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Testing finite differences method for retrieving the slip partitioning of thrust fault branches of the 2008 Wenchuan (Sichuan) China earthquake (MW 7.9)

Abstract : The Sichuan province is an active tectonic area at the boundary between the eastern Tibetan Plateau and the Sichuan basin. It was strongly affected on May 12, 2008 by an earthquake of magnitude Mw= 7.9. During that event, several faults of the Longmenshan fault system ruptured over more than 230 kilometers. This event has already been of interest to many research teams (e.g. Qi et al 2011, Hubbard and Shaw 2008): field campaigns were carried out, and several rupture models have been proposed by the scientific community. Although the geological observation shows parallel surface traces (Beichuan and Guanian-Anxian faults) along several kilometers and these are taken into account in some inversions, it is difficult to identify the slip partitioning on such faults. Furthermore, it is considered that these faults coincide at depth. However, it is important to understand the role of such branching fault systems. Thus the aim of our work is to investigate the slip partitioning using mainly the ALOS interferogram showing the surface deformation in the near fault area. We also use a meta-heuristic (genetic) algorithm to avoid uncertain prior information. The displacement field from the ALOS interferogram that we have calculated and many punctual GPS measurements are used. Moreover, the 3D surface displacement vectors, covering an area around the fault trace about 30km wide, which we computed from the ALOS and Envisat SAR amplitude offsets data (de Michele et al., 2010), are also used. This is very useful for characterizing the shallowest part of the fault system. Green functions for an elementary source are prepared using a finite difference model, as we aimed to further develop our inversion scheme including the temporal change and complex medium. The fault is discretized into a set of equally distributed elementary sources. We choose a relatively small number of points where the fault slip is to be given, and then we use bilinear interpolation to obtain a more smooth slip distribution for calculation. Consequently, our inversion focuses on large-scale slip pattern, which is sufficient to understand the role of the segmented faults. We test different realistic geometric configurations for the fault systems. We find that the approach used to constrain the solutions is key: in fact, when the constraint is not sufficient, the inversion could fit the observations, but the solution is physically inconsistent. Hence, we have to at least restrain the rake direction over the fault in the interval roughly 45° from the rake direction indicated by centroid moment tensor inversion.
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https://hal-brgm.archives-ouvertes.fr/hal-00701929
Contributeur : Thomas Ulrich <>
Soumis le : lundi 28 mai 2012 - 11:49:34
Dernière modification le : jeudi 9 avril 2015 - 15:16:44

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  • HAL Id : hal-00701929, version 1

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Thomas Ulrich, Marcello de Michele, Hideo Aochi. Testing finite differences method for retrieving the slip partitioning of thrust fault branches of the 2008 Wenchuan (Sichuan) China earthquake (MW 7.9). European Seismological Commission 33-rd General Assembly (ESC 2012), Aug 2012, Moscow, Russia. ⟨hal-00701929⟩

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