract
The time derivative of cross-correlation
functions (CCF) of ambient noise fields recorded by two stations can be approximated
as the Green's Function (GF) between the stations. The CCFs are thus used as Peudo-GFs
(dominated by surface waves) to invert for group velocity structure in eastern
North America. Stations from two regional networks deployed to monitor the New
Madrid Seismic Zone and eastern Tennessee seismic zone, together with stations
of the US National Seismic Network, greatly improve tomographic ray coverage.
The short period (T = 5 s) group velocity | map
shows strong correlations with the depth to Precambrian basement. Many subtle
local structures can be clearly identified from the velocity map, including the
Ozark uplift, Cincinnati Arch, Nashville Dome and the Blue Ridge province of the
Appalachians showing relatively high group velocity. The long period (T = 15 s)
group velocity map shows strong correlations with regional geology. Ancient rift
basins, such as the Mid-Continent Rift (MCR) system, the Reelfoot rift, the Oklahoma
Aulacogen and the Eastern Continent Rift, are associated with low velocity belts
along their rift axes. We also find that all major seismic zones in eastern North
America, such as the New Madrid seismic zone, Eastern Tennessee seismic zone as
well as Ouachita Orogen seismic zone, are approximately located at transition
zones separating velocity highs and lows. This observation suggests that those
seismic zones may reflect the reactivation of ancient faults associated with continental
rift and collision zones. |
|
Marzo
de 2008
High-definition analysis of fluid-induced seismicity related
to the mesoscale hydromechanical properties of a fault zone
Authors:
Yves Guglielmi, Frédéric Cappa et al
Link: Click here
Abstract
Interactions between pore pressure,
strain and seismicity were analyzed in a 30-m-thick shallow fault zone that was
subjected to a small overpressure of 80 kPa to produce small changes in effective
stress. Simultaneous pressure-strain | and
seismic measurements taken in the fractured damage zone during the pressurization
indicated that seismicity is triggered along low-permeable, highly rigid, low-dip
angle, mesoscale-inherited fractures where deformation is controlled by the hydromechanical
behavior of the surrounding high-permeability, not-so-rigid, aseismic, sub-vertical,
fault-related fractures. Using a three-dimensional distinct-element representation
of the fractures network, we show that the seismicity observed is mainly due to
shear-slip along inherited planes in response to the mechanical loading induced
by the opening by a few microns of the surrounding subfaults. |
Marzo
de 2008
Gas hydrate dissociation in sediments: Pressure-temperature
evolution
Authors: Tae-Hyuk Kwon, Gye-Chun Cho et al
Link: Click
here
Abstract
Hydrate-bearing sediments may destabilize
spontaneously as part of geological processes, unavoidably during petroleum drilling/production
operations or intentionally as part of gas extraction from the hydrate itself.
In all cases, high pore fluid pressure generation is anticipated during hydrate
dissociation. A comprehensive formulation is derived for the prediction of fluid
pressure evolution in hydrate-bearing sediments subjected to thermal stimulation
without mass transfer. The formulation considers pressure- and temperature-dependent
volume changes in all phases, effective stress-controlled sediment compressibility,
capillarity, and the relative solubilities of fluids. Salient implications are | explored
through parametric studies. The model properly reproduces experimental data, including
the PT evolution along the phase boundary during dissociation and the effect of
capillarity. Pore fluid pressure generation is proportional to the initial hydrate
fraction and the sediment bulk stiffness; is inversely proportional to the initial
gas fraction and gas solubility; and is limited by changes in effective stress
that cause the failure of the sediment. When the sediment stiffness is high, the
generated pore pressure reflects thermal and pressure changes in water, hydrate,
and mineral densities. Comparative analyses for CO2 and CH4 highlight the role
of gas solubility in excess pore fluid pressure generation. Dissociation in small
pores experiences melting point depression due to changes in water activity, and
lower pore fluid pressure generation due to the higher gas pressure in small gas
bubbles. Capillarity effects may be disregarded in silts and sands, when hydrates
are present in nodules and lenses and when the sediment experiences hydraulic
fracture. |
Marzo
de 2008
A semi-empirical model of the contribution from sporadic meteoroid
sources on the meteor input function in the MLT observed at Arecibo
Authors: Jonathan T. Fentzke & Diego Janches et
al
Link: Click here
Abstract
In this paper, we present a modeling
and observational study of the micrometeor input function with a focus on understanding
how each of the extraterrestrial sporadic meteoroid sources contributes to the
observed meteoric flux in the Mesosphere and Lower Thermosphere (MLT) atmospheric
region. For this purpose, we expand the model presented by Janches et al. (2006)
using a Monte Carlo technique and incorporating: 1) a widely accepted global mass
flux, which is divided into different proportions among the known sporadic meteoroid
sources as the initial input above Earth’s atmosphere; 2) contemporary knowledge
on the source's velocity and radiant distributions; and 3) the full integration
of the canonical meteor equations that describe the | meteoroid
entry and ablation physics. In addition, we constrain the initial input through
a comparison of our modeled results with meteor observations obtained with the
430 MHz High Power and Large Aperture (HPLA) Arecibo radar in Puerto Rico that
covers all seasons. The predicted meteor rates and velocity distributions are
in excellent agreement with the observed ones without the need for any additional
normalization factor. Our results indicate that although the Earth’s Apex
centered radiant source, which is characterized by high geocentric speeds (~ 55
km/s), appears to be ~ 33% of the meteoroids in the Solar System at 1 AU, it accounts
for ∼60% of the meteors observed by the Arecibo HPLA radar in the atmosphere.
