Mayo
de 2007
Electrical resistivity image of the Jingsutu Graben at the NE margin
of the Ejina Basin (NW China) and implications for the basin development
Authors: M. Becken, S. Hölz et al
Link: Click Here Abstract
Magnetotelluric and transient electromagnetic data reveal the Graben geometry
of a NNE striking depression within crystalline basement at the NE | margin
of the Ejina Basin. We interpret this structure as a Late Mesozoic pull-apart
basin, which is delineated by NNE-striking normal faults to the east and west.
Satellite imagery, SRTM elevation data and previously published geophysical data
of our work group suggest that the Graben is part of normal-fault system that
continues at least 140 km farther to the south, thus forming the eastern margin
of the Ejina Basin. NNE striking normal faults are atypical in a regional context,
where NW striking and ENE striking fault systems prevail. Thus, this feature marks
a distinct difference of the Ejina Basin when compared to other basin structures
in the China-Mongolia border region. | Mayo
de 2007
Analysis of Global Positioning System (GPS) radio occultation measurement
errors based on Satellite de Aplicaciones Cientificas-C (SAC-C) GPS radio occultation
data recorded in open-loop and phase-locked-loop mode
Author: Martin
S. Lohmann
Link: Click Here Abstract
The error characteristics of Global Positioning System (GPS) radio occultation
(RO) measurement errors are studied based on Satellite de Aplicaciones Cientificas-C
(SAC-C) GPS radio occultation data tracked in both open-loop (OL) and phase-locked-loop
(PLL) mode. The error characteristics are derived by applying dynamical error
estimation, i.e., without using any external data. The computed error profiles
show that the mean measurement errors are the smallest in the height range between
about 5-7 | km
and 20-25 km, about 0.2-1% for bending angles and 0.1-0.2% for refractivity at
all latitudes. The largest measurement errors are found in the lower troposphere,
where the mean bending angle measurement errors are within the range from 1 to
6%, whereas the mean refractivity measurement errors are within the range from
0.2% to 1%. From the error distributions, it is found that the occultation-to-occultation
variability of the measurement errors generally spans one order of magnitude.
The bending angle error correlation length is about 1 km and 100 m, at high and
low altitudes, respectively, corresponding approximately to the cutoff frequency
of the applied noise filters. The widths of the refractivity error autocorrelation
functions are notably broader. The variability and the magnitude of the OL measurement
errors are larger than for the PLL measurement errors. This is mainly attributed
to the ability of OL tracking to track RO signals under atmospheric conditions
for which PLL tracking fails. | Mayo
de 2007
Viscosity of the Earth's fluid core and torsional oscillations
Authors: Jon Mound, Bruce Buffett et al
Link: Click Here Abstract
Viscous effects on the temporal behavior and spatial structure of torsional oscillation
normal modes of the Earth's fluid core are examined. If the viscosity of the fluid
core is equal to that inferred from theoretical and experimental studies of liquid
metals, then viscous effects on torsional oscillations are unlikely to be important;
however, if the viscosity is as high as allowed from observational constraints,
then Ekman layers at | the
fluid core boundaries may play an important role in damping torsional oscillations.
The observationally inferred decay time of decadal torsional oscillations leads
to an upper bound on the viscosity of the fluid core at the core-mantle boundary
of the order of 10-2 m2/s. A sufficiently large viscosity would result in torsional
oscillation normal modes that undergo pure decay, as opposed to damped oscillatory
motion. The viscosity value at which the temporal behavior switches between these
regimes depends on the period of the normal mode. Numerical geodynamo models must
reach an Ekman number of the order of 10-7 for free-slip boundaries or 10-10 for
no-slip boundaries to accurately model the temporal behavior of decadal period
torsional oscillations. | Mayo
de 2007
Laboratory sandbox validation of transient hydraulic tomography
Authors: X. Liu, W. A. Illmanet al
Link: Click Here Abstract
Hydraulic tomography is a method that images the hydraulic heterogeneity of the
subsurface through the inversion of multiple pumping or cross-hole hydraulic test
data. Transient hydraulic tomography is different from steady state hydraulic
tomography in that it utilizes transient hydraulic head records to yield the distribution
of hydraulic conductivity (K) as well as specific storage (S s ) of an aquifer.
In this paper we demonstrate the robustness of transient hydraulic tomography
through the use of hydraulic head data obtained from multiple cross-hole pumping
tests conducted in a laboratory sandbox with deterministic heterogeneity. We utilize
the algorithm developed by Zhu and Yeh (2005) to | conduct
the transient inversions and validate the K and S s tomograms using a multimethod
and multiscale validation approach previously proposed by Illman et al. (2006).
Validation data consist of cross-hole tests not used in the inversion as well
as other hydraulic tests that provided local (core, single-hole tests) as well
as large-scale (unidirectional flow-through tests) estimates of hydraulic parameters.
Results show that the algorithm is able to yield consistent estimates that agree
with independently collected local as well as large-scale hydraulic parameter
data. In addition, we find that the transient hydraulic tomography requires a
fewer number of pumping tests to estimate a similar quality K tomogram when compared
with steady state hydraulic tomography, as the former approach utilizes more data
from each pumping test. Overall, we find that transient hydraulic tomography is
a robust subsurface characterization technique that can delineate the subsurface
heterogeneity in both K and S s from multiple pumping or cross-hole hydraulic
tests. | Mayo
de 2007
Methane thermodynamics in nanoporous ice: A new methane reservoir
on Titan
Authors: L. F. Voss, B. F. Henson et al
Link: Click Here Abstract
A porous, icy regolith has been proposed to house a methane/ethane/nitrogen liquid
reservoir on Titan. The thermodynamics of such a mesoporous reservoir with a pore
diameter fixed at 2 mm are equivalent to the previously proposed surface ocean.
This work shows that if such a porous hydrocarbon/nitrogen reservoir contains
a significant volume within pores of nanometer dimension, then the modification
of methane thermodynamics through the Kelvin effect significantly changes the
reservoir-atmosphere interaction on Titan. We measure the capillary uptake of
pure methane liquid into well-characterized ice grown at temperatures and | partial
pressures simulating Titan at present. From these results, a model of a nanoporous
reservoir housing the methane/ethane/nitrogen solution is constructed. Modifying
a previously published analytical climate model for Titan to incorporate such
a reservoir, we report calculations of the coupled reservoir-atmosphere system.
