Temporal variation of vP/vS, vP and vS before and after 2008 Wenchuan earthquake

On May 12, 2008, a great earthquake of MS8.0 occurred on Longmenshan fault, Sichuan, China. With the stress change near-source region before and after the mainshock, would there have been corresponding physical property change of crust media? It became one of the hot questions in the research of Wenchuan earthquake. In this study we collected seismic phase data from January 1, 2001, to May 31, 2010, reported by Sichuan Digital Station Network of China Earthquake Administration (CEA), and the phase data from May 12 to September 30, 2008, reported by the Mobile Digital Seismic Network of Institute of Geophysics, CEA. We processed these data using the Wadati method. Based on the phase arrival times of selected direct wave Pg and Sg we paid special attention to the study of temporal variation of wave apparent velocity, and compared the velocity variation with the temporal variation of wave velocity ratio. To ensure the accuracy and stability of the calculated result, we carefully winnowed the data. Our research result shows that, in Longmenshan fault zone and its surrounding area, there was a 4 years' obvious decrease process of P-and S-wave apparent velocity before the Wenchuan earthquake, and at the time of half year before the earthquake an abrupt return appeared. Different from apparent velocity, the anomalous variation of wave velocity ratio showed a low amplitude decrease and lasted only for more than 2 years. But both wave velocity ratio and apparent velocity decrease returned fast at the time of half year before the mainshock.