The remaining 40% of observed meteors originate mostly from the Helion and Anti-Helion
sources, with a very small, but constant during the day, contribution of the South
and North Toroidal sources. These results also suggest that particles smaller
than ~ 10−3μg with slow velocities (<30 km/s) will not significantly
ablate and never become observable meteors. The motivation of this effort is to
construct a new and more precise MIF model needed for the subsequent modeling
of the atmospheric phenomena related to the meteoric flux. |
Marzo
de 2008
Electrowetting on mineral and rock surfaces
Authors:
F. M. Francisca, D. Fratta et al
Link: Click here
Abstract
Electrowetting was investigated
as a method to influence fluid movement on mineral or rock surfaces. The experimental
setup was to place a small drop of water containing an electrolyte on a solid
surface directly or on the same solid surface coated with a hydrophobic film.
The contact angle decreased with an applied electric field until it reached a
saturation value associated with a | critical
voltage. The electrowetting experiments were conducted again after surfactant
was added to the water drop. In all cases the contact angle decreased with increasing
voltage and surfactant concentration. Electrowetting enhanced the wetting effect
over that of the surfactant alone both by lowering the contact angle and by promoting
rupture of the hydrophobic film. Four zones of behavior were identified based
on the surfactant concentration and the voltage level. The results open the possibility
that electrowetting can be used to enhance mobilization of one fluid relative
to another in hydrocarbon recovery or contaminant remediation, as interfacial
tensions control multiphase fluid flow. |
Marzo
de 2008
Use of FLUXNET in the Community
Land Model development
Authors: R. Stöckli,
D. M. Lawrence et al
Link: Click here
Abstract
The Community Land Model version
3 (CLM3.0) simulates land-atmosphere exchanges in response to climatic forcings.
CLM3.0 has known biases in the surface energy partitioning as a result of deficiencies
in its hydrological and biophysical parameterizations. Such models, however, need
to be robust for multidecadal global climate simulations. FLUXNET now provides
an extensive data source of carbon, water and energy exchanges for investigating
land processes, and it encompasses a global range of ecosystem-climate interactions.
Data from 15 FLUXNET sites are used to identify and improve model deficiencies.
Including a prognostic | aquifer,
a bare soil evaporation resistance formulation and numerous other changes in the
model result in a significantly improved soil hydrology and energy partitioning.
