Marzo
de 2009
Principles of volcanic risk metrics: Theory and the case study of
Mount Vesuvius and Campi Flegrei, Italy
Authors: Warner
Marzocchi and Gordon Woo
Link: Click Here
Abstract
Despite volcanic risk having been defined
quantitatively more than 30 years ago, this risk has been managed without being
effectively measured. The recent substantial progress in quantifying eruption
probability paves the way for a new era of rational science-based volcano risk
management, based on what may be termed “volcanic risk metrics” (VRM).
In this paper, we propose the basic principles of VRM, based on coupling probabilistic
volcanic hazard | assessment
and eruption forecasting with cost-benefit analysis. The VRM strategy has the
potential to rationalize decision making across a broad spectrum of volcanological
questions. When should the call for evacuation be made? What early preparations
should be made for a volcano crisis? Is it worthwhile waiting longer? What areas
should be covered by an emergency plan? During unrest, what areas of a large volcanic
field or caldera should be evacuated, and when? The VRM strategy has the paramount
advantage of providing a set of quantitative and transparent rules that can be
established well in advance of a crisis, optimizing and clarifying decision-making
procedures. It enables volcanologists to apply all their scientific knowledge
and observational information to assist authorities in quantifying the positive
and negative risk implications of any decision. |
Marzo
de 2009
Miocene-Pliocene exhumation along the west Salton detachment fault,
southern California, from (U-Th)/He thermochronometry of apatite and zircon
Authors: Catherine R. Shirvell, Daniel F. Stockli et al
Link: Click Here
Abstract
The Salton Trough is the northernmost
segment of the active Gulf of California oblique rift system. The main rift-related
structure in the western Salton Trough is the low-angle west Salton detachment
fault (WSDF). Footwall and hanging wall apatite and zircon (U-Th)/He ages record
a significant contrast in thermal history across the WSDF, confirming Pliocene
normal slip along the fault. Apatite (U-Th)/He ages record rapid | exhumational
cooling from ~ 5 to ~ 2 Ma, consistent with the timing of accelerated tectonic
subsidence recorded in WSDF hanging wall sedimentary strata and consistent with
the initiation age of dextral plate boundary slip on the southern San Andreas
fault in the Salton Trough. Our results indicate that the WSDF caused at least
2.3–4 km of exhumation and >8–10 km of approximately E directed horizontal
extension since ~ 5 Ma. Slip rate of the WSDF at Yaqui Ridge was likely in the
range of ?2.3–5 km/Ma from ~ 7 to ~ 2 Ma. The WSDF may have been active well
before ~ 5 Ma. Middle Miocene apatite (U-Th)/He ages from samples at high elevations
have steep age-elevation gradients and suggest that exhumation may have initiated
by ~ 12 Ma, about when subduction ceased along a large segment of the Baja California
margin and consistent with the age of onset of extension in northeastern Baja
California. |
Marzo
de 2009
Phase relations of Fe-Si alloy in Earth's core
Authors: Jung-Fu Lin and Henry P. Scott et al
Link: Click Here
Abstract
Phase relations of an Fe0.85Si0.15 alloy
were investigated up to 240 GPa and 3000 K using in situ X-ray diffraction in
a laser-heated diamond anvil cell. An alloy of this composition as starting material
is found to result in a stabilized mixture of | Si-rich
bcc and Si-poor hcp Fe-Si phases up to at least 150 GPa and 3000 K, whereas only
hcp-Fe0.85Si0.15 is found to be stable between approximately 170 GPa and 240 GPa
at high temperatures. Our extended results indicate that Fe0.85Si0.15 alloy is
likely to have the hcp structure in the inner core, instead of the previously
proposed mixture of hcp and bcc phases. Due to the volumetric dominance of the
hcp phase in the hcp + bcc coexistence region close to the outer-core conditions,
the dense closest-packed Fe-Si liquid is more relevant to understanding the properties
of the outer core. |
Marzo
de 2009
Intraoceanic thrusts in the Nankai Trough off the Kii Peninsula:
Implications for intraplate earthquakes
Authors: Takeshi
Tsuji, Jin-Oh Park et al
Link: Click Here
Abstract
We identified intraoceanic thrusts developed
as imbricate structures within the subducting Philippine Sea plate off the Kii
Peninsula in central Japan manifesting as strong-amplitude reflections observed
in an industry-standard three-dimensional (3D) seismic reflection data set. | These
imbricate intraoceanic thrusts cut through the oceanic crust as a discontinuous
thrust plane striking approximately parallel to the trench. In our survey area,
large intraplate earthquakes with moment magnitudes (Mw) over 7 occurred on 5
September 2004, causing strong ground motions on the islands of Japan and tsunami
waves. The locations of the intraoceanic thrusts recognized in the seismic data
are distributed around the estimated hypocenters of the mainshocks and aftershocks
of the 2004 earthquakes. Furthermore, their geometry extracted from the 3D seismic
data could explain the kind of complex rupture pattern observed during the 2004
events. Therefore we propose that the intraoceanic thrusts are seismogenically
active. |
Marzo
de 2009
Methodology for tomographic imaging ahead of mining using the shearer
as a seismic source
Authors: Andrew King and Xun Luo
Link: Click Here
Abstract
Poor rock conditions in a coal longwall
panel can result in roof collapse when a problematic zone is mined, significantly
interrupting mine production. The ability to image rock conditions - stress and
degree of fracturing - ahead of the face gives the miners the ability to respond
proactively to such problems. This method uses the energy from mining machinery,
in this case a coal shearer, to produce an image of the rock velocity ahead of
the mining face without interrupting mining. Data from an experiment illustrates
the concept. Geophones | installed
in gate-road roofs record the noise generated by the shearer after it has traversed
the panel ahead of the mining face. A generalized crosscorrelation of the signals
from pairs of sensors determines relative arrival times from the continuous seismic
noise produced by the shearer. These relative times can then be inverted for a
velocity structure. The crosscorrelations, performed in the frequency domain,
are weighted by a confidence value derived from the spectral coherence between
the traces. This produces stable crosscorrelation lags in the presence of noise.
The errors in the time-domain data are propagated through to the relative traveltimes
and then to the final tomographic velocity image, yielding an estimate of the
uncertainty in velocity at each point. This velocity image can then be used to
infer information about the stress and fracture state of the rock, providing advance
warning of potentially hazardous zones. |
Marzo
de 2009
Stacking seismic data using local correlation
Authors: Guochang Liu, Sergey Fomel et al
Link: Click Here
Abstract
Stacking plays an important role in improving
signal-to-noise ratio and imaging quality of seismic data. However, for low-fold-coverage
| seismic profiles, the result
of conventional stacking is not always satisfactory. To address this problem,
we have developed a method of stacking in which we use local correlation as a
weight for stacking common-midpoint gathers after NMO processing or common-image-point
gathers after prestack migration. Application of the method to synthetic and field
data showed that stacking using local correlation can be more effective in suppressing
random noise and artifacts than other stacking methods. |
|
Marzo
de 2009
Energy taken up by co-seismic chemical reactions during a large
earthquake: An example from the 1999 Taiwan Chi-Chi earthquake
Authors: Yohei Hamada, Tetsuro Hirono et al
Link: Click Here
Abstract
Frictional heat in a fault zone during
earthquake slip transiently induces chemical reactions that may use energy released
during the earthquake. We estimated the energy used by such reactions (E C ) by
a numerical analysis incorporating | frictional
heat, thermal diffusion, chemical kinetics, and energy conservation, and found
that E C has an auto-feedback effect that inhibits temperature rise in fault zone.
During the 1999 Taiwan Chi-Chi earthquake, estimated E C was 0.43 MJ/m2, corresponding
to 0.79% of the frictional heat generated. This low percentage probably reflects
the low initial concentrations of reactive materials. However, in the case of
a fault with abundant reactive materials, E C could reach >50% of the frictional
heat and the auto-feedback effect could be large. At this case E C is a nonnegligible
component on earthquake energy budget and can affect fault mechanics. |
Marzo
de 2009
An analysis of shear waves generated by the Sterling explosion
Authors: Heming Xu, Jeffry L. Stevens et al
Link: Click Here
Abstract
We show that the near-field shear waves
from the decoupled explosion Sterling were likely caused by the cavity shape,
which was approximately spherical except for a flat floor due to melted and recrystallized
salt. We model the impact of the explosion shock wave on the cavity walls using
a two-dimensional (2-D) Eulerian finite difference code that simulates the evolution
of the air shock in the cavity coupled with 2-D Lagrangian finite difference code
that simulates the nonlinear region outside the cavity. Calculated shear waves
generated by the asymmetric impact of the shock wave on the cavity walls match
the observed initial | shear
wave amplitudes and radiation pattern. The observations also have substantial
P and S coda, which are not reproduced by the calculations. Similarity of coda
waveforms with distance indicates that their source is at or very near the cavity.
Longer time modeling of the air shock evolution appears to produce a more realistic
source function and provide a better match to the coda and indicates that the
coda are caused by air reverberations between the top and bottom of the cavity.
We assess the likelihood that fractures created by the tamped explosion that formed
the cavity Salmon were reopened by Sterling. Modeling of hydrofracture propagation
driven by the Sterling explosion, by coupling stress wave dynamics in rock with
fluid mechanics in the fractures, shows that the cavity pressure is insufficient
to overcome the overburden to propagate fractures into rock, except for the area
immediately below the explosion on the cavity floor. |
Marzo
de 2009
Rupture parameters of the 2003 Zemmouri (Mw 6.8), Algeria, earthquake
from joint inversion of interferometric synthetic aperture radar, coastal uplift,
and GPS
Authors: Samir Belabbès, Charles Wicks
et al
Link: Click Here
Abstract
We study the surface deformation associated
with the 21 May 2003 (M w = 6.8) Zemmouri (Algeria) earthquake, the strongest
seismic event felt in the Algiers region since 1716. The thrust earthquake mechanism
and related surface deformation revealed an average 0.50 m coastal uplift along
~55-km-long coastline. We obtain coseismic interferograms using Envisat advanced
synthetic aperture radar (ASAR) (IS2) and RADARSAT standard beam (ST4) data from
both the ascending and descending orbits of Envisat satellite, whereas the RADARSAT
data proved useful only in the descending mode. While the two RADARSAT interferograms
cover the earthquake area, Envisat data cover only the western half of | the
rupture zone. Although the interferometric synthetic aperture radar (InSAR) coherence
in the epicenter area is poor, deformation fringes are observed along the coast
in different patches. In the Boumerdes area, the maximum coseismic deformation
is indicated by the high gradient of fringes visible in all interferograms in
agreement with field measurements (tape, differential GPS, leveling, and GPS).
To constrain the earthquake rupture parameters, we model the interferograms and
uplift measurements using elastic dislocations on triangular fault patches in
an elastic and homogeneous half-space. We invert the coseismic slip using first,
a planar surface and second, a curved fault, both constructed from triangular
elements using Poly3Dinv program that uses a damped least square minimization.
The best fit of InSAR, coastal uplift, and GPS data corresponds to a 65-km-long
fault rupture dipping 40° to 50° SE, located at 8 to 13 km offshore with
a change in strike west of Boumerdes from N60°-65° to N95°-105°.
The inferred rupture geometry at depth correlates well with the seismological
results and may have critical implications for the seismic hazard assessment of
the Algiers region. |
Marzo
de 2009
Upper mantle anisotropy beneath Australia and Tahiti from P wave
polarization: Implications for real-time earthquake location
Authors: Fabrice R. Fontaine, Guilhem Barruol et al
Link: Click
Here
Abstract
We report measurements of long-period
P wave polarization (P pol) in Australia and Tahiti made by combining modeling
of the polarization deviation and harmonic analysis. The analysis of the deviation
of the horizontal polarization of the P wave as a function of event back azimuth
may be used to obtain information about (1) sensor misorientation, (2) dipping
discontinuities, (3) seismic anisotropy, and (4) velocity heterogeneities beneath
a seismic station. The results from harmonic analysis and a grid search using
Snell's law suggest the presence of a dipping seismic discontinuity beneath stations |
CTAO and CAN in Australia. These results are consistent with published receiver
function studies for these stations. The P pol fast axis orientation is close
to the N-S absolute plate motion direction at station TAU (Tasmania), which may
be due to plate-motion-driven alignment of olivine crystals in the asthenosphere.
