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Septiembre de 2010
Características de la ruptura del año 2005
ocurrida en Tarapacá , norte de Chile: ¿Evidencia
de distribución heterogénea de fluído
a través de la placa oceánica de subducción?
Autores: Keiko Kuge, Yuko Kase et al
Link: Click aquí
Abstract
Examinamos la ruptura asociada al terremoto ocurrido en el
año 2005 en Tarapacá, norte de Chile, en una
profundidad de 110 Km respecto a las características
cinemáticas y dinámicas mediante la utilización
de formas de onda regionales y telesísmicas. El terremoto
tuvo un mecanismo focal tensional en descenso. La ruptura
subhorizontal está caracterizada por dos parches de
gran pendiente y mucha tensión, los que están
alineados cerca de la dirección este-oeste, siendo
perpendiculares a la dirección de la fosa chilena.
La ruptura se inició en el parche oriental y se propagó
hacia el occidental. Entre los dos parches existe una región
de tensión no-positiva y de exceso de tensión,
el cual puede causar una ruptura de sub-corte que se propagaría
de este a oeste, pero radiaría pequeñas ondas
sísmicas. La radiación de energía sísmica
del terremoto tiende a ser baja, lo que es consistente con
la tensión no-positiva y con el exceso de tensión
entre los dos parches. Mientras el mecanismo físico
de los terremotos de profundidad intermedia es aún
controversial, las hipótesis están relacionadas
con la deshidratación de las placas de subducción.
Las características de ruptura del terremoto de Tarapacá
pueden estar relacionadas con una distribución heterogenea
de fluídos debido a la deshidratación. La separación
espacial y la tensión dominante de los dos grandes
parches de deslizamiento coinciden con la zona doble sísmica
reportada previamente bajo Chile. Los dos parches pueden ser
la manifestación de una doble zona sísmica donde
las reacciones de deshidratación pueden emitir fluído.
Utilizando una simulación numérica 3D de la
dinámica de la ruptura, nosotros mostramos que el debilitamiento
debido al fluído puede explicar las característcas
de la ruptura del terremoto de Tarapacá.
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Septiembre de 2010
Rupture characteristics of the 2005 Tarapaca, northern
Chile, intermediate-depth earthquake: Evidence for heterogeneous
fluid distribution across the subducting oceanic plate?
Authors: Keiko Kuge, Yuko Kase et al
Link: Click here
Abstract
We examined the rupture of the 2005 Tarapaca, northern Chile,
earthquake at about 110 km depth with respect to both kinematic
and dynamic characteristics by using regional and teleseismic
waveforms. The earthquake has a downdip tensional focal mechanism.
The subhorizontal rupture is characterized by two patches
of large slip and high stress drop which are aligned nearly
in the east-west direction, being perpendicular to the direction
of the Chile Trench. Rupture initiated in the eastern patch
and then propagated to the western patch. Between the two
patches, there exists a region of nonpositive stress drop
and high strength excess, which can cause subshear rupture
to propagate from the eastern to the western patches but radiates
little seismic waves. Seismic radiation energy from this earthquake
tends to be low, which is consistent with the nonpositive
stress drop and high strength excess between the two patches.
While the physical mechanism of intermediate-depth earthquakes
is still controversial, current leading hypotheses are associated
with dehydration within subducting plates. The rupture characteristics
of the Tarapaca earthquake can be related to heterogeneous
fluid distribution due to the dehydration. The spatial separation
and dominant stress of the two large-slip patches agree with
the characteristics of the previously reported double seismic
zone beneath Chile. The two patches may be the manifestation
of the double seismic zone where dehydration reactions can
release fluid. Using a numerical simulation of 3-D dynamic
rupture, we have shown that weakening due to fluid can account
for the rupture characteristics of the Tarapaca earthquake.
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Septiembre de 2010
Does hydrologic circulation mask frictional heat on faults
after large earthquakes?
Authors: Patrick M. Fulton, Robert N. Harris
et al
Link: Click here
Abstract
Knowledge of frictional resistance along faults is important
for understanding the mechanics of earthquakes and faulting.
The clearest in situ measure of fault friction potentially
comes from temperature measurements in boreholes crossing
fault zones within a few years of rupture. However, large
temperature signals from frictional heating on faults have
not been observed. Unambiguously interpreting the coseismic
frictional resistance from small thermal perturbations observed
in borehole temperature profiles requires assessing the impact
of other potentially confounding thermal processes. We address
several issues associated with quantifying the temperature
signal of frictional heating including transient fluid flow
associated with the earthquake, thermal disturbance caused
by borehole drilling,
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and
heterogeneous thermal physical rock properties. Transient fluid
flow is investigated using a two-dimensional coupled fluid flow
and heat transport model to evaluate the temperature field following
an earthquake. Simulations for a range of realistic permeability,
frictional heating, and pore pressure scenarios show that high
permeabilities (>10-14 m2) are necessary for significant
advection within the several years after an earthquake and suggest
that transient fluid flow is unlikely to mask frictional heat
anomalies. We illustrate how disturbances from circulating fluids
during drilling diffuse quickly leaving a robust signature of
frictional heating. Finally, we discuss the utility of repeated
borehole temperature profiles for discriminating between different
interpretations of thermal perturbations. Our results suggest
that temperature anomalies from even low friction should be
detectable at depths >1 km 1 to 2 years after a large earthquake
and that interpretations of low friction from existing data
are likely robust. |
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Septiembre de 2010
Magnetotelluric pulses generated by volcanic lightning
at Sakurajima volcano, Japan
Authors: Koki Aizawa, Akihiko Yokoo et
al
Link: Click here
Abstract
Continuous magnetotelluric (MT) measurements were conducted
at Sakurajima volcano, Japan, revealing syn-eruption electric
pulses (and sometimes accompanying geomagnetic pulses). Movies
of the eruptions, recorded with timing provided by a GPS clock,
show a large number of volcanic
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lightning flashes. Some MT pulses
occurred simultaneously with lightning flashes. Pulses were
observed more than 10 seconds after the onset of the eruption,
and tend to occur during eruptions that emit volcanic ash to
high altitudes. Pulses were more common during mild eruptions
rather than during Vulcanian eruptions. The observations suggest
that the dominant mechanism of volcanic lighting is similar
to that of lightning in thunderstorms, in that it requires the
collision of particles and subsequent separation of positive
and negative charge. |
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Septiembre de 2010
A multiscale approach to estimating topographically correlated
propagation delays in radar interferograms
Authors: Yu-nung Nina Lin, Mark Simons
et al
Link: Click here
Abstract
When targeting small amplitude surface deformation, using
repeat orbit Interferometric Synthetic Aperture Radar (InSAR)
observations can be plagued by propagation delays, some of
which correlate with topographic variations. These topographically-correlated
delays result from temporal variations in vertical stratification
of the troposphere. An approximate model assuming a linear
relationship between topography and interferometric phase
has been used to correct observations with success in a few
studies. Here, we present a robust approach to estimating
the transfer function, K, between topography and phase that
is relatively insensitive to confounding processes (earthquake
deformation, phase ramps from orbital errors, tidal loading,
etc.). Our approach takes advantage of a multiscale perspective
by using a band-pass decomposition of both topography and
observed phase. This decomposition into several spatial scales
allows us
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to determine the bands wherein correlation between topography
and phase is significant and stable. When possible, our approach
also takes advantage of any inherent redundancy provided by
multiple interferograms constructed with common scenes. We define
a unique set of component time intervals for a given suite of
interferometric pairs. We estimate an internally consistent
transfer function for each component time interval, which can
then be recombined to correct any arbitrary interferometric
pair. We demonstrate our approach on a synthetic example and
on data from two locations: Long Valley Caldera, California,
which experienced prolonged periods of surface deformation from
pressurization of a deep magma chamber, and one coseismic interferogram
from the 2007 Mw 7.8 Tocapilla earthquake in northern Chile.
