Finite-Frequency Tomography in a 3D Crustal Environment: Application to the Western Gulf of Corinth

Description

We investigate finite-frequency effects in local tomography. For this purpose, we developed an inversion method including the sensitivity kernels in an 3D tomographic method. The influence of the frequency content is analyzed when considering variable velocity structure. Both travel-time and amplitude estimations are required to compute the sensitivity kernels. In this approach, the seismic tool uses (1) the graph theory and an additional bending to estimate accurately both rays and travel-times between source/receiver and a complete grid of diffraction points and (2) the paraxial theory to estimate the amplitude along theses rays. We invert both the velocity and the hypocenter parameters, using these so-called banana-doughnut kernels and the LSQR iterative solver. We apply finite-frequency tomography to image the intermediate structures beneath the Gulf of Corinth (Greece), which has long been recognised as the most active continental rifting zone in the Mediterranean region. Although many studies have already been devoted to this area, the nature of the rifting process remains an open question. Our dataset consists of 451 local events with 9236 P- and 7523 S- first-arrival times recorded in the western part of the Gulf (Aigion area) in the framework of the 3F-Corinth European project. Ray- theoretical tomographic images show a complex velocity model and a low-dip surface that may accommodate the deformation. An accurate velocity model will provide additional constraints on the rifting process. In order to improve the resolution of the mid-crust images, we applied the new method to the same dataset. The starting model is the 3D model obtained earlier from the ray theory. This is the first time the finite-frequency method is used with a 3D starting model, and as a first step we analyze the influence of finite frequency effects at a very small scale in a variable velocity structure. Surprisingly - because the kernels spread the information over a volume - finite frequency tomography results in a sharpening of layer boundaries. We present a full comparison of the finite-frequency tomography models with previous tomographic results to assess the improvement of the mid-crust images.

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