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Enero de 2007
Topography and self-gravitation interaction in elastic-gravitational
modeling
Authors: M. Charco, F. Luzón et al
Link: Click here
Abstract
Changes in gravity due to volcanic loading of the crust are
influenced by topography. We investigate the relative importance
of topography and self-gravitation in the interpretation of
gravity changes. It is shown that modeling of gravity changes
can be more precise with the introduction of topographic relief,
although it is neglected self-
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gravitation
of the medium. This paper exploits this result by suggesting a
mathematical simplification that could be useful in the future
development of a numerical technique to accurately include topographic
effects in the modeling of deformation and gravity changes. Finally,
we perform an inversion of the gravity changes observed at Mayon
volcano (Philippines) between December 1992 and December 1996
including topographic effects by varying the depth of the source.
Failure to account for topographic influences can bias estimates
of source parameters particularly when the lateral extension of
the relief is of the same order of magnitude as the source depth.
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Enero
de 2007
Mw 7.8 Tarapaca intermediate depth earthquake of 13 June 2005
(northern Chile): Fault plane identification and slip distribution
by waveform inversion
Authors: Bertrand Delouis and Denis Legrand
Link: Click here
Abstract
The Mw 7.8 Tarapaca earthquake of 13 June 2005 occurred between
90 and 115 km depth within the subducting slab of northern Chile.
The fault plane could be identified using two independent
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approaches:
by locating accurately one week of aftershocks and by modeling
the strong-motion waveforms. The rupture occurred on a low-angle
(15°) west-dipping plane in pure normal faulting. The slip
distribution obtained by the joint inversion of strong-motion
and teleseismic data indicates a compact slip zone of dimension
50 × 40 km2 with a maximum slip of 13 m. The aftershock
distribution slightly extends beyond the slip zone dimension in
the NS direction but slip extends slightly more towards the west
than do the aftershocks. The Tarapaca earthquake can be classified
as an intraslab down-dip tensional event, in the context of a
mature seismic gap. |
Enero
de 2007
Ferromagnetic resonance spectroscopy for assessment of magnetic
anisotropy and magnetostatic interactions: A case study of mutant
magnetotactic bacteria
Authors: Robert E. Kopp, Cody Z. Nash et al
Link: Click here
Abstract
Ferromagnetic resonance spectroscopy (FMR) can be used to measure
the effective magnetic field within a sample, including the
contributions of both magnetic anisotropy and magnetostatic
interactions. One particular use is in the detection of magnetite
produced by magnetotactic bacteria. These bacteria produce single-domain
particles with narrow size and shape distributions that are
often elongated and generally arranged in chains. All of these
features are detectable through FMR.
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Here,
we examine their effects on the FMR spectra of magnetotactic bacteria
strains MV-1 (which produces chains of elongate magnetite crystals),
AMB-1 (which produces chains of nearly equidimensional magnetite
crystals), and two novel mutants of AMB-1: mnm13 (which produces
isolated, elongate crystals) and mnm18 (which produces nearly
equidimensional crystals that are usually isolated). Comparison
of their FMR spectra indicates that the positive magnetic anisotropy
indicated by the spectra of almost all magnetotactic bacteria
is a product of chain alignment and particle elongation. We also
find correlations between FMR properties and magnetic measurements
of coercivity and magnetostatic interactions. FMR thus provides
a rapid method for assessing the magnetic properties of assemblages
of particles, with applications including screening for samples
likely to contain bacterial magnetofossils. |
Diciembre
de 2006
An application of Bayesian analysis and Markov chain Monte
Carlo methods to the estimation of a regional trend in annual
maxima
Authors: B. Renard, V. Garreta et al
Link: Click here
Abstract
Bayesian analysis is becoming increasingly popular in a number
of fields, including hydrology. It appears to be a convenient
framework for deriving complex models in agreement with both
physical reality and statistical requirements. The aim of this
paper is to present an application to the regional frequency
analysis of extremes in a nonstationary context. A nonstationary
regional
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model
is thus proposed, together with the related hypotheses. The Bayesian
inference of this model is then described. Markov chain Monte
Carlo (MCMC) methods are needed for this purpose because of the
dimensionality of the model and are described in this paper. The
usefulness of such a model is then illustrated on a hydrological
case study concerning annual maximum discharges of several sites.