In both examples, the corrected interferograms show improvements
in regions of high relief, independent of whether or not we
pre-correct the data for a source model. We believe that most
of the remaining signals are predominately due to heterogeneous
water vapor distribution that requires more sophisticated correction
methods than those described here. |
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Septiembre de 2010
Upper mantle electrical resistivity structure beneath the
central Mariana subduction system
Authors: Tetsuo Matsuno, Nobukazu Seama
et al
Link: Click here
Abstract
This paper reports on a magnetotelluric (MT) survey across
the central Mariana subduction system, providing a comprehensive
electrical resistivity image of the upper mantle to address
issues of mantle dynamics in the mantle wedge and beneath
the slow back-arc spreading ridge. After calculation of MT
response functions and their correction for topographic distortion,
two-dimensional electrical resistivity structures were generated
using an inversion algorithm with a smoothness constraint
and with additional restrictions imposed by the subducting
slab. The resultant isotropic electrical resistivity structure
contains several key features. There is an uppermost resistive
layer with a thickness of up to 150 km beneath the Pacific
Ocean Basin, 80-100 km beneath the Mariana Trough, and 60
km beneath the Parece Vela Basin along with a conductive mantle
beneath the resistive layer. A resistive region down to 60
km depth and a conductive region at greater depth are inferred
beneath the volcanic arc in the mantle wedge. There is no
evidence for a conductive
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feature beneath the back-arc spreading
center. Sensitivity tests were applied to these features through
inversion of synthetic data. The uppermost resistive layer is
the cool, dry residual from the plate accretion process. Its
thickness beneath the Pacific Ocean Basin is controlled mainly
by temperature, whereas the roughly constant thickness beneath
the Mariana Trough and beneath the Parece Vela Basin regardless
of seafloor age is controlled by composition. The conductive
mantle beneath the uppermost resistive layer requires hydration
of olivine and/or melting of the mantle. The resistive region
beneath the volcanic arc down to 60 km suggests that fluids
such as melt or free water are not well connected or are highly
three-dimensional and of limited size. In contrast, the conductive
region beneath the volcanic arc below 60 km depth reflects melting
and hydration driven by water release from the subducting slab.
The resistive region beneath the back-arc spreading center can
be explained by dry mantle with typical temperatures, suggesting
that any melt present is either poorly connected or distributed
discontinuously along the strike of the ridge. Evidence for
electrical anisotropy in the central Mariana upper mantle is
weak. |
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Septiembre de 2010
A geologically constrained Monte Carlo approach to modeling
exposure ages from profiles of cosmogenic nuclides: An example
from Lees Ferry, Arizona
Authors:Alan J. Hidy, John C. Gosse et
al
Link: Click here
Abstract
We present a user-friendly and versatile Monte Carlo simulator
for modeling profiles of in situ terrestrial cosmogenic nuclides
(TCNs). Our program (available online at http://geochronology.earthsciences.dal.ca/downloads-models.html)
permits the incorporation of site-specific geologic knowledge
to calculate most probable values for exposure age, erosion
rate, and inherited nuclide concentration while providing
a rigorous
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treatment
of their uncertainties. The simulator is demonstrated with 10Be
data from a fluvial terrace at Lees Ferry, Arizona. Interpreted
constraints on erosion, based on local soil properties and terrace
morphology, yield a most probable exposure age and inheritance
of 83.9-14.1+19.1 ka, and 9.49-2.52+1.21 × 104 atoms g-1,
respectively (2s). Without the ability to apply some constraint
to either erosion rate or age, shallow depth profiles of any
cosmogenic nuclide (except for nuclides produced via thermal
and epithermal neutron capture, e.g., 36Cl) cannot be optimized
to resolve either parameter. Contrasting simulations of 10Be
data from both sand- and pebble-sized clasts within the same
deposit indicate grain size can significantly affect the ability
to model ages with TCN depth profiles and, when possible, sand-not
pebbles-should be used for depth profile exposure dating. |
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Septiembre de 2010
An enhanced nonparametric streamflow disaggregation model
with genetic algorithm
Authors: T. Lee, J. D. Salas et al
Link: Click here
Abstract
Stochastic streamflow generation is generally utilized for
planning and management of water resources systems. For this
purpose, a number of parametric and nonparametric models have
been suggested in literature. Among them, temporal and spatial
disaggregation approaches play an important role particularly
to make sure that historical variance-covariance properties
are preserved at various temporal and spatial scales. In this
paper, we review the underlying features of existing nonparametric
disaggregation methods,
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identify some of their pros and
cons, and propose a disaggregation algorithm that is capable
of surmounting some of the shortcomings of the current models.
The proposed models hinge on k-nearest neighbor resampling,
the accurate adjusting procedure, and a genetic algorithm. The
models have been tested and compared to an existing nonparametric
disaggregation approach using data of the Colorado River system.
It has been shown that the model is capable of (1) reproducing
the season-to-season correlations including the correlation
between the last season of the previous year and the first season
of the current year, (2) minimizing or avoiding the generation
of flow patterns across the year that are literally the same
as those of the historical records, and (3) minimizing or avoiding
the generation of negative flows. In addition, it is applicable
to intermittent river regimes. |
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Agosto de 2010
Detecting low-frequency earthquakes within non-volcanic
tremor in southern Taiwan triggered by the 2005 Mw8.6 Nias
earthquake
Authors: Chi-Chia Tang, Zhigang Peng et
al
Link: Click here
Abstract
We use a matched filter technique to detect 41 low-frequency
earthquakes (LFEs) within 700-s of triggered tremor signals
in the Southern Central Range in Taiwan during the surface
waves of the
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2005
Mw8.6 Nias earthquake off the coast of northern Sumatra. The
depth distributions of LFEs after double-difference relocations
concentrate at the depth range of 12-38 km below the background
seismicity and above the Moho depth inferred from receiver function
studies. The locations of LFEs are close to the downward extension
of the steep-dipping Chaochou-Lishan fault with only modestly
high Vp/Vs ratios (1.75-1.85). Our observation indicates that
at least portions of triggered tremor consists of many LFEs,
similar to ambient tremor observed at other major plate boundary
faults. |
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Agosto de 2010
Intrusions and anomalous Vp/Vs ratios associated with the
New Madrid seismic zone
Authors: Christine A. Powell, Mitchell
M. Withers et al
Link: Click here
Abstract
Detailed P wave velocity (Vp) and S wave velocity (Vs) models
and Vp/Vs ratios were determined for a major portion of the
New Madrid seismic zone using arrival times recorded by the
New Madrid seismic network and Portable Array for Numerical
Data Acquisition (PANDA) stations. We performed a simultaneous
inversion for P and S wave velocities and hypocentral locations,
yielding the most detailed tomographic image of the upper
portion of the crust to date. Low Vp and high Vs anomalies
resulted in low Vp/Vs ratios that
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correspond to the major arms
of seismicity north of the intersection of the Cottonwood Grove-Blytheville
Arch (CG-BA) fault with the Reelfoot fault. The unusual low
Vp/Vs values can be attributed to the presence of quartz-rich
rocks. Two regions contain anomalous Vp and Vs values and Vp/Vs
ratios that cannot be attributed to variations in rock composition
and are probably produced by overpressured fluids. One region
is located on the hanging wall of the northern portion of the
Reelfoot fault and is aseismic. The other region corresponds
to a portion of the southern Reelfoot fault that experiences
swarm activity. A distinct velocity contrast exists across the
CG-BA fault at depths exceeding 4.65 km; basement rocks southeast
of the fault have Vp values that are 4%-6% slower than values
for rocks located to the northwest. The most logical explanation
is that the fault follows a preexisting structural feature or
lithologic change in basement rocks. |
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Agosto de 2010
Land-Level Changes Produced by the Mw 8.8 2010 Chilean
Earthquake
Authors: Marcelo Farías, Gabriel
Vargas et al
Link: Click here
Abstract
We observed vertically displaced coastal and river markers
after the 27 February 2010 Chilean earthquake [moment magnitude
(Mw) 8.8]. Land-level changes range between 2.5 and -1 meters,
evident along an ~500-kilometers-long segment identified here
as the maximum length of coseismic rupture. A hinge line located
120 kilometers from the trench separates uplifted areas, to
the west, from subsided regions. A
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simple elastic dislocation model fits these
observations well; model parameters give a similar seismic
moment to seismological estimates and suggest that most
of the plate convergence since the 1835 great earthquake
was elastically stored and then released during this event.
