固体地球物理研究生学术报告第二期（1）

With the development of shale gas in the Changning-Zhaotong play in the southern Sichuan basin of China, which is the largest shale gas prospect in China, the frequency and magnitude of earthquakes in this region have increased significantly in recent years. Several earthquakes withM≥4.0occurred in the area including two destructive earthquakes, theM5.7 Xingwen earthquake in December 2018 and theM5.3 Gongxian earthquake in January 2019, which caused serious property loss and potential impact on shale gas development in Sichuan Basin. To further understand the local seismic activity and triggering mechanism, we conducted a two-phase dense array seismic monitoring with about 200 Zland 3C and SmartSolo 3C 5 Hz seismic nodes, from late February to early May, 2019 for a period of 70 days. The survey consists of roughly 340 deployments at 240 sites, with an average interstation distance of 1.5 km, covering 500 km² in total. We have processed seismic records from nearly two months of monitoring, and picked some 800,000 P- and S-wave arrival times from about 10,000 detected local earthquakes. The earthquake hypocenters and the subsurface velocity structure of the Changning-Zhaotong area are inverted for using the double-difference tomography method. The relocation results show that the majority of hypocenters were located at depths ranging from 1.0km to 4.0km, in the proximity of the horizontal hydraulic fracturing wells. The tomographic results (< 3 km) correlate well with the known surface geological units, and most earthquakes occurred along the velocity discontinuities, likely characterizing a large hidden fault which, interestingly, is where the January 2019 M5.3 occurred. Our study is very important for understanding the seismic potentials in this area, and should provide useful information for the shale gas development in this region and other areas in China with similar geological, tectonic and stress conditions.