This modification leads to a very sensitive dependence of the state of the atmosphere
on the total reservoir volume. In particular, assuming a nanoporous volume distribution
reported here, calculations show that such a reservoir must comprise at least
1.5% of a 10 km deep ice regolith in order to yield a climate consistent with
that currently observed on Titan. In contrast to the mesoporous model, it reduces
the sensitivity of the system to temperature, mitigating the runaway greenhouse
effect observed in the previous model. Furthermore, the model indicates that in
a past epoch a surface ocean subsided into the reservoir as the total surface
liquid volume reduced with time. | Mayo
de 2007
The 2004 Parkfield earthquake: Test of the electromagnetic precursor
hypothesis
Authors: Stephen K. Park and William Dalrymple et al
Link:
Click Here Abstract
A controversy has existed for 30 years concerning the possibility of earthquake
prediction using electromagnetic precursors. Long-term electromagnetic monitoring
prior to, during, and after the M6.0 earthquake at Parkfield, California, on 28
September 2004 now provides a definitive test of this hypothesis. During the earthquake
our instruments recorded clearly documented electrical signals from an earthquake:
impulsive changes of up to +2.5 mV at an electrode located 250 m from the rupture
zone followed by a | transient
decay lasting at least 3 1/4 hours. Similar signals (-1.5 mV) were recorded with
the only two M > 5 aftershocks, but their transient decays lasted for only
~17 min. These signals are unambiguously a result of the earthquake and can be
useful in studies of fluid flow in faults, a major current topic in earthquake
physics. Patterns in the distribution of transient voltages differ from static
stress changes, but are consistent with a small coseismic pressure drop on the
fault and fluid flow inward. Signals at a distance of 1 km from the fault are
less than 1 mV and are statistically no different from zero in our analysis. Transient
voltages for the aftershocks have opposite polarity from those recorded with the
main shock and are again maximal close to the fault, suggesting subsequent outward
flow from the fault. Finally, there is no evidence of any precursory signal, strongly
calling into question previously suggested signals preceding smaller or more distant
earthquakes. | Abril
de 2007
Teoría de Periodo de Recurrencia de Terremotos
Autores:
Alexander Saichev y Didier Sornette
Link: Click Aquí Abstract
La estadística del periodo de recurrencia realizada en grandes áreas
ha probado obedecer leyes universales de escala, ya sea que ésta se realice
en áreas únicas homógeneas o promediando sobre múltiples
regiones. Esas leyes de escala unificadas son caracterizadas por leyes de potencia
asintóticas. Por otro lado, Molchan (2005) ha presentado una prueba matemática
donde se afirma que si tal ley universal existe, ésta debe ser necesariamente
exponencial, lo que está en obvia contradicción con los hechos.
Nosotros primeramente generalizamos el argumento de Molchan para mostrar que se
puede encontrar una ley aproximadamente unificada y que a la vez es compatible
con los hechos cuando incorporamos la Ley de Inducción de Terremotos
de Omori-Utsu. A continuación desarrollamos la teoría de la estadística
del tiempo entre terremotos en el marco del modelo de inducción sísmica
de la Secuencia Post-Terremoto Tipo-Epidémica (ETAS) y mostramos que las
observaciones empíricas pueden ser totalmente explicadas. Nuestra
expresión teórica ajusta perfectamente la estadística
empírica sobre el rango completo de periodos de recurrencia, contabilizado
para distintos tipos de regimenes mediante la única utilización
de la física de la inducción, la que es cuantificada mediante
la Ley de Omori-Utsu. La descripción estadística del periodo de
recurrencia sobre múltiples regiones requiere una adicional y sutil derivación
matemática a partir de los mapas de la geometría fractal
de los epicentros de los terremotos considerando las razones sísmicas
promedio en múltiples regiones. Esto nos lleva a predicciones en excelente
acuerdo con los datos empíricos para razonables valores de la dimensión
fractal d ˜ 1.8, de la razón promedio del agrupamiento n ˜ 0.9
y el exponente de productividad a ˜ 0.9 veces el valor b de la Ley de Gutenberg-Richter.
| Theory
of earthquake recurrence times
Authors: Alexander Saichev and Didier Sornette
Link: Click Here Abstract
The statistics of recurrence times in broad areas have been reported to obey universal
scaling laws, both for single homogeneous regions and when averaged over multiple
regions. These unified scaling laws are characterized by intermediate power law
asymptotics. On the other hand, Molchan (2005) has presented a mathematical proof
that if such a universal law exists, it is necessarily an exponential, in obvious
contradiction with the data. First, we generalize Molchan's argument to show that
an approximate unified law can be found which is compatible with the empirical
observations when incorporating the impact of the Omori-Utsu law of earthquake
triggering. We then develop the theory of the statistics of interevent times in
the framework of the Epidemic-Type Aftershock Sequence (ETAS) model of triggered
seismicity and show that the empirical observations can be fully explained.
Our theoretical expression well fits the empirical statistics over the
whole range of recurrence times, accounting for different regimes by using only
the physics of triggering quantified by the Omori-Utsu law. The description of
the statistics of recurrence times over multiple regions requires an additional
subtle statistical derivation that maps the fractal
geometry of earthquake epicenters onto the distribution of the average seismic
rates in multiple regions. This yields a prediction in excellent agreement
with the empirical data for reasonable values of the fractal dimension d ˜
1.8, the average clustering ratio n ˜ 0.9, and the productivity exponent
a ˜ 0.9 times the b value of the Gutenberg-Richter law. | Abril
de 2007
Statistics of fracture strength and fluid-induced microseismicity
Authors: Elmar Rothert and Serge A. Shapiro
Link: Click Here Abstract
In this paper we develop an approach for estimating the strength of rocks by analyzing
fluid-induced microseismicity. The strength corresponds to the value of critical
pressure in the pore space that must be exceeded in order to activate preexisting
fractures, i.e., to trigger earthquakes. We assume that during hydraulic injection
experiments in boreholes, microseismicity is mainly triggered by a diffusive process
of pore pressure perturbation. An analytical solution can be applied to find time-dependent
pore pressure perturbations in rocks caused by fluid injections. Characteristics
of the spatiotemporal evolution of microseismic clouds can be then used to estimate
minimum and maximum pressures necessary to trigger earthquakes. Moreover, we present
a method for | reconstruction
of the full spectrum of rock strength; that is, we show how to estimate the probability
density function of the critical pressure. We verify the approach using numerical
data and apply it to real data of injection-induced microseismicity from two Hot
Dry Rock tests in crystalline rocks and one hydraulic fracturing experiment in
a sedimentary rock. The results show that such an analysis of microseismicity
is able to provide us with a completely new feature of natural fractured rocks
in situ, namely, the statistics of their strength. We find that very low critical
pressures, in the range 0.001-1 MPa, characterize the strength of preexisting
cracks for all data sets analyzed. The range of critical pressures is broadly
distributed within 3 to 4 orders of magnitude. However, the probability density
functions of critical pressure change very quickly from the zero level to significant
values and from significant values to the zero level at the lower and upper limits
of their nonvanishing value ranges, respectively. The lower bound of critical
pressure is possibly defined by the magnitude of tidal-induced stresses, permanently
occurring and relaxing in the Earth. | Abril
de 2007
Earthquake source characteristics from dynamic rupture with constrained
stochastic fault stress
Authors: J. Ripperger and J.-P. Ampuero et al
Link: Click Here Abstract
One of the challenging tasks in predicting near-source ground motion for future
earthquakes is to anticipate the spatiotemporal evolution of the rupture process.