Interestingly, measurements of SKS splitting at Tahiti (French Polynesia) show
an apparent isotropy, whereas an inversion of P pol observations at PPTL seismic
station located in Tahiti suggests the presence of two anisotropic layers. The
fast axis azimuth is oriented E-W in the upper layer, and it is close to the NW-SE
orientation in the lower layer. Since P pol orientations are used for real-time
earthquake locations, especially in poorly instrumented areas such as the South
Pacific, we show that the bias from anisotropy and sensor misorientation determined
here can be corrected to improve the location accuracy, which yields fundamental
data for rapid location necessary for effective tsunami warning. |
Marzo
de 2009
Stress change and effective friction coefficient along the Sumatra-Andaman-Sagaing
fault system after the 26 December 2004 (Mw = 9.2) and the 28 March 2005 (Mw =
8.7) earthquakes
Authors: R. Cattin, N. Chamot-Rooke
et al
Link: Click Here
Abstract
The 2004 Aceh and 2005 Nias events are
the two greatest earthquakes of the past 40 years with a total rupture of 1700
km long and a coseismic slip reaching up to 25 m. These two earthquakes have caused
large stress perturbations which significantly altered seismic activity in the
Sumatra-Andaman region. Using both detailed mapping of failure planes and various
slip distributions, we calculate this stress change along the Sumatra-Andaman-Sagaing
fault system from central Sumatra to southern Myanmar. The static Coulomb stress
change Delta CFF and the observed seismic activity are in very good agreement
with a Coulomb index ~ 20% greater than the one obtained for random events. Compared
to previous studies, this high Coulomb Index confirms two important issues on
the use of static stress change criterion: unsuited to study | near-field
aftershocks and only relevant for aftershocks analysis on large and mature faults
at a time scale of several months. The calculated Delta CFF distribution suggests
that the 2004 and 2005 earthquakes inhibit failure on the North Andaman rift and
on the Sagaing fault, while failure is encouraged along the transform Andaman
zone, the central Andaman rift, the West Andaman fault, the Sumatra fault system,
and the offshore thrust faults west of Sumatra Island. The maximum value of ~15-20
bar (1.5-2 MPa) for Delta CFF is reached in the northern part of the Sumatra fault
system. This high value together with the lack of major earthquake in the last
170 years result in a high seismic hazard for this region. Our results are also
consistent with temporal evolution of both earthquakes' location and focal mechanism
prior to and after the events. In particular, we explain the occurrence and the
mechanism of seismic swarms observed in the central Andaman rift and along the
west Andaman fault. Finally, our calculations reveal that the seismicity in the
Andaman rift zone can only be explained if µ' > 0.5. This result leads
to two end-member models: one with a constant and high fault friction and one
with spatial variations, for which friction may depend on either the nature of
the lithosphere (oceanic versus continental) or the fault type. |
Marzo
de 2009
Geoid and gravity anomaly data of conjugate regions of Bay of Bengal
and Enderby Basin: New constraints on breakup and early spreading history between
India and Antarctica
Authors: K. S. Krishna, Laju Michael
et al
Link: Click Here
Abstract
Timing of breakup of the Indian continent
from eastern Gondwanaland and evolution of the lithosphere in the Bay of Bengal
still remain as ambiguous issues. Geoid and free-air gravity data of Bay of Bengal
and Enderby Basin are integrated with shipborne geophysical data to investigate
the early evolution of the eastern Indian Ocean. Geoid and gravity data of the
Bay of Bengal reveal five N36°W fracture zones (FZs) and five isolated NE-SW
structural rises between the Eastern Continental Margin of India (ECMI) and the
85°E Ridge/86°E FZ. The FZs meet the 86°E FZ at an angle of ~39°.
The rises are associated with low-gravity and geoid anomalies and are oriented
nearly orthogonal to the FZs trend. The geoid and gravity data of the western
Enderby Basin reveal a major Kerguelen FZ and five N4°E FZs. The FZs discretely
converge to the Kerguelen FZ at an angle of ~37°. We interpret the FZs identified
in Bay of Bengal and western Enderby Basin as | conjugate
FZs that trace the early Cretaceous rifting of south ECMI from Enderby Land. Structural
rises between the FZs of Bay of Bengal may either represent fossil ridge segments,
possibly have extinct during the early evolution of the Bay of Bengal lithosphere
or may have formed later by the volcanic activity accreted the 85°E Ridge.
Two different gravity signatures (short-wavelength high-amplitude negative gravity
anomaly and relatively broader low-amplitude negative gravity anomaly) are observed
on south and north segments of the ECMI, respectively. The location of continent-ocean
boundary (COB) is at relatively far distance (100-200 km) from the coastline on
north ECMI than that (50-100 km) on the south segment. On the basis of geoid,
gravity, and seismic character and orientation of conjugate FZs in Bay of Bengal
and western Enderby Basin, we believe that transform motion occurred between south
ECMI and Enderby Land at the time of breakup, which might have facilitated the
rifting process in the north between combined north ECMI-Elan Bank and MacRobertson
Land and in the south between southwest Sri Lanka and Gunnerus Ridge region of
East Antarctica. Approximately during the period between the anomalies M1 and
M0 and soon after detachment of the Elan Bank from north ECMI, the rifting process
possibly had reorganized in order to establish the process along the entire eastern
margins of India and Sri Lanka. |
Marzo
de 2009
Mirror-mode storms: STEREO observations of protracted generation
of small amplitude waves
Authors: C. T. Russell, X.
Blanco-Cano et al
Link: Click Here
Abstract
In the solar wind, STEREO occasionally
observes mirror-mode storms, periods in which small | amplitude
waves suddenly appear and persist for hours. Two triggers of these storms are
high-plasma beta and weak shocks, both consistent with conditions for the growth
of mirror-mode waves. The appearance of these waves is quite distinct from the
isolated mirror-mode structure that is frequently seen in the solar wind. They
have not been reported previously. Here we show three examples of their occurrence.
|
Marzo de 2009
Coronal
magnetic field analysis with Faraday rotation observations of Alfven waves
Authors: E. A. Jensen and C. T. Russell
Link: Click Here
Abstract
Faraday rotation observations of an Alfven
wave at 4 solar radii during the January 9, 1983 superior conjunction of the Helios
spacecraft were simultaneously obtained in Goldstone, California and Canberra,
Australia allowing the measurement of the Alfven wave speed. We find |
that the T. Hoeksema et al. (1982) potential field source surface model performs
better than the M. Paetzold et al. (1987) magnetic field magnitude model for predicting
the magnitude of the magnetic field in the region based on the observed Alfven
speed. Assuming the Alfven wave is monochromatic, coherent, and radially propagating,
we determine the minimum magnetic field fluctuation required to reproduce the
observations. We use the minimum magnetic field fluctuation to calculate the minimum
Alfven wave energy flux of 6 × 1019 W; this is approximately 20% of the
wave energy required to accelerate the solar wind. |
Marzo
de 2009
Identification and spatiotemporal organization of aftershocks
Authors: M. Bottiglieri, E. Lippiello et al
Link: Click Here
Abstract
We propose a fast method able to discriminate
between Poissonian independent earthquakes and aftershocks. The method is based
on the evaluation of the variability coefficient, defined as the ratio between
the standard deviation and the | average
value of the interoccurrence time between two successive earthquakes. We apply
this technique to the California and a properly constructed synthetic catalogue
in order to estimate the level of background seismicity and identify seismic sequences.
We then investigate the spatiotemporal organization of aftershocks focusing on
the distributions of interevent times and interevent distances between two successive
events. We find evidence for the existence of a characteristic spatial length
scale, related to the size of the aftershock zone, whereas no typical timescale
is detected. |
Marzo
de 2009
A sporadic layer in the Venus lower ionosphere of meteoric origin
Authors: M. Pätzold, S. Tellmann et al
Link: Click Here
Abstract
The Venus Express Radio Science (VeRa)
experiment aboard Venus Express has detected, by means of radio occultation, distinct,
low-lying layers of electron density below the base (115 km altitude) of the ionosphere
of Venus. A plausible origin of these lowest layers is ionization by the influx
of meteoroids into the atmosphere. The layers appeared only occasionally during
the 2006 | and 2007 Venus Express
occultation seasons, could be identified only on the dayside and seem to be geographically
localized as they usually occur in either the northern or southern hemisphere
of the same orbit; they are detected at all latitudes, but only at solar zenith
angles between 55° and 90°. Typical peak plasma densities of 1010 m-3
are reached between 110 and 120 km altitude. Peak meteor layer electron densities
increase with decreasing solar zenith angle. Layer shapes are symmetric with respect
to peak altitude. The present observational statistics and lack of dedicated models
prevents definite statements to be made on the origin of the source meteoroids. |
Marzo
de 2009
Reflection seismic waveform tomography
Authors: Yanghua Wang and Ying Rao
Link: Click Here
Abstract
In seismic waveform tomography, if using
reflection data with limited source-receiver offsets, it is difficult to reconstruct
the deep part of the subsurface velocity model. We present two approaches to tackle
this problem: layer stripping and weighted updating. In a layer-stripping procedure,
we replace the top portion of seismic data with synthetics generated from the
previous-layer inversion and make the current inversion focus on the minimization
of the data misfit corresponding to the deep part of the model. To improve efficiency,
we use only sparsely sampled frequency data in the deeper-layer inversions, unlike
the first-layer inversion where we use densely sampled frequency data as usual.
The sparsely sampled frequencies together have the | full
wave number coverage for effective imaging. Combined use of dense and sparse sampling
in frequency is a compromise between resolution and efficiency, as it reduces
the number of iterations needed in layer-stripping inversion while still producing
a good image. In the second scheme, we apply depth-dependent weights to model
updates in order to improve the convergence in an iterative solution. The weighting
is inversely proportional to the ray density variation along the depth and is
mathematically equivalent to the application of an inverse Hessian matrix which
sharpens the gradient vector for model updating. For real seismic data, we transfer
point source shot records to line source records, by partial amplitude compensation
and phase adjusting, before inputting it to the waveform tomography. We perform
traveltime inversion to generate a reliable layered velocity model and then waveform
tomography to produce a high-resolution image of the subsurface model through
frequency domain iteration. |
Marzo
de 2009
Sudden stratospheric warmings seen in MINOS deep underground muon
data
Authors: S. Osprey, J. Barnett et al
Link:
Click Here
Abstract
The rate of high energy cosmic ray muons
as measured underground is shown to be strongly | correlated
with upper-air temperatures during short-term atmospheric (10-day) events. The
effects are seen by correlating data from the MINOS underground detector and temperatures
from the European Centre for Medium Range Weather Forecasts during the winter
periods from 2003-2007. This effect provides an independent technique for the
measurement of meteorological conditions and presents a unique opportunity to
measure both short and long-term changes in this important part of the atmosphere. |
Marzo
de 2009
Real-time earthquake detection and hazard assessment by ElarmS across
California
Authors: Richard M. Allen, Holly Brown et
al
Link: Click Here
Abstract
ElarmS is a network-based methodology
for rapid earthquake detection, location and hazard assessment in the form of
magnitude estimation and peak ground motion prediction. The methodology is currently
being tested as part of the real-time seismic system in California leveraging
the resources of the California Integrated Seismic Network (CISN) and the |
Advanced National Seismic System. A total of 603 velocity and acceleration sensors
at 383 sites across the state stream waveform data to ElarmS processing modules
at three network processing centers where waveforms are reduced to a few parameters.
These parameters are then collected and processed at UC Berkeley to provide a
single statewide prediction of future ground shaking that is updated every second.
The system successfully detected the Mw 5.4 Alum Rock earthquake in northern California
for which it generated an accurate hazard prediction before peak shaking began
in San Francisco. It also detected the Mw 5.4 Chino Hills earthquake in southern
California. The median system latency is currently 11.8 sec; the median waveform
data latency is 6.5 sec. |
Marzo
de 2009
Evidence of long-term weakness on seismogenic faults in western
North America from dynamic modeling
Authors: Elliot
C. Klein, Lucy M. Flesch et al
Link: Click Here
Abstract
We investigate the long-term strength
of faults within the plate boundary zone of western North America by quantifying
the depth-integrated deviatoric stress field acting within the seismogenic portion
of the crust. Forcings in the depth-integrated force balance equations are the
horizontal gradients in gravitational potential energy per unit area (GPE). Seismic
velocity data define the densities we use to determine GPE. We also solve for
stress field boundary conditions that, when added to the contribution from GPE
differences, provides a best fit to stress indicators. | We
estimate that the long-term depth-integrated total stress differences within the
approximately 20 km thick seismogenic layer are of the order of 0.1-1.4 ×
1012 N m-1. Using these stress differences as a proxy for depth integrals of fault
strength within the actively deforming regions, we infer that the long-term values
of coefficients of friction on faults within the Basin and Range of Nevada and
Utah, and most of California, are 0.1-0.2 under long-term hydrostatic pore pressure
conditions. We test the sensitivity of these results by considering a range of
maximum depths of integration. We show that for depths of integration in excess
of 20 km below sea level, there is diminishing contribution to the depth-integrated
stress differences, and by proxy depth-integrated fault strength. This is consistent
with a brittle-ductile transition in the plate boundary zone at depths less than
20 km below sea level, and with a weaker lower crust. |
Febrero
de 2009
Effect of bacterial adsorption on low frequency electrical properties
of clean quartz sands and iron-oxide coated sands
Authors:
Gamal Abdel Aal, Estella Atekwana et al
Link: Click Aquí
Resumen
Low frequency electrical measurements
(0.1-1000 Hz) were conducted to investigate the adsorption effect ofPseudomonas
aeruginosa cells onto clean quartz sands and iron-oxide coated sands. The clean
quartz sands showed a gradual increase in the microbial adsorption to mineral
| grains,
concurrent with an increase of 13% in the imaginary conductivity component (s?).
