Temporal v_P/v_S variation characteristics in different zones of China's Capital Circle area before and after Wen'an earthquake

On July 4, 2006, an earthquake of M_S5.1 took place in Wen'an, Hebei Province, just at the south center of China's Capital Circle area digital seismograph network. It is the strongest event recorded ever since the network went into operation in 2002. We processed the vast amounts of phase data yielded by the 107 digital seismic stations between 2002∼2007 using Wadati method. In order to improve the precision and stability of shear and compressional wave velocities (v_P/v_S) calculation, we impose a number of restrictions on the computation environment and condition, e.g., the earthquakes are densely concentrated, selected stations are limited in range, the number of stations involved in the computation is larger than 5 and linear fitting features high precision and small error. Under these restrictions, the study shows that v_P/v_S in and around Wen'an and Tangshan underwent a normal-low-normal process one year before Wen'an earthquake, v_P/v_S became obviously low and the low ratio lasted for about one year, meanwhile, little variation of v_P/v_S was seen in Xingtai, northwest of Beijing, southwest of Beijing, Beijing-Tianjin and Beijing; after the quake, the v_P/v_S returned normal in Wen'an and Tangshan. Error and stability analysis of the calculated result for v_P/v_S shows it is convincible that anomaly appeared in and around Wen'an and Tangshan before Wen'an earthquake.