The final size of an event but also its temporal properties (propagation velocity,
slip velocity) depend on the distribution of shear stress on the fault plane.
Though these incipient stresses are not known for future earthquakes, they might
be sufficiently well characterized in a stochastic sense. We examine the evolution
of dynamic rupture in numerical models of a fault | subjected
to heterogeneous stress fields with varying statistical properties. By exploring
the parameter space of the stochastic stress characterization for a large number
of random realizations we relate generalized properties of the resulting events
to the stochastic stress parameters. The nucleation zone of the simulated earthquake
ruptures in general has a complex shape, but its average size is found to be independent
of the stress field parameterization and is determined only by the material parameters
and the friction law. Furthermore, we observe a sharp transition in event size
from small to system-wide events, governed mainly by the standard deviation of
the stress field. A simplified model based on fracture mechanics is able to explain
this transition. Finally, we find that the macroscopic rupture parameters (e.g.,
moment, moment rate, seismic energy) of our catalog of model quakes are generally
consistent with observational data. | Abril
de 2007
Seasonal modulation of seismicity in the Himalaya of Nepal
Authors: L. Bollinger, F. Perrier et al
Link: Click Here Abstract
For the period 1995-2000, the Nepal seismic network recorded 37 ± 8% fewer
earthquakes in the summer than in the winter; for local magnitudes ML > 2 to
ML > 4 the percentage increases from 31% to 63% respectively. We show the probability
of observing this by chance is | less
than 1%. We find that most surface loading phenomena are either too small, or
have the wrong polarity to enhance winter seismicity. We consider enhanced Coulomb
failure caused by a pore-pressure increase at seismogenic depths as a possible
mechanism. For this to enhance winter seismicity, however, we find that fluid
diffusion following surface hydraulic loading would need to be associated with
a six-month phase lag, which we consider to be possible, though unlikely. We favor
instead the suppression of summer seismicity caused by stress-loading accompanying
monsoon rains in the Ganges and northern India, a mechanism that is discussed
in a companion article. | Abril
de 2007
Sensitivity of U.S. surface ozone to future emissions and climate
changes
Authors: Zhining Tao and Allen Williams et al
Link: Click
Here Abstract
The relative contributions of projected future emissions and climate changes to
U.S. surface ozone concentrations are investigated focusing on California, the
Midwest, the Northeast, and Texas. By 2050 regional average ozone concentrations | increase
by 2-15% under the IPCC SRES A1Fi ("dirty") scenario, and decrease by
4-12% under the B1 (relatively "clean") scenario. However, the magnitudes
of ozone changes differ significantly between major metropolitan and rural areas.
These ozone changes are dominated by the emissions changes in 61% area of the
contiguous U.S. under the B1 scenario, but are largely determined by the projected
climate changes in 46% area under the A1Fi scenario. In the ozone responses to
climate changes, the biogenic emissions changes contribute strongly over the Northeast,
moderately in the Midwest, and negligibly in other regions. | Abril
de 2007
Bounds on the viscosity coefficient of continental lithosphere
from removal of mantle lithosphere beneath the Altiplano and Eastern Cordillera
Authors: Peter Molnar and Carmala N. Garzione
Link: Click Here Abstract
The rapid rise of the central Andean plateau between ~10 and 6.8 Ma implies that
mantle lithosphere, including eclogitized lower crust, was removed from beneath
the region in that time interval; we infer from that removal that the average
viscosity coefficient of mantle lithosphere was quite low when removal occurred.
