| | Noviembre
de 2008
Excess degassing from volcanoes and its role on eruptive and intrusive
activity
Author: Hiroshi Shinohara
Link: Click
here Abstract
Volcanoes
emit larger amounts of volcanic gas than can be dissolved in the volume of erupted
magma during a variety of volcanic processes, including explosive and effusive
eruption and noneruptive continuous degassing. Degassing of unerupted magma with
a much larger volume than that of erupted magma caused such a large | degassing;
erupted magma represents only a small portion of the magma that drives volcanic
activity. Evaluation of the magma-gas differentiation process causing the excess
degassing is necessary to understand eruption processes, magma chamber evolution,
and crustal growth by magma intrusion. Three mechanisms are proposed to explain
various degassing modes, including eruption of bubble-accumulated magma, degassing
of a convecting magma column, and permeable gas transportation from a deep magma
chamber. Examples of large degassing in excess of the erupted magma are common
in subduction zone volcanism but are rare in rift- and hot spot-associated volcanism.
| Noviembre
de 2008
Spatiotemporal correlations of earthquakes in the continuum limit
of the one-dimensional Burridge-Knopoff model
Authors:
Takahiro Mori and Hikaru Kawamura
Link: Click here Abstract
Spatiotemporal
correlations of the one-dimensional spring-block (Burridge-Knopoff) model of earthquakes,
either with or without the viscosity term, are studied by means of numerical computer
simulations. The continuum limit of the model is examined by systematically investigating
the model properties by varying the block-size parameter a toward a ? 0. The Kelvin
viscosity term is introduced so that the model dynamics | possesses
a sensible continuum limit. In the presence of the viscosity term, many of the
properties of the original discrete BK model are kept qualitatively unchanged
even in the continuum limit, although the size of minimum earthquakes gets smaller
as a gets smaller. One notable exception is the existence/nonexistence of the
doughnutlike quiescence prior to the mainshock. Although large events of the original
discrete BK model accompany seismic acceleration together with a doughnutlike
quiescence just before the mainshock, the spatial range of the doughnutlike quiescence
becomes narrower as a gets smaller, and in the continuum limit, the doughnutlike
quiescence might vanish altogether. The doughnutlike quiescence observed in the
discrete BK model is then a phenomenon closely related to the short-length cutoff
scale of the model. | Noviembre
de 2008
Differences between spontaneous and triggered earthquakes: Their
influences on foreshock probabilities
Authors: Jiancang
Zhuang, Annemarie Christophersen et al
Link: Click here Abstract
In
this study we investigate the foreshock probabilities calculated from earthquake
catalogs from Japan, southern California, and New Zealand. Unlike conventional
studies on foreshocks, we use a probability-based declustering method to separate
each catalog into stochastic versions of family trees, such that each event is
classified as either having been triggered by a preceding event or being a spontaneous
event. The probabilities are determined from parameters that provide the best
fit of the real catalogue using a space-time epidemic-type aftershock sequence
(ETAS) model. The model | assumes
that background and triggered earthquakes have the same magnitude-dependent triggering
capability. A foreshock here is defined as a spontaneous event that has one or
more larger descendants, and a triggered foreshock is a triggered event that has
one or more larger descendants. The proportion of foreshocks in spontaneous events
of each catalog is found to be lower than the proportion of triggered foreshocks
in triggered events. One possibility is that this is due to different triggering
productivity in spontaneous versus triggered events, i.e., a triggered event triggers
more children than a spontaneous events of the same magnitude. However, further
analysis on simulated data shows that such difference might be caused by the events
triggered by smaller events below the magnitude threshold of the catalog. Even
if the physical interpretation is unclear, a clustering model where spontaneous
events and triggered events have different triggering behaviors can be used to
assess the risk of foreshocks, and to avoid overpredicting. | Noviembre
de 2008
Multiarrival wavefront tracking and its applications
Authors: J. Hauser, M. Sambridge et al
Link: Click here Abstract
Multivalued
traveltimes have traditionally not been used in seismic imaging, with only a handful
of notable exceptions in the field of exploration geophysics. For studies at local
and regional scales (e.g., local earthquake/teleseismic tomography), the focus
has largely been on first arrivals, with numerous ray- and grid-based schemes
developed for their calculation. However, later arrivals often contribute to the
length and shape of a recorded wave train, particularly in regions of complex
geology. These arrivals are likely to contain additional information about seismic
structure, as their two point path differs from that of the first arrival; in
particular, they are more amenable to sampling regions of lower | velocity.
In this work the wavefront construction principle is used as the basis of a new
scheme for computing multivalued traveltimes that arise from smooth variations
in both velocity structure and interface geometry. The idea is to represent the
wavefront as a set of points in reduced phase space and use local ray tracing
and interpolation to advance the wavefront in a series of time steps. The scheme
is robust in the presence of strong velocity heterogeneity and interface curvature,
with phases comprising multiple reflections, refractions, and triplications successfully
tracked. Outside the field of exploration seismology wavefront construction techniques
are rarely used, yet they hold great potential for addressing problems in other
areas of seismology. This paper demonstrates the viability of the new wavefront
construction scheme by applying it to a range of scenarios, including multiarrival
body and surface wave tomography, teleseismic receiver function prediction using
Gaussian beams, and the tracking of global phases such as PcP. | Noviembre
de 2008
Laboratory impact experiments and numerical simulations on shock
pressure attenuation in water ice
Authors: Kei Shirai,
Manabu Kato et al
Link: Click here Abstract
High-velocity
impact experiments of water ice were conducted to measure the shock pressure profile
at various distances from the impact point. A numerical simulation of shock wave
propagation in water ice was also conducted for comparison with the experimental
results. The numerical model was improved to fit the measured profiles, and it
was found that a tensile strength of Y c = 1 MPa was necessary to reproduce the
shock pressure profiles above the Hugoniot Elastic Limit. This improved numerical
model was then used to study the shock pressure attenuation in water ice at various
impact conditions and to | refine
the crater scaling law. The late-stage effective energy (LE) is the product of
initial shock pressure (P 0) and the third power of the projectile size (L p 3).
The impact conditions with the same late-stage effective energy can produce the
same shock pressure distribution far from the impact point (so-called late-stage
equivalence). These impact conditions were investigated by numerical calculations
with different projectiles and impact velocities. As a result of our calculation
for water ice impacts, we found that a power law index of 2.2 instead of 3, as
adopted by previous studies, is suitable for reproducing the late-stage equivalence
in water ice (i.e., LE is proportional to P 0 · L p 2.2). By using this
improved LE, we can reconcile the inconsistency between the crater size and the
LE indicated by previous studies. By using this improved LE, the crater volume
V cr formed on water ice is expressed by the following equation, V cr = 1.0 exp
(l n/2450), with V cr in cm3, wherein l n (in Pa m2.2) is a constant derived from
a fit to the data. | Noviembre
de 2008
Electrical characterization of non-Fickian transport in groundwater
and hyporheic systems
Authors: Kamini Singha, Adam
Pidlisecky et al
Link: Click here Abstract
Recent
work indicates that processes controlling solute mass transfer between mobile
and less mobile domains in porous media may be quantified by combining electrical
geophysical methods and electrically conductive tracers. Whereas direct geochemical
measurements of solute preferentially sample the mobile domain, electrical geophysical
methods are sensitive to changes in bulk electrical conductivity (bulk EC) and
therefore sample EC in both the mobile and immobile domains. Consequently, the
conductivity difference between direct geochemical samples | and
remotely sensed electrical geophysical measurements may provide an indication
of mass transfer rates and mobile and immobile porosities in situ. Here we present
(1) an overview of a theoretical framework for determining parameters controlling
mass transfer with electrical resistivity in situ; (2) a review of a case study
estimating mass transfer processes in a pilot-scale aquifer storage recovery test;
and (3) an example application of this method for estimating mass transfer in
watershed settings between streams and the hyporheic corridor. We demonstrate
that numerical simulations of electrical resistivity studies of the stream/hyporheic
boundary can help constrain volumes and rates of mobile-immobile mass transfer.
We conclude with directions for future research applying electrical geophysics
to understand field-scale transport in aquifer and fluvial systems subject to
rate-limited mass transfer. | Noviembre
de 2008
A model of earthquake triggering probabilities and application to
dynamic deformations constrained by ground motion observations
Authors: Joan Gomberg and Karen Felzer
Link: Click here Abstract
We
have used observations from Felzer and Brodsky (2006) of the variation of linear
aftershock densities (i.e., aftershocks per unit length) with the magnitude of
and distance from the main shock fault to derive constraints on how the probability
of a main shock triggering a single aftershock at a point, P(r, D), varies as
a function of distance, r, and main shock rupture dimension, D. We find that P(r,
D) becomes independent of D as the | triggering
fault is approached. When r » D P(r, D) scales as D m where m~2 and decays
with distance approximately as r -n with n = 2, with a possible change to r -(n-1)
at r > h, where h is the closest distance between the fault and the boundaries
of the seismogenic zone. These constraints may be used to test hypotheses about
the types of deformations and mechanisms that trigger aftershocks. We illustrate
this using dynamic deformations (i.e., radiated seismic waves) and a posited proportionality
with P(r, D). Deformation characteristics examined include peak displacements,
peak accelerations and velocities (proportional to strain rates and strains, respectively),
and two measures that account for cumulative deformations. Our model indicates
that either peak strains alone or strain rates averaged over the duration of rupture
may be responsible for aftershock triggering. | Noviembre
de 2008
Generación de terremotos mediante la inyección de
agua a ~ 3 Km de profundidad en el campo de gas de Rongchang, Chongqing, China
Autores: Xinglin Lei, Guozheng Yu et al
Link:
Click aquí
Abstract
Inyectamos cerca de 1 millón de
m3 de agua no deseada de un modo intermitente mediante bombeo a presión
a 2.1-2.9 MPa (hidrostática) a 2.6-2.9 Km de profundidad en el campo de
gas de Rongchang, en Chongqing, China, a partir de julio de 1988. La inyección
de agua ha producido más de 32.000 registros de sismos superficiales, incluyendo
dos de magnitud M L = 5, catorce de magnitud M L = 4, y más de 100 de magnitud
M L = 3 contabilizados hasta finales del 2006. Examinamos el modelo de secuencia
del tipo epidémica después del sismo (ETAS) y la evolución
temporal de la estadística de la secuencia de los sismos. El componente
aleatorio del modelo ETAS puede ser considerado como un indicador de la actividad
fluída dirigida, mientras que los sismos posteriores al principal del tipo
Ley de Omori indica generación de tensiones de los sismos precedentes.
Nosotros observamos tres fases de actividad, las cuales coincidían con
diferentes periodos de inyección. La fase I corresponde a leves inyecciones
iniciales de agua (durante un gran intervalo de tiempo) y muestra la mayor cantidad
de eventos del tipo Ley de Omori y sólo 8% de actividad forzada por agentes
externos. La fase II corresponde al periodo de grandes inyecciones de agua y muestra
una intensa actividad caracterizada por un gran porcentaje (45%) de sismos fluído-dirigidos.