However, iron-oxide coated sands (20-100% by weight) showed a rapid increase in
microbial adsorption with s? reaching a maximum of 37 % for the 80-100% iron coated
sands. No significant changes were observed in the real conductivity component
(s') due to microbial adsorption. A power law dependency was observed between
the adsorbed cells and s?. We suggest that the polarization results from the increase
in the surface roughness and surface area of the grain due to bacteria sorption.
These results suggest that low frequency electrical measurements can play an important
role in assessing microbial transport in subsurface environments. |
Febrero
de 2009
Estimating porosity with ground-penetrating radar reflection tomography:
A controlled 3-D experiment at the Boise Hydrogeophysical Research Site
Authors: John H. Bradford, William P. Clement et al
Link: Click
Aquí
Resumen
To evaluate the uncertainty of water-saturated
sediment velocity and porosity estimates derived from surface-based, ground-penetrating
radar reflection tomography, we conducted a controlled field experiment at the
Boise Hydrogeophysical Research Site (BHRS). The BHRS is an experimental well
field located near Boise, Idaho. The experimental data set consisted of 3-D multioffset
radar acquired on an orthogonal 20 × 30 m surface grid that encompassed
a set of 13 boreholes. Experimental control included (1) 1-D vertical velocity
functions determined from traveltime inversion of vertical radar profiles (VRP)
and (2) neutron porosity logs. We estimated the | porosity
distribution in the saturated zone using both the Topp and Complex Refractive
Index Method (CRIM) equations and found the CRIM estimates in better agreement
with the neutron logs. We found that when averaged over the length of the borehole,
surface-derived velocity measurements were within 5% of the VRP velocities and
that the porosity differed from the neutron log by less than 0.05. The uncertainty,
however, is scale dependent. We found that the standard deviation of differences
between ground-penetrating-radar-derived and neutron-log-derived porosity values
was as high as 0.06 at an averaging length of 0.25 m but decreased to less than
0.02 at length scale of 11 m. Additionally, we used the 3-D porosity distribution
to identify a relatively high-porosity anomaly (i.e., local sedimentary body)
within a lower-porosity unit and verified the presence of the anomaly using the
neutron porosity logs. Since the reflection tomography approach requires only
surface data, it can provide rapid assessment of bulk hydrologic properties, identify
meter-scale anomalies of hydrologic significance, and may provide input for other
higher-resolution measurement methods. |
Febrero
de 2009
GRACE gravity evidence for an impact basin in Wilkes Land, Antarctica
Authors: Ralph R. B. von Frese, Laramie V. Potts et al
Link: Click
Aquí
Resumen
New details on the east Antarctic gravity
field from the Gravity Recovery and Climate Experiment (GRACE) mission reveal
a prominent positive free-air gravity anomaly over a roughly 500-km diameter subglacial
basin centered on (70°S, 120°E) in north central Wilkes Land. This regional
inverse correlation between topography and gravity is quantitatively consistent
with thinned crust from a giant meteorite impact underlain by an isostatically
disturbed mantle plug. The inferred impact crater is nearly three times the size
of the Chicxulub crater and presumably formed before the Cretaceous formation
of the east Antarctic coast that cuts the projected ring faults. It | extensively
thinned and disrupted the Wilkes Land crust where the Kerguelen hot spot and Gondwana
rifting developed but left the adjacent Australian block relatively undisturbed.
The micrometeorite and fossil evidence suggests that the impact may have occurred
at the beginning of the greatest extinction of life on Earth at ~260 Ma when the
Siberian Traps were effectively antipodal to it. Antipodal volcanism is common
to large impact craters of the Moon and Mars and may also account for the antipodal
relationships of essentially half of the Earth's large igneous provinces and hot
spots. Thus, the impact may have triggered the "Great Dying" at the
end of the Permian and contributed to the development of the hot spot that produced
the Siberian Traps and now may underlie Iceland. The glacial ice up to a few kilometers
thick that has covered the crater for the past 30-40 Ma poses formidable difficulties
to sampling the subglacial geology. Thus, the most expedient and viable test of
the prospective crater is to survey it for relevant airborne gravity and magnetic
anomalies. |
Febrero
de 2009
Latitudinal signature of Earth's magnetic field variation over the
last 5 million years
Autor: Christopher G. A. Harrison
Link: Click Aquí
Resumen
In order to investigate the latitudinal
effect of the geomagnetic field variation, a new data set consisting of virtual
geomagnetic poles (VGPs) from all latitudes has been produced. Since the updated
data set was limited to data with VGPs within 45° of the geographic poles,
data from lava flows with low-latitude VGPs were added. More rigorous criteria
were used to winnow the data. The data were divided into groups from different
observation latitudes. In each group it was shown that the distribution of VGP
latitudes could be described by a predominance of poles (average 82%) following
a Fisher distribution with the rest following a distribution that would produce
a uniform number of poles as a function of latitude. A distribution composed of
two Fisher distributions also fit the data very well. For the case using a Fisher
distribution plus a uniform distribution, the Fisher distribution changed such
| that the angular standard
deviation (ASD) of VGPs from a set of observations taken at the equator is about
10° and the ASD at 60° observation latitude is about 19°. These results
are similar to some results seeking to determine the ASD of VGPs as a function
of observation latitude using other methods, which have been recently published,
but there are also discrepancies. The results allow us to model inclination distributions
as a function of observation latitude for comparison with data in which only the
inclination is known, such as data from drill holes. It is shown that in order
not to have doubt about the polarity of the inclination data, a drill hole has
to be located at an absolute latitude greater than 27° for there to be less
than a 5% error. This has major importance for the location of the "Mission
to the MOHO" of the Integrated Ocean Drilling Program. The results also confirm
that the sources of the nondipole field located in the outer core have to be more
than twice as strong at high latitudes than at low latitudes so as to produce
the observed increase in VGP scatter with observation latitude. The model of the
secular variation proposed here is in no way a theory about how the secular variation
happens, but it does allow those who wish to develop such a theory to have a model
distribution with which to check their predictions. |
Febrero
de 2009
Anisotropic structure beneath central Java from local earthquake
tomography
Autores: Ivan Koulakov, Andrey Jakovlev et
al
Link: Click Aquí
Resumen
In this study we present the new tomographic
code ANITA which provides 3-D anisotropic P and isotropic Svelocity distribution
based on P and S traveltimes from local seismicity. For the P anisotropic model,
we determine four parameters for each parameterization cell. This represents an
orthorhombic anisotropy with one predefined direction oriented vertically. Three
of the parameters describe slowness variations along three horizontal orientations
with azimuths of 0°, 60°, and 120°, and one is a perturbation along
the vertical axis. The nonlinear iterative inversion procedure is similar to that
used in the LOTOS code. We have implemented this algorithm for the updated data
set of central Java, part of which was previously used for the isotropic inversion.
It was obtained that the crustal and uppermost mantle velocity structure beneath
central Java is strongly anisotropic with 7-10% of maximal difference |
between slow and fast velocity in different directions. In the forearc (area between
southern coast and volcanoes), the structure of both isotropic and anisotropic
structure is strongly heterogeneous. Variety of anisotropy orientations and highly
contrasted velocity patterns can be explained by a complex block structure of
the crust. Beneath volcanoes we observe faster velocities in vertical direction,
which is probably an indicator for vertically oriented structures (channels, dykes).
In the crust beneath the middle part of central Java, north to Merapi and Lawu
volcanoes, we observe a large and very intense anomaly with a velocity decrease
of up to 30% and 35% for P and S models, respectively. Inside this anomaly E-W
orientation of fast velocity takes place, probably caused by regional extension
stress regime. In a vertical section we observe faster horizontal velocities inside
this anomaly that might be explained by layering of sediments and/or penetration
of quasi-horizontal lenses with molten magma. In the mantle, trench parallel anisotropy
is observed throughout the study area. Such anisotropy in the slab entrained corner
flow may be due to presence of B-type olivine having predominant axis parallel
to the shear direction, which appears in conditions of high water or/and melting
content. |
Febrero
de 2009
Topographic control on lava flow paths at Mount Etna, Italy: Implications
for hazard assessment
Autores: Massimiliano Favalli,
Francesco Mazzarini et al
Link: Click Aquí
Resumen
Assessment of the hazard from lava flow
inundation at the active volcano of Mount Etna, Italy, was performed by calculating
the probability of lava flow inundation at each position on the volcano. A probability
distribution for the formation of new vents was calculated using geological and
volcanological data from past eruptions. The simulated lava flows from these vents
were emplaced using a maximum expected flow length derived from geological data
on previous lava flows. Simulations were run using DOWNFLOW, a | digital-elevation-model-based
model designed to predict lava flow paths. Different eruptive scenarios were simulated
by varying the elevation and probability distribution of eruptive points. Inundation
maps show that the city of Catania and the coastal zone may only be impacted by
flows erupted from low-altitude vents (<1500 m elevation) and that flank eruptions
at elevations >2000 m preferentially inundate the northeast and southern sectors
of the volcano as well as the Valle del Bove. Eruptions occurring in the summit
area (>3000 m elevation) pose no threat to the local population. Discrepancies
between the results of simple, hydrological models and those of the DOWNFLOW model
show that hydrological approaches are inappropriate when dealing with Etnean lava
flows. Because hydrological approaches are not designed to reproduce the full
complexity of lava flow spreading, they underestimate the catchment basins when
the fluid has a complex rheology. |
Febrero
de 2009
Un estudio de los algoritmos de alerta del sistema de alerta temprana
de terremotos de Estambul (Turquía)
Autores:
Hakan Alcik, Oguz Ozel et al
Link: Click Aquí
Resumen
Los terremotos del 17 de agosto (Mw 7.4)
y del 12 de noviembre de 1999 (Mw 7.2) causaron gran interés sobre las
futuras ocurrencias de terremotos en Estambul y en la Región de Marmara.
Estudios de transferencia de tensiones han renovado los estudios del tipo probabilísticos,
indicando una probabilidad de 2% anual para un terremoto superior a 7 Mw en el
Mar de Marmara. Como parte de los preparativos para el esperado terremoto de Estambul,
en el año 2002 se creó un sistema de alerta temprana y que consiste
en un simple y robusto algoritmo, basado en la superación de amplitudes
en el dominio del tiempo y niveles de velocidad absoluta acumulada (CAV). Se han
determinado niveles de umbrales razonables relacionados con un nuevo modelo CAV
del tipo agrupamiento de ventana (BCAV-W), basados en conjuntos de datos de registros
de fuertes movimientos, con fallas inferiores a 100 Km, han generado tres niveles
de alarma relacionados con 0.2 m/s, 0.4 m/s y 0.7 m/s, los cuales serán
incorporados en el sistema de alerta temprana de terremotos de Estambul. NOTA:
Esta idea fue sugerida por nosotros en el año 2005 | Ver
artículo | Febrero
de 2009
A study on warning algorithms for Istanbul earthquake early warning
system
Authors: Hakan Alcik, Oguz Ozel et al
Link:
Click here
Abstract
17 August (Mw 7.4) and 12 November 1999
(Mw 7.2) earthquakes have caused major concern about future earthquake occurrences
in Istanbul and in the Marmara Region. Stress transfer studies and renewal model
type probabilistic investigations indicate about 2% annual probability for a M
w = 7+ earthquake in the Marmara Sea. As part of the preparations for the expected
earthquake in Istanbul, an early warning system has been established in 2002.
A simple and robust algorithm, based on the exceedance of specified threshold
time domain amplitude and the cumulative absolute velocity (CAV) levels, is implemented
for this system. Rational threshold levels related to new bracketed CAV window
approach (BCAV-W) are determined, based on dataset of strong ground motion records
with fault distances of less than 100 km, as 0.2 m/s, 0.4 m/s and 0.7 m/s related
to three alarm levels which will be incorporated in the Istanbul earthquake early
warning system. |
Febrero
de 2009
Analysis of electrical activity and seismicity in the natural time
domain for the volcanic-seismic swarm activity in 2000 in the Izu Island region,
Japan
Authors: S. Uyeda, M. Kamogawa et al
Link:
Click here
Abstract
Seismicity and geoelectric potential
changes, possibly associated with the seismic swarm activity in 2000 in the Izu
Island region, Japan, are analyzed in the framework of the natural time x, which
has been recently introduced as an index of the kth event x = k/N, where N is
the total number of events. The Izu 2000 swarm activity lasted for | about
2 months with some 7000 shocks with magnitude M = 3 and five M = 6 shocks, and
was preceded by a pronounced electrical activity with innumerable signals that
started 2 months prior to the swarm onset. It is shown, first, that the seismicity
subsequent to the electrical activity approaches to a critical stage a few days
before the occurrence of the first M = 6 shock and, second, that the electrical
signals also have the properties characteristic to the critical stage. Despite
the big differences in time scale and numbers of electric signals and earthquakes,
these features are found similar to those in Greece. The present results suggest
that both in Greece and Japan, the electrical activity as well as the subsequent
seismicity may have a self-similar structure and exhibits similar dynamic evolution
toward critical stage. |
Febrero
de 2009
Wave gradiometry for USArray: Rayleigh waves
Authors: Chuntao Liang and Charles A. Langston et al
Link: Click here
Abstract
Wave gradiometry (WG) is a new array
data processing technique to extract phase velocity, wave directionality, geometrical
spreading, and radiation pattern from spatial gradients of waveforms. A weighted
inversion method and a reducing velocity method are introduced to compute spatial
gradients accurately for irregular arrays. Numerical experiments are conducted
to test techniques and to evaluate the parameters determined from the WG method.