Using scaling laws for the growth of perturbations to the thickness of a dense
layer over an inviscid substratum (Rayleigh-Taylor instability), we place bounds
on the average viscosity coefficient for central Andean lithosphere. When compared
with laboratory measurements of flow laws for olivine and eclogite, the allowed
range of viscosity coefficients yields bounds on the temperature of ~500-800°C
at the Moho beneath this region and suggests that mean stresses across mantle
lithosphere during continental deformation are less than ~50 MPa. This range of
temperature is comparable with, if a slightly lower than, we might expect for
lithosphere approximately doubled in thickness and not yet equilibrated with the
doubled crustal radioactivity. The mean deviatoric stress is comparable to that
associated with stresses that drive plates and hence shows that lithospheric material
is not too strong to prevent removal of its mantle part. | Abril
de 2007
Flank instability on Mount Etna: Radon, radar interferometry, and
geodetic data from the southwestern boundary of the unstable sector
Authors:
Marco Neri, Francesco Guglielmino et al
Link: Click Here Abstract
Understanding Etnean flank instability is hampered by uncertainties over its western
boundary. Accordingly, we combine soil radon emission, interferometric synthetic
aperture radar (InSAR), and electronic distance measurement (EDM) data to study
the Ragalna fault system (RFS) on the SW flank of the volcano. Valuable synergy
developed between our differing techniques, producing consistent results and serving
as a model for other studies of partly obscured active faults. The RFS, limited
in its surface expression, is revealed as a complex interlinked structure ~14
km long that extends from the edifice base toward the area of summit rifting,
possibly linking northeastward to the Pernicana fault system (PFS) to define the
unstable sector. Short-term deformation rates on the RFS from InSAR data reach
~7 mm yr-1 in the satellite line of sight on the upslope segment and ~5 mm yr-1
on the prominent central segment. Combining this with EDM data confirms the central
segment of the RFS as a dextral transtensive structure, with strike-slip and dip-slip
components of ~3.4 and ~3.7 mm yr-1, respectively. We measured thoron (220Rn,
half-life 56 s) as well as radon, and probably because of its limited diffusion
range, this appears to be a more sensitive but previously unexploited isotope
for pinpointing active near-surface faults. Contrasting activity of the PFS and
RFS reinforces proposals that the instability they bound is divided into at least
three subsectors by intervening faults, while, in section, fault-associated basal
detachments also form a nested pattern. Complex temporal and spatial movement
interactions are expected between these structural components of the unstable
sector. | Abril
de 2007
Space weather at Venus and its potential consequences for atmosphere
evolution
Authors: J. G. Luhmann, W. T. Kasprzak et al
Link: Click
Here Abstract
Space weather storms at the Earth are dominated by the magnetosphere's response
to coronal mass ejections, or CMEs, whose disturbances propagate through the solar
wind to 1 AU, and to a lesser extent by the pressure ridges associated with the
interactions of solar wind streams with different properties. The interplanetary
signatures of these events include high solar wind dynamic pressures and high
interplanetary magnetic fields, the same parameters that cause compression and/or
magnetization of the ionosphere at weakly magnetized Venus. The nature of Venus'
response to these events is also | expected
to include increased atmosphere erosion by the solar wind interaction, a matter
of potential interest for historical extrapolations of atmosphere escape. We consider
the possible evidence for enhanced escape during these disturbances in the data
from the Pioneer Venus Orbiter (PVO). Available magnetometer and plasma analyzer
results are used to identify periods of interest and the interplanetary characteristics
of the disturbed periods. It is found that the suprathermal (>36 eV) ion measurements
from the Pioneer Venus neutral mass spectrometer (ONMS) provide convincing evidence
of the related enhanced escape of mostly O+ suprathermal ions. The results provide
the first direct demonstration that space weather has played an important role
in Venus (and other weakly magnetized planet) atmosphere loss through time. It
is important to pursue study of these effects with the new measurements from Venus
Express and, in light of the approaching solar activity cycle rising phase, extremely
timely. | Abril
de 2007
Hydrometeorological application of a microwave link: 2. Precipitation
Authors: H. Leijnse and R. Uijlenhoet et al
Link: Click Here Abstract
A method to estimate areal evaporation using a microwave link (radio wave scintillometer)
in combination with an energy budget constraint is proposed. This radio wave scintillometry-energy
budget method (RWS-EBM) is evaluated for its applicability in different meteorological
conditions and for its sensitivity to various variables (the structure parameter
of the refractive index of air C n 2, the total available energy R n - G, the
wind velocity u, the effective average vegetation height h 0, and the correlation
coefficient between the | temperature
and humidity fluctuations r TQ ). The method is shown to be best suited for use
in wet to moderately dry conditions, where the latent heat flux is at least a
third of the total available energy (i.e., Bowen ratio =2). It is important to
accurately measure the total available energy and the wind velocity as the RWS-EBM
is most sensitive to these variables. The Flevoland field experiment has provided
the data, obtained with a 27-GHz radio wave scintillometer (over 2.2 km), a large-aperture
scintillometer (also 2.2 km), and four eddy covariance systems, which are used
to test the RWS-EBM. Comparing 92 daytime measurements (30-min intervals) of the
evaporation estimated using the RWS-EBM to that determined in alternative manners
(eddy covariance and two-wavelength scintillometry) leads to the conclusion that
the method provides consistent estimates (coefficient of determination r 2 = 0.85
in both cases) under relatively wet conditions. | Abril
de 2007
Improved finite-sample Hurst exponent estimates using rescaled
range analysis
Author: Khaled H. Hamed
Link: Click Here Abstract
Rescaled range analysis is one of the classical methods used for detecting and
quantifying long-term dependence in time series. However, rescaled range analysis
has been shown in several studies to give biased estimates of the Hurst exponent
in finite samples. A new estimator based on a modified expression of the expected
value of the rescaled range is proposed. A comparison of the modified estimator
with other |
alternatives shows that the modified estimator offers a great improvement over
the classical rescaled range estimator in terms of bias and root mean square error,
which makes it comparable to some of the leading estimators. The application of
the proposed modified rescaled range estimator to a group of temperature, rainfall,
river flow, and tree-ring time series in the Midwest USA demonstrates the extent
to which classical rescaled range analysis can give misleading results. Based
on a statistical test of significance, the number of time series exhibiting the
Hurst effect is reduced from 36 to only 11 out of 56 temperature series, and from
23 to 7 out of 60 rainfall series. On the other hand, the number of time series
exhibiting the Hurst effect marginally increased from 8 to 9 out of 49 river flow
series, and from 49 to 54 out of 88 tree-ring series. | Abril
de 2007
Analysis of glacial earthquakes
Authors: Victor C. Tsai
and Göran Ekström
Link: Click Here Abstract
In 2003, Ekström et al. reported on the detection of a new class of earthquakes
that occur in glaciated regions, with the vast majority being in Greenland. The
events have a characteristic radiation pattern and lack the high-frequency content
typical of tectonic earthquakes. It was proposed that the events correspond to
large and sudden sliding motion of glaciers. Here we present an analysis of all
184 such events detected in Greenland between 1993 and 2005. Fitting the teleseismic
long-period surface waves to a landslide model of the source, we obtain improved
locations, timing, force amplitudes, and force directions. After relocation, the
events cluster into seven regions, |
all of which correspond to regions of very high ice flow and most of which are
named outlet glaciers. These regions are Daugaard Jensen Glacier, Kangerdlugssuaq
Glacier, Helheim Glacier, the southeast Greenland glaciers, the northwest Greenland
glaciers, Rinks Isbrae, and Jakobshavn Isbrae. Event amplitudes range from 0.1
to 2.0 × 1014 kg m. Force directions are consistent with sliding in the
direction of glacial flow over a period of about 50 s. Each region has a different
temporal distribution of events. All glaciers are more productive in the summer,
but have their peak activity in different months. Over the study period, Kangerdlugssuaq
has had a constant number of events each year, whereas Jakobshavn had most events
in 1998-1999, and the number of events in Helheim and the northwest Greenland
glaciers has increased substantially between 1993 and 2005. The size distribution
of events in Kangerdlugssuaq is peaked above the detection threshold, suggesting
that glacial earthquakes have a characteristic size. | Abril
de 2007
Similar microearthquakes observed in western Nagano, Japan, and
implications for rupture mechanics
Authors: Xin Cheng, Fenglin Niu et
al
Link: Click Here Abstract
We have applied a waveform cross correlation technique to study the similarity
and the repeatability of more than 21,000 microearthquakes (0 < M < 4.5)
in the aftershock zone of the 1984 western Nagano earthquake in central Japan.