The advantage of regional analysis compared to at-site estimation
is thus highlighted. Moreover, the Bayesian framework allows for
a direct and comprehensive inference based on the posterior distribution
and is able to take into account modeling uncertainties, which
is particularly useful when the stationarity of a series can neither
be ensured nor be totally rejected. |
Diciembre
de 2006
Injection mechanism of clay-rich sediments into dikes during
earthquakes
Authors: Tsafrir Levi, Ram Weinberger et al
Link: Click here
Abstract
Clastic dikes may form by simultaneous fracture propagation
in rocks and injection of clastic material into the fractures
resulting from strong seismic shaking. We studied the mechanisms
of clastic-dike formation within the seismically active Dead
Sea basin, where hundreds of clastic dikes cross-cut the soft
rock of the late Pleistocene lacustrine Lisan Formation. We
analyzed the anisotropy of magnetic susceptibility (AMS) of
dikes with known formation mechanisms and defined the characteristic
AMS signatures, mainly of dikes developed by injection process.
Most of the dikes were emplaced due to fluidization of clay-rich
sediment and are characterized by triaxial
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AMS
ellipsoids. The dominant triaxial AMS ellipsoids along the dike
widths suggest that the fluidization mechanism of clay-rich sediment
is different from the known liquefaction process of sand. The
AMS analysis supported by field evidence indicates that the injection
of clay-rich sediment is characterized by two main regimes: (1)
Vertical flow characterized by subvertical V2 axes and subhorizontal
V1 and V3 axes. The V2 axes may indicate the flow directions during
fast flow. (2) Horizontal slow flow characterized by subvertical
V3 axes and subhorizontal V1 and V2 axes. A streaked AMS pattern
mainly composed of V2 and V3 axes represents a turbulent flow
that generated local eddies simultaneously with the clastic transport.
The AMS parameters along the dikes and possible grain imbrications
along dike walls support organization of grains under high strain
rates. This application of the AMS method provides a petrofabric
tool for identifying seismites and inferring their flow kinematics
in complex geologic areas. |
Diciembre
de 2006
Curie temperatures of synthetic titanomagnetites in the Fe-Ti-O
system: Effects of composition, crystal chemistry, and thermomagnetic
methods
Authors: Dominique Lattard, Ralf Engelmann et al
Link: Click here
Abstract
The present study is aimed at improving the calibration of the
compositional dependence of the Curie temperature (T C ) of
titanomagnetite (Tmt) on the basis of temperature-dependent
magnetic susceptibility (?-T) curves measured on synthetic Tmts
in the Fe-Ti-O system. In order to assess the possible influence
of high-temperature cation vacancies onto the T C values, we
have synthesized two types of assemblages in subsolidus conditions
at 1 bar, 1100°C and 1300°C, under controlled oxygen
fugacity conditions. Tmts synthesized in equilibrium with ilmenite-hematitess
(Ilmss) are expected to have
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the
highest vacancy concentrations, those in equilibrium with wüstite
(Wus) the lowest. The composition and homogeneity of the synthetic
Tmts were carefully checked with a scanning electron microscope
(SEM) and an electron microprobe (EMP). T C was determined from
Xi-T curves using a kappabridge and, for comparison, from M s
-T curves measured with a variable field translation balance.
Our data set shows systematically higher T C values for Tmt coexisting
with Ilmss than for Tmt coexisting with Wus. Most Xi-T curves
are nonreversible, whereby the largest Xi T C (40 K) concern Tmt(+Ilmss)
of intermediate compositions synthesized at 1300°C. Nonreversibility
is interpreted as reflecting cation reordering in Tmt during the
high-temperature Xi-T measurements. T C values obtained from M
s -T curves are higher than those obtained from the Xi-T curves,
whereby the difference regularly increases (up to 40 K) with increasing
Ti content, up to X Usp = 0.6. Our new calibration curves are
suitable to retrieve Tmt compositions in basalts that were rapidly
cooled and not oxidized by deuteric or hydrothermal fluids. |
Diciembre
de 2006
M = 7.0 earthquake recurrence on the San Andreas fault from
a stress renewal model
Author: Tom Parsons
Link: Click here
Abstract
Forecasting M = 7.0 San Andreas fault earthquakes requires an
assessment of their expected frequency. I used a three-dimensional
finite element model of California to calculate volumetric static
stress drops from scenario M = 7.0 earthquakes on three San
Andreas fault sections. The ratio of stress drop to tectonic
stressing rate derived from geodetic displacements yielded recovery
times at points
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throughout
the model volume. Under a renewal model, stress recovery times
on ruptured fault planes can be a proxy for earthquake recurrence.