We apply this method to USArray data for the western United States. In this study,
Rayleigh waves from nine earthquakes with varying azimuths are analyzed. The stability
of this method is shown by the similarity between the results from two nearly
collocated earthquakes from the Kurile Islands. The error check shows the WG results
are stable for ambient noise level as high as 10%. Phase velocities determined
by WG and two station (TS) methods are statistically consistent, while these determined
from beam forming method are |
systematically higher for wavelength larger than one quarter of the array diameter.
Our results show that, first, the average phase velocities of Rayleigh waves range
from 3.8 to 4.1 km/s for periods from 60 s to 150 s. This is consistent with average
earth models. The prominent feature on the phase velocity map is that the Basin
and Range province is dominated by velocity lows while the west coast of the United
States and the north and northeastern Snake River plain are dominated by velocity
highs. The Snake River plain appears to be a primary tectonic boundary. Second,
azimuthal variations represent the accumulated wave directionality changes along
the raypath. A velocity contrast of 0.25 km/s across the oceanic-continental lithosphere
boundary along the west coast of the United States is needed to explain the negative
azimuth variations. Third, geometrical spreading is slightly anticorrelated with
phase velocity, which may suggest that amplitude variations in radial directions
are subject to surface wave focusing and defocusing. Fourth, similar to the wave
directionality, radiation pattern variations also exhibit strong path dependence.
Further theoretical and experimental studies will be conducted to understand the
two amplitude parameters: geometrical spreading and radiation pattern and their
relations with the local geophysical properties. |
Febrero
de 2009
Three-dimensional joint inversion of traveltime and gravity data
across the Chicxulub impact crater
Authors: P. M. Vermeesch,
J. V. Morgan et al
Link: Click here
Abstract
In 2005 an extensive new seismic refraction
data set was acquired over the central part of the Chicxulub impact crater, allowing
us to image its structure with much better resolution than before. However, models
derived from traveltime data are limited by the available ray coverage and the
nonuniqueness that is inherent to all geophysical methods. Therefore, many different
models can fit the data equally well. To address these issues, we have developed
a new method to simultaneously invert traveltime and gravity data to | obtain
an integrated model. To convert velocity to density, we use a linear relationship
derived from measurements on core from the Chicxulub impact basin, thus providing
a reliable conversion equation that is typical for lithologies of the central
part of this crater. Prior to utilizing the inversion on the observed data, we
have run a suite of tests to establish the optimum weighting between traveltime
and gravity constraints, using a synthetic model of central crater structure and
the real experimental geometry. These synthetic tests indicate which inversion
parameters lead to the best recovery of subsurface structure, as well as which
parts of the model are well resolved. We applied the method to all existing gravity
data and to seismic refraction data acquired in 1996 and the new, higher-resolution
seismic refraction data acquired in 2005. We favor the traveltime model wherever
we have sufficient ray coverage and the joint model where we have no ray coverage.
|
Febrero
de 2009
Three-dimensional simulation of tsunami generation and propagation:
Application to intraplate events
Authors: Tatsuhiko
Saito, Takashi Furumura et al
Link: Click here
Abstract
A parallel finite difference numerical
simulation program based on the Navier-Stokes (NS) equations is developed for
simulating 3-D tsunami generation and propagation. We can simulate tsunami propagation
over more than 1000 km using this program, although such tsunami propagation over
long distances has usually been conducted on the basis of on 2-D simulations.
Some 2-D simulations have assumed that the initial tsunami distribution is identical
to the sea bottom deformation caused by | the
earthquake. The 3-D simulations, however, indicate that this assumption is inappropriate
when the source width 2a x is less than 10 times the sea depth h (2a x < 10h)
for a source process time of 20 s. Dispersion of tsunami appears when the source
is of small size, and the dispersion is more apparent in a direction perpendicular
to the fault strike. We also conduct tsunami simulations for two intraplate events:
the 2004 off-Kii Peninsula (M 7.4) and the 2007 off-Kuril Islands (M 8.1) events.
The 3-D tsunami simulation has successfully simulated the offshore tsunami, and
we obtained good agreement between the observations and the calculations for both
events. In particular, for the 2004 off-Kii Peninsula event, although the 2-D
linear long-wave theory cannot simulate important characteristics in the record
such as the arrival time of the peak amplitude and the dispersive tsunami, the
3-D NS simulation is well able to model those characteristics. |
Febrero
de 2009
Nanofabrication of two-dimensional arrays of magnetite particles
for fundamental rock magnetic studies
Authors: David
Krása, Chris D. W. Wilkinson et al
Link: Click here
Abstract
Magnetic measurements of samples with
precisely controlled magnetic mineralogy, grain size, and interparticle spacing
are needed to provide crucial experimental rock magnetic underpinning for paleomagnetic
studies. We report a novel nanofabrication method for producing two-dimensional
arrays of cylindrical synthetic magnetite particles with well-defined composition,
particle size, and interparticle spacing. The samples are fabricated by writing
dot arrays with electron beam lithography, transferring these patterns into sputtered
Fe thin films by | reactive
ion etching in a CO/NH3 plasma, and oxidizing the resulting Fe particles in a
controlled atmosphere to form magnetite. Scanning electron microscopy and transmission
electron microscopy have been used to monitor the fabrication process and to determine
the particle geometry. The particle sizes of our samples range between 100 nm
and 265 nm with center-to-center spacings between 180 nm and 310 nm. Low-temperature
magnetic remanence data confirm the stoichiometry of the magnetite. We present
magnetic hysteresis data and first-order reversal curve diagrams for our samples
and compare these with previously published data from other synthetic and natural
magnetite samples. The ability to independently control particle size and interparticle
spacing of magnetite grains makes our synthetic samples ideal for studying the
influence of magnetostatic interactions on the paleomagnetic recording fidelity
of naturally occurring magnetite in rocks. |
Febrero
de 2009
The 16 November 2006 flank collapse of the south-east crater at
Mount Etna, Italy: Study of the deposit and hazard assessment
Authors: Gianluca Norini, Emanuela De Beni et al
Link: Click here
Abstract
On 16 November 2006 a flank collapse
affected the unstable eastern slope of the South-East Crater (SEC) of Mount Etna.
The collapse occurred during one of the paroxysmal events with sustained strombolian
activity that characterized the August-December 2006 eruption and was triggered
by erosion of loose, hydrothermally altered material of the steep south-east sector
of SEC from the outpour of lava. The collapse produced a debris avalanche that
involved both lithic and juvenile material and resulted in a | deposit
emplaced on the eastern flank of the volcano up to 1.2 km away from the source.
The total volume of the deposit was estimated to be in the order of 330,000-413,000
m3. The reconstruction of the collapse event was simulated using TITAN2D software
designed to model granular avalanches and landslides. This approach can be used
to estimate areas that may be affected by similar collapse events in the future.
The area affected by the 16 November 2006 lateral collapse of SEC was a small
portion of the Mount Etna summit area, but the fact that no one was killed or
injured should be considered fortuitous. The summit and adjacent areas of the
volcano, in fact, are usually visited by many tourists who are not prepared to
face this type of danger. The 16 November 2006 collapse points to the need to
be prepared for similar events through scientific investigation (analysis of flank
instability, numerical simulation of flows) and development of specific civil
protection plans. |
Febrero
de 2009
Home seismometer for earthquake early warning
Authors: Shigeki Horiuchi, Yuko Horiuchi et al
Link: Click here
Abstract
The Japan Meteorological Agency (JMA)
has started the practical service of Earthquake Early Warning (EEW) and a very
dense deployment of receiving units is expected in the near future. The receiving/alarm
unit of an EEW system is equipped with a CPU and memory and is on-line | via
the internet. By adding an inexpensive seismometer and A/D converter, this unit
is transformed into a real-time seismic observatory, which we are calling a home
seismometer. If the home seismometer is incorporated in the standard receiving
unit of EEW, then the number of seismic observatories will be drastically increased.
Since the background noise inside a house caused by human activity may be very
large, we have developed specialized software for on-site warning using the home
seismometer. We tested our software and found that our algorithm can correctly
distinguish between noise and earthquakes for nearly all the events. |
Febrero
de 2009
North polar deposits of Mars: Extreme purity of the water ice
Authors: Cyril Grima, Wlodek Kofman et al
Link: Click here
Abstract
The polar layered deposits are the largest
reservoir of water on the surface of Mars. The physical properties of the ice
and their spatial distribution are largely unknown. 140,000 data points from the
sounding radar SHARAD on the | Mars
Reconnaissance Orbiter were analyzed over the Gemina Lingula region, one-fourth
of the north polar layered deposits area. Maps of the dielectric properties of
the bulk ice were drawn up. There is no basal melting signature. A drop of the
dielectric constant in north-west of Gemina Lingula could be explained by an abrupt
250-meter uplift of the base. The bulk ice of the studied region has an average
dielectric constant of 3.10 (s = 0.12) and a loss tangent <0.0026 (s = 0.0005).
Analytic interpretations shown the volume of ice is pure at =95%. The impurities
have a radial distribution, with higher concentrations at margins. |
Enero
de 2009
Estimating the uncertain mathematical structure of a water balance
model via Bayesian data assimilation
Authors: Nataliya
Bulygina and Hoshin Gupta
Link: Click here
Abstract
When constructing a hydrological model
at the macroscale (e.g., watershed scale), the structure of this model will inherently
be uncertain because of many factors, including the lack of a robust hydrological
theory at that scale. In this work, we assume that a suitable conceptual model
structure for the hydrologic system has already been determined; that is, the
system boundaries have been specified, the important state variables and input
and output fluxes to be included have been selected, the major hydrological processes
and geometries of their interconnections have been identified, and the continuity
equation (mass balance) has been assumed to hold. The remaining structural identification
problem that remains, then, is to select the mathematical form of the dependence
of the output on the inputs and state variables, so that a computational model
can be constructed for making simulations and/or predictions of the system input-state-output
behavior. The conventional approach to this problem is to preassume some fixed
(and possibly erroneous) mathematical forms for the model output equations. We
show instead how Bayesian data assimilation can be used to directly estimate (construct)
the form of these | mathematical
relationships such that they are statistically consistent with macroscale measurements
of the system inputs, outputs, and (if available) state variables. The resulting
model has a stochastic rather than deterministic form and thereby properly represents
both what we know (our certainty) and what we do not know (our uncertainty) about
the underlying structure and behavior of the system. Further, the Bayesian approach
enables us to merge prior beliefs in the form of preassumed model equations with
information derived from the data to construct a posterior model. As a consequence,
in regions of the model space for which observational data are available, the
errors in preassumed mathematical form of the model can be corrected, improving
model performance. For regions where no such data are available the "prior"
theoretical assumptions about the model structure and behavior will dominate.