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Agosto de 2010
Lower plate structure and upper plate deformational segmentation
at the Sunda-Banda arc transition, Indonesia
Authors: L. Planert, H. Kopp et al
Link: Click here
Abstract
The Sunda-Banda arc transition at the eastern termination
of the Sunda margin (Indonesia) represents a unique natural
laboratory to study the effects of lower plate variability
on upper plate deformational segmentation. Neighboring margin
segments display a high degree of structural diversity of
the incoming plate (transition from an oceanic to a continental
lower plate, presence/absence of an oceanic plateau, variability
of subducting seafloor morphology) as well as a wide range
of corresponding fore-arc structures, including a large sedimentary
basin and an accretionary prism/outer arc high of
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variable
size and shape. Here, we present results of a combined analysis
of seismic wide-angle refraction, multichannel streamer and
gravity data recorded in two trench normal corridors located
offshore the islands of Lombok (116°E) and Sumba (119°E).
On the incoming plate, the results reveal a 8.6-9.0 km thick
oceanic crust, which is progressively faulted and altered when
approaching the trench, where upper mantle velocities are reduced
to ~7.5 km/s. The outer arc high, located between the trench
and the fore-arc basin, is characterized by sedimentary-type
velocities (Vp < 5.5 km/s) down to the top of the subducting
slab (~13 km depth). The oceanic slab can be traced over 70-100
km distance beneath the fore arc. A shallow serpentinized mantle
wedge at ~16 km depth offshore Lombok is absent offshore Sumba,
where our models reveal the transition to the collisional regime
farther to the east and to the Sumba block in the north. Our
results allow a detailed view into the complex structure of
both the deeper and shallower portions of the eastern Sunda
margin. |
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Agosto de 2010
Linking regional sources and pathways for submarine groundwater
discharge at a reef by electrical resistivity tomography,
222Rn, and salinity measurements
Authors: M. Bayani Cardenas, Peter B. Zamora
et al
Link: Click here
Abstract
Submarine groundwater discharge (SGD) is an important component
of the hydrologic cycle connecting terrestrial to marine environments.
SGD in fringing reefs and its consequences on biogeochemistry
and ecology remain mostly
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unexplored. The 222Rn activity
and salinity of seawater indicate a substantial groundwater
contribution throughout most of the 20 km2 studied tropical
reef in Pangasinan, Philippines. Over 30 km of electrical resistivity
profiles with a penetration depth of 12.5 m shows widespread
zones within the reef that are much more resistive than porous
reef rocks or sediment saturated with typical seawater. Some
discrete resistive areas are located close to where seawater
has 222Rn peaks and where geologic lineaments are likely located
suggesting that these are preferential pathways for fresher
groundwater discharging to the reef. SGD at the site could be
a major ecological factor connecting the reef to the subsurface
environment which in turn may lead to connections to land. |
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Agosto de 2010
Satellite thermal observations of the Bezymianny lava dome
1993-2008: Precursory activity, large explosions, and dome
growth
Authors: S. M. van Manen, J. Dehn et al
Link: Click here
Abstract
Fifteen years worth of Advanced Very High Resolution Radiometer
(AVHRR) data is presented and used to quantitatively assess
processes occurring at Bezymianny. This andesitic volcano
is one of Kamchatka's most dangerous volcanoes with 16 eruptions
in the last decade that have dispersed ash into North Pacific
air routes. All known episodes of increased activity for which
data were available were detected in band 3 (3.53-3.93 µm)
AVHRR thermal data. Twenty-three peaks can be seen in the
data; nineteen peaks correspond to known explosions, while
the remaining three peaks correspond to known phases of dome
growth that were not believed to have been accompanied by
explosive activity. Start and end dates of extrusive phases
defined by the thermal data are presented. Repose times between
phases of extrusion vary from four months to just over two
and half years
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with
an average of just less than a year. Using rank-order statistics
a 'maximum' time interval between consecutive explosions of
1288 ± 170 days is determined; this could serve as a
cut-off time for declaring the current dome-growth activity
over. The calculated cumulative erupted volume (0.28 km3) and
time-averaged extrusion rate (0.6 m3 s-1) from 1993 to 2008
corresponds to values found at Bezymianny from 1956 to 1976,
showing that the satellite-based methodology provides a good
way of quantitatively assessing dome growth. Three different
types of precursors to explosive behavior have been identified
at Bezymianny: (1) values that cluster around the mode of the
data set prior to explosion, potentially due to endogenous dome
growth, (2) upward trends that commence 15-20 days prior to
explosion and reach sensor saturation levels are due to significant
extrusion, and (3) a gradual upward trend that starts 5 days
prior to explosion, probably due to ramping up of extrusion.
This work shows that retrospectively analyzing and modeling
of a volcano's thermal signal provides increased insight into
its characteristic behavior. The methods used in this paper
can be used at other dome-building volcanoes around the world.
The insights presented here can be used to improve monitoring
capabilities to aid in providing early warnings to large explosions
at Bezymianny |
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Agosto de 2010
Are spontaneous earthquakes stationary in California?
Authors: Qi Wang, David. D. Jackson et
al
Link: Click here
Abstract
Aftershocks and some main shocks are triggered, with timing
controlled by preceding events. The remaining spontaneous
earthquakes presumably respond to tectonic stresses. We consider
whether triggered events can be reliably identified, whether
the rest are stationary, and whether external phenomena control
them. To all three questions, some studies of earthquake physics
and hazard assume answers. Many suggest that stress changes
from large distant earthquakes can alter the local spontaneous
earthquake rate.
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We demonstrate significant differences
in the apparent earthquake rate after declustering by different
methods, and we present criteria for assessing the influence
of such distant events. The estimated spontaneous earthquake
rate depends on the lower magnitude threshold of included events,
whether spontaneity is treated in binary or probabilistic form,
as well as assumptions about catalog completeness. Different
statistical tests give different answers to the question of
stationarity. We examine a reported rate change in southern
California and the suggestion that it might have resulted from
the 1960 Chile and 1964 Alaska earthquakes. The rate change
itself is questionable. If it has occurred, it was probably
not caused by those distant events, because the rate change
is not present in northern California or in all parts of southern
California. |
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Agosto de 2010
Three-dimensional resistivity tomography of Vulcan's forge,
Vulcano Island, southern Italy
Authors: A. Revil, T. C. Johnson et al
Link: Click here
Abstract
9,525 DC resistivity measurements were taken along 9 profiles
crossing the volcanic edifice of La Fossa di Vulcano (the
forge of God Vulcan in ancient Roman mythology), Vulcano Island
(Italy) using a total of 958 electrode locations. This unique
data set has been inverted in 3D by minimizing the L2 norm
of the data misfit using a Gauss-Newton approach. The true
3D inversion
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was
performed using parallel processing on an unstructured tetrahedral
mesh containing 75,549 finite-element nodes and 398,208 elements
to accurately model the topography of the volcanic edifice.
The 3D tomogram shows a very conductive body (>0.1 S/m) comprised
inside the Pietre Cotte crater with conductive volumes that
are consistent with the position of temperature and CO2 anomalies
at the ground surface. This conductive body is interpreted as
the main hydrothermal body. It is overlaid by a resistive and
cold cap in the bottom of the crater. The position of the conductive
body is consistent with the deformation source responsible for
the observed 1990-1996 deflation of the volcano associated with
a decrease of hydrothermal activity. |
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Agosto de 2010
Broadband seismic monitoring of active volcanoes using
deterministic and stochastic approaches
Authors: Hiroyuki Kumagai, Masaru Nakano
et al
Link: Click here
Abstract
We systematically used two approaches to analyze broadband
seismic signals for monitoring active volcanoes: one is waveform
inversion of very-long-period (VLP) signals assuming possible
source mechanisms; the other is a source location method of
long-period (LP) events and tremor using their amplitudes.
