Temporal variations of non-volcanic tremor (NVT) locations in the Mexican subduction zone: Finding the NVT sweet spot.

International audienceEpicentral locations of non-volcanic tremors (NVT) in the Mexican subduction zone are determined from the peak of the energy spatial distribution and examined over time. NVT is found to occur persistently at a distance of ∼215 km from the trench, which we term the "Sweet Spot" because this region probably has the proper conditions (i.e., temperature, pressure, and fluid content) for the NVT to occur with minimum shear slip. High-energy NVT episodes are also observed every few months, extending ∼190 km to ∼220 km from the trench with durations of a few weeks. During the 2006 slow slip event (SSE) the duration and the recurrence rate of the NVT episodes increased. Low-energy episodes were also observed, independent from the high-energy episodes, ∼150 km to ∼190 km from the trench during the 2006 SSE. Both the high and low energy episodes were made up of many individual NVT's that had a range of energy-release-rates. However, the highest energy-release-rates of the high-energy episodes were consistently double those of the low-energy episodes and the persistent activity at the Sweet Spot. We suggest that all of the high-energy episodes are evidence of small, short repeat interval SSE. Given this model, the increased recurrence rate of the high-energy NVT episodes during the 2006 long-term SSE implies that short-term SSE's also increase during the SSE and are therefore triggered by the SSE

Some aspects concerning the dynamics of stochastic chemostats

In this paper we study a simple chemostat model influenced by white noise which makes this kind of models more realistic. We use the theory of random attractors and, to that end, we first perform a change of variable using the OrnsteinUhlenbeck process, transforming our stochastic model into a system of differential equations with random coefficients. After proving that this random system possesses a unique solution for any initial value, we analyze the existence of random attractors. Finally we illustrate our results with some numerical simulations.Fondo Europeo de Desarrollo RegionalMinisterio de Economía y CompetitividadJunta de Andalucí

Modelling Seismic Wave Propagation for Geophysical Imaging

International audienceThe Earth is an heterogeneous complex media from the mineral composition scale (10−6m) to the global scale ( 106m). The reconstruction of its structure is a quite challenging problem because sampling methodologies are mainly indirect as potential methods (Günther et al., 2006; Rücker et al., 2006), diffusive methods (Cognon, 1971; Druskin & Knizhnerman, 1988; Goldman & Stover, 1983; Hohmann, 1988; Kuo & Cho, 1980; Oristaglio & Hohmann, 1984) or propagation methods (Alterman & Karal, 1968; Bolt & Smith, 1976; Dablain, 1986; Kelly et al., 1976; Levander, 1988; Marfurt, 1984; Virieux, 1986). Seismic waves belong to the last category. We shall concentrate in this chapter on the forward problem which will be at the heart of any inverse problem for imaging the Earth. The forward problem is dedicated to the estimation of seismic wavefields when one knows the medium properties while the inverse problem is devoted to the estimation of medium properties from recorded seismic wavefields

Rupture dynamique des failles non-planaires en différences finies

Ce travail est divisé en deux parties: la première est consacrée à la propagation des ondes et la seconde à la rupture dynamique des séismes. Dans la première partie je propose une révision des concepts de base des méthodes en différences finies (DF). Un accent particulier est fait sur la méthode choisie en DF, basée sur une grille partiellement en quinconce (GPQ), utilisée dans la deuxième partie du travail pour étudier la rupture des séismes. Les conditions aux limites sur la surface libre ainsi qu'à l'intérieur d'une fracture plane sont traitées, en discutant certaines approches déjà établies dans chaque cas. La formulation et la mise au point des conditions de radiation " Perfectly Matched Layer " (PML) sont abordées en trois dimensions (3D). Des tests d'absorption sont ainsi présentés en milieux de propagation homogènes et hétérogènes. Dans la deuxième partie, je propose premièrement une autre révision, cette fois-ci sur la mécanique de la rupture. Une fois ainsi posé le problème central de ce travail, j'aborde la modélisation numérique de la rupture dynamique de failles non-planaires en deux dimensions (2D). Une nouvelle approche en DF permettant de résoudre ce problème complexe est donc introduite. Une loi d'échelle, reliant le pas de discrétisation et le nombre de points à l'intérieur de la source, permet d'augmenter la précision des solutions en réduisant les oscillations numériques. Le passage à la géométrie 3D est fait. Sa précision et sa convergence sont analysées en termes de la résolution de la zone de cohésion. La méthode est validée dans le cas de ruptures spontanées planes et non-planaires (i.e. curvilignes) grâce à la comparaison des résultats avec ceux obtenus par d'autres méthodes numériques indépendantes. Finalement, la méthode est appliquée le long de failles non-planaires sous l'effet d'une charge tectonique biaxiale. La géométrie de la faille s'est avérée un facteur déterminant dans la propagation de la rupture. L'analyse de la rupture du séisme de Landers (1992) illustre ce fait, où des simulations faites en tenant compte de la vraie géométrie du séisme sont fondamentalement différentes de celles réalisées par d'autres auteurs où la faille a été considérée comme plane.This work is separated into two parts: the first part is devoted to wave propagation, and the second part treats the dynamic rupture of earthquakes. In the first part I propose a general review of the basic concepts concerning finite difference (FD) methods. Based on a partly staggered grid, the FD technique used in the second part of the work to study the rupture of earthquakes is analyzed in detail. The free surface and the planar crack boundary conditions are treated in separated sections. Several approaches, previously introduced by different authors, are discussed for each problem. The formulation and implementation of the Perfectly Matched Layer (PML) absorbing boundary conditions in three dimensions (3D) are described. Different tests to illustrate the performance of these boundary conditions are presented in homogeneous and heterogeneous media. In the second part I firstly provide an overview of rupture mechanics. After formulating the main problem of this study, I tackle the numerical modeling of nonplanar faults in two dimensions (2D). A new approach allowing these kinds of complex simulations is then introduced. A scaling law, relating the grid size to the amount of grid points discretizing the source, allows an increase in the precision of solutions by reducing the numerical oscillations. The extension of the rupture model to 3D is performed. Both numerical precision and convergence are analyzed in terms of the cohesive zone resolution. The model is validated for the spontaneous rupture along planar and nonplanar (i.e. curvilinear) faults by comparing solutions with those yielded by two independent numerical approaches. Finally, the method is applied to the case of nonplanar faults loaded in a biaxial tectonic stress regime. The fault geometry becomes an extremely important factor for rupture propagation under these conditions. A dynamic rupture analysis of the 1992 Landers earthquake confirms the importance of the fault geometry, since fundamentally different results are obtained when considering the real rupture surface with respect to those reported by previous authors where the fault was assumed to be planar.NICE-BU Sciences (060882101) / SudocSudocFranceF

