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:

An M_S4.9 earthquake occurred at 08:17 on the December 27, 2016 in Rongchang District, Chongqing, and the epicenter is located in the north central section of Huayingshan basement fault system on the eastern margin of Sichuan Basin. The seismicity shown in the historical earthquake catalogue was originally very weak in this area. Since the late 1980s, due to the impact of waste water reinjection in the natural gas field, earthquakes of magnitude ≥4.0 occurred frequently and 14 earthquakes with M_S≥4.0 have occurred, the largest of which was Rongchang M_S5.0 earthquake in 1997. In this paper, the fine three-dimensional P-wave velocity structures and relocation results of seismic events in Rongchang and its surrounding areas are inversed by double difference tomography method, based 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)The distribution of high-velocity and low-velocity zones within 4km depth is significantly different from that below 7~13km depth. The P-wave high-velocity zone at 4km depth is mainly distributed in Renyi-Rongchang area, where there are four water injection wells, a major concentration area of continuous water injection in Rongchang since 2008. The range of Renyi-Rongchang high velocity zone significantly gets narrowed at the 7km depth and is obviously different from that at the shallow surface. The velocity structures on the east and west sides of Huayingshan basement fault vary greatly from 7 to 13km. The P-wave velocity structures of different sections across Huayingshan basement fault all indicate that the depth of the interface between the sedimentary cover and crystalline basement is 12km in Rongchang area, which is basically consistent with the previous research results and the characteristics of seismic reflection profiles in Rongchang area. The inversed velocity structures well mirror the shape of Luoguanshan fold, and further confirm the reliability of our results. 2)The lateral difference of P-wave velocity structure in the shallow layer of Rongchang area varies greatly. There is a high-velocity zone near the Luo2^# water injection well at the axis of Luoguanshan anticline and the depth distribution is 3~7km. The hidden fault in the north wing of Luoguanshan anticline with buried depth of 1.7km is developed near well Luo2^#, and the high velocity zone is distributed along the dip of the 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 7km, resulting in a large velocity difference between the two sides of the fault. The M_S4.9 earthquake on December 27, 2016 and the M_S4.0 earthquake on December 28, 2016 are just distributed in the velocity transition zone. Obvious high-velocity body was not found below 3km in Luo4^# water injection well, which may be related to the cessation of water injection in Luo4^# well in February 2001. 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 believed that the seismogenic fault of earthquakes above M_S4.0 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