Earthquakes in the Weiyuan area in China: insights from numerical simulation

SUMMARY Shale gas extraction could produce underground stress perturbation and local seismicity, which could put a threat human casualty. The Weiyuan area in the Sichuan province, China, underwent massive gas production and a significant increase of earthquake since 2015. In this study, we focus on human-induced subsurface hydrofracturing, calculate cumulative underground Coulomb-stress changes using a 3-D numerical model and probe the main cause of recent seismic activity in the Weiyuan area based on continuous regional stress/displacement loading. The simulation reveals the regional extent of positive Coulomb stress change with fractures matches the distribution of the moderate and microseismicity in the past ten years. Background regional tectonic stress in the vicinity of the active fault likely resulted in earthquake preparation within and around the active faults; hydraulic fracturing changes mainly the displacement and stress pattern in the vicinity of the fracturing wells, and enhanced fracturing intensity [fracture volume-to-model volume ratio (θ)], causes more obvious difference; faults may be locked prior to fracturing and even small fracturing intensity may trigger the earthquakes near faults and fracturing wells; the seismic risk will be significantly increased near the two faults and fracturing wells in the next 50 yr.