The deterministic approach of the waveform inversion is useful
to constrain the source mechanism and location but is basically
only applicable to VLP signals with periods longer than a
few seconds. The source
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location method assumes isotropic
radiation of S waves and uses seismic amplitudes corrected for
site amplifications. This simple approach provides reasonable
source locations for various seismic signals such as a VLP event
accompanying LP signals, an explosion event, and tremor associated
with lahars and pyroclastic flows observed at five or fewer
stations. Our results indicate that a frequency band of about
5-12 Hz and a Q factor of about 60 are appropriate for the determination
of the source locations. In this frequency band the assumption
of isotropic radiation may become valid because of the path
effect caused by the scattering of seismic waves. The source
location method may be categorized as a stochastic approach
based on the nature of scattering waves. Systematic use of these
two approaches provides a way to better utilize broadband seismic
signals observed at a limited number of stations for improved
monitoring of active volcanoes. |
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Agosto de 2010
Monochromatic infrasonic tremor driven by persistent degassing
and convection at Villarrica Volcano, Chile
Authors: M. Ripepe, E. Marchetti et al
Link: Click here
Abstract
Infrasonic data collected at Villarrica volcano (Chile) in
March 2009 show a sustained, continuous, infrasonic oscillation
(tremor) with a monochromatic low frequency content at ~0.75
Hz. This tremor is extremely stable in time both at the
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summit
and at a distal (~4 km) small aperture array. Infrasonic tremor
is characterized by discrete high amplitude bursts with a cyclic
recurrence time of ~40 s and is well correlated (0.93) with
seismic tremor. These new data are compared with previous datasets
collected in 2002 and 2004 during different levels of activity.
All data show the same persistent infrasonic tremor and have
the same strong correlation with seismic tremor. The stability
and correlation of infrasonic and seismic tremor indicate the
existence of a sustained and continuous process, which we suggest
is related to the gravity-driven bubble column dynamics responsible
for conduit convection. |
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Agosto de 2010
Postseismic relaxation following the 1989 MS7.1 Loma Prieta
earthquake, central California
Authors: J. C. Savage and J. L. Svarc
Link: Click here
Abstract
The postseismic relaxation (postseismic displacement less
displacement that would have occurred at the preseismic rate)
measured by GPS and leveling following the 1989 MS7.1 Loma
Prieta earthquake is reexamined. The temporal dependence of
the relaxation over the first 1200 days postseismic is well
described by 1 - e-t/t, where t = 414 ± 92 days. (Quoted
uncertainties are standard deviations.) That temporal dependence
appears to be a linear function of the cumulative number of
M > 2.5 aftershocks that have occurred.
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The relaxation is attributed to
afterslip (1.56 ± 0.20 m dextral strike slip and 0.60
± 0.04 m reverse slip) on the downdip extension (depth,
16-21 km) of the coseismic rupture plus a collapse (0.11 ±
0.02 m fault-normal displacement) of the rupture zone (depth,
5-16 km). Because the postseismic uplift was determined by leveling
over a route with relief in excess of 1000 m, an allowance (18
± 2 ppm of height above the base elevation) for excess
(beyond corrections already applied) height-dependent error
in the measured uplift was estimated simultaneously with the
afterslip and collapse parameters. This new solution for afterslip
and collapse on the plane of the rupture provides an alternative
explanation to the suggestion by Bürgmann et al. (1997)
that the postseismic deformation was due to afterslip on the
coseismic rupture and on a nearby, shallow, 30° dipping
thrust. |
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Agosto de 2010
Average properties of the magnetic reconnection ion diffusion
region in the Earth's magnetotail: The 2001-2005 Cluster observations
and comparison with simulations
Authors: J. P. Eastwood, T. D. Phan et
al
Link: Click here
Abstract
Magnetic reconnection plays a key role in the circulation
of plasma through the Earth's magnetosphere. As such, the
Earth's magnetotail is an excellent natural laboratory for
the study of reconnection and in particular the diffusion
region. To address important questions concerning observational
occurrence rates and average properties, the Cluster data
set from 2001-2005 has been systematically examined for encounters
with reconnection X lines and ion diffusion regions in the
Earth's magnetotail. This survey of 175
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magnetotail passes resulted in a sample of 33 correlated field
and flow reversals. Eighteen events exhibited electric and magnetic
field perturbations qualitatively consistent with the predictions
of antiparallel Hall reconnection and could be identified as
diffusion region encounters. The magnitudes of both the Hall
magnetic and electric field were found to vary from event to
event. When normalized against the inflow magnetic field and
the current sheet number density the average peak Hall magnetic
field was found to be 0.39 ± 0.16, the average peak Hall
electric field was found to be 0.33 ± 0.18, and the average
out of plane (reconnection) electric field was found to be ~0.04.
Good quantitative agreement was found between these results
and a large, appropriately renormalized particle-in-cell simulation
of reconnection. In future missions, the magnitude of the total
DC electric field may be a useful tool for automatically identifying
ion diffusion region encounters. |
|
Agosto de 2010
Numerical modeling of landquakes
Authors: P. Favreau, A. Mangeney et al
Link: Click here
Abstract
The Thurwieser landslide that occurred in Italy in 2004 and
the seismic waves it generated have been simulated and compared
to the seismic signal recorded a few tens of kilometers from
the landslide source (i.e., landquake). The main
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features of the low frequency
seismic signal are reproduced by the simulation. Topography
effects on the flowing mass have a major impact on the generated
seismic signal whereas they weakly affect low-frequency wave
propagation. Simulation of the seismic signal makes it possible
to discriminate between possible alternative scenarios for flow
dynamics and to provide first estimates of the rheological parameters
during the flow. As landquakes are continuously recorded by
seismic networks, our results provide a new way to collect data
on the dynamics and rheology of natural flows. |
|
Agosto de 2010
Quantifying the concentration of ferrimagnetic particles
in sediments using rock magnetic methods
Authors: Ioan Lascu, Subir K. Banerjee
et al
Link: Click here
Abstract
We have developed a quantification method that uses mainly
room temperature rock magnetic measurements to calculate concentrations
of ferrimagnetic particles in sediments. Our method uses saturation
magnetization (Ms) as a total ferrimagnetic concentration
proxy, the saturation remanence ratio (Mrs/Ms) as a magnetic
grain-size proxy, the anhysteretic remanence ratio (Xi a/Mrs)
to estimate inter-particle magnetostatic
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interactions,
and the normalized susceptibility of the ferrimagnetic fraction
(Xif/Ms) to calculate the proportion of ultrafine, superparamagnetic
particles. This approach eliminates the effect of dilution of
the magnetic properties by weakly magnetic matter, and allows
the calculation of direct concentrations (or fluxes for dated
sedimentary profiles) of constituent ferrimagnetic components.
We test our method on a short sediment core from an urban Minnesota
lake, for which we calculate ferrimagnetic fluxes of four magnetic
components, and compare their pre- and post-European settlement
values. Our quantification technique can be applied for reconstructing
past environmental changes in a range of sedimentary environments,
and is particularly useful for large sets of samples, where
detailed magnetic unmixing methods are unfeasible due to time
or instrument constraints. |
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Agosto de 2010
Volcanism and resurfacing on Venus at the full resolution
of Magellan SAR data
Authors: Peter M. Grindrod, Ellen R. Stofan
et al
Link: Click here
Abstract
We examine the importance of localized volcanism in resurfacing
on Venus by analyzing the results of geologic mapping of a
12° × 12° area at the full resolution of Magellan
SAR data. Resurfacing due to corona-, ridge-, and small
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volcano-related volcanism accounts
for 27%, 6%, and 10% respectively of the mapped area. Mapping
at the resolution of Magellan data, rather than a regional scale,
gives corona-related flow unit areas that can differ individually
by almost an order of magnitude, with a total increase of 28%,
and more than three times as many identifiable units. A total
of 2919 small volcanoes or vents less than 10 km in diameter
were identified in the F-Map, with a mean diameter of 1.59 (s.d.