La fase III muestra sismicidad decreciente como resultado de una baja tasa de
inyección de agua y donde más del 70% de los sismos ocurrieron aleatoriamente
en el tiempo, indicando el predominio de agentes externos. El mecanismo físico
que induce la secuencia de sismos de Rongchang fue un cambio en las tensiones
de la Falla de Coulomb, causada por difusión porosa de la presión
y transformada en el deslizamiento previo al sismo. Un incremento de presión
de 2.1 a 2.9 MPa en el pozo de inyección es más que suficiente
para inducir fallas en las formaciones locales. | Noviembre
de 2008
Earthquakes induced by water injection at ~3 km depth within the
Rongchang gas field, Chongqing, China
Authors: Xinglin
Lei, Guozheng Yu et al
Link: Click here
Abstract
Unwanted water, amounting more than 1
million m3, has been injected intermittently at a pumping pressure of 2.1-2.9
MPa (over hydrostatic) at 2.6-2.9 km depth within the Rongchang gas field, western
Chongqing, China, since July 1988. The injections have induced more than 32,000
surface-recorded earthquakes, including 2 of M L = 5, 14 of M L = 4, and more
than 100 of M L = 3 up until the end of 2006. We examined the epidemic-type aftershock
sequence (ETAS) model and the temporal evolution of several statistics of the
earthquake sequence. The random component in the ETAS model can be considered
as an indicator of fluid-driven activity, while the rate of Omori-law-type aftershocks
indicates stress triggering of the preceding earthquakes. We observed three distinct
seismic activity phases that were coincident with different injection periods.
Phase I corresponds to initial injections with low injection rate (long time interval)
and shows a vast majority of Omori-type events and only 8% externally forced activity.
Phase II corresponds to a period of high injection rates and shows intense earthquake
activity characterized by a high percentage (45%) of fluid-driven earthquakes.
Phase III demonstrates decreasing seismicity as a result of a lowering in the
injection rate; more than 70% of earthquakes occurred randomly over time, indicating
the dominance of external triggering. The physical mechanism that induced the
Rongchang earthquake sequence was a change in Coulomb's failure stress caused
by pore pressure diffusion and transformed from seismic slip of preceding earthquakes.
A pressure increase of 2.1-2.9 MPa at the injection well is more than sufficient
to induce failure within the local formations. | Octubre
de 2008
An EGF technique to infer the rupture velocity history of a small
magnitude earthquake
Authors: S. de Lorenzo, M. Filippucci
et al
Link: Click here
Abstract
An empirical Green's function (EGF) technique
has been developed to detect the rupture velocity history of a small earthquake.
The assumed source model is a circular crack that is characterized by a single
and unipolar moment rate function (MRF). The deconvolution is treated as an inverse
problem in the time domain, which involves an assumed form of the moment rate
function (MRF). The source parameters of the MRF are determined by adopting a
global nonlinear inversion scheme. A thorough synthetic study on | both
synthetic and real seismograms allowed us to evaluate the degree of reliability
of the retrieved model parameters. The technique was applied to four small events
that occurred in the Umbria-Marche region (Italy) in 1997. To test the hypothesis
of a single rupture process, the inversion results were compared with those arising
from another EGF technique, which assumes a multiple rupture process. For each
event, the best fit model was selected using the corrected Akaike Information
Criterion. For all the considered events the most interesting result is that the
selected best fit model favors the hypothesis of a single faulting process with
a clear variability of the rupture velocity during the process. For the studied
events, the maximum rupture speed can even approach the P-wave velocity at the
source, as theoretically foreseen in studies of the physics of the rupture and
recently observed for high-magnitude earthquakes. | Octubre
de 2008
Complex slab subduction beneath northern Sumatra
Authors:J. D. Pesicek, C. H. Thurber et al
Link: Click here
Abstract
New data provided by the 2004-2005 Sumatra-Andaman
great earthquake sequences allow us to image with improved detail the P-wave velocity
| structure
beneath Sumatra and adjacent regions. Below northern Sumatra, we find that the
slab is folded at depth, exhibiting geometry similar to that of the volcanic arc
and the trench at the surface. We speculate that this fold plays a major role
in the segmentation of the Sumatra megathrust, and may impede rupture propagation
in the region. North of Sumatra, significant slab material in the mantle transition
zone is imaged for the first time, and we infer the presence of a major tear between
the upper mantle and transition zone there. | Octubre
de 2008
Atmospheric correction of SeaWiFS ocean color imagery in the presence
of absorbing aerosols off the Indian coast using a neuro-variational method
Authors: J. Brajard, C. Moulin et al
Link: Click here
Abstract
This paper presents a comparison of the
atmospheric correction accuracy between the standard sea-viewing wide field-of-view
sensor (SeaWiFS) algorithm and the NeuroVaria algorithm for the ocean off the
Indian coast in March 1999. NeuroVaria is a general method developed to retrieve
aerosol optical properties and water-leaving reflectances for all types of | aerosols,
including absorbing ones. It has been applied to SeaWiFS images of March 1999,
during an episode of transport of absorbing aerosols coming from pollutant sources
in India. Water-leaving reflectances and aerosol optical thickness estimated by
the two methods were extracted along a transect across the aerosol plume for three
days. The comparison showed that NeuroVaria allows the retrieval of oceanic properties
in the presence of absorbing aerosols with a better spatial and temporal stability
than the standard SeaWiFS algorithm. NeuroVaria was then applied to the available
SeaWiFS images over a two-week period. NeuroVaria algorithm retrieves ocean products
for a larger number of pixels than the standard one and eliminates most of the
discontinuities and artifacts associated with the standard algorithm in presence
of absorbing aerosols. | Octubre
de 2008
Characterization of the shallow structures of active fault zones
using 3-D ground-penetrating radar data
Authors: Alastair
F. McClymont, Alan G. Green et al
Link: Click here
Abstract
Where they can be correlated with geological
exposures and trenches, 3-D ground-penetrating radar (GPR) data can contribute
critical subsurface information to paleoseismic investigations. Because active
faults are typically characterized by complicated near-surface structures that
vary with the styles of faulting and the types of rock that are ruptured, GPR
data can be difficult to interpret. We have acquired 3-D GPR data sets across
three active fault zones within New Zealand that have different deformation styles:
the strike-slip Wellington fault zone, reverse faults of the Ostler fault zone,
and normal faults of the Maleme fault zone. To improve our | interpretation
of the processed GPR volumes, we employed two suites of geometric attributes.
The first suite was computed using a coherence-based algorithm. It provided estimates
of the coherency, azimuth, and dip of reflections. The second suite quantified
the volumetric textures of reflections, which allowed different reflection facies
to be defined objectively. We have demonstrated how some attributes were more
successful at visualizing certain structural or depositional characteristics than
others. For example, the coherency attribute was an excellent tool for highlighting
normal faults within volcanic deposits of the Maleme fault zone, whereas the texture-based
attributes were most useful for discriminating between the gravel and metasediment
units juxtaposed by the Wellington fault zone. Our GPR data sets and associated
attribute volumes showed details of near-surface fault geometry that were not
obvious from surface mapping, and they revealed evidence of off-fault deformation,
gravitational collapse, and topple structures. | Octubre
de 2008
A Poisson model for earthquake frequency uncertainties in seismic
hazard analysis
Authors: J. Greenhough and I. G. Main
Link: Click here
Abstract
Frequency-magnitude distributions, and
their associated uncertainties, are of key importance in statistical seismology.
When fitting these distributions, the assumption of Gaussian residuals is invalid
since event numbers are both discrete and of unequal variance. In general, the
observed number in any given magnitude range is | described
by a binomial distribution which, given a large total number of events of all
magnitudes, approximates to a Poisson distribution for a sufficiently small probability
associated with that range. In this paper, we examine four earthquake catalogues:
New Zealand (Institute of Geological and Nuclear Sciences), Southern California
(Southern California Earthquake Center), the Preliminary Determination of Epicentres
and the Harvard Centroid Moment Tensor (both held by the United States Geological
Survey). Using independent Poisson distributions to model the observations, we
demonstrate a simple way of estimating the uncertainty on the total number of
events occurring in a fixed time period. | Octubre
de 2008
Maximum entropy production and earthquake dynamics
Authors: Ian G. Main and Mark Naylor
Link: Click here
Abstract
We examine the consistency of natural
and model seismicity with the maximum entropy production hypothesis for open,
slowly-driven, steady-state, dissipative systems. Assuming the commonly-observed
power-law feedback between remote boundary stress and strain rate at steady state,
| several natural observations
are explained by the system organizing to maximize entropy production in a near
but strictly sub-critical state. These include the low but finite seismic efficiency
and stress drop, an upper magnitude cut-off that is large but finite, and the
universally- observed Gutenberg-Richter b-value of 1 in frequency-magnitude data.
In this state the model stress field organizes into coherent domains, providing
a physical mechanism for retaining a finite memory of past events. This implies
a finite degree of predictability, strongly limited theoretically by the proximity
to criticality and practically by the difficulty of directly observing Earth's
stress field at an equivalent resolution. | Octubre
de 2008
Correlations between SO2 flux, seismicity, and outgassing activity
at the open vent of Villarrica volcano, Chile
Authors:
José Luis Palma, Eliza S. Calder et al
Link: Click Aquí
Abstract
The characteristics of the open vent
activity of Villarrica volcano, Chile, were studied in detail by integrating visual
observations of the lava lake, analysis of the seismic tremor, and measurements
of SO2 flux. The outgassing activity comprises a persistent gas plume emission
from the bottom of the crater as well as frequent explosive events. Three main
styles of bubble bursting were identified at the surface of the active lava lake:
seething magma, small short-lived lava fountains, and Strombolian explosions.
Seething magma consists of continual burst of relatively small bubbles (a few
meters in | diameter)
with varying strength over the entire surface of the lava lake. Small lava fountains,
seen as a vigorous extension of seething magma, commonly have durations of 20-120
s and reach 10-40 m high above the lava lake. Correlations between seismicity
and visual observations indicate that the seismic tremor is mostly caused by the
explosive outgassing activity. Furthermore, for different periods between 2000
and 2006, during which the activity remained comparable, the real-time seismic
amplitude measurement system (RSAM) and SO2 emission rates show a very good correlation.
Higher SO2 emissions appeared to be related to higher levels of the lava lake,
stronger bubble bursting activity, and changes in the morphology and texture of
the crater floor. Background (low) levels of activity correspond to a lava lake
located >80 m below the crater rim, small and/or blocky morphology of the roof,
seismic amplitude (RSAM) lower than 25 units, few volcano-tectonic earthquakes,
and daily averages of SO2 emissions lower than 600 Mg/d. | Octubre
de 2008
Rupture process of four medium-sized earthquakes that occurred
in the Gulf of California
Authors: Héctor E.
Rodríguez-Lozoya, Luis Quintanar et al
Link: Click Aquí
Abstract
Four medium-sized earthquakes (M w Global
CMT project 5.5, 5.6, 5.9, 6.3; hereinafter named Topolobampo, Angel de la Guarda,
San Lorenzo, and Loreto earthquakes, respectively) located in the Gulf of California
Extensional Province were studied to obtain their kinematic rupture processes.
