Analysis of Seismicity Risk Increase Triggered by Longtan Reservoir, China, Using a Fully Coupled Poroelastic Model

Abstract:

Impoundment of reservoirs can lead to increase in crustal pore pressure and Coulomb stress, and promotes nearby faults to fail. Abnormal seismicity increase around reservoir is often thought to be triggered by the water impounded behind the dam. In this study, we analyze the impact of Longtan reservoir in Guangxi province, southwest China, on local seismicity, which has increased significantly since the initial impoundment in October, 2006. Most of the earthquakes cluster around the reservoir according to local seismic network and CEA reports, and the greatest earthquake is of magnitude 4.2. We introduce an improved Bayesian method to show that the temporal variation of b-value is inversely correlated with the seasonal fluctuation of reservoir water level. Upon extracting background seismicity using the Epidemic Type Aftershock Sequence (ETAS) model, we show that background seismicity variation is highly related to the filling rate and volume We further investigate the mechanism of seismicity increase and reservoir impoundment by performing a fully coupled 3-D poroelastic model with the reservoir loading history as a dynamic boundary condition. An optimally oriented fault plane is chosen according to the regional stress field and GPS deformation data when calculating the induced Coulomb stress. Our results demonstrate that most earthquakes occurred within positive Coulomb stress regime, and the seismicity rate is highly related to the rate of pore pressure increase. In the next step we plan to apply the coupled poroelastic model to understand hydraulic fracturing induced earthquakes in shale gas exploration, such as the recent Fox Creek event cluster, in the western Canada sedimentary basin.