= 1.08) km and densities of up to 36 small volcanoes per 50
km2. Taken together, coronae, ridge eruptions, and small volcanoes
probably make a significant contribution to resurfacing on Venus.
|
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Agosto de 2010
Blocking Moving Window algorithm: Conditioning multiple-point
simulations to hydrogeological data
Authors: Andres Alcolea and Philippe Renard
Link: Click here
Abstract
Connectivity constraints and measurements of state variables
contain valuable information on aquifer architecture. Multiple-point
(MP) geostatistics allow one to simulate aquifer architectures,
presenting a predefined degree of global connectivity. In
this context, connectivity data are often disregarded. The
conditioning to state variables is usually carried out by
minimizing a suitable objective function (i.e., solving an
inverse problem). However, the discontinuous nature of lithofacies
distributions and of the corresponding objective function
discourages the use of traditional sensitivity-based inversion
techniques. This work presents the Blocking Moving Window
algorithm (BMW), aimed at overcoming these limitations by
conditioning MP simulations to hydrogeological data such as
connectivity and
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heads.
The BMW evolves iteratively until convergence: (1) MP simulation
of lithofacies from geological/geophysical data and connectivity
constraints, where only a random portion of the domain is simulated
at every iteration (i.e., the blocking moving window, whose
size is user-defined); (2) population of hydraulic properties
at the intrafacies; (3) simulation of state variables; and (4)
acceptance or rejection of the MP simulation depending on the
quality of the fit of measured state variables. The outcome
is a stack of MP simulations that (1) resemble a prior geological
model depicted by a training image, (2) honor lithological data
and connectivity constraints, (3) correlate with geophysical
data, and (4) fit available measurements of state variables
well. We analyze the performance of the algorithm on a 2-D synthetic
example. Results show that (1) the size of the blocking moving
window controls the behavior of the BMW, (2) conditioning to
state variable data enhances dramatically the initial simulation
(which accounts for geological/geophysical data only), and (3)
connectivity constraints speed up the convergence but do not
enhance the stack if the number of iterations is large. |
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Julio de 2010
Internal structure of the western flank of the Cumbre Vieja
volcano, La Palma, Canary Islands, from land magnetotelluric
imaging
Authors: X. Garcia and A. G. Jones
Link: Click here
Abstract
Large-scale mass wasting is a natural part of the evolution
of volcanic islands, where deformation and indications of
flank instability, such as large-scale faulting and seismic
and aseismic slip are common. The Cumbre Vieja volcano on
the island of La Palma (Canary Islands) provides an ideal
setting to address fundamental questions about the structure,
evolution and stability of island volcanoes. The island of
La Palma is still in a shield-building stage, and it has been
postulated that the western side of the island lies over a
pre-existing zone of weakness that can nucleate ruptures.
We undertook an audiomagnetotelluric (AMT) survey over the
proposed unstable western
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flank to try to image structures
that may be associated with the zone of weakness. The magnetotelluric
method (MT) is a geophysical technique used to map the presence
of fluids or image important structural contrasts. The goals
of this study were (1) to delineate the unstable flank, (2)
to map the structures underneath and (3) to determine the presence
and geometry of fluids. The results show a 1 km thick top resistive
layer overlaying an area of reduced resistivity (enhanced conductivity),
interpreted as a layer consisting of an alteration zone and
also fluids. Our results confirm previous studies that suggested
the existence of a western flank lying over collapse debris
material and hyaloclastites, and also they allow us to map part
of the subaerial southern extent of the Cumbre Nueva units that
lie beneath the more recent Cumbre Vieja rocks. In addition,
dimensionality analysis maps the rotation of the dike emplacement
off ridge, along the western flank in an en echelon fashion.
|
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Julio de 2010
Connecting near-field and far-field earthquake triggering
to dynamic strain
Authors: Nicholas J. van der Elst and Emily
E. Brodsky
Link: Click here
Abstract
Any earthquake can trigger more earthquakes. This triggering
occurs in both the classical aftershock zone as well as the
far field. These populations of triggered earthquakes may
or may not be distinct in terms of triggering mechanism. Here
we look for a distinction between the populations by examining
how the observed intensity of triggering scales with the amplitude
of the triggering strain in each. To do so, we apply a new
statistical metric based on earthquake interevent times to
a large data set and measure earthquake triggering as a function
of dynamic strain amplitude, where strain is estimated from
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empirical
ground motion regressions. This method allows us to identify
triggering at dynamic strain amplitudes down to 3 × 10-9,
orders of magnitude smaller than previously reported. This threshold
appears to be an observational limit and shows that extremely
small dynamic strains can trigger faults that are sufficiently
near failure. Using a probabilistic model to transform measured
interevent times to seismicity rate changes, we find that triggering
rates in the far field scale with peak dynamic strain. This
scaling, projected into the near field, accounts for 15%-60%
of earthquakes within 6 km of magnitude 3-5.5 earthquakes. Statistical
seismicity simulations validate the interevent time method and
show that the data are consistent with the number of far-field
triggered earthquakes being linearly proportional to peak dynamic
strain. We interpret the additional near-field component as
reflecting either static stress triggering, more effective dynamic
triggering at higher frequencies, or a concentration of aftershock
nucleation sites very near main shocks. |
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Julio de 2010
Variations of fluid pressure within the subducting oceanic
crust and slow earthquakes
Authors: Aitaro Kato, Takashi Iidaka et
al
Link: Click here
Abstract
We show fine-scale variations of seismic velocities and converted
teleseismic waves that reveal the presence of zones of high-pressure
fluids released by progressive metamorphic dehydration reactions
in the subducting Philippine Sea plate in Tokai district,
Japan. These zones have a strong correlation with the distribution
of slow earthquakes, including long-term slow slip
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(LTSS) and low-frequency earthquakes
(LFEs). Overpressured fluids in the LTSS region appear to be
trapped within the oceanic crust by an impermeable cap rock
in the fore-arc, and impede intraslab earthquakes therein. In
contrast, fluid pressures are reduced in the LFE zone, which
is deeper than the centroid of the LTSS, because there fluids
are able to infiltrate into the narrow corner of the mantle
wedge, leading to mantle serpentinization. The combination of
fluids released from the subducting oceanic crust with heterogeneous
fluid transport properties in the hanging wall generates variations
of fluid pressures along the downgoing plate boundary, which
in turn control the occurrence of slow earthquakes. |
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Julio de 2010
A new climate era in the sub-Arctic: Accelerating climate
changes and multiple impacts
Authors: Terry V. Callaghan, Fredrik Bergholm
et al
Link: Click here
Abstract
Climate warming in the Swedish sub-Arctic since 2000 has reached
a level at which statistical analysis shows for the first
time that current warming has exceeded that in the late 1930's
and early 1940's, and has significantly crossed the 0°C
mean annual temperature threshold which causes many cryospheric
and ecological impacts. The accelerating temperature increase
trend has driven similar trends in the century-long increase
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in
snow thickness, loss of lake ice, increases in active layer
thickness, lake water TOC (total organic carbon) concentrations
and the assemblages of diatoms, and changes in tree-line location
and plant community structure. Some of these impacts were not
evident in the first warm period of the 20th Century. Changes
in climate are associated with reduced temperature variability,
particularly loss of cold winters and cool summers, and an increase
in extreme precipitation events that cause mountain slope instability
and infrastructure failure. The long term records of multiple,
local environmental factors compiled here for the first time
provide detailed information for adaptation strategy development
while dramatic changes in an environment particularly vulnerable
to climate change highlight the need to adopt global mitigation
strategies. |
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Julio de 2010
A microbial fuel cell in contaminated ground delineated
by electrical self-potential and normalized induced polarization
data
Authors: R. Doherty, B. Kulessa et al
Link: Click here
Abstract
There is a growing interest in the use of geophysical methods
to aid investigation and monitoring of complex biogeochemical
environments, for example delineation of contaminants and
microbial activity related to land contamination. We combined
geophysical monitoring with chemical and microbiological analysis
to create a conceptual biogeochemical model of processes around
a contaminant plume within a manufactured gas plant site.
