Three-dimensional P-wave velocity structure tomography in the source area of Rushan earthquake swarm

Based on the observation data recorded by the fixed stations in Shandong seismic network and by portable stations of Rushan array, the three-dimensional P-wave velocity structure of the shallow crust within 15 km depth in the source region of Rushan earthquake swarm and its surrounding areas is studied by using the double difference tomographic method. The results show that the uplifted area （such as Duogushan and Dagushan） and the ultrahigh pressure rock mass near the Haiyangsuo town are high-velocity areas, and the white sand beach between the two areas is characterized by low-velocity, the Rushan sequence occurred on the side of the high-velocity of the high-low velocity transitional zone. The velocity structure section shows that the crust beneath the Rushan sequence has an obvious ellipse-like relatively low velocity region, the sequence is basically located in the high-velocity interlayer between the low-velocity area and the Quaternary cap rock. Considering the sequence distribution, regional geological structure and the position relationship between high and low velocity rock masses, it is speculated that under the background of regional stress adjustment, the instability of local medium plays an important role in the occurrence of Rushan earthquake sequence.