Alexandrine Gesret

Slab top dips resolved by teleseismic converted waves in the Hellenic subduction zone

The variations of the arrival times and polarities with backazimuth and distance of teleseismic P-to-S converted waves at interfaces bounding the slab crust under the upper plate mantle are used to constrain the depth, dip angle and azimuth of the slab of the Hellenic subduction zone. A grid search is designed to estimate the model parameters. Dip values of 16-18°, with an azimuth of 20° to 40°, are thus derived at 3 sites aligned over 50 km along the eastern coast of Peloponnesus. They are consistent with the variation from 54 to 61 km of the slab top depths constrained below each receiver. North of the Gulfs of Corinth and Evvia, a similar depth for the top of the slab is found at a distance from the subduction at least 100 km larger. This suggests flatter subduction of a different slab segment. Such a variation in slab attitude at depth across the region from south of the eastern Gulf of Corinth to north of Evvia is a candidate for the control of the recent or active localized crustal thinning of the upper plate we documented in earlier work, and of the surface deformation.

Microseismic Monitoring - Consequences of Velocity Model Uncertainties on Event Location Uncertainties

Among many factors that contribute to microseismic location errors, the largest contribution is due to the lack of knowledge of the wave-propagation medium. In spite of efforts to build the “best” velocity model derived from surface seismic and/or logging data, these models are very often not adapted to the microseismic context and are characterized by numerous uncertainties. These uncertainties are often enhanced due to the poor aperture of the microseismic monitoring networks. Precise location of hypocenters requires deriving a very accurate velocity model using calibration shots; the inversion to obtain this model is a difficult task but cannot be neglected. We propose a tomography algorithm using calibrations shots that does not produce only a unique “best” velocity model but all velocity models that explain the observed data within the traveltime picking uncertainties. This approach allows deriving location uncertainties associated to velocity model uncertainties. These maps show that the commonly used probability associated to the picking uncertainties must not be used to represent the probability associated to the velocity model uncertainties

First Arrival Travel Time Tomography - Bayesian Approach

First arrival time tomography aims at determining the propagation velocity of seismic waves from experimental measurements of their first arrival time. This problem is usually ill-posed and is classically tackled by considering various iterative linearised approaches. However, these methods can yield wrong seismic velocity for highly nonlinear cases and they fail to estimate the uncertainties associated to the model. In our study, we rely on a Bayesian approach coupled with an interacting Markov chain-Monte Carlo (MCMC) algorithm to estimate the wave velocity and the associated uncertainties. The main difficulty associated to this approach is that traditional MCMC algorithms can be inefficient when multimodal probability distributions or complex velocity models involving a great number of parameters come into play. Therefore, a first step toward an efficient implementation of the Bayesian approach is to properly parametrize the model to reduce its dimension and to select adequate prior distribution for the parameters. In this paper, we present a ten layers probabilistic model for the velocity, that we illustrate on tomography results.

Fast Method for Amplitude Computation on High Contrast Velocity Models

The amplitude of seismic waves is a very important wave componant. It is usefull for many seismic and seismological applications such as preserved amplitude prestack depth migration or even tomography. In this article we will present a new method to compute amplitudes based on the transport equation. It consists of a combination of two schemes: first for a given point in the medium the amplitude is predicted locally from neighboring points (local scheme) and then a global scheme is applied to propagate amplitudes across the whole medium. For clarity we present the method in a 2D model, extension to 3D models is straight forward. The main feature of this new algorithm is its ability to compute accurate amplitudes in velocity models with very strong contrasts.