Analysis on the seismogenic mechanism of Changdao earthquake swarm in Shandong province

Abstract

The Changdao earthquake swarm in 2017-2018 was relocated by using the data of the seismic observation report of Shandong Seismological Network, and the focal mechanism solutions of the significant earthquakes in the swarm were calculated by FOCMEC method. The results show that the overall distribution of the Changdao earthquake swarm is NWW. The epicentral location of the north earthquake swarm is obviously distributed along the Dazhushan-Weihai North Fault, which is a nearly upright high-angle fault, inclining to NNE, with a dip angle of about 80°, and a strike-slip type. The focal mechanism solutions of five earthquakes with ML4.0 or higher show that, the northern earthquake swarm was formed by the activity of the branch faults of Dazhudao-Weihai North Fault under the action of local tectonic stress field, and the southern earthquake swarm was related to Shendaokou Fault and Changdao-Zhifudao Fault. The data above ML 2.0 in the earthquake swarm were selected, using the double difference wave velocity ratio method to calculate the wave velocity ratio in Changdao area. The average values of wave velocity ratio of North earthquake swarm and South earthquake swarm in Changdao are 1.7301 and 1.7217, respectively, and the fluctuation range is between 1.69 and 1.78. During the active period of the earthquake swarm or before the occurrence of a large earthquake, the ratio of wave velocity increased gradually and decreased after the occurrence of the earthquake. The calculation of the source spectrum parameters shows that the high frequency attenuation coefficient γ is generally higher than 2, which shows that the seismogenic mechanism of the Changdao earthquake swarm is not simply the fault rupture, the high-pressure fluid intrusion may play an important role, which is in line with the process of expansion-saturation-occurrence. The average value of stress drop in this region is lower than the average level of Jiaodong Peninsula, and the faults are densely distributed and broken, which makes it difficult to accumulate large stress. The continuous activity of earthquake swarm belongs to the process of low energy rupture, which forms the activity characteristics of more moderate and small earthquakes but less moderate and strong earthquakes in this region, and has no background of strong earthquakes in the short term.