We find that the seismicity in this particular intraplate fault essentially consists
of no repeating earthquakes that occurred on the same patch of the fault in a
quasiperiodic manner in the study period between 1995 and 2001. On the other hand,
we identify a total of 278 doublets and 62 multiplets (807 events) that occurred
consecutively within seconds to days. On the | basis
of the relative arrival times of the P and S waves, we have obtained precise relative
locations of these consecutive events with an error between several meters to
a few tens of meters. There is a clear lower bound on the distances measured between
these consecutive events and the lower bound appears to be proportional to the
size of the first events. This feature is consistent with what Rubin and Gillard
[2000] have observed near the San Juan Bautista section of the San Andreas Fault.
Shear stress increases at the edge of an earthquake rupture, and the rupture edge
becomes the most likely place where the second events are initiated. The observed
minimum distance thus reflects the rupture size of the first events. The minimum
distance corresponds to the rupture size calculated from a circular fault model
with a stress drop of 10 MPa. We found that using different time windows results
in a slight difference in the delay time estimates and the subsequent projection
locations, which may reflect the finite size nature of earthquake ruptures. | Abril
de 2007
Correlaciones entre terremotos y grandes erupciones volcánicas
de lodos
Autores: R. Mellors, D. Kilb et al
Link: Click Aquí Abstract
Nosotros examinamos la relación entre la potencial inducción de
grandes terremotos a partir de las erupciones volcánicas del tipo metano
y lodo. Nuestra base de datos consistió en un catálogo de 191 años
(1810-2001) abarcando erupciones de 77 volcanes en Azerbaijan, en el Asia Central,
los que fueron suplementados con reportes de erupciones volcánicas de lodo
en Japón, Rumania, Pakistán y las islas Andaman. Nosotros comparamos
la ocurrencia de terremotos históricos regionales (M > 5)
con la ocurrencia de erupciones volcánicas de lodo en Azerbaijan y encontramos
que el número de pares terremotos/erupciones ocurridos el mismo día
es significativamente mayor que lo esperado si las erupciones y terremotos
fuesen procesos independientes (los que son modelados por la Ley de Poisson).
La correlación temporal entre terremotos y erupciones es más pronunciada
para terremotos cercanos (dentro de un radio de ~ 100 Km) que producen intensidades
sísmicas iguales o superiores a 6 Mercalli en la ubicación de la
erupción. Esto puede significar que la relación magnitud/distancia
para inducción de terremotos observada en Azerbaijan es consistente con
la relación documentada entre terremotos y erupciones volcánicas
de lodo. También encontramos una débil correlación que intensifica
el número de erupciones volcánicas de lodo que ocurren dentro de
un año después del gran terremoto. La distribución de erupciones
anuales se aproxima fuertemente a procesos tipo Poisson. Sin embargo, el tiempo
de reposo parece favorecer algún tipo de razón de falla no homogénea,
lo que implica que los volcanes necesitan de cierto tiempo para recargarse después
de una erupción. La inducción de la erupción parece resultar
del paso de una onda sísmica, aunque el mecanismo preciso no es claro.
| Abril
de 2007
Correlations between earthquakes and large mud volcano eruptions
Authors: R. Mellors, D. Kilb et al
Link: Click Here Abstract
We examine the potential triggering relationship between large earthquakes and
methane mud volcano eruptions. Our data set consists of a 191-year catalog (1810-2001)
of eruptions from 77 volcanoes in Azerbaijan, central Asia, supplemented with
reports from mud volcano eruptions in Japan, Romania, Pakistan, and the Andaman
Islands. We compare the occurrence of historical regional earthquakes (M >
5) with the occurrence of Azerbaijan mud volcano eruptions and find that the
number of same-day earthquake/eruption pairs is significantly higher than expected
if the eruptions and earthquakes are independent Poisson processes. The temporal
correlation between earthquakes and eruptions is most pronounced for nearby earthquakes
(within ~100 km) that produce seismic intensities of Mercalli 6 or greater at
the location of the mud volcano. This assumed magnitude/distance relationship
for triggering observed in the Azerbaijan data is consistent with documented earthquake-induced
mud volcano eruptions elsewhere. We also find a weak correlation that heightened
numbers of mud volcano eruptions occur within 1 year after large earthquakes.
The distribution of yearly eruptions roughly approximates a Poisson process, although
the repose times somewhat favor a nonhomogenous failure rate, which implies that
the volcanoes require some time after eruption to recharge. The volcanic triggering
likely results from some aspect of the seismic wave's passage, but the precise
mechanism remains unclear. | Abril
de 2007
Magnetic field visualization of magnetic minerals and grain boundary
regions using magneto-optical imaging
Authors: Alexander V. Bobyl, Yuri
Y. Podladchikov et al
Link: Click Here Abstract
Magneto-optical imaging based on the Faraday effect has been used to characterize
magnetic minerals embedded in a nonmagnetic matrix. We have studied magnetite
grains and magnetite-magnetite grain boundary regions in samples of skarns and
serpentinites. Distributions of the | remanent
magnetic field were measured across at the surface of polished thin sections kept
at room temperature. The magneto-optical images resolve directly magnetic structures
on length scales ranging from millimeter down to micrometer, thereby revealing
the shape and arrangement of the magnetite grains and allow determination of the
grain magnetization. We find that (1) for the skarns the intergrain interactions
do not affect the magnetic properties of magnetite grains within 0.6-60 µm
of each other, while the saturation remanence decreases weakly with increasing
grain size from 40 µm to 0.6 mm, and (2) for the serpentinites the magnetic
properties of the stripes are size-dependent due to variations in chemical composition.
| Abril
de 2007
Earthquake swarms driven by aseismic creep in the Salton Trough,
California
Authors: R. B. Lohman and J. J. McGuire
Link: Click Here Abstract
In late August 2005, a swarm of more than a thousand earthquakes between magnitudes
1 and 5.1 occurred at the Obsidian Buttes, near the southern San Andreas Fault.