A network of broadband seismic stations located around the Gulf of California
recorded the events (Network of Autonomously Recording Seismographs-Baja and Red
Sísmica de Banda Ancha). Inversion of the seismic moment tensor and body
waveform modeling were used to obtain the fault geometry and slip distribution
on the fault plane, respectively. From these analyses, we obtained source depths
of the order of 5.5 ± 0.5 km. We found also that the | source
rupture processes of the Topolobampo and Angel de la Guarda events have simple
moment rate functions and source time durations of 5.0 ± 1.2 and 4.2 ±
1.2 s, respectively. The Topolobampo event was a right-lateral strike-slip event,
and Angel de la Guarda was a normal event. The San Lorenzo and Loreto shocks show
a rather complex rupture, with source time durations of 7.5 ± 1.2 and 9.0
± 1.2 s, respectively. For these earthquakes, we tested the resolution
of numerical results, performing an extra inversion with smoother waveforms. The
new inversions do not show the separated patches of slip, as in the first analysis,
but the slip distribution has an elongated shape not characteristic of simple
events. We cannot therefore conclude rupture propagation for the San Lorenzo event,
although the extent of the patch for the Loreto earthquake agrees with aftershock
locations. Estimates of source time durations for these earthquakes are at the
upper limit of the values found for earthquakes elsewhere. Directions of P axes
are in the same order of magnitude as the maximum horizontal stress obtained for
the so-called Gulf of California stress province from borehole elongations, focal
plane solutions, and fault slip data. | Octubre
de 2008
The nonseismic tsunami observed in the Bulgarian Black Sea on 7
May 2007: Was it due to a submarine landslide?
Authors:
Boyko Ranguelov, Stefano Tinti et al
Link: Click Aquí
Abstract
A tsunami-like sea disturbance of nonseismic
origin was observed on 7 May 2007 on the Bulgarian Black Sea coast. The maximum
wave amplitude was three meters, and the sea oscillations lasted for several hours.
The anomalous behaviour was seen in many places | of
the coast of Bulgaria and was stronger in the north. Several accounts of eyewitnesses
as well as reports of local port authorities and three tide-gauge recordings are
available and have been collected and analysed. The tsunami could have been produced
by a submarine landslide or by atmospheric pressure pulses. Here, we explore the
former hypothesis, i.e., the possibility that the tsunami was induced by a landslide
that may have occurred on the shelf margin off Bulgaria. By making use of numerical
modelling we find that submarine mass movements taking place within a certain
delimited source area off Bulgaria may have generated tsunamis compatible with
the observations. | Septiembre
de 2008
Experimental determination of the elasticity of iron at high pressure
Authors: Wendy L. Mao, Viktor V. Struzhkin et al
Link:
Click Aquí
Abstract
We present a multitechnique approach
to experimentally determine the elastic anisotropy of polycrystalline hcp Fe at
high pressure. Directional phonon measurements from inelastic X-ray scattering
on a sample with lattice preferred orientation at 52 GPa in a diamond anvil cell
were coupled with X-ray diffraction data to determine the elastic tensor. Comparison
of the results from this | new
method with the elasticity determined by lattice strain analysis of radial X-ray
diffraction measurements showed significant differences, highlighting the importance
of strength anisotropy in hcp Fe. At 52 GPa, we found that a method which combines
results from inelastic scattering and pressure-volume measurements gives a shape
in the velocity anisotropy close to sigmoidal (with a faster c and slower a axis)
a smaller magnitude in the anisotropy and compared to velocities based on the
lattice strain method which gives a bell shape velocity distribution with the
fast direction between the c and a axes. We used additional results from nuclear
resonant inelastic X-ray scattering to constrain errors and provide additional
validation of the accuracy of our results. | Septiembre
de 2008
Magnetospheric amplification and emission triggering by ELF/VLF
waves injected by the 3.6 MW HAARP ionospheric heater
Authors: M. Golkowski, U. S. Inan et al
Link: Click Aquí
Abstract
The HF dipole array of the High Frequency
Active Auroral Research Program (HAARP) in Gakona, Alaska, was recently upgraded
to 180 elements, facilitating operations at a total radiated power level of 3.6
MW and an effective radiated power of ~575 MW. In the first experiments at the
new power level, the HAARP array is used for magnetospheric wave injection. Modulated
heating of auroral electrojet currents in the ionosphere yields radiation in the
ELF/VLF frequency range. The HAARP-generated signals are injected into the magnetosphere,
where they propagate in the whistler mode in field-aligned "ducts,"
allowing them to be observed at the | conjugate
point on a ship-borne receiver and on autonomous buoy platforms. The observation
of the 1-hop signals is accompanied by the observation of associated 2-hop components
in the northern hemisphere, which have reflected from the ionospheric boundary
in the southern hemisphere. The observed signals are accompanied by triggered
emissions and exhibit temporal amplification of 15-25 dB/s and bandwidth broadening
to ~50 Hz. Amplification occurs at injected signal frequencies selected in near
real time on the basis of observations of natural emission activity, and only
certain components of the frequency-time formats transmitted are amplified. Observations
at multiple sites and dispersion analysis show that the signals are injected into
the magnetosphere directly above the HF heater. The duration of echo observation
and the prevalence of 1-hop observations are consistent with statistics from 1986
Siple Station experiments. The particle-trapping wave amplitude near the magnetic
equator is estimated in the range 0.1-0.4 pT and gyroresonance with 10 keV-100
keV electrons. | Septiembre
de 2008
Spectral element modeling of spontaneous earthquake rupture on
rate and state faults: Effect of velocity-strengthening friction at shallow depths
Authors: Y. Kaneko, N. Lapusta et al
Link: Click Aquí
Abstract
We develop a spectral element method
(SEM) for simulating dynamic rupture on rate and state faults and use it to study
how the rupture is affected by a shallow fault region of steady state velocity-strengthening
friction. Our comparison of the developed SEM and a spectral boundary integral
method (BIM) for an antiplane (two-dimensional) test problem shows that for the
finest resolution that we use, the two methods produce virtually identical solutions,
with negligible differences in rupture arrival times and peak slip velocities
(less than 0.05% of their absolute values). The convergence with grid | reduction
of the developed SEM is comparable to that of BIM. We also use the test problem
to compare numerical resolution required for different state evolution laws and
for linear slip-weakening friction. Using our three-dimensional implementation
of the methodology, we find that a shallow velocity-strengthening fault region
can significantly alter dynamic rupture and ground motion. The velocity-strengthening
region suppresses supershear propagation at the free surface occurring in the
absence of such region, which could explain the lack of universally observed supershear
rupture near the free surface. In addition, the velocity-strengthening region
promotes faster falloff of slip velocity behind the rupture front and decreases
final slip throughout the entire fault, causing a smaller average stress drop.
The slip decrease is largest in the shallow parts of the fault, resulting in a
depth profile of slip qualitatively consistent with observations of shallow coseismic
slip deficit. The shallow velocity-strengthening region also reduces the amplification
of strong ground motion due to a low-velocity bulk structure. | Septiembre
de 2008
Inverse groundwater modeling for hydraulic conductivity estimation
using Bayesian model averaging and variance window
Authors: Frank T.-C. Tsai and Xiaobao Li
Link: Click Aquí
Abstract
This study proposes a Bayesian model
averaging (BMA) method to address parameter estimation uncertainty arising from
nonuniqueness in parameterization methods. BMA is able to incorporate multiple
parameterization methods for prediction through the law of total probability and
to obtain an ensemble average of hydraulic conductivity estimates. Two major issues
in applying BMA to hydraulic conductivity estimation are discussed. The first
problem is using Occam's window in usual BMA applications to measure approximated
posterior model probabilities. Occam's window only accepts | models
in a very narrow range, tending to single out the best method and discard other
good methods. We propose a variance window to replace Occam's window to cope with
this problem. The second problem is the Kashyap information criterion (KIC) in
the approximated posterior model probabilities, which tends to prefer highly uncertain
parameterization methods by considering the Fisher information matrix. With sufficient
amounts of observation data, the Bayesian information criterion (BIC) is a good
approximation and is able to avoid controversial results from using KIC. This
study adopts multiple generalized parameterization (GP) methods such as the BMA
models to estimate spatially correlated hydraulic conductivity. Numerical examples
illustrate the issues of using KIC and Occam's window and show the advantages
of using BIC and the variance window in BMA application. Finally, we apply BMA
to the hydraulic conductivity estimation of the "1500-foot" sand in
East Baton Rouge Parish, Louisiana. | Septiembre
de 2008
Impact fracturing and structural modification of sedimentary rocks
at Meteor Crater, Arizona
Authors: P. Senthil Kumar
and David A. Kring
Link: Click Aquí
Abstract
Meteor Crater provides a rare opportunity
to study impact deformation of sedimentary target rocks and isolate those features
from preexisting tectonic deformation and impact-generated reactivation of preexisting
tectonic features. This study reports over 2500 new measurements of orientations
of bedding, faults, and fractures in crater walls and in surrounding bedrock.
Target rocks are characterized by horizontal bedding planes that are cut by at
least three prominent sets of preimpact tectonic fracture systems. The crater
rim is also cut by three distinct groups of fractures: radial, concentric, and
conical fractures. When the crater rim is restored to preimpact |
condition, the radial and concentric fractures resemble preimpact fracture populations,
indicating that crater wall deformation and rim uplift were partly accommodated
by activation of preexisting fractures. In contrast, the conical fractures are
dissimilar to the preimpact fractures and apparently formed as a direct result
of impact deformation. Some of the preimpact fractures were transformed into tear
faults during the impact event, and motion along those faults appears to have
controlled the geometry of the impact deformational features. The crater rim is,
thus, square in plan view rather than circular. Faults occurring in the crater
diagonals are prominent ones, allowing greater vertical displacement. The deformation
pattern of Meteor Crater is different from that at Lonar Crater, which was excavated
in basalt with fewer preimpact fractures. The differences between deformation
at Meteor Crater and Lonar Crater may reflect the same disparities seen in simple
craters produced in different target lithologies on Mars and other planetary surfaces.
| Septiembre
de 2008
Modeling the mechanics of rate and state friction with linear viscoelasticity
Authors: Joseph B. Walsh and David L. Goldsby
Link: Click
Aquí
Abstract
In slide-hold-slide friction experiments,
rock surfaces loaded by normal stress s slide against one another at velocity
V until steady-state conditions prevail; the applied shear stress t required for
steady-state sliding is fs, where f is the friction coefficient. Sliding is then
stopped abruptly for a specified hold time t H , and then resumed at velocity
V. Shear stress t and the separation delta w between the rough surfaces are measured
throughout the test. During the hold period, both applied shear stress and separation
are found to decay with time from their steady-state values. We show that the
constitutive behavior in shear during the hold period is in effect a relaxation
test, and likewise, the closure measurements describe a creep test. Analyzing
the data in detail, we find that they can be modeled | as
the response of a standard linear solid, with a characteristic time T 1 for creep
of 1900 s and a characteristic time T 2 for relaxation of 1400 s. The shear stress
required to initiate sliding at the end of the hold period is found to peak k
f sigma above the steady-state stress f sigma, then decay over a distance D c
to the steady-state value. In our experiments, measurements of separation and
shear load were made for hold periods as long as 106 s, i.e., one hundred times
longer than in similar previous studies. We find that our measured values of delta
f increase gradually with increasing hold times-in agreement with previous measurements
up to about 104 s-but then approach a limiting value asymptotically. In our analysis
we show that delta f is equal to (1 + f ^2) delta sigma H /f lambda, where delta
d H is the total decrease in separation during the hold period and lambda is a
length parameter that describes the roughness of the surfaces. The parameter D
c is found to be given by delta sigma H / delta f. Using measurements from our
experiments, we find that both relationships are valid to a reasonable degree,
considering the uncertainties in the data and the simplicity of the analysis.
| Septiembre
de 2008
Seismicity on volcanoes generated by gas slug ascent
Authors: G. S. O'Brien and C. J. Bean
Link: Click Aquí
Abstract
Very long period (VLP) signals provide
a unique insight into the source process of seismic signals recorded on active
or restless volcanoes. This is due to the large wavelengths that may allow the
structural heterogeneity to be ignored hence | source
inversions can be calculated. The source process involved in such events is thought
to be related to magma transport, gas slug ascent or dike resonance initiated
by a fluid driven process. Numerical modelling of gas slug ascent (constrained
by results from published laboratory experiments) shows that this process is capable
of generating long period and VLP like signals. Inputting the fluid dynamic derived
forces into an elastic medium, with volcano like topography, VLP signals are reproduced.
