Deep structure of the Longmenshan fault zone and mechanism of the 2008 Wenchuan earthquake

Since the occurrence of the 2008 Wenchuan earthquake ( M s8.0), many researchers have conducted extensive seismological and geophysical observations and investigations and obtained important results about the Longmenshan fault zone. Crustal structure inferred from local tomography shows that seismic velocity exhibits significant changes across the Wenchuan earthquake hypocenter from the south to the north. To the south,obvious low-velocity (low-V) anomalies exist, whereas strong lateral heterogeneities are revealed to the north, which may explain why the aftershocks extend northeastward. The Wenchuan earthquake occurred at the boundary between high-velocity (high-V) and low-V anomalies and a significant low-V zone is revealed below the mainshock hypocenter, suggesting that the nucleation of the Wenchuan earthquake was related to partial melts and/or fluid effects and associated with the reduction of effective normal stress on the fault plane, due to hightemperature and highpressure in the Longmenshan fault zone caused by the India-Asia collision. The upper-mantle structure inferred from teleseismic tomography shows that the Longmenshan fault zone is located in the transition zone from low-V anomalies beneath the Songpan-Ganzi block to high-V anomalies beneath the Sichuan basin. This structural feature extends down to 200−300 km depths. High-V anomalies in the mantle transition zone are connected with those in the upper mantle beneath the Burma arc, indicating that the Wenchuan earthquake could be associated with upwelling of hot and wet materials in the big mantle wedge formed by the deep subduction of the Indian plate. These results suggest that the generation of the Wenchuan earthquake was related to structural heterogeneities in not only the crust but also the upper mantle. In addition,high-poisson’s ratio and high-conductivity anomalies are revealed beneath the Wenchuan earthquake source area, which may also reflect lower crustal flow that is compressed and moving eastward. When the eastward flow encountered the strong Sichuan basin block around the Longmenshan fault zone, the flow is further compressed upward along the margin of the Sichuan basin, which could have caused the fault ruptureand generated the Wenchuan earthquake. Some geologists found a vertical co-seismic displacement of 6.5 m in the Longmenshan fault zone, suggesting that there exists an imbricate structure due to the crustal shortening. Such a shortening mechanism could rupture the seismogenic fault leading to the large earthquake. The Zipingpu reservoir is located very close to the Wenchuan earthquake epicenter. It is still debated whether the reservoir triggered the 2008 Wenchuan earthquake or not. Although some simulation results suggest that the Wenchuan earthquake could be triggered by the reservoir, there are large differences in the estimated stress in the hypocentral area due to large uncertainties of the physical parameters adopted in the numerical simulations. Hence, the physical parameters should be determined precisely in future investigations so as to clarify the effect of the reservoir on the seismogenesis. We also discuss the causal mechanism of the 2013 Lushan earthquake ( M s7.0) and future seismic risk in the gap between the aftershock zones of the Wenchuan and Lushan earthquakes.