Search

A 3D shear wave velocity model of the subsurface of Popocatepetl volcano was obtained using ambient noise tomography, employing the principal cross-terms of the empirical Green’s function. Daily cross-correlation waveforms were computed from the data recorded by four seismic broadband stations in 2012. Dispersion curves were estimated from the stack of anti-causal and causal parts for the three ground motion components. In the period ranging between 0.5 and 7 s, the radial component provided the most well-defined group velocity dispersion curves for Rayleigh waves. Dispersion curves for long paths (> 9.5 km) suggested the presence of a layered structure with group velocity values ranging from 0.5 to 3.0 km/s. Dispersion curves for short paths (< 7 km) and close to the volcano showed values similar to those for long paths but with a velocity inversion occurring between 3- and 5-s periods. Despite the scarce ray-path coverage, a tomography inversion provided the means to image an S-wave velocity anomaly (2.5–3.0 km/s) in the northern part of the crater at depths not larger than 6 km. The volcano-tectonic seismicity during 2012–2013 was found to be concentrated in the southern segment of this anomaly, which seems to mark a boundary between a solidified body (possibly a product of ancient volcanic craters) and the softer material.

SUMMARYTo shed light on the prevalently slow, aseismic slip interaction between tectonic plates, we developed a new static slip inversion strategy, the ELADIN (ELastostatic ADjoint INversion) method, that uses the adjoint elastostatic equations to compute the gradient of the cost function. ELADIN is a 2-step inversion algorithm to efficiently handle plausible slip constraints. First it finds the slip that best explains the data without any constraint, and then refines the solution by imposing the constraints through a Gradient Projection Method. To obtain a self-similar, physically consistent slip distribution that accounts for sparsity and uncertainty in the data, ELADIN reduces the model space by using a von Karman regularization function that controls the wavenumber content of the solution, and weights the observations according to their covariance using the data precision matrix. Since crustal deformation is the result of different concomitant interactions at the plate interface, ELADIN simultaneously determines the regions of the interface subject to both stressing (i.e. coupling) and relaxing slip regimes. For estimating the resolution, we introduce a mobile checkerboard analysis that allows to determine lower-bound fault resolution zones for an expected slip-patch size and a given stations array. We systematically test ELADIN with synthetic inversions along the whole Mexican subduction zone and use it to invert the 2006 Guerrero Slow Slip Event (SSE), which is one of the most studied SSEs in Mexico. Since only 12 GPS stations recorded the event, careful regularization is thus required to achieve reliable solutions. We compared our preferred slip solution with two previously published models and found that our solution retains their most reliable features. In addition, although all three SSE models predict an upward slip penetration invading the seismogenic zone of the Guerrero seismic gap, our resolution analysis indicates that this penetration might not be a reliable feature of the 2006 SSE.

Either the triggering of large earthquakes on a fault hosting aseismic slip or the triggering of slow slip events (SSE) by passing seismic waves involve seismological questions with important hazard implications. Just a few observations plausibly suggest that such interactions actually happen in nature. In this study we show that three recent devastating earthquakes in Mexico are likely related to SSEs, describing a cascade of events interacting with each other on a regional scale via quasi-static and/or dynamic perturbations across the states of Guerrero and Oaxaca. Such interaction seems to be conditioned by the transient memory of Earth materials subject to the “traumatic” stress produced by seismic waves of the great 2017 (Mw8.2) Tehuantepec earthquake, which strongly disturbed the SSE cycles over a 650 km long segment of the subduction plate interface. Our results imply that seismic hazard in large populated areas is a short-term evolving function of seismotectonic processes that are often observable., 大地震とスロースリップの相互作用を解明 --メキシコにおける3つの大地震の連鎖的発生のメカニズム--. 京都大学プレスリリース. 2021-04-12.

We have applied instantaneous seismic attributes to a stacked P-wave reflected seismic section in the Tenerife field located in the Middle Magdalena Valley Basin in Colombia to estimate the volume of clay [Formula: see text] and the density [Formula: see text] at seismic scale. The well logs and the seismic attributes associated to the seismic trace closer to one of the available wells (Tenerife-2) is the information used to train some multilayered artificial neural networks (ANN). We perform data analysis via the gamma test, a mathematically nonparametric nonlinear smooth modeling tool, to choose the best input combination of seismic attributes to train ANNs to estimate [Formula: see text] and [Formula: see text]. Once the ANNs are trained, they are applied to predict these parameters along the seismic line. From the continuous estimations of [Formula: see text], we distinguish two facies: sands for [Formula: see text] and shales when [Formula: see text]. These estimations confirm the production of the Mugrosa C-Sands zone, and we draw the brown shale that correlates with the high-amplitude attributes and the yellow sand that correlates with the low-amplitude attributes. Using the well-log information for [Formula: see text] and the facies classification (also in the well log), two cubic polynomials that depend on time (or depth) are obtained, one for sands and the other for shales, to fit the [Formula: see text]. These two cubic polynomials and the facies classification obtained from the [Formula: see text] at the seismic scale enable us to estimate [Formula: see text] at the seismic scale. To validate the 2D [Formula: see text] and [Formula: see text] predicted data, a forward-modeling software (the Kennett reflectivity algorithm) is used. This model calculates synthetic seismograms that are compared with the real seismograms. This comparison indicates a small misfit that suggests that the [Formula: see text] and [Formula: see text] images are representing the reservoir description characteristics and the ANN method is accurate to map these parameters.

