184 research outputs found
Loading rates in California inferred from aftershocks
International audienceWe estimate the loading rate in southern California and the change in stress induced by a transient slip event across the San Andreas fault (SAF) system in central California, using a model of static fatigue. We analyze temporal properties of aftershocks in order to determine the time delay before the onset of the power law aftershock decay rate. In creep-slip and stick-slip zones, we show that the rate of change of this delay is related to seismic and aseismic deformation across the SAF system. Furthermore, we show that this rate of change is proportional to the deficit of slip rate along the SAF. This new relationship between geodetic and seismological data is in good agreement with predictions from a Limited Power Law model in which the evolution of the duration of a linear aftershock decay rate over short time results from variations in the load of the brittle upper crust
Dissipation at the core-mantle boundary on a small-scale topography
International audienceThe parameters of the nutations are now known with a good accuracy, and the theory accounts for most of their values. Dissipative friction at the core-mantle boundary (CMB) and at the inner core boundary is an important ingredient of the theory. Up to now, viscous coupling at a smooth interface and electromagnetic coupling have been considered. In some cases they appear hardly strong enough to account for the observations. We advocate here that the CMB has a small-scale roughness and estimate the dissipation resulting from the interaction of the fluid core motion with this topography. We conclude that it might be significant
Periodicity in fields of elongating dunes
Dune fields are commonly associated with periodic patterns that are among the
most recognizable landscapes on Earth and other planetary bodies. However, in
zones of limited sediment supply, where periodic dunes elongate and align in
the direction of the resultant sand flux, there has been no attempt to explain
the emergence of such a regular pattern. Here, we show, by means of numerical
simulations, that the elongation growth mechanism does not produce a pattern
with a specific wavelength. Periodic elongating dunes appear to be a
juxtaposition of individual structures, the arrangement of which is due to
regular landforms at the border of the field acting as boundary conditions.
This includes, among others, dune patterns resulting from bed instability, or
the crestline reorganization induced by dune migration. The wavelength
selection in fields of elongating dunes therefore reflects the interdependence
of dune patterns over the course of their evolution.Comment: 7 pages, 4 figure
Relation between stress heterogeneity and aftershock rate in the rate-and-state model
We estimate the rate of aftershocks triggered by a heterogeneous stress
change, using the rate-and-state model of Dieterich [1994].We show that an
exponential stress distribution Pt(au) ~exp(-tautau_0) gives an Omori law decay
of aftershocks with time ~1/t^p, with an exponent p=1-A sigma_n/tau_0, where A
is a parameter of the rate-and-state friction law, and \sigma_n the normal
stress. Omori exponent p thus decreases if the stress "heterogeneity" tau_0
decreases. We also invert the stress distribution P(tau) from the seismicity
rate R(t), assuming that the stress does not change with time. We apply this
method to a synthetic stress map, using the (modified) scale invariant "k^2"
slip model [Herrero and Bernard, 1994]. We generate synthetic aftershock
catalogs from this stress change.The seismicity rate on the rupture area shows
a huge increase at short times, even if the stress decreases on average.
Aftershocks are clustered in the regions of low slip, but the spatial
distribution is more diffuse than for a simple slip dislocation. Because the
stress field is very heterogeneous, there are many patches of positive stress
changes everywhere on the fault.This stochastic slip model gives a Gaussian
stress distribution, but nevertheless produces an aftershock rate which is very
close to Omori's law, with an effective p<=1, which increases slowly with time.