This swarm provides the best opportunity to date to assess the mechanisms driving
seismic swarms along |
transform plate boundaries. The recorded seismicity can only explain 20% of the
geodetically observed deformation, implying that shallow, aseismic fault slip
was the primary process driving the Obsidian Buttes swarm. Models of earthquake
triggering by aseismic creep can explain both the time history of seismic activity
associated with the 2005 swarm and the ~1 km/h migration velocity exhibited by
this and several other Salton Trough earthquake swarms. A combination of earthquake
triggering models and denser geodetic data should enable significant improvements
in time-dependent forecasts of seismic hazard in the key days to hours before
significant earthquakes in the Salton Trough. | Abril
de 2007
Effects of topography on the spin-up of a Venus atmospheric model
Authors: A. Herrnstein and T. E. Dowling
Link: Click Here Abstract
We study how topography affects the spin-up from rest of a model of the atmosphere
of Venus. The simulations are performed with the EPIC model using its isentropic,
terrain-following hybrid vertical coordinate, and are forced with the Newtonian-cooling
profile used to achieve superrotation in a Venus model with no topography by Lee
et al. (2005). We are able to | reproduce
their results with our model, which was developed independently and uses a different
vertical coordinate. Both groups use a horizontal resolution of 5°, which
is dictated by the need for reasonable computer runtime and is not a claim of
numerical convergence. We find that the addition of topography substantially changes
both the evolution and end state of the model's spin-up: the magnitude of the
superrotation is diminished from 55 ms-1 to 35 ms-1, and it reaches steady state
faster, in a few years instead of a few decades. A large, stationary eddy associated
with Ishtar Terra forms that has a local horizontal temperature anomaly of order
2 K at the 0.7 bar level; such a feature may be observable in high-resolution
infrared images. | Abril
de 2007
Fractal topography and subsurface water flows from fluvial bedforms
to the continental shield
Authors: Anders Wörman, Aaron I. Packman
et al
Link: Click Here Abstract
Surface-subsurface flow interactions are critical to a wide range of geochemical
and ecological processes and to the fate of contaminants in freshwater environments.
Fractal scaling relationships have been found in distributions of both land surface
topography and solute efflux from watersheds, but the linkage between those | observations
has not been realized. We show that the fractal nature of the land surface in
fluvial and glacial systems produces fractal distributions of recharge, discharge,
and associated subsurface flow patterns. Interfacial flux tends to be dominated
by small-scale features while the flux through deeper subsurface flow paths tends
to be controlled by larger-scale features. This scaling behavior holds at all
scales, from small fluvial bedforms (tens of centimeters) to the continental landscape
(hundreds of kilometers). The fractal nature of surface-subsurface water fluxes
yields a single scale-independent distribution of subsurface water residence times
for both near-surface fluvial systems and deeper hydrogeological flows. | Abril
de 2007
Evidence that floodwaters filled and overflowed Capri Chasma, Mars
Authors: Neil M. Coleman, Cynthia L. Dinwiddie et al
Link: Click Here Abstract
Erosional features on a plateau in the Valles Marineris provide evidence that
a lake filled Capri Chasma until it overflowed its eastern rim, carving two large
spillover channels. The floodwaters surged into the adjacent lowlands of ancestral
Ganges Chasma and eastern Eos Chasma. The | channel
floor elevation and depth of incision at Daga Vallis reveal that a 1200-m-deep
lake water column was drained by the flooding. The width, depth, and steep energy
slope (~0.02) of Daga Vallis and the existence of several cataracts, including
a 500-m-high dry falls, demonstrate the power of the floodwaters. We estimate
a possible peak discharge rate of 1-6 × 108 m3·s-1. The catastrophic
flows may have been triggered by the collapse of topographic barriers in eastern
Coprates Chasma, providing a gateway for lake waters in the central Valles Marineris
to pour eastward toward Capri Chasma and the lowlands beyond. These may have been
among the earliest flows in Simud-Tiu Valles, contributing discharge to a possible
sea in the northern plains of Mars. | Abril
de 2007
Probabilistic envelope curves for design flood estimation at ungauged
sites
Author: Attilio Castellarin
Link: Click Aquí Abstract
Castellarin et al. (2005) introduced the probabilistic regional envelope curves
(PRECs) of flood flows and formulated an empirical estimator of the recurrence
interval T associated with the PREC for M synthetic cross-correlated and concurrent
sequences of annual maxima with length n. In principle, a PREC can be used to
estimate the T-year flood (design flood) for any basin in a given region as a
function of the catchment area alone. A number of fundamental issues hamper the
construction of PRECs and the estimation of their T for historical regional samples
of flood flows. Some of these issues are | addressed
by this study, which presents an algorithm for the application of the empirical
estimator of T to historical annual maximum series of unequal length and assesses
how the selection of a particular cross-correlation formula and plotting position
affects the accuracy of design flood estimates retrieved from PRECs (PREC flood
quantiles). The reliability of the proposed algorithm and the accuracy of the
flood quantiles are discussed through a comprehensive cross validation. The results
show that the reliability of PREC flood quantiles for ungauged sites is comparable
with the reliability of regional estimates produced by the application of the
index flood approach. The results of the cross-validation also show that the selection
of a particular correlation formula or plotting position may produce biased PREC
flood quantiles, and suggest that the accuracy of the approach could be significantly
enhanced by relaxing the assumption that the design flood scales with catchment
area alone. | Abril
de 2007
Using lunar boulders to distinguish primary from distant secondary
impact craters
Authors: Gwendolyn D. Bart and H. J. Melosh
Link: Click
Aquí Abstract
A high-resolution study of 18 lunar craters, including both primary and distant
secondary craters, shows that the secondary craters produce larger ejecta fragments
at a given crater size than do the primary craters. The maximum boulder diameter
(B) increases with crater size (D) | according
to the power law B = KD 2/3; for primary craters, when B and D are in meters,
K is 0.29, whereas for secondary craters, we find that K is 0.46 (60% larger).
