A Possible Mechanism of Reservoir-Induced Earthquakes in the Three Gorges Reservoir, Central China

Induced seismicity has been observed at the Three Gorges Reservoir, central China, since its impoundment in May 2003. According to the data from a local seismic network deployed before the impoundment, more than 4000 earthquakes occurred between September 2008 and September 2017 at the intersection of the Fairy Mount fault and Nine-Brook fault. The earthquakes are relocated using a double-difference method. Based on the relocated hypocenters, the temporal and spatial variations in the earthquakes with water impoundment are investigated. The investigation indicates that the majority of the induced earthquakes before September 2013 occurred in the water-loading stage within a depth of 5 km, which is considered to be initial seismicity. After September 2013, most earthquakes occurred in the water-unloading stage with larger magnitude and greater depth and are considered to be protracted seismicity. The focal mechanisms of the earthquakes that are larger than M 2.0 are calculated and indicate that most earthquakes are associated with the local northwest- and northeast-striking reverse faulting. A few earthquakes have focal mechanisms representing normal faulting and are associated with the tensile fractures caused by the stress adjustment. Finally, a possible mechanism of the reservoir-induced earthquakes in the Three Gorges Reservoir is proposed, which explains the predominant factor for the induced seismicity for different water impoundment stages.