We obtain a good estimation of the stress distribution for realistic catalogs,
when we constrain the shape of the distribution. However, there are probably
other factors which also affect the temporal decay of aftershocks with time. In
particular, heterogeneity of A\sigma_n can also modify the parameters p and c
of Omori's law. Finally, we show that stress shadows are very difficult to
observe in a heterogeneous stress context.Comment: In press in JG
Coarsening Dynamics of 2D Subaqueous Dunes
Abstract: Fluid flow over an initially flat granular bed leads to the formation of a surfaceâwave instability. The sediment bed profile coarsens and increases in amplitude and wavelength as disturbances develop from ripples into dunes. We perform experiments and numerical simulations to quantify both the temporal evolution of bed properties and the relationship between the initial growth rate and the friction velocity uâ. Experimentally, we study underwater bedforms originating from a thin horizontal particle layer in a narrow and counterârotating annular flume. We investigate the role of flow speed, flow depth and initial bed thickness on dune evolution. Bedforms evolve from small, irregular disturbances on the bed surface to rapidly growing connected terraces (2D equivalent of transverse dunes) before splitting into discrete dunes. Throughout much of this process, growth is controlled by dune collisions which are observed to result in either coalescence or ejection (mass exchange). We quantify the coarsening process by tracking the temporal evolution of the bed amplitude and wavelength. Additionally, we perform Large Eddy Simulations (LES) of the fluid flow inside the flume to relate the experimental conditions to uâ. By combining the experimental observations with the LES results, we find that the initial dune growth rate scales approximately as u â 5 . These results can motivate models of finiteâamplitude dune growth from thin sediment layers that are important in both natural and industrial settings
Coexistence of two dune growth mechanisms in a landscape-scale experiment
In landscape-scale experiments at the edge of the Gobi desert, we show that
various dune types develop simultaneously under natural wind conditions. Using
4 years of high-resolution topographic data, we demonstrate that, depending on
sand availability, the same wind regime can lead to two different dune
orientations, which reflect two independent dune growth mechanisms. As periodic
oblique dunes emerge from a sand bed and develop to 2 meters in height, we
analyze defect dynamics that drive the non-linear phase of pattern coarsening.
Starting from conical sand heaps deposited on gravels, we observe the
transition from dome to barchan and asymmetric barchan shapes. We identify a
minimum size for arm elongation and evaluate the contribution of wind reversals
to its longitudinal alignment. These experimental field observations support
existing theoretical models of dune dynamics boosting confidence in their
applicability for quantitative predictions of dune evolution under various wind
regimes and bed conditions
Comparing different models of aftershock rate decay: the role of catalog incompleteness in the first times after main shock
We evaluated the efficiency of various models in describing the time decay of
aftershock rate of 47 simple sequences occurred in California (37) from 1933 to
2004 and in Italy (10) from 1976 to 2004. We compared the models by the
corrected Akaike Information Criterion (AICc) and the Bayesian Information
Criterion (BIC), both based on the log-likelihood function but also including a
penalty term that takes into account the number of independent observations and
of free parameters of each model. We compared the performance of different
models by varying the starting time Ts and the minimum magnitude threshold Mmin
for each sequence. We found that Omori-type models including parameter c are
preferable to those not including it, only for short Ts and low Mmin while the
latters generally perform better than the formers for Ts longer than a few
hours and Mmin larger than the main shock magnitude Mm minus 3 units. This
clearly indicates that a value of parameter c different from zero does not
represent a general property of aftershock sequences in California and Italy
but it is very likely induced in most cases by catalog incompleteness in the
first times after the main shock. We also considered other models of aftershock
decay proposed in the literature: the Stretched Exponential Law in two forms
(including and not including a time shift) and the band Limited Power Law
(LPL). We found that such models perform worse than the Modified Omori Model
(MOM) and other Omori-type models for the large majority of sequences, although
for LPL, the relatively short duration of the analyzed sequences (one year)
might also contribute to its poor performance.Comment: 33 pages, 10 figures, 1 tabl
Spatiotemporal correlations of aftershock sequences
Aftershock sequences are of particular interest in seismic research since
they may condition seismic activity in a given region over long time spans.
While they are typically identified with periods of enhanced seismic activity
after a large earthquake as characterized by the Omori law, our knowledge of
the spatiotemporal correlations between events in an aftershock sequence is
limited. Here, we study the spatiotemporal correlations of two aftershock
sequences form California (Parkfield and Hector Mine) using the recently
introduced concept of "recurrent" events. We find that both sequences have very
similar properties and that most of them are captured by the space-time
epidemic-type aftershock sequence (ETAS) model if one takes into account
catalog incompleteness. However, the stochastic model does not capture the
spatiotemporal correlations leading to the observed structure of seismicity on
small spatial scales.Comment: 31 pages, 5 figure
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