Analysis of stress field and its causes of the Xianyou earthquake swarm in Fujian

This study employed the double-difference location method to relocate earthquakes recorded by the seismic network center in the Xianyou region of Fujian from August 1, 2010, to December 31, 2014. It also used the initial motion of P waves to determine the focal mechanism solutions for 159 events within this seismic cluster. The results reveal a significant inconsistency between the focal mechanism solutions of the cluster and the background tectonic stress field. To further investigate the reasons behind this discrepancy, the obtained focal mechanism solutions were analyzed to calculate the stress field by region and depth. The findings indicate that in the southeastern part, near the Jinzhong Reservoir, the direction of stress deviates more from the background tectonic stress direction, with a stress shape factor less than 0.5, and the intermediate axis showing extensional stress. As depth increases, the direction of the stress field also becomes closer to that of the background tectonic stress field. Additionally, based on previous research, it is inferred that the deviation in stress direction in this area is due to the low-angle subduction and compression of the Pacific Plate at depth, leading to extension at shallow levels, with the stress manifesting as east-west extension. Under the combined influence of these two factors, the stress field results deduced from the focal mechanism solutions differ significantly from the background stress field.