In the oil and gas industry, well test analysis using derivative plots, has been the core technique in examining reservoir and well behavior over the last three decades. Recently, diagnostics plots have gained recognition in the field of hydrogeology; however, this tool is still underused by groundwater professionals. The foremost drawback is that the derivative function must be computed from noisy field measurements, usually based on finite-difference schemes, which complicates the analysis. We propose a B-spline framework for smooth derivative computation, referred to as Constrained Quartic B-Splines with Free Knots. The approach presents the following novelties in relation to methodological precedents: (1) the use of automatic equality derivative constraints, (2) a knot removal strategy and (3) the introduction of a Boolean shape parameter that defines the number of initial knots to choose. These can lead to evaluate both simple (manually recorded drawdown measurements) and complex (transducer measured records) datasets. Furthermore, we propose an additional shape preserving smoothing preprocess procedure, as a simple, fast and robust method to deal with extremely noisy signals. Our framework was tested in four pumping tests by comparing the spline derivative with regards to the Bourdet algorithm, and we found that the latter is rather noisy (even for large differentiation intervals) and the second derivative response is basically unreadable. In contrast, the spline first and second derivative led to smoother responses, which are more suitable for interpretation. We concluded that the proposed framework is a welcome contribution to evaluate reliable aquifer tests using derivative-diagnostic plots.

International audience; We present a new hierarchical time kinematic source inversion method able to assimilate data traces through evolutive time windows. A linear time domain formulation relates the slip rate function and seismograms, preserving the positivity of this function and the causality when spanning the model space: taking benefit of the time‐space sparsity of the rupture model evolution is as essential as considering the causality between rupture and each record delayed by the known propagator operator different for each station. This progressive approach, both on the data space and on the model space, does require mild assumptions on prior slip rate functions or preconditioning strategies on the slip rate local gradient estimations. These assumptions are based on simple physical expected rupture models we foresee. Successful applications of this method to a well‐known benchmark (Source Inversion Validation Exercise 1) and to the recorded data of the 2016 Kumamoto mainshock (Mw=7.0) illustrate the advantages of this alternative approach of a linear kinematic source inversion. This stabilized overparameterized optimization of a linear forward problem has a potential extension to stochastic inversion in a Bayesian framework in the future to assess uncertainties on kinematic source inversions, which is more difficult to investigate under nonlinear formulations.

Finite-difference methods are popular for wave simulation within the seismic exploration community, thanks to their efficiency. However, difficulties arise when encountering complex topography due to the regular grid pattern of the finite-difference schemes. Despite alternatives that can handle the free surface with little effort, such as the spectral element or discontinuous Galerkin’s methods, incorporating a free-surface boundary condition within the finite-difference framework is still appealing, even at the cost of extra algorithm complexity and stronger requirement of computational resources. We present a free-surface boundary treatment within the finite-difference framework, belonging to the family of the immersed-boundary methods. Inherently, the presented boundary treatment is separated from the rest of the wave simulation, which makes it easy to be integrated in existing finite-difference codes. Specifically, we construct an extrapolation operator for each grid point above the free surface, if requested by the finite-difference stencil, to estimate its fictitious wavefield value at each time step. These operators are constructed only once and remain unchanged for all the time steps and source locations. The memory requirement of these operators is significant. Fortunately, grouping together multiple simulations concerning different source locations makes it possible to dilute the memory burden to a negligible level. Additionally, applying these operators incurs numerical noise, which may lead to long time instabilities. In such a scenario, additional numerical procedures, for instance, introducing artificial diffusion, are necessary to control the instability and obtain sensible simulation results. Successful applications of the presented boundary treatment to elastic-wave equations on domains with nontrivial topographies, in 2D and 3D, are presented. Robust and efficient numerical techniques to control high-frequency numerical noise remain to be investigated.

The article of record as published may be found at http://dx.doi.org/10.1785/0220170222 We describe a set of benchmark exercises that are designed to test if computer codes that simulate dynamic earthquake rupture are working as intended. These types of computer codes are often used to understand how earthquakes operate, and they produce simulation results that include earthquake size, amounts of fault slip, and the patterns of ground shaking and crustal deformation. The benchmark exercises examine a range of features that scientists incorporate in their dynamic earthquake rupture simulations. These include implementations of simple or complex fault geometry, off-fault rock response to an earthquake, stress conditions, and a variety of formulations for fault friction. Many of the benchmarks were designed to investigate scientific problems at the forefronts of earthquake physics and strong ground motions research. The exercises are freely available on our website for use by the scientific community. Southern California Earthquake Center (SCEC)