Self-potential, induced polarization and electrical resistivity
techniques were used to monitor the plume. We propose that
an exceptionally strong
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(>800 mV peak to peak) dipolar
SP anomaly represents a microbial fuel cell operating in the
subsurface. The electromagnetic and electrical geophysical data
delineated a shallow aerobic perched water body containing conductive
gasworks waste which acts as the abiotic cathode of microbial
fuel cell. This is separated from the plume below by a thin
clay layer across the site. Microbiological evidence suggests
that degradation of organic contaminants in the plume is dominated
by the presence of ammonium and its subsequent degradation.
We propose that the degradation of contaminants by microbial
communities at the edge of the plume provides a source of electrons
and acts as the anode of the fuel cell. We hypothesize that
ions and electrons are transferred through the clay layer that
was punctured during the trial pitting phase of the investigation.
This is inferred to act as an electronic conductor connecting
the biologically mediated anode to the abiotic cathode. Integrated
electrical geophysical techniques appear well suited to act
as rapid, low cost sustainable tools to monitor biodegradation.
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Julio de 2010
Impact cratering on the H chondrite parent asteroid
Authors: Axel Wittmann, Timothy D. Swindle
et al
Link: Click here
Abstract
This paper reports petrological data for LaPaz Icefield 02240,
03922, 031125, 031173, 031308, 04462, and 04751, which are
meteoritic samples of clast-rich impact melt rocks from the
H chondrite parent asteroid. The size distribution and metallographic
characteristics of Fe-Ni metal in the melts indicate very
rapid 1 to 40°C/s cooling in the temperature range between
>1500 and ~950°C when the clast-melt mixtures were
thermally equilibrating. Cooling slowed to values between
10-3 and 10-2°C/s in the temperature range between 700
and 400°C when the melt
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rocks
were cooling to their surroundings. These data suggest that
the rocks cooled near the surface of the H chondrite asteroid
within suevitic impact deposits. Integrating these data with
the petrologic characteristics of other H chondrite melt rocks
and their radioisotopic ages indicates that the H chondrite
asteroid suffered at least one large impact event while still
cooling from endogenous metamorphism at ~4500 Ma; this impact
must have degraded the asteroid's integrity but did not cause
shattering. Impact events in the era between ~4100 and ~3600
Ma produced melt volumes large enough to allow segregation of
metal and troilite from silicate melts, possibly within continuous
impact melt sheets contained in craters. The impact record after
3600 Ma does not display such assemblages, which suggests a
decrease in the rate of large impact events or a catastrophic
size reduction of the H chondrite parent asteroid at around
this time. |
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Julio de 2010
Three-dimensional inversion of ZTEM data
Authors: Elliot Holtham and Douglas W.
Oldenburg et al
Link: Click here
Abstract
Z-Axis Tipper Electromagnetic Technique (ZTEM) data are airborne
electromagnetic data which record the vertical magnetic field
that results from natural sources. The data are transfer functions
that relate the local vertical field to orthogonal horizontal
fields measured at a reference station on the ground. The
transfer functions depend on frequency and provide information
about the 3-D conductivity structure of the Earth. The practical
frequency range is 30-720 Hz and hence it is possible to see
structures at depths of a kilometre or more if the earth is
of moderate conductivity. This depth of penetration is significantly
greater than that obtained with controlled source EM
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techniques and, when coupled with
rapid spatial acquisition with an airborne system, means that
ZTEM data can be used to map large-scale structures that are
difficult to survey with ground based surveys. We present some
fundamentals about understanding the signatures obtained with
ZTEM transfer functions and then develop a Gauss-Newton algorithm
to invert ZTEM data. The algorithm is applied to synthetic examples
and to a field data set from the Bingham Canyon region in Utah.
The field data set requires a workflow procedure to estimate
appropriate noise levels in individual frequency components.
These noise levels can then be used to invert multiple frequencies
simultaneously. ZTEM data are insensitive to a 1-D conductivity
structures and hence the background can be difficult to estimate.
We provide two methods to determine appropriate background models.
Interestingly, topography, which is usually a hinderance in
field data interpretation, provides a first-order signal in
the ZTEM data and helps with this calibration. |
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Julio de 2010
Evaluating local hydrological modelling by temporal gravity
observations and a gravimetric three-dimensional model
Authors: M. Naujoks , C. Kroner et al
Link: Click here
Abstract
An approach for the evaluation of local hydrological modelling
is presented: the deployment of temporal terrestrial gravity
measurements and gravimetric 3-D modelling in addition to
hydrological point observations. Of particular interest is
to what extent such information can be used to improve the
understanding of hydrological process dynamics and to evaluate
hydrological models. Because temporal gravity data contain
integral information about hydrological mass changes they
can be considered as a valuable augmentation to traditional
hydrological observations. On the other hand, hydrological
effects need to be eliminated from high-quality gravity time-series
because they interfere with small geodynamic signals. In areas
with hilly topography and/or inhomogeneous subsoil, a simple
reduction based on hydrological point measurements is usually
not sufficient. For such situations, the underlying hydrological
processes in the soil and the disaggregated bedrock need to
be considered in their spatial and temporal dynamics to allow
the development of a more sophisticated reduction.
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Regarding
these issues interdisciplinary research has been carried out
in the surroundings of the Geodynamic Observatory Moxa, Germany.
At Moxa, hydrologically induced gravity variations of several
10 nm s-2 are observed by the stationarily operating superconducting
gravimeter and by spatially distributed and repeated high-precision
measurements with transportable relative instruments. In addition,
hydrological parameters are monitored which serve as input for
a local hydrological catchment model for the area of about 2
km2 around the observatory. From this model, spatial hydrological
variations are gained in hourly time steps and included as density
changes of the subsoil in a well-constrained gravimetric 3-D
model to derive temporal modelled gravity variations.
The gravity variations obtained from this combined modelling
correspond very well to the observed hydrological gravity changes
for both, short period and seasonal signals. From the modelling
the amplitude of the impact on gravity of hydrological changes
occurring in different distances to the gravimeter location
can be inferred. Possible modifications on the local hydrological
model are discussed to further improve the quality of the model.
Furthermore, a successful reduction of local hydrological effects
in the superconducting gravimeter data is developed. After this
reduction global seasonal fluctuations are unmasked which are
in correspondence to GRACE observations and to global hydrological
models. |
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Julio de 2010
Modelo de velocidad de onda de corte para la cuenca de
Santiago de Chile derivado a partir de mediciones del ruido
ambiental: una comparación de condiciones sísmicas
y amplificación.
Autores: Marco Pilz, Stefano Parolai
et al
Link: Click aquí
Abstract
Nosotros determinamos un modelo de velocidad 3D de alta resolución
para la onda S en un área de 26 x 12 Km2
en la zona norte de la cuenca de Santiago de Chile. Para alcanzar
este objetivo, utilizamos registros de microtemblores en 125
sitios para obtener las razones espectrales horizontal a vertical
(H/V) que invertimos para recuperar los perfiles locales de
velocidad de la onda S. En el procedimiento de inversión
utilizamos exigencias geológicas y geofísicas
adicionales y valores de espesor de la capa sedimentaria a
partir de mediciones gravimétricas, las cuales varían
apreciablemente en cortas distancias. El resultado del modelo
se derivó de interplolaciones con una técnica
kriging entre los perfiles de la onda S y muestra buenos acuerdos
locales con los pocos perfiles de velocidad pre-existentes,
pero la generalización representa la superficie con
mayor detalle. La riqueza de los datos nos permitió
investigar si existía alguna correlación entre
la velocidad de la onda S en la zona superior de los 30 m
(v30S) y la pendiente de la topografía, una nueva técnica
recientemente propuesta por Wald y Allen. Observamos que mientras
la litología puede provocar una mayor reflexión,
casi no existe correlación entre el gradiente topográfico
y v30S, pero una mejor relación parece existir entre
v30S y la geología local. Finalmente comparamos la
distribución de v30S con las intensidades del MSK del
evento de Valparaíso de 1985, indicando que las altas
intensidades son encontradas donde los valores esperados de
v30S son bajos en la delgada cobertura sedimentaria. Sin embargo,
esta evidencia no puede ser generalizada para todos los posibles
terremotos, lo que indica la influencia de efectos del lugar
modificando el movimiento terrestre cuando los sismos ocurren
en las afueras de la cuenca de Santiago.