3D Dynamic rupture simulations by a finite volume method

International audienceDynamic rupture of a 3D spontaneous crack of arbitrary shape is investigated using a finite volume (FV) approach. The full domain is decomposed in tetrahedra while the surface on which the rupture takes place is discretized with triangles which are faces of tetrahedra. First of all, the elastodynamic equations are described into a pseudo-conservative form for an easy application of the FV discretization. Explicit boundary conditions are given using criteria based on the conservation of discrete energy through the crack surface. Using a stress-threshold criterion, these conditions specify fluxes through those triangles which have suffered rupture. On these broken surfaces, stress follows a linear slip-weakening law although other friction laws can be implemented. For The Problem Version 3 of the dynamic-rupture code verification exercise conducted by the SCEC/USGS, numerical solutions on a planar fault exhibit a very high convergence rate and are in good agreement with the reference one provided by a finite difference (FD) technique. For a nonplanar fault of parabolic shape, numerical solutions agree satisfactorily well with those obtained with a semi-analytical boundary integral method in terms of shear stress amplitudes, stopping phases arrival times and stress overshoots. Differences between solutions are attributed to the low-order interpolation of the FV approach, whose results are particularly sensitive to the mesh regularity (structured/unstructured). We expect this method, which is well adapted for multi-processor parallel computing, to be com- petitive with others for solving large scale dynamic ruptures scenarii of seismic sources in the near future

Seismic evidence of nonlinear crustal deformation during a large slow slip event in Mexico

5p.International audienceRepeated cross-correlations of ambient seismic noise indicate a long-term seismic velocity change associated with the 2006 M7.5 slow-slip event (SSE) in the Guerrero region, Mexico. Because the SSE does not radiate seismic waves, the measured velocity change cannot be associated with the response of superficial soil layers to strong shaking as observed for regular earthquakes. The perturbation observed maximized at periods between 7 s and 17 s, which correspond to surface waves with sensitivity to the upper and middle crust. The amplitude of the relative velocity change (∼10−3) was much larger than the volumetric deformation (∼10−6) at the depths probed (∼5-20 km). Moreover, the time dependence of the velocity perturbation indicated that it was related to the strain rate rather than the strain itself. This suggests that during strong slow-slip events, the deformation of the overlying crust shows significant nonlinear elastic behavior

Seismic velocity changes, strain rate and non-volcanic tremors during the 2009–2010 slow slip event in Guerrero, Mexico

International audienceSeismic velocity changes, strain rate and non-volcanic tremors during the S U M M A R Y We use ambient noise cross-correlations to monitor small but reliable changes in seismic velocities and to analyse non-volcanic tremor (NVT) intensities during the slow slip event (SSE) that occurred in 2009 and 2010 in Guerrero. We test the sensitivity of the seismic velocity to strain variations in absence of strong motions. The 2009-2010 SSE presents a complex slip sequence with two subevents occurring in two different portions of the fault. From a seismic array of 59 seismometers, installed in small antennas, we detect a velocity drop with maximum amplitude at the time of the first subevent. We analyse the velocity change at different period bands and observe that the velocity perturbation associated with the SSE maximizes for periods longer than 12 s. Then a linearized inversion of the velocity change measured at different period bands is applied in order to determine the depth of the portion of the crust affected by this perturbation. No velocity change in the first 10 km is detected. Below, the velocity perturbation increases with depth, affecting the middle and lower crust. Finally, we compute the transient deformation produced by the SSE in an elastic model using the slip evolution recovered from the inversion of continuous GPS. The comparison between the velocity changes and the deformation suggests that the velocity change is correlated with the strain rate rather than with the strain. This result is similar to what was observed during the 2006 SSE in the same region and suggests a non-linear behaviour of the crust. The velocity changes can be interpreted together with other observables such as NVTs. During the 2009-2010 SSE we measure NVT activity using continuous seismic records filtered between 2 and 8 Hz. We observe a correlation between velocity changes (for period band greater than 14 s) and tremor activity whereas no correlation exists between velocity changes and seismic noise energy measured at long periods. These observations suggest that both seismic velocity change and NVT can be used as indication of transient deformation at depth