Geophysical background field and deep dynamics of the Wenchuan-Yingxiu <italic>M</italic><sub>S</sub>8.0 earthquake

Abstract

The Wenchuan-Yingxiu M S8.0 earthquake occurred along the Longmenshan fault system where the tectonic and seismic activities are relatively less inactive than its surroundings. After the great earthquake, a dynamic failure corridor zone was formed at the surface, which was about 300 km-long and 80 km-wide and more than 70000 aftershocks occurred. The earthquake caused serious surface damages and large numbers of casualties, but no confirmed precursor phenomena were reported. In the northeastern Tibetan Plateau, geophysicists had applied many kinds of geophysical methods, including the artificial seismic probing, the natural seismic imaging, the gravity probing, and the electric and magnetic methods. Many researchers believed that the crust or mantle shortening is the main dynamic of the M S8.0 Wenchuan-Yingxiu earthquake. However, the evidences were not enough, due to the disadvantage of the geophysical methods, such as the dubious resolution problem and the ambiguous interpretation from colorful results. Based on the previous results and our extensively study, we made a further study on the Wenchuan-Yingxiu M S8.0 earthquake which mainly concentrating on the three issues, including the highly isostatic equilibrium of the Longmenshan fault system, the fine structure and abrupt deep dynamics of the Longmenshan fault zone, and the force source system, the development and occurrence of the Wenchuan-Yingxiu M S8.0 great earthquake. First, we reviewed the gravity filed and response of the boundary in the northeastern margin of Tibetan Plateau. The free-space and Bouguer gravity anomaly were given. The distribution of the theoretical isostatic crustal thickness and the isostatic thickness difference were analyzed. Considering the topography and gravity field of eastern Tibetan Plateau, the Longmenshan zone was far from gravity isostasy. Second, the electrical structures and the S-wave velocity structures were reviewed. We reanalyzed the four artificial seismic sounding profiles. Third, we showed a diagram of the 15±5 km deep convergence and the columnar force source of the seismogenic fault. Fourth, we summarized the three flow locus of deep matter in the eastern margin of Tibetan Plateau. Meanwhile, a model about the Wenchuan-Yingxiu M S8.0 strong earthquake and deep material motion was proposed, illustrating the opposite motions of crust and upper mantle materials, and the strong collision between the crust of the Songpan-Gantze block and the Sichuan Basin. We believe that the Wenchuan-Yingxiu M S8.0 earthquake was caused by the collision and extrusion of the Indian plate and Eurasian plate which the east tectonic knot was extruding north under the Tibetan Plateau and the large force system intensified the tectonic and seismic activities in the east margin. In summary, we illustrated the processes as follows. (1) The crust and mantle material in the central Tibetan Plateau was blocked in the east which resulted a large-scale stress concentration and strong exchange of deep materials and energy. The variation of the crustal thickness between the northeastern Tibetan Plateau and the Sichuan Basin was large as 15–20 km. (2) The structures and properties of the material in the deep source mutated and leaded to a highly disequilibrium of gravity field. (3) Blocked by the hard crust of the Sichuan Basin, the low velocity crust and mantle materials beneath the western Sichuan Plateau thrusted upward along the steep fault plane of the Longmengshan fault and collided at the convergence spot of the three west-dipped fault planes. (4) The three surficial faults in the Longmenshan fault zone were not the seismogenic faults, but the convergence fault at the depth of 15±5 km. The convergence fault was a columnar focal volume, which had a radius of 5 km and a length of 300 km. The convergence of the fault planes in the deep focus induced the seismogenic fracture of the M S8.0 Wenchuan-Yingxiu earthquake.