关键词: 双差层析成像 P波速度结构 地震定位 荣昌及周边

Abstract: The Ms4.9 earthquake on the 27 December 2016 occurred in Rongchang District, Chongqing, with the epicenter in the middle-north section of Huayingshan basement fault system on the eastern margin of Sichuan Basin. The seismicity in this area was originally very weak in the historical earthquake catalogue. There have been frequent earthquakes of magnitude Ms≥4.0 since the natural gas field wastewater was reinjected in this area in the 1980s. According to statistic, there have been 14 earthquakes of Ms≥4.0, the largest of which was the Ms5.2 Rongchang earthquake in 1997. In this paper, the fine three-dimensional P-wave velocity structure and relocation results of seismic events in Rongchang and its surrounding areas are inversed by double difference tomography method, basing on the P-wave and S-wave arrival time data of 1786 seismic events recorded by Chongqing Regional fixed network, mobile network and Zigong local network from January 2008 to June 2020. The results show that 1）It is 12 km the boundary depth between sedimentary cover and crystalline basement in Rongchang area and the distribution of high and low velocity zones within 4 km depth is obviously different from that of the depth layer below 7~13 km. The velocity structures at the depth of 7~13 km show that the east to the Huayingshan basement fault is low velocity zone and the west is high velocity zone respectively, which are basically consistent with the previous research results. 2) The lateral difference of P wave velocity structure in shallow crust of Rongchang area varies greatly. The high velocity zones are near the water injection wells of Luo2# located at the axis of Luoguanshan anticline, and the depth distribution is 3~7 km. The hidden fault in the north wing of Luoguanshan anticline with buried depth of 1.7 km is developed near well Luo2#, and the high velocity zone is distributed along the dip of hidden fault, which may indicate that the hidden fault may be the main channel for wastewater infiltration. The depth of wastewater infiltration is up to 7 km, resulting in a large velocity difference between the two sides of the fault. The Ms4.9 earthquake on December 27, 2016 and the Ms4.0 earthquake on December 28, 2016 are just distributed in the velocity transition zone. 3) The results of seismic relocation indicate that earthquakes are mainly distributed in the axis of the strongly deformed Luoguanshan anticline, showing obvious stripe distribution in NE direction, and the focal dominant depth is 0~6km. Based on the focal mechanism solution and the regional seismotectonic environment, it is shown that the seismogenic fault of earthquakes above Ms4.0 on the south side of Guangshun transverse fault should be the hidden fault on the south wing of luoguanshan, while the seismicity on the north side of Guangshun transverse fault may be related to the hidden fault on the north wing of Luoguanshan.

Key words: Double difference tomography, P-wave velocity structure, earthquake relocation, Rongchang and adjacent area