The approach, entitled Bayesian estimation of structure, is used to estimate water
balance models for the Leaf River Basin, Mississippi, at annual, monthly, and
weekly time scales, conditioned on the assumption of a simple single-state-variable
conceptual model structure. Inputs to the system are uncertain observed precipitation
and potential evapotranspiration, and outputs are estimated probability distributions
of actual evapotranspiration and streamflow discharge. Results show that the models
estimated for the annual and monthly time scales perform quite well. However,
model performance deteriorates for the weekly time scale, suggesting limitations
in the assumed form of the conceptual model. |
Enero
de 2009
Electron impact excitation of carbon monoxide in comet Hale-Bopp
Authors: L. Campbell and M. J. Brunger
Link: Click here
Abstract
The fourth positive emissions of carbon
monoxide in the coma of comet Hale-Bopp have been assumed to be due mainly to
fluorescence induced by sunlight. Based on this assumption they were used to deduce
the abundance of carbon monoxide in the comet, giving a value higher than in other
comets. Emissions produced | by
electron impact excitation of CO were not considered. Recent measurements and
theoretical calculations of integral cross sections for electron impact excitation
of CO allow the contribution of electron impact to be calculated, giving about
40% of the total. This implies that the abundance of CO in the outer coma of comet
Hale-Bopp was only 60% of that previously deduced. However, as the high proportion
of CO in comet Hale-Bopp was also seen in some other measurements, alternative
explanations are considered. The method of calculation is tested by successfully
predicting the O I emission at 1356 Å, supporting the belief that this line
is due to electron impact excitation. |
Enero
de 2009
Observations of electric fields associated with internal gravity
waves
Authors: Roger H. Varney, Michael C. Kelley et
al
Link: Click here
Abstract
At the Jicamarca Radio Observatory, the
vertical drift component yields a very accurate measure of the eastward electric
field. Occasionally, this drift component displays a downward phase progression,
evidence for a relationship to a gravity wave. We examined the Jicamarca database
for events of this type and made an attempt to determine the properties of the
associated waves. The only measurables we have are the amplitudes, the frequency
in the Earth-fixed frame, and the vertical wavelength. In order to avoid shorting
by the current along | magnetic
field lines, we argue that the propagation must be close to pure zonal. We then
use measurements or models of the zonal plasma drift and argue that the zonal
wind should be in the same direction and about 15% higher. Using this estimate,
we then determine the frequency in the wind frame by solving the dispersion relation
for gravity waves and the Doppler-shift equation simultaneously. Typical values
for the horizontal wavelength, vertical wavelength, and period in the wind frame
are 600 km, 350 km, and 25 min, respectively. The typical gravity wave-induced
vertical drift perpendicular to B in these events is a few meters per second.
This is marginal at best for seeding the Rayleigh-Taylor instability. However,
larger-amplitude events may be masked by the development of the plumes themselves.
All but two events found thus far occurred at night but the daytime cases are
fascinating since the E region is expected to short out such fields. |
Enero
de 2009
Estimating uncertainty of streamflow simulation using Bayesian neural
networks
Authors: Xuesong Zhang, Faming Liang et al
Link: Click here
Abstract
Recent studies have shown that Bayesian
neural networks (BNNs) are powerful tools for providing reliable hydrologic prediction
and quantifying the prediction uncertainty. The reasonable estimation of the prediction
uncertainty, a valuable tool for decision making to address water resources management
and design problems, is influenced by the techniques used to deal with different
uncertainty sources. In this study, four types of BNNs with different treatments
of the uncertainties related to parameters (neural network's weights) and model
structures were applied for uncertainty estimation of streamflow simulation in
two U.S. Department of Agriculture Agricultural Research |
Service watersheds (Little River Experimental Watershed in Georgia and Reynolds
Creek Experimental Watershed in Idaho). An advanced Markov chain Monte Carlo algorithm,
evolutionary Monte Carlo, was used to train the BNNs and to estimate uncertainty
limits of streamflow simulation. The results obtained in these two case study
watersheds show that the 95% uncertainty limits estimated by different types of
BNNs are different from each other. The BNNs that only consider the parameter
uncertainty with noninformative prior knowledge contain the least number of observed
streamflow data in their 95% uncertainty bound. By considering variable model
structure and informative prior knowledge, the BNNs can provide more reasonable
quantification of the uncertainty of streamflow simulation. This study stresses
the need for improving understanding and quantifying methods of different uncertainty
sources for effective estimation of uncertainty of hydrologic simulation using
BNNs. |
Enero
de 2009
Aperiodic recurrence of geologically recorded tsunamis during the
past 5500 years in eastern Hokkaido, Japan
Authors:
Yuki Sawai, Takanobu Kamataki et al
Link: Click here
Abstract
Along Hokkaido's Pacific coast near the
town of Kiritappu, sandy deposits in a muddy lagoon and on a nearby beach-ridge
plain provide evidence for 15 tsunamis between 200 and 6000 years ago. Additional
sand beds at the lagoon probably represent the historical tsunamis of A.D. 1843
and 1894. We observed the sequences of sandy deposits in continuous slices 2 to
4 m deep. Some of the deposits consist of just a single sand bed, whereas others
contain multiple units of sand, muddy sand (or sandy mud), and mud caps |
including plant detritus. We also found at the lagoon a 17th century tsunami deposit
that thickens and thins regardless of elevation or distance inland. We bracketed
the ages of most of the inferred tsunamis by radiocarbon dating of detritus, mainly
seeds and leaves at the lagoon and charcoal at the beach-ridge plain, from pretsunami
and posttsunami beds. Tsunami dates computed from the bracketing ages commonly
have uncertainties spanning 2 to 4 centuries. Within these uncertainties, the
inferred sequence of 15 prehistoric tsunamis at the lagoon, beginning almost 6000
years ago, can be matched tsunami by tsunami with the inferred history at the
beach-ridge plain, 15 km away. The sand sheet extents suggest that most of these
tsunamis were larger than any generated at Hokkaido in the last 200 years. The
intervals between these inferred outsized tsunamis average nearly 400 years but
range widely from about 100 to about 800 years. |
Enero
de 2009
Quantifying uncertainty in mean earthquake interevent times for
a finite sample
Authors: M. Naylor, I. G. Main et al
Link: Click here
Abstract
Seismic activity is routinely quantified
using means in event rate or interevent time. Standard estimates of the error
on such mean values implicitly assume that the events used to calculate the mean
are independent. However, earthquakes can be triggered by other events and are
thus not necessarily independent. As a result, the errors on mean earthquake interevent
times do not exhibit Gaussian convergence with increasing sample size according
to the central limit theorem. In this paper we investigate how the errors decay
with sample size in real earthquake catalogues and | how
the nature of this convergence varies with the spatial extent of the region under
investigation. We demonstrate that the errors in mean interevent times, as a function
of sample size, are well estimated by defining an effective sample size, using
the autocorrelation function to estimate the number of pieces of independent data
that exist in samples of different length. This allows us to accurately project
error estimates from finite natural earthquake catalogues into the future and
promotes a definition of stability wherein the autocorrelation function is not
varying in time. The technique is easy to apply, and we suggest that it is routinely
applied to define errors on mean interevent times as part of seismic hazard assessment
studies. This is particularly important for studies that utilize small catalogue
subsets (fewer than ~1000 events) in time-dependent or high spatial resolution
(e.g., for catastrophe modeling) hazard assessment. |
Enero
de 2009
Postseismic relaxation following the 1992 M7.3 Landers and 1999
M7.1 Hector Mine earthquakes, southern California
Authors:
J. C. Savage and J. L. Svarc
Link: Click here
Abstract
Postseismic relaxation (measured postseismic
deformation less the deformation that would have occurred at the preseismic rate)
has been monitored at the same 10 monuments over ~ 6 years following both the
1992 Landers and the 1999 Hector Mine earthquakes. For both earthquakes the displacement
components of the observed relaxation are well described by y i + a i f 1(t),
where y i and a i are constants peculiar to each component at each monument, t
is the time after the earthquake, and f 1(t) is a temporal function common to
all components and all monuments for that earthquake. The temporal function f
1(t) can be approximated by bt + c log e |
(1 + t/T), where t = 38.7 ± 15.2 days and 25.6 ± 7.7 days for the
Landers and Hector Mine relaxations, respectively. Because the estimated values
of t do not differ significantly, the transient term log e (1 + t/t) in the temporal
function may be the same for both earthquakes. The asymptotic (t --> oo) relaxation
rates a i b are only a few mm/a and do not appear to be significantly different
following the two earthquakes. The asymptotic deformation rates appear to be slightly
greater than the preseismic deformation rates, but the preseismic rates were not
measured directly. Thus, the deformations of the Landers array measured over the
first 5.6 years following the Landers earthquake and over the first 6.4 years
following the Hector Mine earthquake are generally consistent with a simple model
in which the transient postearthquake relaxation depends upon time as log e (1
+ t/t) with t = 28 ± 5 days and the asymptotic postseismic speeds of the
monuments exceed the preseismic speeds by at most only a few millimeters per annum.
|
Enero
de 2009
Atmospheric complexity or scale by scale simplicity?
Authors: S. Lovejoy, D. Schertzer et al
Link: Click here
Abstract
Is the numerical integration of nonlinear
partial differential equations the only way to tackle atmospheric complexity?
Or do cascade dynamics repeating scale after scale lead to simplicity? | Using
1000 orbits of TRMM satellite radiances from 11 bands in the short wave (visible,
infra red) long wave (passive microwave) and radar regions and 8.8 to 20,000 km
in scale, we find that the radiance gradients follow the predictions of cascade
theories to within about ±0.5%, ±1.25%, ±5.9% for the short
waves, long waves and reflectivities respectively and with outer scales varying
between ˜5,000 to ˜32,000 km. Since the radiances and dynamics are strongly
coupled, we conclude that weather can be accurately modeled as a cascade process.
|
Enero de 2009
Seismic
observations of meteors: Coupling theory and observations
Authors: Wayne N. Edwards, David W. Eaton et al
Link: Click here
Abstract
Over the last century, seismic instruments
have recorded, with increasing frequency, the ground motion produced by meteorically
generated shock waves striking the Earth's surface. In this review, the history
of meteor-related seismic signals is discussed, along with documented waveform
characteristics, source mechanisms, air-ground coupling phenomena, and kinematic
methods of | determining meteor
trajectories and event locations. Uncertainties in the mechanics of air-ground
coupling, however, have left methods of measuring meteor source energy underdeveloped.
To date, coupling of acoustic waves directly with the Earth's surface represents
the bulk of the observed meteor-related seismic signals, while precursory and
impact-related seismic waves remain an observational rarity. With proliferation
of infrasound and seismic monitoring systems, new opportunities exist to explore
the relationship between Earth's atmosphere and surface. Continued study of meteor
seismology will lead to new methods to constrain energies, sizes, and fluxes for
moderately (cm to m) sized meteoroids on Earth and potentially on Mars. |
Enero
de 2009
Correlation between crustal high conductivity zones and seismic
activity and the role of carbon during shear deformation
Authors: Paul W. J. Glover, A. Ádám
Link: Click here
Abstract
The electrical conductivity of the lower
crust is anomalously high in many locations around the world. Well-interconnected
grain boundary carbon not only has the potential for increasing the electrical
conductivity of the rock but also would be expected to reduce its shear strength.
We report a new analysis of field observations and new laboratory measurements
consistent with deep | carbon-bearing
rocks causing observed high conductivities and crustal weaknesses associated with
increased seismicity. The field data indicate a correlation between the depths
to a zone of high electrical conductivity observed in Transdanubia in Hungary,
earthquake focal depths, and zones of high seismic attenuation. The laboratory
triaxial deformation experiments show that progressive shearing of a fracture
in carbon-bearing rock can result in a weaker more electrically conductive fracture.
These results provide strong evidence for the role of carbon at depth in both
electrical conduction and seismotectonics, explaining the correlation between
mid-crustal high reflectivities and high conductivities observed at many locations
worldwide. |
Enero
de 2009
Postseismic signature of the 2004 Sumatra earthquake on low-degree
gravity harmonics
Authors: V. Cannelli, D. Melini et
al
Link: Click here
Abstract
We perform an extensive analysis of the
low-degree gravity field harmonics measured by the GRACE mission, in order to
find a signature of the postseismic relaxation following the 2004 Sumatra earthquake.
We find a statistically significant perturbation in the secular trend of low-degree
zonal coefficients (J l ) in correspondence of the 2004 Sumatra earthquake and
a similar perturbation, but with weak associated statistical significance, also
in the nonzonal coefficients. Technical features and results of such analysis
| are discussed. The time-dependent
postseismic evolution of harmonic coefficients is modeled for various asthenosphere
viscosity values, using a theoretical model of global postseismic deformation.