Terrestrial water storage increased by up to 300 mm in warm climates and decreased
in cold climates. Nitrogen control of photosynthesis is revealed as another missing
process in the model. These improvements increase the correlation coefficient
of hourly and monthly latent heat fluxes from a range of 0.5–0.6 to the
range of 0.7–0.9. RMSE of the simulated sensible heat fluxes decrease by
20–50%. Primary production is overestimated during the wet season in mediterranean
and tropical ecosystems. This might be related to missing carbon-nitrogen dynamics
as well as to site-specific parameters. The new model (CLM3.5) with an improved
terrestrial water cycle should lead to more realistic land-atmosphere exchanges
in coupled simulations. FLUXNET is found to be a valuable tool to develop and
validate land surface models prior to their application in computationally expensive
global simulations. |
Marzo
de 2008
Enhanced precision, accuracy, efficiency, and spatial resolution
of U-Pb ages by laser ablation–multicollector–inductively coupled
plasma–mass spectrometry
Authors:
George E. Gehrels, Victor A. Valencia et al
Link: Click here
Abstract
U-Th-Pb geochronology by laser ablation–multicollector–inductively
coupled plasma–mass spectrometry initiated during the | mid
to late 1990s as a reconnaissance tool, capable of generating ages of only moderate
precision from relatively large volumes of zircon. New developments in instrumentation
and experimental methodology, as described herein and by other researchers, now
make it possible it to correct for common Pb accurately (using measured 204Pb),
to acquire geochronologic information rapidly (30–40 unknowns/h), to generate
U-Pb ages with an accuracy of better than 1% for most zircon standards, and to
conduct analyses on much smaller (e.g., 10 μm by 6 μ m)
volumes of material. These capabilities are driving important advances in many
aspects of Earth science research. |
Marzo
de 2008
Evaluation of real-time PM2.5 forecasts and process analysis
for PM2.5 formation over the eastern United States using the Eta-CMAQ forecast
model during the 2004 ICARTT study
Authors: Shaocai Yu, Rohit Mathur
et al
Link: Click here
Abstract
The performance of the Eta-Community
Multiscale Air Quality (CMAQ) modeling system in forecasting PM2.5 and chemical
species is assessed over the eastern United States with the observations obtained
by aircraft (NOAA P-3 and NASA DC-8) and four surface monitoring networks (AIRNOW,
IMPROVE, CASTNet and STN) during the 2004 International Consortium for Atmospheric
Research on Transport and Transformation (ICARTT) study. The results of the statistical
analysis at the AIRNOW sites show that the model was able to reproduce the day-to-day
and spatial variations of observed PM2.5 and captured a majority (73%) of PM2.5
observations within a factor of 2, with normalized mean bias of −21%. The
consistent underestimations in regional PM2.5 forecast at other networks (IMPROVE
and STN) were mainly due to the underestimation of total carbonaceous aerosols
at both urban and rural sites. The significant underestimation of the “other”
category, which predominantly is composed of primary emitted trace elements in
| the current
model configuration, is also one of the reasons leading to the underestimation
of PM2.5 at rural sites. The systematic overestimations of SO42− both at
the surface sites and aloft, in part, suggest too much SO2 cloud oxidation due
to the overestimation of SO2 and H2O2 in the model. The underestimation of NH4+
at the rural sites and aloft may be attributed to the exclusion of some sources
of NH3 in the emission inventory. The systematic underestimations of NO3−
may result from the general overestimations of SO42−. Note that there are
compensating errors among the underestimation of PM2.5 species (such as total
carbonaceous aerosols) and overestimation of PM2.5 species (such as SO42−),
leading to generally better performance of PM2.5 mass. The systematic underestimation
of biogenic isoprene (by ∼30%) and terpene (by a factor of 4) suggests that
their biogenic emissions may have been biased low, whereas the consistent overestimations
of toluene by the model under the different conditions suggest that its anthropogenic
emissions might be too high. The contributions of various physical and chemical
processes governing the distribution of PM2.5 during this period are investigated
through detailed analysis of model process budgets using the integrated process
rate (IPR) analysis along back trajectories at five selected locations in Pennsylvania
and Georgia. The results show that the dominant processes for PM2.5 formation
and removal vary from the site to site, indicating significant spatial variability. |
Marzo
de 2008
Estructural context of the great Sumatra-Andaman Islands earthquake
Authors: Nikolai M. Shapiro, Michael H. Ritzwoller et al
Link: Click here
Abstract
A new three-dimensional seismic
model and relocated regional seismicity are used to illuminate the great Sumatra-Andaman
Islands earthquake of December 26, 2004. The | earthquake
initiated where the incoming Indian Plate lithosphere is warmest and the dip of
the Wadati-Benioff zone is least steep along the subduction zone extending from
the Andaman Trench to the Java Trench. Anomalously high temperatures are observed
in the supra-slab mantle wedge in the Andaman back-arc. The subducting slab is
observed along the entire plate boundary to a depth of at least 200 km. These
factors contribute to the location of the initiation of rupture, the strength
of seismic coupling, the differential rupture properties between the northern
and southern segments of the earthquake, and the cause of convergence in the Andaman
segment. |
Marzo
de 2008
Microwave dielectric constant of liquid hydrocarbons: Application
to the depth estimation of Titan's lakes
Authors: Philippe Paillou,
Karl Mitchell et al
Link: Click here
Abstract
Cassini RADAR reveals the surface
of Titan since flyby Ta acquired on October 2004. The RADAR instrument discovered
volcanic structures, craters, dunes, channels, lakes and seas. In particular,
flyby T16 realized in July 2006 imaged tens of radar-dark features close to Titan's
north pole. They are interpreted as lakes filled with liquid hydrocarbons –
mainly methane, a key material in | the
geologic and climatic history of Titan. In order to perform quantitative analysis
and modeling of the radar response of Titan's lakes, the dielectric constant of
liquid hydrocarbons is a crucial parameter, in particular to estimate the radar
wave attenuation. We present here first measurements of the dielectric constant
of LNG (Liquefied Natural Gas), mainly composed of methane, at Ku-band (10–13
GHz): we obtained a value  =
1.75 – 0.002j. This value is used to model the radar backscattering of lakes
observed during T16 flyby. Using a two-layer scattering model, we derive a relationship
that is used to estimate a minimum depth for Titan's lakes. The proposed relationship
is also coherent with the observation that the larger and then the deeper lakes
are also the darker in radar images. |
Marzo
de 2008
Clay-clast aggregates: A new textural evidence for seismic
fault sliding?