A moment tensor inversion is performed on these synthetic VLP signals retrieving
the correct input source mechanism (a pipe structure). | Septiembre
de 2008
Detection of motion and heterogeneity in Earth's liquid outer core
Authors: Wei Dai and Xiaodong Song
Link: Click Aquí
Abstract
The lateral variations in the fluid outer
core are believed to be very small from fluid dynamics calculations. Seismological
studies on the issue have been limited and controversial. A great challenge is
to sort out influences from heterogeneity in the mantle or the inner core. Using
high-quality earthquake waveform doublets, we found that waves passing through
the fluid | core
over a few years are significantly more variable than those passing through the
mantle only. We interpret the temporal variability as the result of the fluid
motion of the heterogeneous materials in the outer core. The level of heterogeneity
in the fluid outer core is constrained to be ±0.022 s (95% confidence)
in seismic travel times through the core, negligible for most seismological studies.
However, the estimated velocity perturbation, about 10-3 for small-scale heterogeneity
(10 km) or 10-4 for large-scale heterogeneity (1000 km), borders or exceeds the
high-end estimates of the lateral variations that can be supported by dynamic
forces within the fluid core. The source of the heterogeneity is not clear at
present. | Septiembre
de 2008
Multiobjective training of artificial neural networks for rainfall-runoff
modeling
Authors: N. J. de Vos and T. H. M. Rientjes
Link: Click Aquí
Abstract
This paper presents results on the application
of various optimization algorithms for the training of artificial neural network
rainfall-runoff models. Multilayered feed-forward networks for forecasting discharge
from two mesoscale catchments in different climatic regions have been developed
for this purpose. The performances of the multiobjective algorithms Multi Objective
Shuffled Complex Evolution Metropolis-University of Arizona (MOSCEM-UA) and Nondominated
Sorting Genetic Algorithm II (NSGA-II) have been compared to the single-objective
Levenberg-Marquardt and Genetic Algorithm for training of | these
models. Performance has been evaluated by means of a number of commonly applied
objective functions and also by investigating the internal weights of the networks.
Additionally, the effectiveness of a new objective function called mean squared
derivative error, which penalizes models for timing errors and noisy signals,
has been explored. The results show that the multiobjective algorithms give competitive
results compared to the single-objective ones. Performance measures and posterior
weight distributions of the various algorithms suggest that multiobjective algorithms
are more consistent in finding good optima than are single-objective algorithms.
However, results also show that it is difficult to conclude if any of the algorithms
is superior in terms of accuracy, consistency, and reliability. Besides the training
algorithm, network performance is also shown to be sensitive to the choice of
objective function(s), and including more than one objective function proves to
be helpful in constraining the neural network training. | | Septiembre
de 2008
The drying of Titan's dunes: Titan's methane hydrology and its
impact on atmospheric circulation
Author: Jonathan
L. Mitchell
Link: Click Aquí
Abstract
We explore the effect of a finite reservoir
of methane on Titan's atmospheric circulation, precipitation patterns, and surface
methane content. We develop a soil model that accounts for the methane cycle in
the surface-atmosphere system, and we implement this surface model in a two-dimensional
model of the Titan's atmosphere. Seasonal oscillations in latitude of the large-scale
circulation accomplish net drying of the low-latitude surface by diverging methane
vapor from low latitudes to higher latitudes. Simulations with an initially deep
methane |
reservoir indicate this mechanism is able to dry ~1.75 meters of liquid methane
per Titan year from the low-latitude surface. The existence of low-latitude desert
morphologies suggests that the system has had sufficient time to completely remove
the surface methane by this mechanism. We then varied the reservoir size, focusing
on initial depths of 30 meters of liquid methane or less and compared the results
to available observations. The climate system has an abrupt transition to a warmer
state with less precipitation and nearly global surface drying near the level
at which the atmosphere can store the majority of the methane reservoir as vapor
or around 6.5 meters of equivalent liquid methane for our particular choice of
parameters. A comparison of our model results with Huygens' observations suggests
Titan's climate mimics a state in which most of the methane inventory with direct
access to the atmosphere (i.e., excluding underground sources) is stored in the
atmosphere. | Agosto
de 2008
A double branching model for earthquake occurrence
Authors: Warner Marzocchi and Anna Maria Lombardi
Link: Click Aquí
Abstract
The purpose of this work is to put forward
a double branching model to describe the spatiotemporal earthquake occurrence.
The model, applied to two worldwide catalogs in different time-magnitude windows,
shows a good fit to the data, and its earthquake forecasting performances are
superior to what was obtained by the ETAS (first-step branching model) and by
the Poisson model. The results obtained also | provide
interesting insights about the physics of the earthquake generation process and
the time evolution of seismicity. In particular, the so-called background seismicity,
i.e., the catalog after removing short-time clustered events, is described by
a further (second-step model) branching characterized by a longer time-space clustering
that may be due to long-term seismic interaction. Notably, this branching highlights
a long-term temporal evolution of the seismicity that is never taken into account
in seismic hazard assessment or in the definition of reference seismicity models
for a large earthquake occurrence. Another interesting issue is related to the
parameters of the short-term clustering that appear constant in a different magnitude
window, supporting some sort of universality for the generating process. | Agosto
de 2008
Caracterización geofísica y textural de la actividad
basáltica explosiva del volcán Villarrica
Autores: L. Gurioli, A. J. L. Harris et al
Link: Clic Aquí
Abstract
El volcán Villarrica en Chile,
es uno de los volcanes más activos de Sudamérica. Su actividad es
actualmente caracterizada por la continua desgasificación de su cumbre
a través de su conducto de tipo lago de lava y perturbado por eventos explosivos.
Durante un periodo de diez días en noviembre de 2004 se desarrolló
una serie de experimentos multidisciplinarios para caracterizar las emisiones
y deducir la dinámica de su centro basáltico. Las experiencias incluyeron
recolección de datos térmicos, sísmicos e infrasónicos
para describir la actividad interna estándar del cráter, junto con
el muestreo textural y químico del material eyectado durante los eventos
más energéticos. La actividad estándar fue caracterizada
por emisiones gaseosas con una frecuencia de ~ 9 eventos por minuto. Esto incluyó
burbujeo y "reventones" ubicados en su mayoría en los bordes
inferiores de los muros del cráter. La distribución del material
eyectado durante los eventos más energéticos incluyó tanto
escoria como golden pómez. Estos dos tipos de clastos presentan
diferente textura, pero idéntica composición vidriosa, sugiriendo
que experimentaron diferentes historias. La pómez golden es interpretada
como una expansión interna y rápida del magma ascendente. La escoria
se forma en el exterior y por compresión del material desgasificado en
la porción superior del conducto. Así es como se consigue una gran
columna superior desgasificada que alimenta el burbujeo permanente, a través
del cual surge material fresco que ocasionalmente origina eventos de relativamente
alta energía. | Agosto
de 2008
Textural and geophysical characterization of explosive basaltic
activity at Villarrica volcano
Authors: L. Gurioli,
A. J. L. Harris et al
Link: Click Here
Abstract
Villarrica volcano (Chile) is one of
the most active volcanoes in South America. Its activity is currently characterized
by continuous degassing from a summit lava lake/vent punctuated by explosive events.
During November 2004 a multidisciplinary experiment was deployed for a 10-d period
to define the style of emission and infer shallow conduit dynamics at this basaltic
center. This involved collection of thermal, seismic and infrasonic data to describe
the background activity confined inside the crater, and use of samples to texturally
and chemically characterize the ejecta from more energetic explosions able to
attain the crater rim. The background activity was characterized by gas bursting
with a frequency of ~9 events per minute. This involved emission of gas puffs
fed by bubble bursting, with larger bursts emplacing sheets of magma onto the
lower crater walls. The ejecta population from the more energetic events was characterized
by the coexistence of both scoriae and golden pumice. These two types of clasts
have different textures but identical glass compositions, suggesting that they
underwent different conduit histories. The golden pumice is interpreted as the
expanding inner part of a short-lived jet fed by a rapidly ascending, magma batch.
The scoria forms the outer portion of the jet and comprises degassed material
entrained during passage of the fresh batch through material residing in the upper-most
portion of the conduit. We thus have a largely degassed upper column that feeds
persistent bubble bursting, through which fresh batches occasionally rise to feed
events of relatively higher energy. | Julio
de 2008
Gradiometry for polarized seismic waves
Authors: Charles A. Langston and Chuntao Liang
Link: Click Here
Abstract
Wave gradiometry for a generic point
source wave model in cylindrical coordinates is developed to utilize the horizontal
components of seismic wavefields. Attributes of P-SV and SH waves such as horizontal
slowness and its change, the change in geometrical spreading, the change in radiation
pattern, azimuth of propagation, and wave polarization can be determined by measuring
the horizontal strains and rotations of the wavefield along with three components
of displacement. Gradiometer cells composed of an | array
of three-component seismographs can be used to provide estimates of the wave gradients.
Alternatively, a single three-component seismograph collocated with an areal strainmeter
(for P-SV waves) or a rotation sensor (for SH waves) can be used to estimate wave
polarization and the slowness/amplitude behavior of one aspect of the wavefield.
Wave gradients are computed for the 1 April 2007 Solomon Island earthquake using
data from the ANZA Seismic Network in southern California and show excellent consistency
among stations of the network. Azimuth and slowness of various long-period teleseismic
waves are recovered using gradiometry analysis. The consistency of strains computed
from the broadband array suggests that they may be used to calibrate Plate Boundary
Observatory tensor strainmeters in the region. | Julio
de 2008
Sudden termination of Martian dynamo?: Implications from subcritical
dynamo simulations
Authors: W. Kuang, W. Jiang et al
Link: Click Here
Abstract
The crustal magnetism measured by the
Mars Global Surveyor requires that Mars possessed a strong internal field generated
by a core dynamo in its early history. We use a numerical model to |
simulate the early Martian dynamo, focusing on the minimum energy for sustaining
an established dynamo. Our results show that near its end, the Martian dynamo
could reverse frequently, and could be subcritical: the energy to sustain the
dynamo is significantly less than that to excite the dynamo. In addition to a
longer lifetime, the subcritical dynamo implies that it could be terminated suddenly
with a very small perturbation and, once turned off, it could not be reactivated
without substantial increase of the buoyancy force in the Martian core. | Julio
de 2008
Rupture velocity estimation of large deep-focus earthquakes surrounding
Japan
Authors: Sun-Cheon Park and Jim Mori et al
Link: Click Here
Abstract
Rupture velocity is an important source
parameter, which is often difficult to determine, especially for deep-focus earthquakes
where there is usually limited near-source information. To help overcome this
problem, we developed a new method to estimate rupture velocities of deep-focus
earthquakes with better resolution. We first carry out teleseismic P waveform
inversions to determine slip distributions for a range of rupture | velocities
on the two nodal planes. Then forward modeling of regional data is performed using
the slip distributions determined in the teleseismic inversions to estimate the
rupture velocity. Using this method, we attempted to determine the rupture velocities
of large deep-focus earthquakes surrounding Japan, which are well recorded on
teleseismic and regional networks. Empirical Green functions are used for both
the teleseismic and regional analyses. Although it is difficult to determine the
rupture velocity from only the teleseismic data, the analyses including regional
data show clear difference which can resolve the rupture velocity and fault geometry.
