Earthquake Swarms on June 28th of 2010, near Lingyun and Fengshan County, South China: Shallow Seismic Activity of Fault Induced by Surface Fluid in Karst Geology Area

An instantaneous intense rainfall occurred in the Fengshan County area, Guangxi Province of South China on June 27, 2010. One day later, an strong seismicity with shallow depth and magnitudes less than M_L3.0 occurred. It caused severe economic losses. It is not clear whether or not the activity of this shallow earthquake swarm is correlated with the heavy rainfall. Based on the tectonic setting in the study area, the activity characteristics, focal mechanism solutions of the earthquake swarm and its relationship with faults are investigated, and then a stress model is proposed to analysis fault slip triggered by pore-fluid pressure. It is concluded that the activity of this shallow earthquake swarm is highly correlated with the heavy rainfall after a prolonged drought in this region, including the occurrence time, epicenter location and the frequency. Prolonged drought and lack of groundwater in karst areas have a positive effect on the accumulation of stress in the shallow crust. In karst regions, the underground caverns, river and vertical cracks can be propitious to the fast penetration of surface water. Pore pressure changes cause the fault strength weakening, thereby trigger the unlocking and slip on the shallow part of fault. Discrete close overpressure fluid and abnormal stress field formed due to the shallow slip of fault. The shallow seismic activity comes from the slip and collapse of shallow sub-fractures and caves in karst regions.