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Julio de 2010
Shear wave velocity model of the Santiago de Chile basin
derived from ambient noise measurements: a comparison of proxies
for seismic site conditions and amplification
Authors: Marco Pilz, Stefano Parolai et
al
Link: Click here
Abstract
We determined a high-resolution 3-D S-wave velocity model
for a 26 km × 12 km area in the northern part of the
basin of Santiago de Chile. To reach this goal, we used microtremor
recordings at 125 sites for deriving the horizontal-to-vertical
(H/V) spectral ratios that we inverted to retrieve local S-wave
velocity profiles. In the inversion procedure, we used additional
geological and geophysical constraints and values of the thickness
of the sedimentary cover already determined by gravimetric
measurements, which were found to vary substantially over
short distances in the investigated area. The resulting model
was derived by interpolation with a kriging technique between
the single S-wave velocity profiles and shows locally good
agreement with the few existing velocity profile data, but
allows the entire area, as well as deeper parts of the basin,
to be represented in greater detail. The wealth of available
data allowed us to check if any correlation between the S-wave
velocity in the uppermost 30 m ( v30S ) and the slope of topography,
a new technique recently proposed by Wald and Allen, exists
on a local scale. We observed that while one lithology might
provide a greater scatter in the velocity values for the investigated
area, almost no correlation between topographic gradient and
calculated v30S exists, whereas a better link is found between
v30S and the local geology. Finally, we compared the v30S
distribution with the MSK intensities for the 1985 Valparaiso
event, pointing out that high intensities are found where
the expected v30S values are low and over a thick sedimentary
cover. Although this evidence cannot be generalized for all
possible earthquakes, it indicates the influence of site effects
modifying the ground motion when earthquakes occur well outside
of the Santiago basin.
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Julio de 2010
Relating 4D seismics to reservoir geomechanical changes
using a discrete element approach
Authors: Haitham Alassi, Rune Holt et al
Link: Click here
Abstract
A modified discrete element method is briefly introduced and
used for modelling reservoir geomechanical response during
fluid injection and depletion. The modified approach works
as a continuum method until some local failure is initiated,
after which it behaves like a discrete element method on a
polygonal lattice. The method is advantageous for modelling
fracture
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developments
in rocks. It is applied here to synthetic models of two reservoirs
taken from the North Sea (Gullfaks and Elgin-Franklin). For
Gullfaks, two cases of water injection were modelled, one with
low horizontal effective stress and the other with low vertical
effective stress. Vertical fractures are developed in the first
case, whereas horizontal fractures are developed in the second
case. This would not have been seen using traditional methods.
Based on 4D seismics data for the Gullfaks field, one may envision
that horizontal fractures could have been formed. The Elgin-Franklin
synthetic model is used to study various scenarios of changing
stress field around the depleting reservoir. Based on 4D seismics
data from this field, one may see changes that could be interpreted
in terms of possible fault reactivation. |
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Julio de 2010
Regionalización Sísmica
de Chile Mediante la Utilización de una Red Neuronal
de Kohonen
Autores: Jorge Reyes y Victor Cárdenas
Link: Clic
Aquí
Abstract
En este paper presentamos una propuesta de regionalización
sísmica de Chile Continental mediante la utilización
de una red neuronal. Obtuvimos una estructura que muestra
seis regiones sísmicas independientes del tamaño
del vecindario y del alcance de la correlación entre
las celdas de la grilla. A diferencia de los métodos
de regionalización convencionales, nosotros obtuvimos
regiones sísmicas consistentes con la información
tectónica a partir de información sísmica
mímima y no redundante, lo que demuestra la gran utilidad
que tienen los mapas auto-organizativos en el campo de la
Sismología. La alta correlación entre la distribución
de las zonas sísmicas y los datos geológicos
confirma que los campos escogidos para estructurar los vectores
de entrenamiento fueron lo más adecuados.
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Julio de 2010
A Chilean seismic regionalization through a Kohonen
neural network
Authors: Jorge Reyes and Victor Cárdenas
Link: Click here
Abstract
Through this paper we are presenting a study of seismic regionalization
for continental Chile based on a neural network. A scenario
with six seismic regions is obtained, irrespective of the
size of the neighborhood or the range of the correlation between
the cells of the grid. Unlike conventional seismic methods,
our work manages to generate seismic regions tectonically
valid from sparse and non-redundant information, which shows
that the self-organizing maps are a valuable tool in seismology.
The high correlation between the spatial distribution of the
seismic zones and geological data confirms that the fields
chosen for structuring the training vectors were the most
appropriate.
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Julio de 2010
Electrical resistivity and seismic refraction tomography
to detect buried cavities
Authors: Ettore Cardarelli, Michele Cercato
et al
Link: Click here
Abstract
Near-surface cavities can pose serious hazards to human safety,
especially in highly urbanized town centres. The location
of subsurface voids, the estimation of their size and the
evaluation of the overburden thickness are necessary to assess
the risk of collapse.
In this study, electrical resistivity tomography (ERT) and
seismic refraction tomography data are integrated in a joint
interpretation process for cavity location in the city of
Rome.
ERT is a well established and widely employed method for cavity
detection. However, additional
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information provided by seismic
refraction tomography is capable of eliminating some potential
pitfalls in resistivity data interpretation. We propose that
the structure of the cavities defined by ERT can be used as
a base to optimize seismic refraction tomography investigations
within the framework of a joint interpretation process.
Data integration and the insertion of a priori information are
key issues for reducing the uncertainties associated with the
inversion process and for optimizing both acquisition procedures
and computation time.
Herein, the two geophysical methods are tested on both synthetic
and real data and the integration of the results is found to
be successful in detecting isolated cavities and in assessing
their geometrical characteristics. The cavity location inferred
by geophysical non-invasive methods has been subsequently confirmed
by direct inspection. |
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Julio de 2010
Ancient ocean on Mars supported by global distribution
of deltas and valleys
Authors: Gaetano Di Achille & Brian
M. Hynek et al
Link: Click here
Abstract
The climate of early Mars could have supported a complex hydrological
system and possibly a northern hemispheric ocean covering
up to one-third of the planet's surface. This notion has been
repeatedly proposed and challenged over the past two decades,
and remains one of the largest uncertainties in Mars research.
Here, we used global databases of known deltaic deposits,
valley networks and present-day martian topography to
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test
for the occurrence of an ocean on early Mars. The distribution
of ancient martian deltas delineates a planet-wide equipotential
surface within and along the margins of the northern lowlands.
We suggest that the level reconstructed from the analysis of
the deltaic deposits may represent the contact of a vast ocean
covering the northern hemisphere of Mars around 3.5 billion
years ago. This boundary is broadly consistent with palaeoshorelines
suggested by previous geomorphologic, thermophysic and topographic
analyses, and with the global distribution and age of ancient
valley networks. Our findings lend credence to the hypothesis
that an ocean formed on early Mars as part of a global and active
hydrosphere. |
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Article-Julio de 2010
Italy puts seismology in the dock
Author: Nicola Nosengo
Link: Click here
Abstract
The deadly earthquake that struck the central Italian city
of L'Aquila on 6 April 2009, has had a bizarre aftershock:
some of Italy's top seismologists could face charges of manslaughter
for not alerting the population before the disaster. The indictment
has outraged experts around the world, who note that earthquakes
cannot be predicted and who say that the Italian government
neglected to enforce building codes that could have reduced
the toll.