For three deep earthquakes, we obtained rupture velocities of about 1~2 km/s,
which correspond to 20~40% of the shear wave velocity and are much slower than
typical values for shallow earthquakes. | Julio
de 2008
Synthesis of vector-wave envelopes in 3-D random media characterized
by a nonisotropic Gaussian ACF based on the Markov approximation
Author: Haruo Sato
Link: Click Here
Abstract
The earthquake source duration is short;
however, the apparent duration time of observed seismogram increases with travel
distance. The amplitude excitation is observed even on the transverse component
for P waves, on the longitudinal component for S waves. These phenomena are well
explained by scattering due to random velocity inhomogeneities around the global
seismic ray. We directly synthesize vector-wave envelopes in 3-D random elastic
media statistically characterized by a nonisotropic Gaussian autocorrelation function
(ACF). The method uses the Markov approximation in the case that the wavelength
is shorter than the correlation distance and the ray direction is | parallel
to one of the principal axes of the ACF. A spherical outgoing vector wavelet radiated
from a point source in the random elastic media is used as a basic model for high-frequency
seismogram envelopes from micro-earthquakes in the inhomogeneous lithosphere.
The stochastic master equation for the two-frequency mutual coherence function
(TFMCF) of the potential field is analytically solved. The Fourier transform of
TFMCF gives mean square (MS) envelopes of band-pass filtered vector-wave traces.
If the ACF is axially symmetric around the ray direction, MS envelopes of vector
components are analytically solved. The aspect ratio of the correlation distance
in the longitudinal direction to that in the transverse direction is the key parameter
for the envelope broadening and the excitation in the orthogonal component. Envelope
broadening becomes longer and the transverse (longitudinal) component amplitude
increases for a P wavelet (for an S wavelet) when the correlation distance in
the transverse plane becomes smaller. When the vertical correlation distance is
shorter than the horizontal one as seen in the real Earth, the envelope broadening
is larger for horizontal raypaths compared with vertical raypaths. | Julio
de 2008
Coherent electric structures: Vlasov-Ampère simulations
and observational consequences
Authors: C. Briand,
A. Mangeney et al
Link: Click Here
Abstract
Coherent electrostatic structures, like
bipolar electric pulses (also called electrostatic solitary waves) or Langmuir
waves, are frequently observed in many astrophysical plasma of the Earth environment
(plasma sheet boundary layer, Earth bow shock, auroral regions etc.) or in the
solar wind. They are thought to play a crucial role in the energy transfer from
small to large scale and to reconnection processes. Numerous simulations have
studied their emergence and evolution. Most of them are based on the |
development of two stream instabilities. Another mechanism is investigated here:
the plasma is excited by a localized, time dependent modulation of the electron
distribution function (heating of the electrons). The investigation is performed
through a 1D Vlasov-Ampere code, in open boundary conditions. We explore the response
of the plasma to several heating conditions, mass ratio and density gradient.
We find that the heating leads to the development of an extended turbulent domain.
We also show that the history of the electrostatic solitary waves (ESW) strongly
depends on the presence of a density gradient and the mass ratio between species.
If the positive charged neutralizing background is composed of heavy ions, the
ESW turns back to the entrance domain when a density gradient is included. From
the observational point of view, this means that the electric field shows a polarity
reversal with time. | Julio
de 2008
Electrical conductivity beneath the Bolivian Orocline and its relation
to subduction processes at the South American continental margin
Authors: Heinrich Brasse and Diane Eydam
Link: Click Here
Abstract
A long-period magnetotelluric data set
was obtained during 2002 and 2004 in the central Andes to study the deep electrical
conductivity structure in the region of the Bolivian Orocline between latitudes
17°S and 19°S. The profile extends from the Coastal Cordillera in northernmost
Chile, crosses the volcanic arc and the Altiplano high plateau in central Bolivia,
and ends in the Eastern Cordillera. Two-dimensional inversion revealed several
well-defined conductivity anomalies: in upper crustal levels the | conductive
sedimentary basins of the central Altiplano and the resistive Arequipa block beneath
the western Altiplano are imaged. Earlier seismological and magnetotelluric investigations
on the southern Altiplano inferred a large, highly conductive (partially molten)
body in the mid to deep crust. It was assumed that this structure would be underlying
the entire plateau, but this is not the case according to the new models. Instead,
the most prominent feature in the new investigation area is a high-conductivity
zone at upper mantle depths below the high plateau, which may be interpreted as
an image of partial melts and fluids triggered by water supply from the subducting
Nazca slab. This conductor would be in accordance with the standard subduction
scenario; it is, however, laterally offset by almost 100 km from the volcanic
arc. In contrast, the deep crust and upper mantle beneath the arc is moderately
resistive. Both observations may hint at an emerging shift of the magmatic/fluid
system in the central Andes. | Julio
de 2008
Three-dimensional VP and VP/VS structure of the upper crust in
the Erzincan basin (eastern Turkey)
Authors: Bülent
Kaypak
Link: Click Here
Abstract
Using aftershocks of the 13 March 1992
Erzincan earthquake (Ms = 6.8) three dimensional V P and V P/V S velocity structure
of the Erzincan basin and its surroundings were modeled by local earthquake tomography.
Travel times of body waves, belonging to 1025 selected high-quality events recorded
by 58 temporary stations, were inverted iteratively and simultaneously by using
the SIMUL2000 algorithm. Synthetic and resolution tests were performed to analyze
the sensitivity of tomographic results and model parameterization. The resulting
3-D V P and V P/V S tomographic images show velocity anomalies related to geologic
and tectonic structures beneath the Erzincan basin. While low V P anomalies down
to 2-3 km depths are associated | to
thick Neogene sediments deposited in the Erzincan basin, high V P anomalies indicate
bed rocks and intrusive magmatic rocks beneath the mountain ranges. V P/V S anomalies
are an important indicator of local tectonic structures, and the physical, mechanical,
and compositional variations in the rocks. The low V P/V S anomalies (=1.7) observed
at shallow depths (0-4 km) track the main fault systems and thus the weakness
zones in the basin. The high V P/V S anomalies (=1.9) located between 4 and 6
km depth in the southeast of the Erzincan basin may correspond to fluid saturation,
high pore pressure, and carbonate content in the rock matrix. The high V P/V S
ratios play an important role for increasing seismicity in this area because of
the fluid saturation and the high pore pressure. Finally, on the basis of the
3-D tomographic images, basement of the Erzincan basin deepens down to about 10
km depth and possesses an irregular and asymmetric shape. The thick and unconsolidated
sedimentary layer increases seismic risk to Erzincan and other cities located
on the basin as it magnifies ground motion caused by large earthquakes. | Julio
de 2008
Multiobjective genetic algorithm inversion of ground deformation
and gravity changes spanning the 1981 eruption of Etna volcano
Authors: Daniele Carbone, Gilda Currenti et al
Link: Click Here
Abstract
During the last few decades, joint investigations
of microgravity and surface deformation measurements have played an increasingly
important role in studying the internal dynamics of active volcanoes. Deformation
and microgravity observations have been accomplished at Mt Etna since the eighties.
Past data sets collected during important paroxysmal events can be utilized as
case-studies to both (1) test the possibilities of nowadays more powerful inversion
tools and improved analytical formulations to model the source-mechanisms of volcano-related
deformation and gravity changes and (2) in turn obtain new insights into the functioning
of the plumbing system of the volcano. Here we analyze a data set spanning the
March 1981 eruption of Mt. Etna. Large horizontal displacements were | evidenced
on the NE and SW flanks of the volcano through electrooptical distance measurements
(EDM) during two 20-month periods, both encompassing the March 1981 eruption.
Elevation changes, evidenced through leveling measurements, during a 12-month
period spanning the eruption, were in general smaller than horizontal displacements
with important amplitudes only close to the eruptive fissure. Gravity measurements,
carried out together with leveling measurements, evidenced positive changes, spatially
well correlated with elevation changes, but having a larger wavelength. The joint
inversion of the multimethod geophysical data is regarded as a multiobjective
optimization problem and solved through a Genetic Algorithm technique of the nondominated
type. We conclude that a composite intrusive mechanism with two tensile cracks,
each associated to a zone where preexisting microfractures were filled with new
magma, leaded to the 1981 eruption. The results of the present study highlight
the advantages of multiobjective evolutionary algorithms, as a powerful tool to
jointly invert multimethod geophysical data, and pose important issues on the
subject of volcano-monitoring. | Julio
de 2008
The 2005 catastrophic acid crater lake drainage, lahar, and acidic
aerosol formation at Mount Chiginagak volcano, Alaska, USA: Field observations
and preliminary water and vegetation chemistry results
Authors: Janet R. Schaefer, William E. Scott et al
Link: Click Here
Abstract
A mass of snow and ice 400-m-wide and
105-m-thick began melting in the summit crater of Mount Chiginagak volcano sometime
between November 2004 and early May 2005, presumably owing to increased heat flux
from the hydrothermal system, or possibly from magma intrusion and degassing.
In early May 2005, an estimated 3.8 × 106 m3 of sulfurous, clay-rich debris
and acidic water, with an accompanying acidic aerosol component, exited the crater
through a tunnel at the base of a glacier that breaches the south crater rim.
Over 27 km downstream, the acidic waters of the flood inundated an important salmon
spawning drainage, acidifying Mother Goose Lake from surface to depth (approximately
0.5 km3 in volume at a pH of 2.9 to 3.1), killing all aquatic life, and preventing
the annual salmon run. Over 2 | months
later, crater lake water sampled 8 km downstream of the outlet after considerable
dilution from glacial meltwater was a weak sulfuric acid solution (pH = 3.2, SO4
= 504 mg/L, Cl = 53.6 mg/L, and F = 7.92 mg/L). The acid flood waters caused severe
vegetation damage, including plant death and leaf kill along the flood path. The
crater lake drainage was accompanied by an ambioructic flow of acidic aerosols
that followed the flood path, contributing to defoliation and necrotic leaf damage
to vegetation in a 29 km2 area along and above affected streams, in areas to heights
of over 150 m above stream level. Moss species killed in the event contained high
levels of sulfur, indicating extremely elevated atmospheric sulfur content. The
most abundant airborne phytotoxic constituent was likely sulfuric acid aerosols
that were generated during the catastrophic partial crater lake drainage event.
Two mechanisms of acidic aerosol formation are proposed: (1) generation of aerosol
mist through turbulent flow of acidic water and (2) catastrophic gas exsolution.
This previously undocumented phenomenon of simultaneous vegetation-damaging acidic
aerosols accompanying drainage of an acidic crater lake has important implications
for the study of hazards associated with active volcanic crater lakes. | Julio
de 2008
S wave velocity structure of the Arabian Shield upper mantle from
Rayleigh wave tomography
Authors: Yongcheol Park, Andrew
A. Nybladeet al
Link: Click Here
Abstract
The shear wave velocity structure of
the shallow upper mantle beneath the Arabian Shield was modeled by inverting Rayleigh
wave phase velocity measurements between 45 and 140 s together with previously
published Rayleigh wave group velocity measurements between 10 and 45 s. For measuring
phase velocities, we applied a modified array method to data from several regional
networks that minimizes the distortion of raypaths caused by lateral heterogeneity.
