Variation Characteristic of Wave Velocity Ratio before and after Hebei Tangshan M_S4.8 Earthquake in 2012

Based on the observation report of seismic phases recorded by Hebei Digital Seismic Network from Jan. 2008 to Apr. 2018, we analyze the variation of wave velocity ratio before and after the 2012 Tangshan M_S4.8 earthquake using the Wadati method of single station and the multiple-station. The result shows a low value anomaly existing before the earthquake in Douhe and other 7 stations with Wadati method of single station. The wave velocity ratio of M_L ≥ 1.0, M_L ≥ 1.5, M_L ≥ 2.0 and different magnitudes calculated by Wadati method of multiple-station corresponds well to the Tangshan M_S 4.8 earthquake. There is a low wave velocity ratio anomaly existing nearly one year before the earthquake.