The indictments, issued on 3 June by the L'Aquila public prosecutor's
office, name six scientists as
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being investigated for manslaughter
in relation to the earthquake. In Italy, this step usually precedes
a request for a court trial, and is meant to allow the accused
time to prepare their defence. The list comprises Enzo Boschi,
president of the National Institute for Geophysics and Vulcanology
(INGV) in Rome, the main institute in charge of seismic monitoring;
Giulio Selvaggi, director of the National Earthquake Center
based at INGV; Franco Barberi, a volcanologist at the University
of 'Roma Tre'; Claudio Eva, a professor of earth physics at
the University of Genoa; Mauro Dolce, head of the seismic risk
office in the Italian government's Civil Protection Agency;
and Gian Michele Calvi, director of the European Centre for
Training and Research in Earthquake Engineering in Pavia. A
government official, Bernardo De Bernardinis, deputy technical
head of the Civil Protection Agency, is also under investigation. |
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Julio de 2010
Hypothesis of a spin-orbit resonance between the Earth
and Venus's core
Author: Gérard V. Caudal
Link: Click here
Abstract
The observation that the spin period of Venus is extremely
close, although not equal, to the p = -5 spin-orbit resonance
with the Earth makes it very improbable that such a situation
is fortuitous. This leads one to explore hypotheses in which
the Earth spin-orbit resonance plays some role in Venus's
observed spin rate. This paper proposes one such hypothesis.
Venus's core is assumed to be composed of a liquid outer core
surrounding a solid inner core, the latter undergoing a 0.31
degree/year differential rotation with the mantle. Due to
gravitational coupling, however, core- mantle differential
rotation would be impossible,
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unless
isostatic compensation exists with an effectiveness of 99.9998%.
Within that assumption, it is proposed that Venus is trapped
in the p = -5 spin-orbit resonance with the Earth, but that
this resonance concerns the inner core rather than the mantle.
Stable resonance requires that the inner core should depart
significantly from spherical symmetry, while its material should
still be able to sustain the stress differences produced by
such asymmetric mass distributions. Compatibility between those
two conditions is studied, leading to constraints on the size
of the inner core. An appreciable probability of stable resonance
is found to be achievable, provided that the average inner core
radius is larger than a minimum, which is estimated as 1300
km within the heating at the ground hypothesis for atmospheric
thermal tide. That condition would become considerably less
stringent if solar heat absorption in the upper atmosphere rather
than at the ground were assumed. |
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Julio de 2010
Gravity waves propagating into an ice-covered ocean: A
viscoelastic model
Authors: Ruixue Wang and Hayley H. Shen
Link: Click here
Abstract
A viscoelastic model is proposed to describe the propagation
of gravity waves into various types of ice cover. The ice-ocean
system is modeled as a homogeneous viscoelastic fluid overlying
an inviscid layer. Both layers have finite thickness. The
viscosity is imagined to originate from the frazil ice or
ice floes much smaller than the wavelength, and the elasticity
from ice floes which are relatively large compared to the
wavelength. A compact form of the dispersion relation is obtained.
Under proper limiting conditions this dispersion relation
can be reduced to several
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previously established models
including the mass loading model, the viscous layer model and
the thin elastic plate model. The full dispersion relation contains
several propagating wave modes under the ice cover. The following
two criteria are used to select the dominant wave mode: (1)
wave number is the closest to the open water value and (2) attenuation
rate is the least among all modes. The modes selected from those
criteria coincide with the ones discussed in previous studies,
which are shown to be limiting cases in small or large elasticity
regimes of the present model. In the intermediate elasticity
regime, however, it appears that there are three wave modes
with similar wavelengths and attenuation rates. Implications
of this intermediate elasticity range remain to be seen. The
general viscoelastic model bridges the gap among existing models.
It also provides a unified tool for wave-ice modelers to parameterize
the polar regions populated with various types of ice cover. |
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Julio de 2010
Convective heat transfer in planetary dynamo models
Authors: Eric M. King, Krista M. Soderlund
et al
Link: Click here
Abstract
The magnetic fields of planets and stars are generated by
the motions of electrically conducting fluids within them.
These fluid motions are thought to be driven by convective
processes, as internal heat is transported outward. The efficiency
with which heat is transferred by convection is integral in
understanding dynamo processes. Several heat transfer scaling
laws have been proposed, but the range of parameter space
to which they apply has not been firmly established. Following
the plane layer convection study by King et al. (2009), we
explore a broad range of buoyancy forcing (Ra) and rotation
strength (E-1) to show that heat transfer (Nu) in spherical
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dynamo
simulations occurs in two distinct regimes. We argue that heat
transfer scales as Nu ~ Ra6/5 in the rapidly rotating regime
and Nu ~ Ra2/7 in the weakly rotating regime. The transition
between these two regimes is controlled by the competition between
the thermal and viscous boundary layers. Boundary layer scaling
theory allows us to predict that the transition between the
regimes occurs at a transitional Rayleigh number, Rat = E-7/4.
Furthermore, boundary layer control of heat transfer is shown
to relate to the interior temperature profiles of the models.
In the weakly rotating regime, the interior fluid is nearly
adiabatic. In the rapidly rotating regime, adverse mean temperature
gradients abide, irrespective of the Reynolds number (Re). Extrapolating
our results to Earth's core, we estimate that core convection
resides in the rapidly rotating regime, with Ra ˜ 2 ×
1024 (Ra/Rat ˜ 0.02), corresponding to a superadiabatic
density variation of ??/?o ˜ 10-7, which is significantly
below the sensitivity of present seismic observations. |
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Julio de 2010
Evaluation of effective groundwater recharge of freshwater
lens in small islands by the combined modeling of geoelectrical
data and water heads
Authors: Jean-Christophe Comte, Olivier
Banton et al
Link: Click here
Abstract
In small islands, a freshwater lens can develop due to the
recharge induced by rain. Magnitude and spatial distribution
of this recharge control the elevation of freshwater and the
depth of its interface with salt water. Therefore, the study
of lens morphology gives useful information on both
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the recharge and water uptake
due to evapotranspiration by vegetation. Electrical resistivity
tomography was applied on a small coral reef island, giving
relevant information on the lens structure. Variable density
groundwater flow models were then applied to simulate freshwater
behavior. Cross validation of the geoelectrical model and the
groundwater model showed that recharge exceeds water uptake
in dunes with little vegetation, allowing the lens to develop.
Conversely, in the low-lying and densely vegetated sectors,
where water uptake exceeds recharge, the lens cannot develop
and seawater intrusion occurs. This combined modeling method
constitutes an original approach to evaluate effective groundwater
recharge in such environments. |
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Julio de 2010
Reflection imaging of deep structure beneath Montserrat
using microearthquake sources
Authors: K. Byerly, L. Brown et al
Link: Click here
Abstract
The SEA-CALIPSO experiment was designed to image structure
related to active volcanism beneath the island of Montserrat
in the Caribbean. As part of that experiment, over 200 Texan
recorders with 5 Hz geophones were deployed in 3 linear arrays
at a nominal spacing of 100m, primarily to record an airgun
source towed
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offshore
around the island. Because the recorders were operating in continuous
mode for three days, a number of shallow microearthquakes under
the active summit of Soufriere Hills Volcano (SHV) were also
recorded. 20 events were sufficiently well recorded and located
that they could be used to identify and map reflections from
deep subsurface structure. Here we report on the processing
of those recordings as multichannel CMP reflection sources,
with emphasis on careful statics correction and coherency enhancement.
The resulting reflection images indicate subhorizontal layering
at depths between 6 and 19km which we interpret as sills. |