The new shear wave velocity model shows a broad low-velocity region to depths
of ~150 km in the mantle across the Shield and a narrower low-velocity region
at depths =150 km localized along the Red Sea coast and Makkah-Madinah-Nafud (MMN)
volcanic line. The velocity reduction in the upper | mantle
corresponds to a temperature anomaly of ~250-330 K. These findings, in particular
the region of continuous low velocities along the Red Sea and MMN volcanic line,
do not support interpretations for the origin of the Cenozoic plateau uplift and
volcanism on the Shield invoking two separate plumes. When combined with images
of the 410 and 660 km discontinuities, body wave tomographic models, a S wave
polarization analysis, and SKS splitting results for the Arabian Peninsula, the
anomalous upper mantle structure in our new velocity model can be attributed to
an upwelling of warm mantle rock originating in the lower mantle under Africa
that crosses through the mantle transition zone beneath Ethiopia and moves to
the north and northwest under the eastern margin of the Red Sea and the Arabian
Shield. In this interpretation, the difference in mean elevation between the Arabian
Platform and Shield can be attributed to isostatic uplift caused by heating of
the lithospheric mantle under the Shield, with the significantly higher elevations
along the Red Sea coast possibly resulting also from lithospheric thinning and
dynamic uplift. | Julio
de 2008
Estimating pore-space gas hydrate saturations from well log acoustic
data
Authors: Myung W. Lee and William F. Waite
Link: Click Here
Abstract
Relating pore-space gas hydrate saturation
to sonic velocity data is important for remotely estimating gas hydrate concentration
in sediment. In the present study, sonic velocities of gas hydrate-bearing sands
are modeled using a three | -phase
Biot-type theory in which sand, gas hydrate, and pore fluid form three homogeneous,
interwoven frameworks. This theory is developed using well log compressional and
shear wave velocity data from the Mallik 5L-38 permafrost gas hydrate research
well in Canada and applied to well log data from hydrate-bearing sands in the
Alaskan permafrost, Gulf of Mexico, and northern Cascadia margin. Velocity-based
gas hydrate saturation estimates are in good agreement with Nuclear Magneto Resonance
and resistivity log estimates over the complete range of observed gas hydrate
saturations. | Julio
de 2008
Effects of shock-induced tensile failure on mb-Ms discrimination:
Contrasts between historic nuclear explosions and the North Korean test of 9 October
2006
Authors: Howard J. Patton and Steven R. Taylor
Link: Click Here
Abstract
Rayleigh wave excitation is studied for
an explosion source model consisting of a superposition of isotropic (monopole),
tensile failure, and tectonic release point sources. The body-force representation
for shock-induced, | deep-seated
tensile failure is a compensated linear vector dipole CLVD, where the relative
strength of the CLVD is given by an index K. Rayleigh wave amplitudes are reduced
owing to destructive interference between an explosive monopole and a CLVD source
with vertical axis of symmetry in extension (K > 1). The effect of tensile
failure on M s is to enhance the explosion-like characteristics on a plot of m
b -M s . This model suggests that the success of the m b -M s discriminant results
from the fact that nuclear tests were conducted under containment practices for
which tensile failure is ubiquitous, while the North Korean nuclear test of 9
October 2006 is a harbinger of poor m b -M s performance when tensile failure
is completely suppressed. | Julio
de 2008
Isotopic fractionation by diffusion in groundwater
Authors: Eric M. LaBolle, Graham E. Fogg et al
Link: Click Here
Abstract
During the last decade, isotopic fractionation
has gained acceptance as an indicator of microbiological and chemical transformations
of contaminants in groundwater. These transformation processes typically favor
isotopically light, compared to isotopically heavy, contaminants, resulting in
enrichment of the latter in the residual aqueous phase. In these isotope applications,
it has been generally presumed that physical transport processes in groundwater
have a negligible effect on isotopic enrichment. It is well known, however, that
aqueous phase diffusion generally proceeds faster for isotopically light, compared
to isotopically heavy, solute molecules, often resulting in isotopic fractionation
in groundwater. This paper considers the potential | for
isotopic fractionation during transport in groundwater resulting from minute isotopic
effects on aqueous diffusion coefficients. Analyses of transport in heterogeneous
systems delimit the viable range of isotopic fractionation by diffusion in groundwater.
Results show that diffusion can result in similar degrees of depletion and enrichment
of isotopically heavy solutes during transport in heterogeneous systems with significant
diffusion rate-limited mass transfer between fast- and slow-flow zones. Additional
analyses and examples explore conditions that attenuate the development of significant
fractionation. Examples are presented for 13C methyl tertiary butyl ether and
deuterated and nondeuterated isopropanol and tertiary butyl alcohol using aqueous
diffusion coefficients measured by the Taylor dispersion method with refractive
index profiling as a part of this study. Examples elucidate the potential for
diffusive fractionation as a confounder in isotope applications and emphasize
the importance of hydrogeologic analysis for assessing the role of diffusive fractionation
in isotope applications at contaminant field sites. | Julio
de 2008
Nonlinear effects of temperature, fluid pressure, and inelastic
porosity on dynamic fault slip and fault tip propagation: Emergence of slip strengthening
and pulse-like fault slip
Authors: Takehito Suzuki,
Teruo Yamashita et al
Link: Click Here
Abstract
We theoretically study dynamic faulting,
taking account of thermoporoelastic effects including inelastic porosity change;
the porosity is assumed to increase with increasing fault slip. We first derive
the analytical solutions for the changes of fluid pressure, temperature, and fault
slip, assuming 1-D fault model. As found in our previous study, the evolution
of these quantities is controlled by a single nondimensional parameter S u , a
quantity proportional to the rate of increase in inelastic porosity multiplied
by the thickness of shear heating zone. If we assume that | temperature
elevation remains below the melting temperature of fault rocks, S u is found be
close to zero or greater than unity. Our 2-D simulation shows that pulse-like
slip appears when S u is greater than unity, which is a necessary consequence
of the emergence of slip strengthening, while slip weakening caused by thermal
pressurization arises when S u is close to zero. The applied shear stress is entirely
released when S u is close to zero, while it is released only partially when S
u is greater than unity, so that the latter will be more reasonable according
to seismological observations. The fault tip stress increases significantly with
the fault extension in classical singular fault model. In contrast, the fault
tip stress is only weakly dependent on the size of extending fault in our model
when S u is greater than unity, so that the fault growth is more vulnerable to
spatial perturbation of model parameters. Our present study indicates the importance
of slip strengthening for the understanding of dynamic faulting. | | Julio
de 2008
Temperature trends at high elevations: Patterns across the globe
Authors: N. C. Pepin and J. D. Lundquist
Link: Click Here
Abstract
Most climate models suggest amplification
of global warming in high mountains, but observations are less clear. Using comprehensive,
homogeneity-adjusted temperature records from over 1000 high elevation stations
across the globe, we examine the causes of changing temperature trends with elevation,
assessing the roles of free atmospheric change, topography (exposure and aspect),
and | cryospheric
feedback. The data show that observed 20th century temperature trends are most
rapid near the annual 0°C isotherm due to snow-ice feedback. Mountain summit
and freely draining slope sites are dominated by free-air advection and thus have
consistent trend magnitudes, with reduced inter-site variance in comparison with
incised valley sites where local factors are more important. Thus, while there
has been no simplistic elevational increase in warming rates, some generalizations
can be made. Water resources and ecosystems near the 0°C isotherm in the extratropics
are at increased risk from accelerated warming. The data also suggest that exposed
mountain summits, away from the effects of urbanization and topographic sheltering,
may provide a relatively unbiased record of the planet's climate. | Julio
de 2008
Ice magnetization in the EPICA-Dome C ice core: Implication for
dust sources during glacial and interglacial periods
Authors: L. Lanci, B. Delmonte et al
Link: Click Here
Abstract
Isothermal remanent magnetization and
insoluble dust content of ice samples from EPICA-Dome C ice core were measured
to characterize the magnetic properties of atmospheric dust. Despite the larger
concentration of dust aerosol during | glacial
stages, the magnetization of the dust fraction was found to be higher during interglacials
and exhibits a larger variability. Changes in magnetic mineralogy of aerosol dust
in ice from different climatic stages were also characterized using coercivity
of remanence. Variations of magnetic properties of dust from glacial to interglacial
stages indicate changes in dust provenance, in agreement with previous results
based on geochemical analysis. However, the extremely large magnetizations of
some interglacial samples also suggest that episodical eolian deposition from
highly magnetic deposits occurred during interglacial periods. | Julio
de 2008
Venting of a separate CO2-rich gas phase from submarine arc volcanoes:
Examples from the Mariana and Tonga-Kermadec arcs
Authors: John Lupton, Marvin Lilley et al
Link: Click Here
Abstract
Submersible dives on 22 active submarine
volcanoes on the Mariana and Tonga-Kermadec arcs have discovered systems on six
of these volcanoes that, in addition to discharging hot vent fluid, are also venting
a separate CO2-rich phase either in the form of gas bubbles or liquid CO2 droplets.
One of the most impressive is the Champagne vent site on NW Eifuku in the northern
Mariana Arc, which is discharging cold droplets of liquid CO2 at an estimated
rate of 23 mol CO2/s, about 0.1% of the global mid-ocean ridge (MOR) carbon flux.
Three other Mariana Arc submarine volcanoes (NW Rota-1, Nikko, and Daikoku), and
two volcanoes on the Tonga-Kermadec Arc (Giggenbach and Volcano-1) also have vent
fields discharging CO2-rich gas bubbles. The vent fluids at these volcanoes have
| very high
CO2 concentrations and elevated C/3He and d 13C (CO2) ratios compared to MOR systems,
indicating a contribution to the carbon flux from subducted marine carbonates
and organic material. Analysis of the CO2 concentrations shows that most of the
fluids are undersaturated with CO2. This deviation from equilibrium would not
be expected for pressure release degassing of an ascending fluid saturated with
CO2. Mechanisms to produce a separate CO2-rich gas phase at the seafloor require
direct injection of magmatic CO2-rich gas. The ascending CO2-rich gas could then
partially dissolve into seawater circulating within the volcano edifice without
reaching equilibrium. Alternatively, an ascending high-temperature, CO2-rich aqueous
fluid could boil to produce a CO2-rich gas phase and a CO2-depleted liquid. These
findings indicate that carbon fluxes from submarine arcs may be higher than previously
estimated, and that experiments to estimate carbon fluxes at submarine arc volcanoes
are merited. Hydrothermal sites such as these with a separate gas phase are valuable
natural laboratories for studying the effects of high CO2 concentrations on marine
ecosystems. | Julio
de 2008
Seasonal variation of short-period (<2 h) gravity wave activity
over Gadanki, India (13.5°N, 79.2°E)
Authors:
Gopa Dutta, Toshitaka Tsuda et al
Link: Click Here
Abstract
We have analyzed the seasonal variation
of short-period (<2 h) gravity wave activity in the troposphere and lower stratosphere
using the wind observations made with VHF radar at Gadanki (13.5°N, 79.2°E),
India. Four years of high resolution (~3 min) wind data (2003-2006) have been
used to study variance and momentum flux of short-period gravity waves. Both horizontal
and vertical variances show annual variation. Variances in the upper troposphere
(12-16 km) and lower stratosphere (18-21 km) are found to | maximize
in the wet months of June-July-August coinciding with the peaks of outgoing longwave
radiation (OLR) and the rain rate (surface and Tropical Rainfall Measuring Mission
[TRMM] 2 km rainfall). An additional strong peak of variance is observed in the
height ranges of 4-12 km during spring which corresponds to TRMM storm height
maximum. The measurements suggest deep convective activity to be the main source
of short period gravity waves. Variances in the UTLS region are found to correlate
well with square of wind shear indicating possible in situ generation of the gravity
waves. Topographic generation of short period gravity waves does not seem to be
important for this site. An interannual variability is observed in the measured
wave activity. Zonal momentum flux shows annual variation with westward preference
in the wet season whereas the meridional flux does not indicate any clear variation.
| Julio de 2008
Potential of 3-D vertical seismic profiles to characterize seismogenic fault
zones
Authors: Roland von Huene, Dirk Klaeschen et
al
Link: Click Here
Abstract
The potential of a 3-D vertical seismic
profile (VSP) to improve resolution of seismogenic plate interfaces was explored
with synthetic modeling. The 3-D VSP modeled is at a proposed site for a 1 to
1.5 km deep open hole that provides background for riser drilling. Three-dimensional
| VSP images
could resolve 30-60 m spaced reflective horizons in a Costa Rican subduction zone.