The observed change in secular trend is found to be consistent with our modeling
results but it cannot be used to discriminate between viscosities. A forward modeling
of the perturbations to time-dependent zonal variation rates following the Sumatra
earthquake for various asthenospheric viscosities is provided. As a result, an
evident signature of the Sumatra earthquake on l time series is expected for asthenospheric
viscosity values below 1018 Pa s. Therefore, long term l time histories from satellite
laser ranging will be able to put constraints on the asthenosphere viscosity,
if such a signature is evidenced from data or, at least, put lower limits if no
significant perturbation will be observed. |
Enero
de 2009
A practical database method for predicting arrivals of "average"
interplanetary shocks at Earth
Authors: X. S. Feng,
Y. Zhang et al
Link: Click here
Abstract
A practical database method for predicting
the interplanetary shock arrival time at L1 point is presented here. First, a
shock transit time database (hereinafter called Database-I) based on HAFv.1 (version
1 of the Hakamada-Akasofu-Fry model) is preliminarily established with hypothetical
solar events. Then, on the basis of the prediction test results of 130 observed
solar events during the period from February 1997 to August 2002, Database-I is
modified to create a practical database method, named Database-II, organized on
a multidimensional grid of source |
location, initial coronal shock speed, and the year of occurrence of the hypothetical
solar event. The arrival time at L1 for any given solar event occurring in the
23rd solar cycle can be predicted by looking up in the grid of Database-II according
to source location, the initial coronal shock speed, and the year of occurrence
in cycle 23. Within the hit window of ±12 h, the success rate of the Database-II
method for 130 solar events is 44%. This could be practically equivalent to the
shock time of arrival (STOA) model, the interplanetary shock propagation model
(ISPM), and the HAFv.2 model. To explore the capability of this method, it is
tested on new data sets. These tests give reasonable results. In particular, this
method's performance for a set of events in other cycles is as good as that of
the STOA and ISPM models. This gives us confidence in its application to other
cycles. From the viewpoint of long-term periodicity for solar activity, it is
expected that the Database-II method can be applicable to the next solar cycle
24. |
Enero de
2009
Predicting the geodetic signature of MW = 8 slow slip events
Authors: Brendan J. Meade and John P. Loveless
Link: Click here
Abstract
Elastic dislocation models of geodetic
measurements above subduction zones have led to the identification of MW ˜
6.0-7.2 slow slip events (SSEs) that release elastic strain over periods of days
to months, but great (MW = 8) | SSEs
have remained unidentified. We extrapolate observations of SSE duration and slip
magnitude to show that slip velocity decreases with event magnitude and predict
that the slip velocity of MW = 8 SSEs is =50 mm/yr. The slip velocity for great
SSEs may never exceed the plate convergence rate and thus never produce a reversal
in trench perpendicular motion. Instead, geodetically constrained estimates of
apparent partial elastic coupling on subduction zone interfaces worldwide may
be direct observations of ongoing MW = 8 silent earthquakes with durations of
decades to centuries. |
Diciembre
de 2008
Modeling the influence of horizontal advection, deformation, and
late uplift on the drainage development in the India-Asia collision zone
Authors: Kurt Stüwe, Jörg Robl et al
Link: Click here
Abstract
We present results of a new model in
which we couple a mechanical model to describe continental indentation in plan
view with a landscape evolution model to describe drainage development. We apply
the model to investigate aspects of the development of drainage systems, | erosion,
and exhumation in the India-Asia collision zone. It is shown that the orogen-scale
distribution of erosion and exhumation can be well matched by the model. However,
the elevation of the Tibetan Plateau and the steep topographic gradients at its
margins are difficult to reproduce. They appear to require an additional cause
of uplift. We show that some of the major observed river capture events can be
reproduced by invoking such a late uplift in much of the orogen, except the region
of the Sichuan Basin. This includes the inferred capture of the Yarlong-Tsangpo
by the Brahmaputra and that of the Upper Yangtze by the Lower Yangtze (thereby
deserting the Red River). |
Diciembre
de 2008
Thermal and structural models of the Sumatra subduction zone: Implications
for the megathrust seismogenic zone
Authors: S. Hippchen,
R. D. Hyndman et al
Link: Click here
Abstract
The 2004 Sumatra-Andaman and 2005 Nias
events provide unique geodetic and seismic data constraining the updip and downdip
rupture extents of great thrust earthquakes. These limits are important for tsunami
generation and earthquake shaking, respectively. There is a consistent downdip
limit of rupture along strike in these great earthquakes at a depth of ~30 km,
and there is an updip limit near the trench that are interpreted to define the
seismogenic limits. Temperature and downdip changes in formation compositions
are controls proposed for these limits. To examine the thermal control, we developed
2-D finite element models of the | Sumatra
subduction zone with smoothly varying subduction dip, variable thermal properties
of the rock units, frictional heating along the rupture planes, and appropriate
thermal state of the incoming plate. The common updip thermal limit for seismic
behavior of 100-150°C occurs close to or at the trench in agreement with the
great earthquake rupture limit. Off central Sumatra the common downdip thermal
limit range of 350-450°C occurs at 30-60 km depth. The 350°C location
is in agreement with the earthquake limits, but 450°C is deeper. North of
Sumatra, 350°C occurs ~14 km deeper than the earthquake rupture limit. The
proposed composition control for the downdip limit, the intersection of the subduction
thrust with the fore-arc mantle, is at a depth of ~30 km, 140-200 km from the
trench, in good agreement with the earthquake limits. These results support the
conclusion that the Sumatra updip seismogenic limit is thermally controlled but
the downdip limit is governed by the intersection of the downgoing plate with
the fore-arc Moho. |
Diciembre
de 2008
First arrival stochastic tomography: Automatic background velocity
estimation using beam semblances and VFSA
Authors: Chaoshun
Hu, Paul Stoffa et al
Link: Click here
Abstract
We present a new tomography method based
on the local beam semblance and the very fast simulated annealing (VFSA) global
optimization method. The data space is the local beam semblance calculated using
local slant stacks for overlapping offset windows, i.e. beam windows, of | the
original common-shot or common-receiver gathers. On each beam semblance panel,
the first coherency peak can be identified with a particular ray parameter, first-arrival
traveltime and beam center position. The forward problem can be solved with any
ray tracer to find arrivals matching the identified peaks. Our inversion scheme
uses VFSA to find the maximum-a-posteriori (MAP) solution and estimates the uncertainty
by applying Bayesian analysis of all the sampled models for a specified model
parameterization. This integration of automatic local semblance evaluation instead
of first-arrival picking and a fast forward modeling method combined with VFSA
to determine the optimal model makes our method robust, efficient and accurate.
|
Diciembre de
2008
Seismicity changes prior to the Ms8.0 Wenchuan earthquake in Sichuan,
China
Author: Qinghua Huang
Link: Click here
Abstract
The Region-Time-Length (RTL) method was
adopted in analyzing the characteristics of the seismicity changes prior to the
M s8.0 Wenchuan earthquake occurred on May 12, 2008. After the pre-analyses (completeness
analysis and aftershock declustering) of the earthquake catalog in China, the
RTL parameters in the investigated | region
were calculated. The temporal variations of the RTL parameters at the epicenter
of the M s8.0 Wenchuan earthquake showed that a seismic quiescence anomaly appeared
during 2006-2007. The close investigation indicated that the above quiescence
anomaly is unlikely an artifact due to the selections of the model parameters.
The investigation of the spatial distribution of seismic quiescence also showed
that the quiescence anomaly appeared around the epicenter of the mainshock during
2006-2007, consistent with the results obtained from the temporal variations of
the RTL parameters. The above characteristics of the seismicity changes may give
better understanding of the seismogenic process of the Wenchuan earthquake. |
Diciembre
de 2008
On the solitary wave paradigm for tsunamis
Authors: Per A. Madsen, David R. Fuhrman et al
Link: Click here
Abstract
Since the 1970s, solitary waves have
commonly been used to model tsunamis especially in experimental and mathematical
studies. Unfortunately, the link to geophysical scales is not well established,
and in this work, we question the geophysical relevance of this paradigm. In part
1, we simulate the evolution of initial rectangular-shaped humps of water propagating
large distances over a constant depth. The objective is to clarify under which
circumstances the front of the wave can develop into an undular bore with a leading
soliton. In this connection, we discuss and test various measures for the threshold
distance necessary for nonlinear and dispersive | effects
to manifest in a transient wave train. In part 2, we simulate the shoaling of
long smooth transient and periodic waves on a mild slope and conclude that these
waves are effectively non-dispersive. In this connection, we discuss the relevance
of finite amplitude solitary wave theory in laboratory studies of tsunamis. We
conclude that order-of-magnitude errors in effective temporal and spatial duration
occur when this theory is used as an approximation for long waves on a sloping
bottom. In part 3, we investigate the phenomenon of disintegration of long waves
into shorter waves, which has been observed, for example, in connection with the
Indian Ocean tsunami in 2004. This happens if the front of the tsunami becomes
sufficiently steep, and as a result, the front turns into an undular bore. We
discuss the importance of these very short waves in connection with breaking and
runup and conclude that they do not justify a solitary wave model for the bulk
tsunami. |
Diciembre
de 2008
Natural gas hydrate investigations by synchrotron radiation X-ray
cryo-tomographic microscopy (SRXCTM)
Authors: M. Mangir
Murshed, Stephan A. Klapp et al
Link: Click here
Abstract
We report the 3D microstructure analyses
of natural gas hydrates sampled from Gulf of Mexico. The samples were characterized
by synchrotron radiation X-ray cryo-tomographic microscopy | (SRXCTM)
using the 'TOMCAT' beam line at the Swiss Light Source (SLS). The SRXCTM demonstrates
its applicability to unlock some microscopic features of the marine hydrates,
in particular of crystallite size and grain boundary network. The gas hydrate
domains are surrounded by a network of pores of typically a few micrometers, which
are largely due to decomposition. Out of the SRXCTM data, the porosity, total
volume of the voids, the void surface area and number of the total gas-filled
voids have been calculated. The results reveal the capability of SRXCTM to access
the 3D microstructure which is of fundamental importance to model the petrophysical
properties of natural gas hydrates. |
Diciembre
de 2008
An analytical solution and case study of groundwater head response
to dual tide in an island leaky confined aquifer
Authors:
Pingping Sun, Hailong Li et al
Link: Click here
Abstract
This paper presents an analytical solution
of groundwater head response to dual-tide fluctuation in the transect of an island
leaky aquifer system comprising a confined aquifer and its overlying semipermeable
confining layer. Both | layers
terminate at the coastlines on two sides of the island. Solution analysis indicates
that the tidal waves from the two sides of the island transect interfere at the
middle of the island and the interference decreases to zero as the horizontal
length of the aquifer increases to infinity. The leakage of the overlying confining
layer enhances the landward attenuation of the tidal head fluctuation and shortens
the time lag between the head and tide fluctuations. The solution agreed well
with the observations in eight piezometers in Garden Island on the continental
shelf of Western Australia reported by Trefry and Bekele (2004). |
Diciembre
de 2008
Developing and applying uncertain global climate change projections
for regional water management planning
Authors: David
G. Groves, David Yates et al
Link: Click here
Abstract
Climate change may impact water resources
management conditions in difficult-to-predict ways. A key challenge for water
managers is how to incorporate highly uncertain information about potential climate
change from global models into local- and regional-scale water management models
and tools to support local planning. This paper presents a new method for developing | large
ensembles of local daily weather that reflect a wide range of plausible future
climate change scenarios while preserving many statistical properties of local
historical weather patterns. This method is demonstrated by evaluating the possible
impact of climate change on the Inland Empire Utilities Agency service area in
southern California. The analysis shows that climate change could impact the region,
increasing outdoor water demand by up to 10% by 2040, decreasing local water supply
by up to 40% by 2040, and decreasing sustainable groundwater yields by up to 15%
by 2040. The range of plausible climate projections suggests the need for the
region to augment its long-range water management plans to reduce its vulnerability
to climate change. |
Diciembre
de 2008
Electrical resistivity imaging of the architecture of substream
sediments
Authors: N. Crook, A. Binley et al
Link:
Click here
Abstract
The modeling of fluvial systems is constrained
by a lack of spatial information about the continuity and structure of streambed
sediments. There are few methods for noninvasive characterization of streambeds.
Invasive methods using wells and cores fail to provide detailed spatial information
on the prevailing architecture and its continuity. Geophysical techniques play
a pivotal role in providing spatial information on subsurface properties and processes
across many other environments, and we have applied the use of one of those techniques
to streambeds. We | demonstrate,
through two examples, how electrical resistivity imaging can be utilized for characterization
of subchannel architecture. In the first example, electrodes installed in riparian
boreholes and on the streambed are used for imaging, under the river bed, the
thickness and continuity of a highly permeable alluvial gravel layer overlying
chalk. In the second example, electrical resistivity images, determined from data
collected using electrodes installed on the river bed, provide a constrained estimate
of the sediment volume behind a log jam, vital to modeling biogeochemical exchange,
which had eluded measurement using conventional drilling methods owing to the
boulder content of the stream. The two examples show that noninvasive electrical
resistivity imaging is possible in complex stream environments and provides valuable
information about the subsurface architecture beneath the stream channels. |
Diciembre
de 2008
Paleointensity of the ancient Martian magnetic field
Authors: Benjamin P. Weiss, Luis E. Fong et al
Link: Click here
Abstract
Mars today has no core dynamo magnetic
field. However, the discovery of remanent magnetization in Martian meteorites
and intense crustal magnetization suggests that Mars once had a global field.
