What causes the spatiotemporal patterns of seismicity in the Three Gorges Reservoir area, central China?

Both artificial and natural hydrosphere changes can impact regional seismicity. In the Three Gorges Reservoir (TGR) area, because of human regulation, the reservoir water level and natural rainfall show opposite seasonal patterns: when the reservoir water level is high, the precipitation amount is low, and vice versa. To investigate whether these artificial and natural factors are related to regional spatiotemporal seismicity and, if they are, to reveal the possible mechanisms, we used statistical analysis and calculated changes in stresses and pore fluid pressures due to these factors. We found that, based on the distances of the epicenters to the TGR, earthquakes can be divided into two groups: those occurring within 0–12 km and those occurring within 32–70 km. Our results show that the reservoir water-level fluctuation influences the seismicity near the TGR (0–12 km), resulting in fewer earthquakes in summer and more in winter. Stress changes caused by the elastic response to water loading play an important role in this trend. In contrast to the seasonal pattern of near-TGR seismicity, earthquakes far from the TGR (32–70 km), to some extent, show opposite pattern: more in summer and less in winter. Furthermore, we found that seismic productivity far from the TGR (32–70 km) is moderately correlated with seasonal precipitation. But it cannot be explained by the precipitation-induced pore pressure variations. • Seismicity is concentrated within 0–12 km and 32–70 km from the reservoir. • The seismic productivities of two groups show different seasonal variations. • Elastic response of impoundment can trigger most earthquakes near the reservoir. • Rainfall-induced pore pressure cannot explain the seismicity far from the reservoir.