Study on Stress Field and Seismogenic Mechanism of the Xianyou Earthquake Swarm in Fujian

From August 2010 to December 31, 2014, a SE-NW-trending earthquake swarm occurred in the Xianyou area of Fujian Province. The current focal mechanism results are obviously inconsistent with the local tectonic stress field. In order to confirm and analyze this phenomenon, more accurate event locations are obtained by using double-difference locating method, and then by using the P-wave initial motion data, the 158 focal mechanism solutions are solved. The dominant P and T axis of the solved focal mechanisms show significantly different from the NEE compressional axis and SSW extensional axis of the previously determined tectonic stress field. In order to further study the change of the stress field along the fault strike, we divided the obtained focal mechanism into different regions and depths, and estimated the stress field respectively. It is found that the stress direction in the southeast of Jinzhong Reservoir is further away from the direction of the background stress field, and the R value is less than 0.5, meaning the middle axis expression of tensile stress, which indicating the effect of the reservoir. The estimated stress fields along the depth show that the direction of the stress field is closer to the direction of the tectonic stress field as the depth increases. This result shows that the impoundment of Jinzhong Reservoir makes the stress field deviate from the original tectonic stress field. However, the direction of the stress field cannot be changed only by the action of the reservoir. In order to reveal the deflection of the stress direction, we propose that the shallow part presents EW extension component due to the low-angle subduction and extrusion of the Pacific plate in the deep part. Under the action of the two factors, it presents a stress state that is quite different from the background tectonic stress.