Variation of crustal seismic velocity in Rushan region of Shandong Province by using repeating earthquakes

In order to explore characteristics of the crustal media variation in Rushan region of Shandong Province, we utilize seismic waveform data of Rushan swarm from October 2013 to June 2015. Based on the principle that the earthquakes recorded by three nearest stations with cross-correlation coefficient c≥0.9 is repeating earthquakes, we firstly identified repeating earthquakes of Rushan earthquake swarm. Secondly, by using the waveform cross-correlation delay method, the position of a double repeat earthquake waveform cross-correlation function reaching the maximum is the travel-time difference. We calculated and analyzed the variation of travel-time difference with time around the stations WED and ZHY. The results show that the waveform similarity of Rushan earthquake swarm is good, and the epicentral locations are concentrated, repeating earthquakes have larger time span, which will improve the time resolution of observations. The short-term low travel-time difference abnormal phenomena appeared before three Rushan earthquakes with M_S>4.0, reflecting that epicenter area of Rushan earthquake swarm always appears the process that velocity of crustal media increases significantly in short-time before moderate and strong earthquakes, that have some indication significance for prediction of moderate and strong earthquakes in this region.