Restudy of the variation of v_P/v_S before and after the Wenchuan earthquake

Based on the seismic phase data recorded by Sichuan Digital Network from 1990 to 2012, this paper analyzes the variation of wave velocity ratio v_P/v_S before and after the 2008 Wenchuan M_S8.0 earthquake by using Wadati method of single station and multi-earthquake. We carefully winnow the date and select 13 stations with relatively abundant and continuous data. The results show that the wave velocity ratio obviously decreased at four stations before the Wenchuan earthquake in the western side of middle-northern Longmenshan fault, which lasts for almost seven years; while the ratio did not show abnormal varaiton at the other nine stations. The position of the four stations with anomalous wave velocity ratio are consistent with Wenchuan earthquake seismogenic zone, providing sufficient evidence for whether the temporal variation process of crustal media characteristics exists before the Wenchuan earthquake.