I show curves of magnitude versus stress recovery time for three
San Andreas fault sections. When stress recovery times were converted
to expected M = 7.0 earthquake frequencies, they fit Gutenberg-Richter
relationships well matched to observed regional rates of M = 6.0
earthquakes. Thus a stress-balanced model permits large earthquake
Gutenberg-Richter behavior on an individual fault segment, though
it does not require it. Modeled slip magnitudes and their expected
frequencies were consistent with those observed at the Wrightwood
paleoseismic site if strict time predictability does not apply
to the San Andreas fault. |
Diciembre
de 2006
Schumann resonances: Interpretation of local diurnal intensity
modulations
Authors: O. Pechony and C. Price
Link: Click here
Abstract
A technique suggested by Sentman and Fraser (1991) for separating
the global and the local contributions to the observed Schumann
resonance (SR) field power variations is applied to simulated
SR field power calculated in a uniform cavity. The spatial/temporal
distribution of
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lighting
strokes included in the computations is based on satellite lightning
data. It is shown that the local intensity modulation function
calculated with a uniform model is similar to the function obtained
by Sentman and Fraser with experimental data. Since the local
modulation function computed from a uniform model simulation cannot
be induced by ionosphere day-night asymmetry, the local modulation
function cannot represent diurnal ionosphere height variation.
It is shown that the local modulation function depends primarily
on the source-receiver distance geometry and that Sentman and
Fraser have developed an efficient technique of removing source-receiver
distance effects from SR records. |
Diciembre
de 2006
Magnetic properties of atmospheric particulate matter from
automatic air sampler stations in Latium (Italy): Toward a definition
of magnetic fingerprints for natural and anthropogenic PM10 sources
Authors: Leonardo Sagnotti, Patrizia Macrì et al
Link: Click here
Abstract
Environmental problems linked to the concentration of atmospheric
particulate matter with dimensions less than 10 µm (PM10)
in urban settings have stimulated a variety of scientific researches.
This study reports a systematic analysis of the magnetic properties
of PM10 samples collected by six automatic stations installed
for air quality monitoring through the Latium Region (Italy).
We measured the low-field magnetic susceptibility of daily air
filters collected during the period July 2004 to July 2005.
For each station, we derived an empirical linear correlation
linking magnetic susceptibility to the concentration
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of
PM10 produced by local sources (i.e., in absence of significant
inputs of exogenous dust). An experimental approach is suggested
for estimating the percentage of nonmagnetic PM10 transported
from natural far-sided sources (i.e., dust from North Africa and
marine aerosols). Moreover, we carried out a variety of additional
magnetic measurements to investigate the magnetic mineralogy of
selected air filters spanning representative periods. The results
indicate that the magnetic fraction of PM10 is composed by a mixture
of low-coercivity, magnetite-like, ferrimagnetic particles with
a wide spectrum of grain sizes, related to a variety of natural
and anthropogenic sources. The natural component of PM10 has a
characteristic magnetic signature that is indistinguishable from
that of eolian dust. The anthropogenic PM10 fraction is mostly
originated from circulating vehicles and is a mixture of prevailing
fine superparamagnetic particles and subordinate large multidomain
grains; the former are more directly related to exhaust, whereas
the latter may be associated to abrasion of metallic parts. |
Diciembre
de 2006
Characterization of hematite (a-Fe2O3), goethite (a-FeOOH),
greigite (Fe3S4), and pyrrhotite (Fe7S8) using first-order reversal
curve diagrams
Authors: Andrew P. Roberts, Qingsong Liu et al
Link: Click here
Abstract
First-order reversal curve (FORC) diagrams have become a standard
tool in rock magnetism, yet magnetite is the only magnetic mineral
that is well characterized using FORC diagrams. We present FORC
diagrams for predominantly single-domain (SD) synthetic aluminous
hematite (a-Fe2-xAlxO3) and goethite (a-(FeAl)OOH) and natural
greigite (Fe3S4) and pyrrhotite (Fe7S8) to constrain interpretation
of FORC diagrams from natural samples. Hematite and goethite
have low spontaneous magnetizations and negligible magnetic
interactions, while greigite and pyrrhotite have higher spontaneous
magnetizations and can have strong magnetic interactions. The
coercivity
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of
hematite systematically increases with Al content only for samples
produced using the same synthesis method, but it is variable for
samples produced with different methods even for similar Al content.
