Abstract:

Xianyou area is located in the middle part of the Fujian southeastern coastal region. Historical seismicity events around Xianyou are relatively rare, and the area belongs to a weak seismic area with few earthquakes. However, since Jinzhong Reservoir in this area started to operate in May 2010, earthquake activities became more frequent near the reservoir area. To deeper our understanding in the seismicity, seismic distribution characteristics, and search for local concealed faults, based on the seismic travel time data provided by the China Seismological Bureau, the double-difference relocation method was used to relocate the earthquakes in the Xianyou area in the past decade, and more accurate source positions were obtained. According to our results, fitting deep faults to search for concealed faults. The results also show that: (1) The relocated hypocenters are more concentrated. They can be divided into four active regions according to earthquake occurrence time, and are mainly distributed along the NW direction on both sides of the secondary Shicang fault at Shaxian-Nanridao. (2) Focal depth of Xianyou earthquake swarm after relocation is mainly 8-11 km. The focal depth of the seismic belt on the left side of the Shicang fault is 6-12 km; The seismic strip on the right side shows obvious stratification. The earthquakes on the northwestern side of the upper layer are relatively disperse, while those on the southeastern side are relatively intensive. The focal depth is 6-12 km on the left side, while there are fewer earthquakes on the lower layer, with focal depth of 14-23 km. (3) According to the relocated source position, three deep fault planes are fitted by the singular value decomposition method, and they dip to the southwest and strike northwest, consistent with the dip and strike of the Shicang and Tongguan faults. Based on previous studies and our results, we speculated that Shicang fault is not the main seismogenic fault, but the deep steeply concealed faults on both sides of it, which are indirectly affected by the reservoir impoundment, triggering earthquakes.

Key words: Xianyou, Fujian, double difference relocation, reservoir-induced earthquake, fitting deep fault, concealed fault