A STUDY OF DEEP SEISMOGENIC ENVIRONMENT IN LUSHAN MS7.0 EARTHQUAKE ZONE BY WIDE‐ANGLE SEISMIC REFLECTION/REFRACTION PROFILE

Abstract

AbstractAfter the Lushan earthquake in April, 2013, the Lushan earthquake scientific investigations headquarter was quickly established by China Earthquake Administration to investigate the deep structure and seismogenic environment of the earthquake. Therefore, an about 410‐km‐long artificial seismic high‐resolution wide‐angle reflection/refraction profile was carried out from September to November in the source area of Lushan earthquake by Geophysical Exploration Center of China Earthquake Administration, and the artificial seismic detection data with high signal‐to‐noise ratio was obtained by means of seismic ray tracing and travel time forward fitting solution. The crustal and upper mantle two‐dimensional P wave velocity structure model in the area was set up. The results show that the features of velocity structure in the Yangtze and the Songpan‐Garzê blocks are greatly different. The crust thickens gradually from south to north. The thickness of the sedimentary cover in the Sichuan Basin is up to 7.8 km, while in the Songpan‐Garzê block the thinnest thickness is only hundreds of meters. In the mid‐upper crust, the average velocity of the Yangtze block is 0.2 km·s−1 faster than that of the Songpan‐Garzê block. To the north of the coupling area between the basin and the plateau, about 20 km deep, there exists a weak layer/low‐velocity layer whose thickness is 8.0 km. The velocity is 5.80 km·s−1, apparently lower than the average velocity of the surrounding media, which is 6.00∼6.10 km·s−1. In the tectonic transfer zone, the seismic phase is unclear, disordered and cannot be compared sequentially. The crustal interface is discontinuous from the earth surface to the top of the upper mantle. The velocity structure is in an anomalous disorder and characterized by low velocity anomaly. In the mid‐lower crust, the velocity along the profile varies with a positive gradient and increases vertically. The fluctuation of the crustal interface is not great and the interface tends to deepen abruptly to the north of the transfer zone. The fluctuation of the Moho is especially clear. Within the scope of 50 km, the depth of the interface varies from 36.2 km in the Yangtze block to 45.8 km beneath the Songpan‐Garzê block, forming an abrupt change belt. The seismic source of Lushan Ms7.0 earthquake is located in a region in which the two‐dimension velocity structure is in an anomalous disorder and the fluctuation of the interface is obvious. The study shows that the differences of the interface and the velocity structure and the features of fluctuation have a close relation to the seismic activity in this region.