This precludes use of magnetic coercivity alone to quantify the
Al content of natural hematites. Goethite has much higher coercivity
than hematite for all measured samples. SD and superparamagnetic
(SP) behavior is common in natural greigite samples, with peak
coercivities ranging from ~70 mT (SD) to zero (SP). This range
overlaps with that of lower-coercivity minerals, which can make
greigite identification ambiguous at room temperature. Fine-grained
SD pyrrhotite has slightly higher coercivities than greigite,
which progressively decreases with increasing grain size within
the SD size range and overlaps the range for greigite. While FORC
diagrams are useful for magnetic characterization, care is needed
in interpretation because of overlaps in the broad range of magnetic
properties, which result from variations in domain state, for
any magnetic mineral with respect to other minerals. |
Diciembre
de 2006
Nonstationary flow and nonergodic transport in random porous
media
Authors: G. Darvini, P. Salandin et al
Link: Click here
Abstract
Transport processes taking place in natural formations are often
characterized by the spatial nonstationarity of flow field and
the nonergodic condition of solute spreading. The former may
originate from statistical inhomogeneity of porous media properties
or from the influence of boundary conditions as well as from
conditioning procedures on measured data, while the latter is
generally due to the finite size of the solute source. In real-world
applications these aspects sometimes dominate the transport
phenomena, and known literature results based on statistical
homogeneity and ergodicity assumptions cannot be properly applied.
In this paper a method is proposed to handle different and concurrent
causes of the flow field nonstationarity and to describe nonergodic
transport of inert solutes by spatial moments in a domain of
finite size. The
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goal
is reached by expanding the steady state flow equation in Taylor
series limited to the first-order and by the recursive application
of finite element (FE) method. The unknowns are the piezometric
head mean values and its derivatives in respect to the fluctuating
porous media hydraulic conductivity. From the latter the velocity
statistics are derived on the basis of Darcy's law according to
the first-order approximation. By neglecting the pore-scale dispersion
the spatial moments of a solute plume are a posteriori obtained
by a consistent Lagrangian analysis starting from the knowledge
of the velocity field covariance matrices without any restriction
regarding the statistical homogeneity of flow and/or ergodicity
conditions. The proposed method is here applied to investigate
the influence of boundary conditions on the solute transport developing
in a limited domain. The results obtained from different test
cases give a velocity field behavior in agreement with literature
findings and allow us to define the nonergodic transport in bounded
domains. The comparison of the results with some ad hoc developed
Monte Carlo simulations ensures the applicability of the proposed
approach at least to the small heterogeneity cases here considered.
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Diciembre
de 2006
On determination of the Curie point from thermomagnetic curves
Authors: E. Petrovský And A. Kapicka
Link: Click here
Abstract
In many rock magnetic studies, information on magnetic mineralogy
is of crucial importance. Besides standard analytical methods,
such as X-ray spectroscopy, more sensitive thermomagnetic analyses
are often used. Temperature dependence of magnetic parameters
can serve as basis for determination of magnetic second-order
phase transition temperatures. Although limited by several drawbacks,
the most serious being thermally induced transformations of
the original minerals, this method provides useful information
not only about the presence of magnetic minerals, but also additional
knowledge
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on,
e.g., the prevailing grain size distribution or degree of substitution.
In thermomagnetic analysis, temperature dependence of two parameters,
induced magnetization and magnetic susceptibility, is mostly used.
However, let us say because of historical reasons, the same approach
for the Curie point determination has been often used in analyzing
the two parameters. In our contribution, we discuss the physical
principles of the two parameters, showing that the methods developed
and used for induced magnetization cannot be used also for temperature
dependence of magnetic susceptibility because there is no physical
justification to do so. Otherwise, the error in determining the
Curie point can be some few degrees but can reach also several
tens of degrees. Such an error has serious consequences for further
interpretation of the data, e.g., in terms of degree of Ti substitution
in Ti magnetite. |
Diciembre
de 2006
Improved hydrogeophysical characterization using joint inversion
of cross-hole electrical resistance and ground-penetrating radar
traveltime data
Authors: Niklas Linde , Andrew Binley et al
Link: Click here
Abstract
Appropriate regularizations of geophysical inverse problems
and joint inversion of different data types improve geophysical
models and increase their usefulness in hydrogeological studies.