Here we present high resolution maps of the magnetic field of Martian meteorite
| ALH 84001. These maps are
the most sensitive yet quantitative study of natural remanent magnetization (with
resolved anomalies as weak as 1 × 10-14 Am2). ALH 84001 likely contains
a 4 billion year old (Ga) thermoremanence partially overprinted by one or more
poorly understood secondary components. Our data suggest that the paleointensity
of the local paleofield was within an order of magnitude of that of the present-day
Earth. If this field were global in extent, it should have played a key role in
Martian atmospheric and climatic evolution. However, it is still too weak to easily
explain the intensity of Martian crustal paleomagnetic anomalies. |
Diciembre
de 2008
Motion on upper-plate faults during subduction zone earthquakes:
Case of the Atacama Fault System, northern Chile
Authors:
J. P. Loveless and M. E. Pritchard
Link: Click here
Abstract
Motion on the Atacama Fault System (AFS)
in northern Chile is driven by Andean subduction zone processes. We use two approaches,
observational and theoretical, to evaluate how the AFS and other forearc faults
responded to coseismic stress induced by one well-studied megathrust earthquake,
the 1995 M w = 8.1 Antofagasta event. We use synthetic aperture radar interferometry
(InSAR) to search for small-scale coseismic and postseismic deformation on individual
faults. The InSAR data are ambiguous: some images show offset consistent with
coseismic faulting on the Paposo segment of the | AFS
and others lack such signal. The fact that we do not observe the fault-like displacement
in all coseismic interferograms suggests that atmospheric contamination, not tectonic
deformation, is responsible for the signal. To explore the capacity of the earthquake
to trigger motion on upper plate faults, we use seven published slip maps constrained
by geodetic and/or seismic data to calculate static and dynamic Coulomb stress
change (CSC) on faults in the Antofagasta region. The static CSC field varies
between models and depends on the distribution of coseismic interplate slip. On
the basis of the CSC distribution predicted by our preferred model constrained
by all available data, we suggest it was unlikely that the Antofagasta earthquake
directly triggered normal motion on the AFS, and the InSAR data are consistent
with this null result. Field reports of normal faulting related to the earthquake
may reflect recent (but not coseismic) motion or highly localized behavior not
representative of the regional coseismic stress field. |
Diciembre
de 2008
Low-frequency acoustic-gravity waves from coseismic vertical deformation
associated with the 2004 Sumatra-Andaman earthquake (Mw = 9.2)
Authors: Takeshi Mikumo, Takuo Shibutani et al
Link: Click here
Abstract
Atmospheric pressure perturbations from
the 2004 Sumatra-Andaman earthquake (M w = 9.2) were observed by sensitive microbarographs
at several global stations. Among these observations, very low-frequency acoustic-gravity
waves (~1.4-2.8 mHz) with a group velocity around 300-314 m/s and amplitudes ranging
between ~1 and 12 Pa can be clearly identified through data processing at four
stations on the Japanese Islands and also at four International Monitoring System
(IMS) stations around the Indian Ocean. Assuming several seismic source parameters
for | this great thrust earthquake,
we produce synthetic barograms using a realistic thermal structure in the atmosphere
up to an altitude of 220 km. For this modeling, we incorporate the source dimensions
in different zones, the expanding velocity of the source region, the vertical
displacements of uplift and subsidence, and their time constants. Combinations
of these source parameters provide synthetic waveforms consistent with the general
features of the observed low-frequency records. The results clearly indicate that
the recorded waves may have been generated by large-scale coseismic uplift and
subsidence of the sea bottom and associated swelling and depression of the sea
surface over the source region extending for 1500 km. The uplift in the south-central
zone of the Andaman-Nicobar regions may be substantially larger than in the other
zones. The time constant of the coseimic vertical deformation is found to be in
the range of 1.0-1.5 min, which may correspond to the time elapsed shortly before
the generation of tsunami waves. |
Diciembre
de 2008
A Bayesian approach to the real-time estimation of magnitude from
the early P and S wave displacement peaks
Authors:
M. Lancieri and A. Zollo
Link: Click here
Abstract
It has been shown that the initial portion
of P and S wave signals can provide information about the final earthquake magnitude
in a wide magnitude range. This observation opens the perspective for the real-time
determination of source parameters. In this paper we describe a probabilistic
evolutionary approach for the real-time magnitude estimation which can have a
potential use in earthquake early warning. The technique is based on empirical
prediction laws correlating the low-frequency peak ground displacement measured
in a few seconds after the P and/or S phase arrival and the final event magnitude.
The evidence for such a correlation has been found through the analysis of 256
shallow crustal events in the magnitude range M jma 4-7.1 located over the entire
Japanese archipelago. The peak displacement measured in a 2-s window from the
first P phase arrival correlates with magnitude in the range M = [4-6.5]. While
a possible saturation effect above M 6.5 is observed, it is less evident in | an
enlarged window of 4 s. The scaling of S peaks with magnitude is instead also
observed at smaller time lapses (i.e., 1 s) after the first S arrival. The different
scaling of P and S peaks with magnitude when measured in a 2-s window is explained
in terms of different imaged rupture surface by the early portion of the body
wave signals. We developed a technique to estimate the probability density function
(PDF) of magnitude, at each time step after the event origin. The predicted magnitude
value corresponds to the maximum of PDF, while its uncertainty is given by the
95% confidence bound. The method has been applied to the 2007 (M jma = 6.9) Noto
Hanto and 1995 (M jma = 7.3) Kobe earthquakes. The results of this study can be
summarized as follows: (1) The probabilistic algorithm founded on the predictive
model of peak displacement versus final magnitude is able to provide a fast and
robust estimation of the final magnitude. (2) The information available after
a few seconds from the first detection of the P phase at the network can be used
to predict the peak ground motion at a given regional target with uncertainties
which are comparable to those derived from the attenuation law. (3) The near-source
S phase data can be used jointly with P data for regional early warning purposes,
thus increasing the accuracy and reliability of magnitude estimation. |
Noviembre
de 2008
Ranking of several ground-motion models for seismic hazard analysis
in Iran
Author: H Ghasemi et al
Link: Click here
Abstract
In this study, six attenuation relationships
are classified with respect to the ranking scheme proposed by Scherbaum et al
(2004 Bull. Seismol. Soc. Am. 94 1-22). First, the strong motions recorded during
the 2002 Avaj, 2003 Bam, 2004 Kojour and 2006 Silakhor earthquakes are consistently
processed. Then the normalized residual sets are determined for each selected
ground-motion model, considering the strong-motion records chosen. The main advantage
of these records is that corresponding information about the causative fault plane
has been well studied for the selected events. Such information |
is used to estimate several control parameters which are essential inputs for
attenuation relations. The selected relations (Zare et al (1999 Soil Dyn. Earthq.
Eng. 18 101-23); Fukushima et al (2003 J. Earthq. Eng. 7 573-98); Sinaeian (2006
PhD Thesis International Institute of Earthquake Engineering and Seismology, Tehran,
Iran); Boore and Atkinson (2007 PEER, Report 2007/01); Campbell and Bozorgnia
(2007 PEER, Report 2007/02); and Chiou and Youngs (2006 PEER Interim Report for
USGS Review)) have been deemed suitable for predicting peak ground-motion amplitudes
in the Iranian plateau. Several graphical techniques and goodness-of-fit measures
are also applied for statistical distribution analysis of the normalized residual
sets. Such analysis reveals ground-motion models, developed using Iranian strong-motion
records as the most appropriate ones in the Iranian context. The results of the
present study are applicable in seismic hazard assessment projects in Iran. |
Noviembre
de 2008
A model for flash weakening by asperity melting during high-speed
earthquake slip
Authors: A. W. Rempel & S. L. Weaver
Link: Click here
Abstract
Recent results from laboratory experiments
on a broad range of mineral systems exhibit dramatic drops in the effective friction
coefficient µ once the slip rate exceeds a critical level V w , which is
typically O(0.1) m/s. This "flash weakening" has been attributed to
the effects of localized heating at highly stressed microscopic asperities. We
extend previous phenomenological treatments to assess whether melting at asperity
contacts can explain the observed changes in strength. Using physical parameters
obtained from the literature on the phase behavior and mechanical properties of
|
quartz, albite, dolomite, gabbro, Westerly granite, and serpentinite, the predictions
of our simplified model are in reasonable agreement with available experimental
data. We derive approximate analytical expressions that suggest that strength
changes are insensitive to the melt viscosity under conditions that likely include
those during earthquake slip along major fault systems. Instead, the primary controls
on µ are the ratio of slip rate V to V w and the Stefan number S, which
is defined as the ratio of the latent heat of fusion to the sensible heat required
to raise the temperature from ambient levels. The phase behavior during the short
lifetimes and at the high confining pressures of asperity contacts is a significant
source of uncertainty in the parameter choices, as are the presence and availability
of water. Nevertheless, our results are encouraging for further efforts to incorporate
the microphysics of fault zone processes into earthquake simulations. |
Noviembre
de 2008
Reduction of electron temperature in low-latitude ionosphere at
600 km before and after large earthquakes
Authors:
Koh-Ichiro Oyama, Yoshihiro Kakinami et al
Link: Click here
Abstract
We examine ionospheric electron temperatures
(Te) observed by HINOTORI satellite during three earthquakes; M6.6 occurred in
November 1981, M7.4 and M6.6 in January 1982 over Philippine, respectively. It
is found that Te around the epicenters significantly decreases in the afternoon
periods within 5 days before and after the three earthquakes. The region of ionosphere
| disturbance extends to 80-120
degrees in longitude. A tendency exists that duration of the disturbance becomes
longer as the increase of earthquake magnitude. F2 peak frequency, foF2 and virtual
height, h'F from a chain of 4 ionosonde stations located in the longitude zone
of 120°E-130°E are used together with electron density(Ne), that is observed
simultaneously onboard HINOTORI satellite to find possible cause mechanisms of
the abnormal reduction of electron temperatures. Behavior of HINOTORI Te/Ne and
ionosonde foF2/h'F implies the existence of westward electric field over epicentre.
Our finding suggests that simple two plasma instruments might be able to play
a fundamental role to study ionosphere disturbance associated with earthquake,
if the constellation of small/mini satellites is organized and the orbits are
properly chosen. |
Noviembre
de 2008
Deformation, seismicity, and fluids: Results of the 2004/2005 water
injection experiment at the KTB/Germany
Authors: T.
Jahr, G. Jentzsch et al
Link: Click here
Abstract
During the past decades, the research
into fluid-controlled geodynamic processes in the upper crust of the Earth is
put foreward regarding the stress accumulation, deformation, and seismicity. In
a large-scale injection experiment at the deep borehole site KTB (Kontinentale
Tiefbohrung der Bundesrepublik Deutschland) in Germany, more than 84.000 m3 fresh
water was injected in 4000 m depth over 10 months, and a lot of geoscientific
investigations were connected to this injection test. The pore-pressure change
of more than 10 MPa yielded an induced deformation, which was | detected
by a tiltmeter array. The used five borehole tiltmeters of the ASKANIA type with
a resolution of better than 1 nrad belong to the most sensitive tiltmeters worldwide.
The poro-elastic finite-element modeling used for the interpretation of the observed
tilts revealed an uplift of 3.1 mm above the injection point. Furthermore, it
was shown that the induced stress and deformation fields depend mainly on the
fluids inside the fault zones, particularly the SE2-reflector which is the Franconian
fault zone, and the local Nottersdorf fault: While the maximum pore pressure is
concentrated close to the injection point at the SE2 zone, the maximum deformation
of about 3 cm is located at the intersection line of both faults in 4 km depth.
The area of the modeled maximum deformation is highly correlated with the region
of the observed induced seismicity, thus linking seismicity and deformation. The
connection of tilt observation and modeling provides a useful tool for the investigation
of fluid coupled geoprocesses. |
Noviembre
de 2008
Biomorphodynamics: Physical-biological feedbacks that shape landscapes
Authors: A. B. Murray, M. A. F. Knaapen et al
Link: Click
here
Abstract
Plants and animals affect morphological
evolution in many environments. The term "ecogeomorphology" describes
studies that address such effects. In this opinion article we use the term "biomorphodynamics"
to characterize a subset of ecogeomorphologic studies: those that investigate
not only the effects of organisms on physical processes and morphology but also
how the biological processes depend on morphology and physical forcing. The two-way
coupling precipitates feedbacks, leading to interesting modes of behavior, much
like the coupling between flow/sediment transport and morphology leads to rich
morphodynamic behaviors. Select examples illustrate how even the basic aspects
of some systems cannot be understood without considering biomorphodynamic coupling.
Prominent examples include the dynamic interactions | between
vegetation and flow/sediment transport that can determine river channel patterns
and the multifaceted biomorphodynamic feedbacks shaping tidal marshes and channel
networks. These examples suggest that the effects of morphology and physical processes
on biology tend to operate over the timescale of the evolution of the morphological
pattern. Thus, in field studies, which represent a snapshot in the pattern evolution,
these effects are often not as obvious as the effects of biology on physical processes.
However, numerical modeling indicates that the influences on biology from physical
processes can play a key role in shaping landscapes and that even local and temporary
vegetation disturbances can steer large-scale, long-term landscape evolution.
