The pore pressure diffusion characteristics of Xianyou earthquake swarm in Fujian Province

From August 2010 to December 2015, a series of small earthquake swarms occurred in the Xianyou area of Fujian Province. Based on the pore pressure diffusion mecha-nism, the small earthquakes data of precise location is analyzed, and it is found that the Xianyou earthquake swarm sequences are cluster distributed in spatial terms and have obvious zoned phenomenon. The origin time is obviously influenced by the modulation effect of water level change in Jinzhong reservoir. The pore pressure diffusion coefficients of five subregions （A to E） were 0.04, 0.08, 0.07, 0.12 and 0.05 m²/s, and along with the seismicity intensity changing from strong to weak, the pore pressure diffusion coefficients changed from increasing to decreasing. The Xianyou earthquake swarm is located in the area where the fault structures developed, and the predominant direction of epicentral distribution is in accordance with the Shicang fault, and the pore pressure diffusion coefficient along the fault strike is the maximum, which shows that the active tendency of the underground fluid is along the Shicang fault from northwest to northeast. The fluid pore pressure coefficient in each subregion is closely related to the maximum diffusion distance, the maximum induced earthquake and the energy released by earthquakes. In addition, when the water level of Jinzhong reservoir falls, the distance from the first small earthquake （trigger source） to the triggered earthquake in the subregion has a decreasing trend, with the range of earthquake groups shrinking, and earthquake frequency and magnitude increasing. This could lead to the conclusion that, when the water level drops, fluid pore pressure diffusion acted repeatedly at the same site, which caused the stress level easier to reach the saturation or critical level in the area near the trigger source, and then triggered a larger earthquake, this is also a possible explanation for the occurrence of stronger earthquakes during the drawdown of water levels. This study will be of reference significance to the prediction of seismicity in Xianyou area in the future, and provide a scientific basis for quantitatively determining risk of reservoir induced seismicity.