We have developed an efficient method to calculate stochastic
regularization operators for given geostatistical models. The
method, which combines circulant embedding and the diagonalization
theorem of circulant matrices, is applicable for stationary
geostatistical models when the grid discretization, in each
spatial direction, is uniform in the volume of interest. We
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also
used a structural approach to jointly invert cross-hole electrical
resistance and ground-penetrating radar traveltime data in three
dimensions. The two models are coupled by assuming, at all points,
that the cross product of the gradients of the two models is zero.
No petrophysical relationship between electrical conductivity
and relative permittivity is assumed but is instead obtained as
a by-product of the inversion. The approach has been applied to
data collected in a U.K. sandstone aquifer in order to improve
characterization of the vadose zone hydrostratigraphy. By analyzing
scatterplots of electrical conductivity versus relative permittivity
together with petrophysical models a zonation could be obtained
with corresponding estimates of the electrical formation factor,
the water content, and the effective grain radius of the sediments.
The approach provides greater insight into the hydrogeological
characteristics of the subsurface than by using conventional geophysical
inversion methods. |
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Diciembre de 2006
Three-dimensional micromagnetic modeling of randomly oriented
magnetite grains (0.03-0.3 µm)
Authors: Koji Fukuma and David J. Dunlop
Link: Click here
Abstract
We report three-dimensional micromagnetic structures obtained
for magnetite cubes with 100 edges in the size range 0.03-0.3
µm around the single-domain threshold size. We employed
the Metropolis algorithm to find equilibrium micromagnetic structures
starting from uniformly magnetized initial states. Directions
of initial magnetizations were randomly set so as to mimic
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a
randomly oriented assemblage. Small grains (<0.07 µm)
exhibit single-domain equilibrium structures parallel to a magnetic
easy axis 111 irrespective of initial magnetization directions.
Vortex structures emerge for small pseudosingle-domain grains
above 0.07 µm. Axes of the vortices are parallel to 100
in the threshold size range but become randomly oriented with
increasing grain size. Neither lamellar domain structures nor
multiple vortex structures were found for grains smaller than
0.3 µm. Reduced saturation remanent magnetization decreased
down to ~0.1 with increasing grain size over a relatively narrow
grain size range (0.07-0.15 µm). The grain size variation
of reduced magnetization depends on microscopic magnetization
structures inside a single grain and the degree of randomness
of grain magnetization directions in an assemblage. |
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Diciembre de 2006
Competition of magnetic and hydrodynamic forces on ellipsoidal
particles under shear: Influence of the Earth's magnetic field
on particle alignment in viscous media
Authors:Josef Jezek and Stuart A. Gilder
Link: Click here
Abstract
We present a model that describes the rotation of ellipsoidal
magnetic particles in a viscous fluid under the influence of
hydrodynamic and magnetic forces, with an aim to better understand
how sediments acquire their remanent magnetizations. Analyses
of the governing equations elucidate how magnetic particles
will rotate for different values of leading parameters including
particle shape, remanent and induced magnetic intensity, magnetic
field intensity and direction, strain rate, shear direction,
and viscosity. Numerical solution of the governing equations
makes it possible to visualize the
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rotation
path and the magnetic direction of a particle through time. Thus
the model can discern the timescales and trajectories of magnetic
particles rotating due to torque of the magnetic field couple
while simultaneously entrained in a velocity gradient. For example,
in a layer of viscosity 104 Pa s, prolate magnetite starting at
any initial orientation and subjected to simple shear with a strain
rate of 3.17 × 10-8 s-1 needs 4 months to rotate within
3° of the Earth's field direction. Under the same conditions,
hydrodynamic forces will govern the orientation of oblate hematite
whose moment will be perpetually randomly oriented with respect
to the magnetic field direction. When applied to laboratory experiments,
the viscous model successfully matches the observed data, particularly
after accounting for mechanical interaction and flocculation effects.
Magnetic anisotropies calculated from multiparticle systems of
hematite yield typical sedimentary fabrics with relatively low
percentages of anisotropy (<5%) and maximum principal axes
that lie in the sedimentation plane. |