The prevalence of biomorphodynamic research is burgeoning in recent years, driven
by societal need and a confluence of complex systems-inspired modeling approaches
in ecology and geomorphology. To make fundamental progress in understanding the
dynamics of many landscapes, our community needs to increasingly learn to look
for two-way, biomorphodynamic feedbacks and to collect new types of data to support
the modeling of such emergent interactions. |
Noviembre
de 2008
Groundwater flow through anisotropic fault zones in multiaquifer
systems
Authors: E. I. Anderson and M. Bakker
Link:
Click here
Abstract
Vertical faults through the shallow crust
are commonly believed to act as either barriers to horizontal groundwater flow
normal to the fault, conduits to horizontal flow tangential to the fault, or a
combination of both. In addition, enhanced vertical permeability has been identified
as a common feature. We investigate the effects of vertical anisotropy of a fault
zone on the distribution of hydraulic head within the fault, using an analytic
solution. We conclude that anisotropy ratios greater than 100 result in nearly
|
hydrostatic conditions within the fault zone, despite the existence of significant
vertical flow rates. Under these conditions, the Dupuit approximation is adequate
for predicting the flow from one side of the fault to the other. We then present
explicit analytical solutions to problems of steady groundwater flow in a multiaquifer
system cut by a single vertical fault. The fault is linear and of negligible width,
is infinite in length, and acts as a conduit for vertical fluid flow. The fault
may act as a leaky barrier to horizontal flow normal to the fault, as a conduit
to horizontal flow tangential to the fault, or a combination of both. Examples
are presented that highlight the effects of enhanced vertical permeability of
a fault on aquifer interaction in a multiaquifer system. Particle tracking is
used to investigate the effects of the fault on pathlines. |
Noviembre
de 2008
Testing probabilistic seismic hazard estimates by comparison with
observations: an example in Italy
Authors: Dario Albarello
and Vera D'Amico
Link: Click here
Abstract
An evaluation of the actual reliability
of probabilistic seismic hazard (PSH) assessments, provided by existing numerical
techniques, is mandatory to orientate new researches and improvements. Two procedures
devoted to this task are proposed, which are based on the comparison of the hazard
estimates with empirical observations (e.g. strong-motion data). | These
procedures have been applied to the estimates provided by the methodology adopted
for most recent seismic hazard evaluations in Italy. The analysis shows that a
significant mismatch exists between peak ground acceleration values characterized
by an exceedence probability of 10 per cent in 30 yr and what has actually been
observed at 68 accelerometric stations located on stiff soil, where continuous
seismicity monitoring has been performed in the last 30 yr. Although this finding
should be considered with caution, it suggests that a future re-examination of
the adopted PSH computational model could be useful to avoid possible underestimates
of seismic hazard in Italy. |
|
Noviembre
de 2008
Crosswell seismic imaging for deep gas reservoir characterization
Authors: Gang Yu, Bruce Marion et al
Link: Click here
Abstract
A gas discovery in the Shengping area
of the Daqing Oilfield in China was made recently in a large-scale volcanic depositional
environment. Because gas in the heterogeneities of formations broken by tectonic
activity and localized volcanic eruptions is not common, researchers sought a
more detailed reservoir characterization before developing the field. Crosswell
seismic data were used to augment existing 3D surface seismic, log, and core data.
This provided data at five times the resolution of the surface seismic data to
bridge | the
gap in resolution between surface seismic and well data. Crosswell seismic data
were acquired in two wells, 832 m apart, and processed to provide images of reflectivity,
velocity, and formation properties from sections produced by amplitude-versus-angle
(AVA) inversion. The state of the art in crosswell seismic is summarized briefly,
reviewing progress in data acquisition and data processing over several decades
of crosswell technology development. A detailed description of the data acquisition
and data processing applied to the data from the Shengping area is also given.
An integrated interpretation of the crosswell images with the surface seismic
and log data was used to produce a more detailed geologic model. The enhanced
geologic model is being used to plan strategic development of the reservoir and
to evaluate possible infill well locations. |
Noviembre
de 2008
Adaptive finite element modelling of two-dimensional magnetotelluric
fields in general anisotropic media
Author: Yuguo Li
and Josef Pek
Link: Click here
Abstract
An adaptive unstructured mesh finite
element (FE) procedure is presented for improving the quality of numerical solutions
to the magnetotelluric forward problem in a general 2-D anisotropic conductivity
structure. We implement a self-adaptive, goal-oriented grid refinement algorithm
in which a finite element analysis is performed on a sequence of refined meshes.
The mesh refinement process is guided by a dual error estimate weighting to bias
refinement towards elements that affect the solution at the EM receiver locations.
We validate |
the finite element code against a layered 1-D model with a sea water layer. Further,
we compare the FE results with those obtained by a finite-difference (FD) scheme
for both a block seamountain and a sea bottom hill model. Both FE and FD schemes
show very good agreement for the block seamountain model. For the sea bottom hill
model, however, only on the flat seafloor segments both the FE and FD solutions
fit very well, but on the seafloor slope, FD results are oscillating due to a
simplistic staircase approximation of the bathymetric undulations. The FD scheme
for 2-D anisotropic conductors, developed primarily for the modelling of magnetotelluric
data on a flat Earth surface, is thus not an adequate tool for dealing with structures
with sloping bathymetry and topography, whereas the FE method with adapting mesh
can easily handle such structures at almost any level of complexity. |
Noviembre
de 2008
Defining the translational velocity of the reference frame of Earth
Author: Donald F. Argus
Link: Click here
Abstract
Earth's centre is fundamental to geodesy
and geoscience because motions of sites on the surface are estimated relative
to it. International Terrestrial Reference Frames ITRF2000 and ITRF2005 are defined
by the centre of mass of Earth's system (CM), consisting of solid Earth, the ice
sheets, the oceans, and the atmosphere. Satellite LAGEOS rotates about CM; satellite
laser ranging (SLR) is used to estimate the velocity of CM relative to sites on
the surface. However, ITRF2000 and ITRF2005 differ by 1.8 mm yr-1, suggesting
that the velocity of CM is constrained poorly by SLR.
In this study, we define
Earth's reference frame with the centre of mass of solid Earth (CE). Site velocities
estimated using SLR, VLBI, GPS and DORIS are corrected for a postglacial rebound
model and inverted for the rotational velocities of the plates and the rotational
and translational velocities of the four space techniques. Because the postglacial
rebound predictions are relative to | CE,
the velocity of CE relative to sites on the surface is estimated. Because the
input SLR site velocities are relative to CM, the output SLR translational velocity
is the velocity of CM relative to CE.
The estimated velocity of CE does not
depend strongly on the postglacial rebound model corrected for. Equal within uncertainties
and having a root mean square of 0.5 mm yr-1 are estimates of the velocity of
CE determined assuming that plate interiors are deforming radially as predicted
by three postglacial rebound models and an estimate of the velocity of CE determined
assuming that parts of plate interiors neither beneath nor along the margins of
the late Pleistocene ice sheets are not deforming laterally.
The velocity of
CE equals within uncertainties (probability greater than 5 per cent) the velocity
of CM in ITRF2000. The velocity of CE differs significantly (0.05 per cent probability)
from the velocity of CM in ITRF2005. Earth's reference frame (and, we believe,
ITRF's) should be defined with the tightly constrained velocity of CE, not with
the poorly constrained velocity of CM. Because CE is believed to be moving relative
to CM no faster than 0.5 mm yr-1, the velocity of CE estimated in this study is
likely to be nearer the true velocity of CM than is the velocity of CM estimated
using SLR. |
Noviembre
de 2008
Long-period earthquakes and co-eruptive dome inflation seen with
particle image velocimetry
Authors: Jeffrey B. Johnson,
Jonathan M. Lees et al
Link: Click here
Abstract
Dome growth and explosive degassing are
fundamental processes in the cycle of continental arc volcanism. Because both
processes generate seismic energy, geophysical field studies of volcanic processes
are often grounded in the interpretation of volcanic earthquakes. Although previous
seismic studies have provided important constraints on volcano dynamics, such
inversion results do not uniquely constrain magma source dimension and material
properties. Here we report combined optical geodetic and seismic observations
that robustly constrain the sources of long-period volcanic earthquakes coincident
with | frequent explosive eruptions
at the volcano Santiaguito, in Guatemala. The acceleration of dome deformation,
extracted from high-resolution optical image processing, is shown to be associated
with recorded long-period seismic sources and the frequency content of seismic
signals measured across a broadband network. These earthquake sources are observed
as abrupt subvertical surface displacements of the dome, in which 20-50-cm uplift
originates at the central vent and propagates at 50 m s-1 towards the 200-m-diameter
periphery. Episodic shifts of the 20-80-m thick dome induce peak forces greater
than 109 N and reflect surface manifestations of the volcanic long-period earthquakes,
a broad class of volcano seismic activity that is poorly understood and observed
at many volcanic centres worldwide. On the basis of these observations, the abrupt
mass shift of solidified domes, conduit magma or magma pads may play a part in
generating long-period earthquakes at silicic volcanic systems. |
Noviembre
de 2008
Broadband simulations for Mw 7.8 southern San Andreas earthquakes:
Ground motion sensitivity to rupture speed
Authors:
Robert W. Graves, Brad T. Aagaard et al
Link: Click here
Abstract
Using the high-performance computing
resources of the Southern California Earthquake Center, we simulate broadband
(0-10 Hz) ground motions for three Mw 7.8 rupture scenarios of the southern San
Andreas fault. The scenarios incorporate a kinematic rupture description with
the average | rupture
speed along the large slip portions of the fault set at 0.96, 0.89, and 0.84 times
the local shear wave velocity. Consistent with previous simulations, a southern
hypocenter efficiently channels energy into the Los Angeles region along the string
of basins south of the San Gabriel Mountains. However, we find the basin ground
motion levels are quite sensitive to the prescribed rupture speed, with peak ground
velocities at some sites varying by over a factor of two for variations in average
rupture speed of about 15%. These results have important implications for estimating
seismic hazards in Southern California and emphasize the need for improved understanding
of earthquake rupture processes. |
Noviembre
de 2008
Amplitude-frequency dependencies of wave attenuation in single-crystal
quartz: Experimental study
Author: E. I. Mashinskii
Link: Click here
Abstract
The experiments have been conducted to
investigate the effect of strain amplitude and frequency on the compressional
and shear wave attenuation in quartz samples of three types: the intact quartz,
fractured quartz, and smoky quartz. The measurements were performed using the
reflection method on a pulse frequency of 1 MHz with changing strain in the range
0.3 = = 2.0 µstrain under a confining pressure of 10 MPa and at ambient
temperature. The essential difference in amplitude-frequency characteristics of
wave attenuation in three quartz types has been detected. The intact quartz shows
the more "simple" behavior in comparison with the fractured | and
smoky quartz. The attenuation (the inverse quality factor Q) depends on strain
amplitude as Q -1( ) ~ -n , where n 0.005-0.085, with the greatest decrease in
the smoky and fractured quartz reaching of about 15%. Relaxation spectra of attenuation
are presented in the frequency range from 0.4 to 1.4 MHz. The dependence Q p -1(f)
~ f -1.2 characterizes the intact and fractured quartz, whereas the smoky quartz
has the relaxation peak. The dependence Q s -1(f) ~ f -0.84 presents S wave relaxation
spectrum in the intact quartz; in the fractured and smoky quartz, the attenuation
peaks take place. The strain amplitude variation exerts influence on the relaxation
strength, the peak frequency, and the width of the relaxation peak. Such behavior
of attenuation can be explained by a joint action of viscoelastic and microplastic
mechanisms. These results can be considered as a contribution for providing the
experimental background to the theory of attenuation in rocks. They can also be
used in solving applied problems in material science, seismic prospecting, etc.
|
Noviembre
de 2008
Natural electromagnetic ULF noise due to fluctuations of ionospheric
currents
Authors: V. V. Surkov and M. Hayakawa
Link: Click here Abstract
An
origin of natural electromagnetic noise observed on the ground surface in the
frequency range 10-4-10-2 Hz was examined. Following a recent paper by Surkov
and Hayakawa (2007), a flicker noise or 1/f noise, provided by random currents,
is treated as a possible source of the ULF electromagnetic noise. In contrast
to the cited paper, MHD wave incident on the ionosphere and neutral gas flow in
the altitude range of conducting E layer of the ionosphere are considered to be
a candidate mechanism for random current | fluctuations,
which in turn produce random electromagnetic fluctuations in the atmosphere. The
main emphasis is on the flicker noise provided by the wind-driven ionospheric
currents, which is assumed to be steady, uniform, and isotropic random fields
inside the ionosphere. A correlation radius of random ionospheric fields is supposed
to be controlled by neutral gas transfer and by acoustic/gravity wave propagation
inside the E layer. A correlation matrix and power spectra of the random electromagnetic
fields on the ground surface were calculated. The predicted spectral index of
the power spectrum of the ULF magnetic noise was found to be 3, which is consistent
with ground-based observations. The experimental data were demonstrated to be
sandwiched between two theoretical lines, which correspond to daytime and nighttime
ionospheric parameters. |