Built-up on top of ancient lake deposits, Mexico City experiences some of the largest seismic site effects worldwide. Besides the extreme amplification of seismic waves, duration of intense ground motion from large subduction earthquakes exceeds three minutes in the lake-bed zone of the basin, where hundreds of buildings collapsed or were seriously damaged during the magnitude 8.0 Michoacán earthquake in 1985. Different mechanisms contribute to the long lasting motions, such as the regional dispersion and multiple-scattering of the incoming wavefield from the coast, more than 300 km away the city. By means of high performance computational modeling we show that, despite the highly dissipative basin deposits, seismic energy can propagate long distances in the deep structure of the valley, promoting also a large elongation of motion. Our simulations reveal that the seismic response of the basin is dominated by surface-waves overtones, and that this mechanism increases the duration of ground motion by more than 170% and 290% of the incoming wavefield duration at 0.5 and 0.3 Hz, respectively, which are two frequencies with the largest observed amplification. This conclusion contradicts what has been previously stated from observational and modeling investigations, where the basin itself has been discarded as a preponderant factor promoting long and devastating shaking in Mexico City.

Despite its popularity among the seismic community, finite difference method encounters difficulty for applications involving complex topography. Incorporating a free surface boundary treatment within the finite difference framework is appealing due to its efficiency and the effort-saving from existing finite difference codes. We present a free surface boundary treatment within the finite difference framework, with origin from the immersed boundary methods. Inherently, the presented boundary treatment is separated from the rest of the wave simulation, making it suitable for modularized code design. Specifically, we construct an extrapolation operator for each missing grid point to estimate its associated wavefield value at each time step. Although complicated in its mathematical expression, these operators only need to be constructed once for all time steps and source locations. Their associated memory consumption can be significant. Fortunately, simulating multiple shots together can dilute this memory cost. Application of these operators may incur numerical noise, leading to long time instability. In this case, additional numerical procedures such as introducing artificial diffusion are necessary to control the instability. The presented boundary treatment is shown to be capable of modeling both the body wave and surface wave accurately and has the potential on full waveform inversion applications.

International audience; For considering elastic seismic wave propagation in large domains, efficient absorbing boundary conditions are required with numerical modelling in finite domains. Since their introduction by Bérenger, the perfectly matched layers (PML) has become the state-of-the art method because of its efficiency and ease of implementation. However, for anisotropic media, theoretical analysis and numerical experiments show that the PML method is amplifying, that is it exhibits numerical instabilities. Numerical experiments can also exhibit numerical instabilities of the PML when dealing with long time simulations even for isotropic media, especially for finite element methods in unstructured grids. Recently, a new method, called SMART layers approach, has been proposed. This method is shown to be stable even for anisotropic media. The drawback is that the SMART layers are not perfectly matched. We have implemented this new approach in a discontinuous Galerkin method and we illustrate that this method does not exhibit numerical instabilities while PML do for an isotropic elastodynamic simulation. We show that this approach is also competitive with respect to the PML method in terms of efficiency and computational cost.

[1] We introduce a novel scheme, DGCrack, to simulate dynamic rupture of earthquakes in three dimensions based on an hp-adaptive discontinuous Galerkin method. We solve the velocity-stress weak formulation of elastodynamic equations on an unstructured tetrahedral mesh with arbitrary mesh refinements (h-adaptivity) and local approximation orders (p-adaptivity). Our scheme considers second-order fault elements (P2) where dynamic-rupture boundary conditions are enforced throughad hocfluxes across the fault. To model the Coulomb slip-dependent friction law, we introduce a predictor-corrector scheme for estimating shear fault tractions, in addition to a special treatment of the normal tractions that guarantees the continuity of fault normal velocities. We verify the DGCrack by comparison with several methods for two spontaneous rupture tests and find excellent agreement (i.e., rupture times RMS errors smaller than 1.0%) provided that one or more fault elements resolve the fault cohesive zone. For a quantitative comparison, we introduce a methodology based on cross-correlation measurements that provide a simple way to quantify the similarity between solutions. Our verification tests include a 60° dipping normal fault reaching the free surface. The DGCrack method reveals convergence rates close to those of well-established methods and a numerical efficiency significantly higher than that of similar discontinuous Galerkin approaches. We apply the method to the 1992 Landers-earthquake fault system in a layered medium, considering heterogeneous initial stress conditions and mesh adaptivities. Our results show that previously proposed dynamic models for the Landers earthquake require a reevaluation in terms of the initial stress conditions that take account of the intricate fault geometry.

International audience; The 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.

A quadratic programming formulation for multiclass image segmentation is investigated. It is proved that, in the convex case, the non-negativity constraint on the recent reported quadratic Markov measure field model can be neglected and the solution preserves the probability measure property. This allows one to design efficient optimization algorithms. Additionally, it is proposed a (free parameter) inter-pixel affinity measure which is more related with classes memberships than with color or gray gradient based standard methods. Moreover, it is introduced a formulation for computing the pixel likelihoods by taking into account local context and texture properties.