Next we show that impact fracture theory predicts that secondary craters, because
of their lower impact velocity, will produce larger ejecta fragments than primary
craters. This result provides an opportunity for distinguishing between primary
and secondary craters in high resolution planetary images. The ability to identify
distant secondary craters will help constrain primary production rates of small
craters and improve surface age determination of small areas based on small crater
counts. | Abril
de 2007
A theoretical model of the inner proton radiation belt
Authors: R. S. Selesnick, M. D. Looper et al
Link: Click Aquí Abstract
A theoretical calculation provides inner radiation belt proton intensities as
a function of time and of the three adiabatic invariants, M, K, and L, in the
kinetic energy range from ~10 MeV to ~4 GeV and the L range from 1.1 to 2.4. Long
residence times for trapped protons of up to several thousand years require similarly
long input time series for the geomagnetic field, solar activity, and solar proton
fluences. Additional inputs include galactic cosmic ray spectra, nuclear scattering
cross sections, and the neutral and plasma densities in the atmosphere, ionosphere,
and plasmasphere. Trapped proton sources are cosmic ray albedo | neutron
decay (CRAND), calculated from a Monte Carlo particle transport simulation, and
solar proton injection using a derived empirical injection efficiency that is
~10-4 at 10 MeV. Radial diffusion provides inward transport of injected solar
protons. Calculated intensities at energies 100 MeV and for L 1.3 are dominated
by solar protons, CRAND being the dominant source otherwise. Losses are by ionization
of the neutral atmosphere, energy transfer to plasma electrons, and inelastic
nuclear scattering. Numerical trajectory tracing determines trapping limits and
drift shell averages of the albedo neutron intensity and of neutral and plasma
densities for loss rate calculations. Geomagnetic secular variations cause adiabatic
energy and drift shell changes. Intensities are greater than they would be in
a constant geomagnetic field by factors up to ~10, a result of long proton residence
times and the presently decreasing geomagnetic dipole moment. | Abril
de 2007
Interpreting in situ pressure and hydraulic properties with borehole
penetrometers in ocean drilling: DVTPP and Piezoprobe deployments at southern
Hydrate Ridge, offshore Oregon
Authors: H. Long, P. B. Flemings et al
Link: Click Aquí Abstract
Two borehole penetrometers, Fugro-McClelland's Piezoprobe and the Ocean Drilling
Program's (ODP) DVTPP, were deployed 50 m below seafloor at Site 1244 on ODP Leg
204 to measure formation pressure at southern Hydrate Ridge, offshore Oregon.
Pore pressure is interpreted to be hydrostatic and the sediment's coefficient
of consolidation is interpreted to lie between 6.92 and 7.8 × 10-7 m2/s,
which is in approximate | agreement
with laboratory measurements. The Piezoprobe pressure reaches 90% of dissipation
14 times sooner than the DVTPP. The observed and modeled pore pressure responses
illustrate how penetrometer geometry impacts our ability to interpret in situ
properties and demonstrate under what conditions these tools can be effectively
used. Because of its narrow tip, the Piezoprobe disturbs a narrower zone than
the DVTPP does. This generates a narrower zone of pressure increase around the
piezoprobe, which dissipates much faster than the DVTPP. As consolidation proceeds,
pressure dissipation of the Piezoprobe is retarded and forms a "bench,"
or flat spot, on the dissipation curve. Owing to its distinct two-radius geometry,
it is possible to apply a consistent method to estimate in situ pressure from
partial dissipation record based on the position of the "bench." | Abril
de 2007
Groundwater noble gas, age, and temperature signatures in an Alpine
watershed: Valuable tools in conceptual model development
Authors: Andrew
H. Manning, Jonathan Saul Caine et al
Link: Click Here Abstract
Bedrock groundwater in alpine watersheds is poorly understood, mainly because
of a scarcity of wells in alpine settings. Groundwater noble gas, age, and temperature
data were collected from springs and wells with depths of 3-342 m in Handcart
Gulch, an alpine watershed in Colorado. Temperature profiles indicate active groundwater
circulation to a maximum depth (aquifer thickness) of about 200 m, or about 150
m below the water table. Dissolved noble gas data show unusually high excess air
concentrations (>0.02 cm3 STP/g, ?Ne > 170%) in the bedrock, | consistent
with unusually large seasonal water table fluctuations (up to 50 m) observed in
the upper part of the watershed. Apparent 3H/3He ages are positively correlated
with sample depth and excess air concentrations. Integrated samples were collected
from artesian bedrock wells near the trunk stream and are assumed to approximate
flow-weighted samples reflecting bedrock aquifer mean residence times. Exponential
mean ages for these integrated samples are remarkably consistent along the stream,
four of five being from 8 to 11 years. The tracer data in combination with other
hydrologic and geologic data support a relatively simple conceptual model of groundwater
flow in the watershed in which (1) permeability is primarily a function of depth;
(2) water table fluctuations increase with distance from the stream; and (3) recharge,
aquifer thickness, and porosity are relatively uniform throughout the watershed
in spite of the geological complexity of the Proterozoic crystalline rocks that
underlie it. | Marzo
de 2007
Chemical composition of Earth's primitive mantle and its variance:
1. Method and results
Authors: Tanya Lyubetskaya and Jun Korenaga
Link: Click Aquí Abstract
We present a new statistical method to construct a model for the chemical composition
of Earth's primitive mantle along with its variance. Earth's primitive mantle
is located on the melting trend exhibited by the global compilation of mantle
peridotites, using cosmochemical constraints on the relative abundances of refractory
lithophile elements (RLE). This so-called pyrolite approach involves the least
amount of assumptions, thereby being probably most satisfactory compared to other
approaches. Its previous implementations, however, suffer from questionable statistical
treatment of noisy geochemical data, leaving the uncertainty of model composition
poorly |
quantified. In order to properly take into account how scatters in peridotite
data affect this geochemical inference, we combine the following statistical techniques:
(1) modeling a nonlinear melting trend in the multidimensional compositional space
through the principal component analysis, (2) determining the primitive mantle
composition on the melting trend by simultaneously imposing all of cosmochemical
constraints with least squares, and (3) mapping scatters in original data into
the variance of the final model through the bootstrap resampling technique. Whereas
our model is similar to previous models in terms of Mg, Si, and Fe abundances,
the RLE contents are at ~2.16 ± 0.37 times the CI chondrite concentration,
which is lower than most of previous estimates. The new model is depleted by >20%
in a number of incompatible elements including heat-producing elements, U, Th,
and K, and this depleted nature is further amplified (up to 60%) in terms of predicted
composition for the present-day mantle. | Marzo
de 2007
First-principles study of density, viscosity, and diffusion coefficients
of liquid MgSiO3 at conditions of the Earth's deep mantle
Authors: J.
