Teleseismic P-wave tomography of the crust and upper mantle in Longmenshan area, west Sichuan

The 3D P-wave velocity structure of the crust and upper mantle within the depth range of 400 km was obtained by using teleseismic traveltimes data recorded by West Sichuan Seismic Array distributed in Longmenshan region and non-linear tomography inversion technique. For adapting to the complicated structure, the fast marching traveltime calculation method and Tarantola s inversion method was used. Our results show the tectonic differences of the crustal and upper-mantle structure among the blocks of Chuandian, Songpan-Garze and Sichuan basin. Our results show that: 1) the crustal structure of the study area correlates with the surface geological features. The Sichuan basin is imaged as a high-velocity feature, while the SongpanGarze and Chuandian block as low-velocity feature. The lithosphere thickness of Sichuan basin has lateral variations from 250 km in south part to 100 km in north part. There exist upwelling features under Songpan-Garze block. 2) Sichuan basin vertically contacts with Chuandian block, but the thickness of Sichuan basin front in Longmenshan area decreases from east to west with the feature of Songpan-Garze block incursion into Sichuan basin at upper mantle. This feature shows the difference in dynamic mechanism between Sichuan basin with Chuandian block and Sichuan with Songpan-Garze block. 3) The Longmen Shan faults belt is divided into two parts at Yingxiu: the south part and north part. The great Wenchuan earthquake and aftershocks distribute in the north part of Longmen Shan faults. The eastward extrusion of Tibetan plateau and upper mantle upwelling caused the uplifting of Songpan-Garze block. Because Yingxiu is located in the south edge of north part of Longmenshan faults belt, it's possible to accumulate high stress. This should be the deep dynamic background of the Wenchuan earthquake. Based on our results, the velocity structure does not support the subduction of the Sichuan basin downward and channel flow.