Anisotropic Tomographic Inversion of the P-Travel Times in the Northwestern Sichuan, China

The velocity and anisotropy image of the crust and upper mantle nearby the eastern Tibetan edge is important for the Tibetan plateau dynamics. The Wenchuan earthquake occurred on May 12, 2008 makes this more interesting. Up to now, however, the resolution of the former results about this area is still poor. The dense movable seismic array deployed recently in the western Sichuan (100°~ 105°E, 26°~ 32°N) provides a good opportunity to insight into the crust and upper mantle structure with high-resolution. In this presentation, the P-travel time tomographic technique has been extended to the case of anisotropy. In our method, under assumption of weak anisotropy, the P-travel times are calculated in terms of the fast marching method. The anisotropic inversion of the P-travel times in the northwestern Sichuan (100°~ 105°E, 29°~ 33°N) is performed by using this technique. Our results show: 1) The lithosphere beneath the Sichuan Basin has a high-velocity structure, but a low-velocity layer exists in the upper mantle at depth range of 150-00 km. The lithosphere underneath the Tibetan Plateau has a lower P-wave velocity than that of the Sichuan Basin, but we did not find intracontinental subduction. Especially, according to the crustal velocity structure, the crust in the Longmen Shan fault belt can be separated into two parts bounded with the Wenchuan earthquake region; both of them have high- velocity perturbation up to 3%. Beneath the intersection of the Longmen Shan and Xianshui He faults, the lithosphere structure is very complicated, suggesting that the large deformation was taking place in this area. 2) In the eastern Tibetan Plateau, the fast-direction of the P-waves is toward the NE direction, which is consistent with the GPS observations. However, the fast-direction is toward the SE in the Sichuan basin. Nearby the Longmen Shan faults belt, it is perpendicular to the faults at the north segment and parallel to the faults at the southern segment, suggesting that these two segments could have completely different deformation manner and dynamical mechanism.