Authors: Sébastien Boutareaud, Dan-Gabriel
Calugaru et al
Link: Click here
Abstract
To determine the processes responsible
for slip-weakening in clayey gouge zones, rotary-shear experiments were conducted
at seismic slip rates (equivalent to 0.9 and 1.3 m/s) at 0.6 MPa normal stress
on a natural clayey gouge for saturated and non-saturated initial conditions.
The mechanical behavior of the simulated faults shows a | reproducible
slip-weakening behavior, whatever initial moisture conditions. Examination of
gouge obtained at the residual friction stage in saturated and non-saturated initial
conditions allows the definition of two types of microstructures: a foliated type
reflecting strain localization, and a non-foliated type composed of spherical
aggregates. Friction experiments demonstrate that liquid-vapor transition of water
within gouge due to frictional heating has a high capacity to explain the formation
of spherical aggregates in the first meters of displacement. This result suggests
that the occurrence of spherical aggregates in natural clayey fault gouges can
constitute a new textural evidence for shallow depth pore water phase transition
at seismic slip velocity and consequently for past seismic fault sliding. |
Marzo de 2008
Formation of maximum electric potential at the geomagnetic equator by
the disturbance dynamo
Authors: C. M. Huang & M. Q. Chen
Link: Click here
Abstract
The effect of a disturbance dynamo
during geomagnetic activity on the equatorial ionospheric electric fields is investigated,
using model results from the NACR/TIEGCM (National Center for Atmospheric Research
Thermosphere Ionosphere Electrodynamics General Circulation Model). Model runs
are made for different time-lengths of geomagnetic activity, for different seasons,
and for different solar activities to investigate how and where the maximum electric
potential forms. Model results show that the maximum electric potential is located
at around | 300
km altitude and at local time after the pre-reversal enhancement at equinox for
high solar activity. For the case at solstice, without pre-reversal enhancement,
the location moves to around 110 km altitude and to the local time close to midnight.
Giving various parameters in the model runs to simulate different background conditions,
many important output quantities are used to study the distributions of perturbed
electric potential at the geomagnetic equator. Model investigation indicates that
normal quiet time electrodynamics, at different seasons with different solar activities,
significantly affect the distribution of perturbed current density associated
with geomagnetic activity. Furthermore, model results also reveal that significant
perturbed zonal electric fields tend to build up six hours after the onset of
geomagnetic activity, except at regions close to sunset and sunrise, and the perturbed
vertical electric fields increase with the time length of geomagnetic activity.
|
Marzo
de 2008
Constraints on core-mantle electromagnetic
coupling from torsional oscillation normal modes
Authors: M. Dumberry & J. E. Mound
Link: Click
here
Abstract
Decadal axial angular momentum variations
in the Earth's core are believed to be carried by the normal modes of torsional
oscillations. Coupling with the mantle transfers angular momentum to the latter,
leading to changes in length of day (LOD). Electromagnetic stresses at the core-mantle
boundary (CMB) may be an important coupling mechanism as well as a source of dissipation
for torsional oscillations. In this work, we investigate whether the observed
spectra of fluid core velocities and LOD variations can be both explained in terms
of the normal modes of | torsional
oscillations when the only coupling with the mantle is through electromagnetic
stresses. We show that this explanation may be true when the magnetic field at
the CMB is based on a downward continuation of surface observations, provided
the conductance at the bottom of the mantle does not greatly exceed 108 S and
small wavelength field features do not contribute more than approximately 25%
of the total radial field at the CMB. A larger conductance or a higher amplitude
radial magnetic field results in a damping of the normal modes of torsional oscillation
that is sufficiently large that they should not be detectable. In particular,
we show that this is the case for the conductance and radial magnetic field that
are inferred from the Earth's forced nutations. If these constraints are correct,
the decadal periodicities in the fluid velocity and LOD must then represent the
preferred frequencies of the excitation mechanism of torsional oscillations rather
than the signature of the free modes. |