It can record a great amount of high-fidelity S wave data to invert for physical
properties, directions of strain, and pore pressure above and below the plate
interface fault. A 6 km × 12 km grid of shots with a surface ship will illuminate
a ~4 km × 7 km area of the plate interface fault zone with a high data density.
Acquisition adds 5 to 9 days to drill ship time on site and a shooting ship. Seismic
image resolution falls between that of borehole information and 3-D surface ship
seismic images. A multiple-kilometer 3-D volume of high-fidelity S wave data is
an exceptional addition not available with other techniques. | Julio
de 2008
Upper lithospheric structure of the subduction zone offshore of
southern Arauco peninsula, Chile, at ~38°S
Authors:
Eduardo Contreras-Reyes, Ingo Grevemeyer et al
Link: Click Here
Abstract
A joint interpretation of swath bathymetric,
seismic refraction, wide-angle reflection, and multichannel seismic data was used
to derive a detailed tomographic image of the Nazca-South America subduction zone
system offshore southern Arauco peninsula, Chile at ~38°S. Here, the trench
basin is filled with up to 2.2 km of sediments, and the Mocha Fracture Zone (FZ)
is obliquely subducting underneath the South American plate. The velocity model
derived from the tomographic inversion consists of a ~7-km-thick oceanic crust
and shows P wave velocities typical for mature fast spreading crust in the seaward
section of the profile, with uppermost mantle velocities >8.4 km s-1. In the
trench-outer rise area, the top of incoming oceanic plate is pervasively fractured
and likely hydrated as shown by extensional faults, horst-and-graben structures,
and a reduction |
of both crustal and mantle velocities. These slow velocities are interpreted in
terms of extensional bending-related faulting leading to fracturing and hydration
in the upper part of the oceanic lithosphere. The incoming Mocha FZ coincides
with an area of even slower velocities and thinning of the oceanic crust (10-15%
thinning), suggesting that the incoming fracture zone may enhance the flux of
chemically bound water into the subduction zone. Slow mantle velocities occur
down to a maximum depth of 6-8 km into the upper mantle, where mantle temperatures
are estimated to be 400-430°C. In the overriding plate, the tomographic model
reveals two prominent velocity transition zones characterized by steep lateral
velocity gradients, resulting in a seismic segmentation of the marine fore arc.
The margin is composed of three main domains: (1) a ~20 km wide frontal prism
below the continental slope with Vp = 3.5 km s-1, (2) a ~50 km area with Vp =
4.5-5.5 km s-1, interpreted as a paleoaccretionary complex, and (3) the seaward
edge of the Paleozoic continental framework with Vp = 6.0 km s-1. Frontal prism
velocities are noticeably lower than those found in the northern erosional Chile
margin, confirming recent accretionary processes in south central Chile. | Julio
de 2008
Probabilistic tephra hazard maps for the Neapolitan area: Quantitative
volcanological study of Campi Flegrei eruptions
Authors:
G. Mastrolorenzo, L. Pappalardo et al
Link: Click Here
Abstract
Tephra fall is a relevant hazard of Campi
Flegrei caldera (Southern Italy), due to the high vulnerability of Naples metropolitan
area to such an event. Here, tephra derive from magmatic as well as phreatomagmatic
activity. On the basis of both new and literature data on known, past eruptions
(Volcanic Explosivity Index (VEI), grain size parameters, velocity at the vent,
column heights and erupted mass), and factors controlling tephra dispersion (wind
velocity and direction), 2D numerical simulations of fallout dispersion and deposition
have been performed for a large number of case events. A bayesian inversion has
been applied to retrieve the best values of critical parameters (e.g., vertical
mass distribution, diffusion coefficients, velocity at the |
vent), not directly inferable by volcanological study. Simulations are run in
parallel on multiple processors to allow a fully probabilistic analysis, on a
very large catalogue preserving the statistical proprieties of past eruptive history.
Using simulation results, hazard maps have been computed for different scenarios:
upper limit scenario (worst-expected scenario), eruption-range scenario, and whole-eruption
scenario. Results indicate that although high hazard characterizes the Campi Flegrei
caldera, the territory to the east of the caldera center, including the whole
district of Naples, is exposed to high hazard values due to the dominant westerly
winds. Consistently with the stratigraphic evidence of nature of past eruptions,
our numerical simulations reveal that even in the case of a subplinian eruption
(VEI = 3), Naples is exposed to tephra fall thicknesses of some decimeters, thereby
exceeding the critical limit for roof collapse. Because of the total number of
people living in Campi Flegrei and the city of Naples (ca. two million of inhabitants),
the tephra fallout risk related to a plinian eruption of Campi Flegrei largely
matches or exceeds the risk related to a similar eruption at Vesuvius. | Julio
de 2008
Structural uplift beneath the Chicxulub impact structure
Authors: Peggy M. Vermeesch and Joanna V. Morgan
Link:
Click Here
Abstract
Models of the central structure of large
impact craters are poorly constrained, partly because of the lack of well-preserved
terrestrial examples, and partly because of the extreme nature of impact events.
Even large impact craters take only a few minutes to form, during which time rocks
from the deep crust move upward many kilometers, interacting with impact melts
and breccias before settling to their final position. We construct a new model
of central uplift beneath the Chicxulub crater, based upon a well-constrained
3-D velocity model, obtained by jointly inverting seismic traveltime and gravity
data. The input tomographic | data
set has good resolution, and many rays cross the central uplift in many directions.
We use laboratory measurements to convert between velocity and density. Our velocity
model possesses a high-velocity zone near the crater center, and velocity gradually
decreases outside this zone. We use regional refraction data to interpret these
velocities in terms of a broad 80-km-wide zone of structural uplift, in which
the central rocks originate from the lower crust, and the surrounding rocks from
the midcrust and upper crust. This is in contrast with previous models in which
the zone of central uplift is either 40-50 km or 150 km wide. Our interpretation
is consistent with scaling laws, Yucatán basement lithology, other velocity
data, observations at similar-sized terrestrial craters, and dynamic modeling
of peak ring formation. Our model of the uplift at Chicxulub can be used to help
distinguish between competing models of effective target strength in numerical
models of crater formation. | Julio
de 2008
VLF wave generation by amplitude-modulated HF heater waves at Gakona,
Alaska
Authors: Spencer P. Kuo, Yen-Liang Wu et al
Link: Click Here
Abstract
Experiments conducted at Gakona, Alaska,
using the intensity-modulated HF heating waves to interact with electrojet currents
for the generation of VLF waves, are reported. An unexpected large increasing
rate from 4 to 8 kHz in the frequency dependency of the VLF radiation intensity
was | observed.
The peak value at 8 kHz was intense (about 7.5 dB above that of the 2 kHz signal
used as a marker) and the wave intensity from 5 to 17 kHz appeared to be abnormally
high (i.e., stronger than that at 2 kHz). In the experiments, we also observed
the enhancement of spread-E irregularities at electrojet current altitudes due
to the amplitude-modulated heater wave. These results and theoretical analyses
suggest that temporally modulated electrojet currents mix with heater wave-excited
density irregularities to form whistler mode currents, which generate VLF whistler
waves directly with much larger intensities and better directivity than a Hertzian
dipole can. | Julio
de 2008
Integrated interpretation of physical properties of rocks of the
borehole Yaxcopoil-1 (Chicxulub impact structure)
Authors:
S. I. Mayr, A. Wittmann et al
Link: Click Here
Abstract
The borehole Yaxcopoil-1, drilled within
the Chicxulub meteoritic impact structure (Mexico), was completely cored from
404 to 1511 m through postimpact Tertiary limestones underlain by impactites.
The impactites comprise impact melt-rich, suevitic breccia followed by megablocks
of Cretaceous limestones, calcarenites, dolomites, and anhydrites. Measurements
of porosity, density, and thermal parameters on 450 samples (equidistant sampling,
complete depth range) and of ultrasonic velocities and electric resistivity on
80 representative samples are used to investigate the physical properties of carbonate
rocks and to study the influence of the impact. Experiments under elevated pressure,
calculations using frequency-dependent Biot-Gassmann theory, and | cross-checking
with borehole logs, where available, show that ultrasonic laboratory and sonic
in situ data correspond. Sonic and electric quasi-continuous logs are obtained
from empirical correlations with thermal conductivity, density, and porosity and
consideration of mineralogical composition and microstructure. These data give
constraints on interpretation and geophysical modeling of, e.g., seismic and gravity
data. In the Tertiary postimpact limestone section, the rock fabric (porosity)
influences the physical properties. The upper boundary of the impactites is distinctly
determined by the high inhomogeneity factor and anisotropy coefficient of thermal
conductivity and by the temperature gradient from high-resolution borehole temperature
measurements. All physical properties indicate that the upper part of the suevitic
breccia can be distinguished from the lower suevite unit. In the Cretaceous megablocks,
a high variability of all properties (particularly, thermal conductivity, density
of solid material, and temperature gradient) due to the high variability in the
mineral composition (calcite, dolomite, anhydrite) is observed. | Julio
de 2008
Assessing the applicability of the spatial autocorrelation method:
A theoretical approach
Authors: Ikuo Cho, Taku Tada
et al
Link: Click Here
Abstract
We present a rigorous theoretical framework
that allows one to assess the range of applicability of the spatial autocorrelation
(SPAC) method, a technique of microtremor exploration that is widely used to infer
phase velocities of Rayleigh waves using vertical-motion records from a circular
array of seismic sensors. The magnitude of systematic errors (biases) that depend
on the number of seismic sensors deployed around the circle, and the magnitude
of systematic errors that arise from | the
presence of incoherent noise, are both evaluated analytically, and their general
properties are discussed. The relationship between the magnitude of stochastic
errors, inherent in the analysis results, and the duration of measurement (or
to put it more accurately, the data's degree of freedom) is also elucidated. The
validity of our theory is corroborated by checks against the results of both real
data analysis and numerical experiments, and an example is given of how the theory
can be adapted to account for practical situations encountered in the field. Discussions
on the range of applicability of the SPAC method, which have heretofore often
fallen back on empirical observations, have now obtained a theoretical ground
on which to stand, providing a basis for strategies to make maximal use of the
SPAC method's capabilities. | Julio
de 2008
Posteruptive phenomena in coronal mass ejections and substorms:
Indicators of a universal process?
Authors: K. K. Reeves,
T. B. Guild et al
Link: Click Here
Abstract
We examine phenomena associated with
eruptions in the two different regimes of the solar corona and the terrestrial
magnetosphere. We find striking similarities between the speeds of shrinking magnetic
field lines in the corona and dipolarization fronts traversing the | magnetosphere.
