Investigation on the seismogenic mechanism of the 2014 earthquake swarm in Shanxi reservoir, Zhejiang Province

We relocated 4184 earthquakes that occurred in Shanxi reservoir, Zhejiang Province from 12 September to 30 December, 2014 by using double-difference algorithm and determined the focal mechanism solutions of 11 M_L≥3.0 earthquakes using the method of CAP in this paper. Based on the analysis of the high-resolution hypocenters and focal mechanisms, we investigated the activity features of the earthquake swarm and its relationship with the fault. On the mapping view, the epicenters of the 2014 earthquake swarm occurred in the extended northwest direction of 2006 seismic swarm belt, which forms a linear belt, running parallel to the south of the Shuangxi-Jiaoxiyang fault. The relocated hypocenters mainly distribute in layers from 0.7 to 6 km in depth. The cross section profile which is perpendicular to the seismic belt strike shows that the seismic plane dips to southwest with a high angle. Most focal mechanisms are strike-slip type. The nodal planes of the focal mechanism solutions that share the same orientation with the trend of the seismic belt distribution of NW show the feature of dextral strike-slip. Taking the fault location error into consideration, the earthquake swarm may align along the rupture plane of the Shuangxi-Jiaoxiyang fault. The high-resolution hypocenters and focal mechanisms delineate the structure and activity nature of the fault. Since there is no distinctive main shock in the earthquake swarm and most earthquakes locate above 6 km in depth, so we can conclude that the earthquake swarm was not caused by the activity of the Shuangxi-Jiaoxiyang fault, but induced by the reservoir water storage. However, it is possible that the Shuangxi-Jiaoxiyang fault may be activated by the water storage of reservoir to generate tectonic earthquake in the future. No evidence shows that there is obvious relationship between the level of the reservoir water storage and seismicity, and the swarm activity becomes greater with time, which may be caused by the pore pressure increasement due to the long term penetration of the reservoir water to the fault plane.