T. K. Wan, T. S. Duffy et al
Link: Click Here Abstract
Constant-pressure constant-temperature ab initio molecular dynamics simulations
at high temperatures have been used to study MgSiO3, the major constituent of
the Earth's lower mantle to conditions of the Earth's core-mantle boundary. The
calculated equilibrium volumes and densities are compared with simulations using
an orthorhombic perovskite configuration under the same conditions. For molten
MgSiO3, we have determined the diffusion coefficients and shear viscosities at
different thermodynamic conditions. Our results provide new constraints on the
properties of molten MgSiO3 at conditions near | the
core-mantle boundary. The volume of the liquid is greater than that of the solid
throughout the pressure-temperature conditions examined, and the volume change
on fusion ranges from 5% at 88 GPa and 3500 K to 2.9% at 120 GPa and 5000 K. Existing
experimental constraints on solid-liquid partition coefficients for Fe suggest
that Fe is preferentially partitioned into the liquid. Such enrichment of Fe increases
the density of the liquid, thus allowing the possibility of negatively buoyant
melts from (Mg,Fe)SiO3 perovskite compositions at deep lower mantle conditions
for plausible values of solid-liquid partition coefficients for Fe. At 120 GPa
and 4500-5000 K, the diffusion coefficient of liquid MgSiO3 is 2-3 × 10-5
cm 2/s and the diffusion rates of the different chemical species are similar.
The shear viscosity is estimated using Zwanzig's formula to be 19-31 cP under
these conditions. On the basis of our calculated diffusivities, MgSiO3 is above
the glass transition temperature at 120 GPa and 4500 K. | Marzo
de 2007
El Próximo Gran Terremoto
Author: Robert McCaffrey
Link: Click Aquí Abstract
El geólogo Robert McCaffrey, del Rensselaer Polytechnic Institute, ha advertido
en la revista Science de los riesgos del próximo Gran Terremoto. Tal como
explica al respecto un comunicado del citado instituto, estos episodios
geológicos están asociados a las así llamadas zonas de subducción,
lugares donde se encuentran las placas tectónicas y donde una placa se
desliza sobre la otra. Estos fenómenos ocurren cada 200 o 1.000 años.
El problema es que, aunque los sismólogos intentan anticipar la evolución
previsible de estas zonas de subducción, con la finalidad de prevenir el
próximo Gran Terremoto, la realidad es que están muy lejos de conseguirlo:
el terremoto Sumatra-Andaman, de diciembre de 2004 (más de 250.000 muertos),
estaba entre las zonas con más bajas posibilidades de ocurrencia establecidas
por los sismólogos. Eso quiere decir que cualquier zona de subducción
puede darnos la sorpresa en cualquier momento, ya sea en el cinturón de
fuego del Pacífico, en el Mar de China Meridional o en la placa de Nazca,
bajo la cordillera de los Andes (...) McCafrey plantea la necesidad de elaborar
políticas de prevención en todas ellas porque muchas están
densamente pobladas y los impactos previsibles pueden ser catastróficos.
Eduardo Martínez - www.tendencias21.net
Tema
relacionado:
Cómo
predecir terremotos - 1
Cómo
predecir terremotos - 2 | The
Next Great Earthquake
Robert McCaffrey 
The
2004 Sumatra-Andaman earthquake occurred at a surprising time and place; the lessons
learned may help costal communities in the future.
The author is in the
Department of Earth and Environmental Sciences, Rensselaer Polytechnic Institute,
Troy, NY 12180, USA. E-mail: mccafr@rpi.edu COMMUNIQUÉ
The
2004 Sumatra-Andaman earthquake and resulting tsunami are now infamous for the
damage they caused, but at the time many scientists believed this area was unlikely
to create a quake of such magnitude. In the March 23 issue of the journal Science,
a geophysicist from Rensselaer Polytechnic Institute urges the public and policy
makers to consider all subduction-type tectonic boundaries to be “locked,
loaded, and dangerous.” | See
More | Marzo
de 2007
A comparison of annual vertical crustal displacements from GPS
and Gravity Recovery and Climate Experiment (GRACE) over Europe
Authors:
T. van Dam , J. Wahr et al
Link: Click here Abstract
We compare approximately 3 years of GPS height residuals (with respect to the
International Terrestrial Reference Frame) with predictions of vertical surface
displacements derived from the Gravity Recovery and Climate Experiment (GRACE)
gravity fields for stations in Europe. An annual signal fit to the residual monthly
heights, corrected for atmospheric pressure and barotropic ocean loading effects,
should primarily represent surface displacements due to | long-wavelength
variations in water storage. A comparison of the annual height signal from GPS
and GRACE over Europe indicates that at most sites, the annual signals do not
agree in amplitude or phase. We find that unlike the annual signal predicted from
GRACE, the annual signal in the GPS heights is not coherent over the region, displaying
significant variability from site to site. Confidence in the GRACE data and the
unlikely possibility of large-amplitude small-scale features in the load field
not captured by the GRACE data leads us to conclude that some of the discrepancy
between the GPS and GRACE observations is due to technique errors in the GPS data
processing. This is evidenced by the fact that the disagreement between GPS and
GRACE is largest at coastal sites, where mismodeling of the semidiurnal ocean
tidal loading signal can result in spurious annual signals. | |
Marzo de 2007
Effect of shock on
the magnetic properties of pyrrhotite, the Martian crust, and meteorites
Authors:
Karin L. Louzada, Sarah T. Stewart et al
Link: Click here Abstract
We performed planar shock recovery experiments on natural pyrrhotite at pressures
up to 6.9 GPa. We find that high-field isothermal remanent magnetization in pyrrhotite
is demagnetized up to 90% by shock due to preferential removal of low coercivity
components of magnetization. Contrary | to
static experiments, we do not observe complete demagnetization. Post shock permanent
changes in magnetic properties include increasing saturation isothermal remanent
magnetization, bulk coercivity and low-temperature memory, and changes in squareness
of hysteresis. These changes are consistent with an increase in the volume fraction
of single domain grains. The lack of magnetic anomalies over large Martian impact
basins is not expected to be solely due to shock demagnetization of the crust.
We find that pyrrhotite-bearing rocks and meteorites can retain records of Martian
magnetic fields even if shocked to pressures approaching 7 GPa. However, some
paleointensity techniques may underestimate this field. |
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