广西凌云-凤山“6·28”震群:岩溶发育区流体触发断层浅层活动机理

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

2010年6月28日,广西凌云-凤山地区在特大强降雨活动之后发生密集的浅源低震级地震活动,造成严重经济损失.该震群活动是否与强降雨有关,强降雨是否可以触发震群活动,仍存在争议.基于该地区地震构造背景,对震群的活动特征、震源机制解及其与断裂构造的关系进行了分析,并建立了断层内流体孔隙压力触发断层滑动的力学模型.该浅层震群活动的发生时间、震中位置和活动频次均与特大强降雨具有密切的相关性.认为岩溶地区长期干旱和地下水缺乏有利于地壳浅层的应力积累.地表流体很难通过下渗扩散触发断层的完全解锁和深部滑动,但岩溶裂隙和管道有利于地表流体快速汇聚下渗,引起断层浅层强度的弱化,导致断层部分解锁滑动.并在断层附近形成局部应力场异常和离散的封闭性超压流体,触发密集的浅层低震级震群活动.

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 ML3.0 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 rain- fall. 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.