We also examine the similarities between supra-arcade downflows observed during
solar flares and bursty bulk flows seen in the magnetotail and find that these
phenomena have remarkably similar speeds, velocity profiles, and size scales.
Thus we show manifest similarities in the magnetic reconfiguration in response
to the ejection of coronal mass ejections in the corona and the ejection of plasmoids
in the magnetotail. The subsequent return of loops to a quasi-potential state
in the corona and field dipolarization in the magnetotail are physical analogs
and trigger similar phenomena such as downflows, which provides key insights into
the underlying drivers of the plasma dynamics. | Julio
de 2008
Double seismic zone of the Nazca plate in northern Chile: High-resolution
velocity structure, petrological implications, and thermomechanical modeling
Authors: Catherine Dorbath, Muriel Gerbault et al
Link:
Click Here
Abstract
This paper presents an interdisciplinary
study of the northern Chile double seismic zone. First, a high-resolution velocity
structure of the subducting Nazca plate has been obtained by the tomoDD double-difference
tomography method. The double seismic zone (DSZ) is observed between 80 and 140
km depth, and the two seismic planes is 20 km apart. Then, the chemical and petrologic
characteristics of the oceanic lithosphere associated with this DSZ are deduced
by using current thermal-petrological-seismological models and are compared to
pressure-temperature conditions provided by a numerical thermomechanical model.
Our results agree with the common hypothesis that seismicity in both upper and
lower planes is related to fluid releases associated with metamorphic dehydration
reactions. In the seismic upper plane located within the upper crust, these reactions
would affect material of basaltic (MORB) composition and document different metamorphic
reactions occurring within high-P (>2.4 GPa) and low-T (<570°C) jadeite-lawsonite
blueschists and, at greater depth (>130 km), lawsonite-amphibole eclogite conditions.
The lower plane lying in the oceanic mantle can be associated with serpentinite
dehydration reactions. The Vp and Vs characteristics of the region in between
both | planes
are consistent with a partially (~25-30 vol % antigorite, ~0-10% vol % brucite,
and ~4-10 vol % chlorite) hydrated harzburgitic material. Discrepancies persist
that we attribute to complexities inherent to heterogeneous structural compositions.
While various geophysical indicators evidence particularly cold conditions in
both the descending Nazca plate and the continental fore arc, thermomechanical
models indicate that both seismic planes delimit the inner slab compressional
zone around the 400°C (±50°C) isotherm. Lower plane earthquakes
are predicted to occur in the slab's flexural neutral plane, where fluids released
from surrounding metamorphic reactions could accumulate and trigger seismicity.
Fluids migrating upward from the tensile zone below could be blocked in their
ascension by the compressive zone above this plane, thus producing a sheeted layer
of free fluids, or a serpentinized layer. Therefore earthquakes may present either
downdip compression and downdip tensile characteristics. Numerical tests indicate
that the slab's thermal structure is not the only factor that controls the occurrence
of inner slab compression. (1) A weak ductile subduction channel and (2) a cold
mantle fore arc both favor inner slab compression by facilitating transmission
of compressional stresses from the continental lithosphere into the slab. (3)
Decreasing the radius of curvature of the slab broadens the depth of inner slab
compression, whereas (4) decreasing upper plate convergence diminishes its intensity.
All these factors indicate that if DSZs indeed contour inner slab compression,
they cannot be linked only to slab unbending, but also to the transmission of
high compressional stresses from the upper plate into the slab. | Julio
de 2008
First identification of mirror mode waves in Venus' magnetosheath?
Authors: M. Volwerk, T. L. Zhang et al
Link: Click Here
Abstract
In this paper first time observations
of mirror mode like structures in Venus' magnetosheath are presented. Using magnetometer
data from the Venus Express spacecraft it is shown that in two regions in the
Venusian magnetosheath strong compressional waves exist, which propagate | nearly
perpendicular to the ambient magnetic field. They are most likely mirror-mode
waves. The waves have periods between 5 and 15 sec, depending on the location
in the magnetosheath. These waves show up just behind the quasi-perpendicular
bow shock, and near the magnetopause during compression of the magnetosheath due
to increased solar wind pressure. The characteristics of the waves are similar
to mirror mode waves found in the Earth's magnetosheath, however, they are down-scaled
in duration and frequency by a factor of 10, comparable with the difference in
size of Venus' and Earth's magnetosheath. | Julio
de 2008
Spring frost risk in a changing climate
Authors: J. R. Rigby and Amilcare Porporato
Link: Click Here
Abstract
While both the mean and variance of daily
temperature are forecasted to increase in future climate scenarios, studies of
spring frost risk to vegetation have relied on changes in mean temperature to
understand frost risk in these | scenarios.
We present a probabilistic model of spring frost risk based on the stochastic-crossing
properties of a coupled temperature-phenology model in which the mean, variance,
and autocorrelation structure of spring temperature may be controlled through
independent parameters. The model results show that frost risk to vegetation is
as sensitive to increases in daily temperature variance (which increases frost
risk) as to increases in the mean temperature (which decreases frost risk). | Julio
de 2008
Dynamics of Mount Etna before, during, and after the July-August
2001 eruption inferred from GPS and differential synthetic aperture radar interferometry
data
Authors: Giuseppe Puglisi, Alessandro Bonforte
et al
Link: Click Here
Abstract
Ground deformation data from GPS and
differential synthetic aperture radar interferometry (DInSAR) techniques are analyzed
to study the July-August 2001 Mount Etna eruption as well as the dynamics preceding
and following this event. Five GPS surveys were carried out on the entire Mount
Etna network or on its southeastern part, from July 2000 to October 2001. Five
ERS-2 ascending passes and three descending ones are used to form five interferograms
spanning periods from a month to 1 year, before and encompassing the eruption.
Numerical and analytical inversions of the GPS and DInSAR data were performed
to obtain analytical models for preeruptive, syneruptive and posteruptive periods. | The
deformation sources obtained were from the Mogi model: (1) pressure sources located
beneath the upper western flank of the volcano, inflating before the eruption
onset and deflating afterward; (2) tensile dislocations to model the intrusion
of a N-S dike in the central part of the volcano; and (3) two sliding and two
normal dislocations to model the eastern and southern flank dynamics. This study
confirms that the lower vents of the eruption were fed by a magma stored at depth
ranging from 9 to 4 km below sea level, as proposed from petrochemical and geophysical
researches. The rising of the magma through the shallow crust started months before
the eruption onset but accelerated on the last day; this study suggests that in
the volcanic pile the path of the rising magma was driven by the volcano topography.
The eastern sliding plane and the interaction between dike intrusion and flank
instability have been better defined with respect to previous studies. The sliding
motion abruptly accelerated with the dike intrusion, and this continued after
the end of the eruption. The acceleration was accompanied by the propagation of
the strain field toward the eastern periphery of the volcano. | Descubren
en Brasil nuevos pozos petroleros
Fuente: www.latercera.cl/contenido/26_21088_9.shtml La
empresa petrolera brasileña Petrobras anunció hoy [13 de junio
de 2008] el descubrimiento de un nuevo pozo de petróleo, denominado
Guará, en la Cuenca de Santos, zona de una serie de hallazgos que
dan al país la posibilidad de convertirse en gran potencia energética.
La
"acumulación" de hidrocarburos se encuentra a más de
5.000 metros de profundidad, debajo de capas de piedra y sal, reportó
la agencia Estado.
Las perforaciones continúan y las estimaciones de
reservas aún no se conocen, pero Guará está situada cerca
del campo Carioca, donde meses atrás se hallaron grandes reservas.
El
director de la Agencia Nacional de Petróleo, | Haroldo
Lima, informó después del descubrimiento del área Carioca,
que esa región puede albergar más de 30 mil millones de barriles
de gas y petróleo.
Carioca es uno de los campos gigantes descubiertos
desde noviembre de 2007.
En la misma zona donde se encuentra el bloque BM-S-9
o Guará, están los bloques BM-S-8, BM-S-21 y BM-S-22,
que aún no fueron explorados.
Petrobrás tiene el 45% de los derechos
de explotación de Guará, mientras el 30% corresponde al BG Group,
de Gran Bretaña, y el 25 % restante a la empresa petrolera española
Repsol.
Analistas sostienen que en esa región del litoral brasileño
hay más de 50.000 millones de barriles de reservas, informó
hoy la agencia Estado. | Junio
de 2008
Multiphase flow predictions from carbonate pore space images using
extracted network models
Authors: Anwar S. Al-Kharusi
and Martin J. Blunt
Link: Click Here
Abstract
A methodology to extract networks from
pore space images is used to make predictions of multiphase transport properties
for subsurface carbonate samples. The extraction of the network model is based
on the computation of the location and sizes of pores and throats to create a
topological representation of the void space of three-dimensional (3-D) rock images,
using the concept of maximal balls. In this work, we follow a multistaged workflow.
We start with a 2-D thin-section image; convert it statistically into a 3-D representation
of the pore space; extract a network model from this image; and finally, simulate
primary drainage, waterflooding, and secondary drainage flow processes using a
pore-scale simulator. We test this workflow for a reservoir carbonate rock. The
network-predicted absolute | permeability
is similar to the core plug measured value and the value computed on the 3-D void
space image using the lattice Boltzmann method. The predicted capillary pressure
during primary drainage agrees well with a mercury-air experiment on a core sample,
indicating that we have an adequate representation of the rock's pore structure.
We adjust the contact angles in the network to match the measured waterflood and
secondary drainage capillary pressures. We infer a significant degree of contact
angle hysteresis. We then predict relative permeabilities for primary drainage,
waterflooding, and secondary drainage that agree well with laboratory measured
values. This approach can be used to predict multiphase transport properties when
wettability and pore structure vary in a reservoir, where experimental data is
scant or missing. There are shortfalls to this approach, however. We compare results
from three networks, one of which was derived from a section of the rock containing
vugs. Our method fails to predict properties reliably when an unrepresentative
image is processed to construct the 3-D network model. This occurs when the image
volume is not sufficient to represent the geological variations observed in a
core plug sample. | Junio
de 2008
Simulation study of the two-dimensional Burridge-Knopoff model
of earthquakes
Authors: Takahiro Mori and Hikaru Kawamura
Link: Click Here
Abstract
Spatiotemporal correlations of the two-dimensional
spring-block (Burridge-Knopoff) model of earthquakes are extensively studied by
means of numerical computer simulations. The model is found to exhibit either
a "subcritical", "supercritical" or "near-critical"
behavior, depending on the values of the model parameters. Transition between
the "subcritical" and "supercritical" regimes is either continuous
or discontinuous. Seismic events in the "subcritical" | regime
and those in the "supercritical" regime at larger magnitudes exhibit
universal scaling properties. In the "supercritical" regime, eminent
spatiotemporal correlations, e.g., a remarkable growth of seismic activity preceding
the main shock, arise in earthquake occurrence, whereas such spatiotemporal correlations
are significantly suppressed in the "subcritical" regime. Seismic activity
is generically suppressed just before the main shock in a close vicinity of the
epicenter of the upcoming event while it remains to be active in the surroundings
(the Mogi doughnut). It is also observed that, before and after the main shock,
an apparent B-value of the magnitude distribution decreases or increases in the
"supercritical" or "subcritical" regimes, respectively. Such
distinct precursory phenomena may open a way to the prediction of the upcoming
large event. | |