Structural Control of Induced Seismicity in the Weiyuan Anticline, Southwestern Sichuan Foreland Thrust Belt, China

AbstractOver the past decade, the development of shale gas in the southwestern Sichuan foreland thrust belt, China, has led to a significant increase in induced seismicity, including one of the world's largest hydraulic fracturing (HF)‐related induced earthquake (ML 5.4), occurring in the Weiyuan anticline. Here, we investigated the structural and mechanical evolution of the Weiyuan anticline and its relationship with HF‐induced earthquakes using seismic interpretation, the discrete‐element method (DEM), and three‐dimensional structural modeling. Interpretation of the seismic reflection profiles revealed a basement‐involved wedge structure and two west‐dipping multi‐bending thrust ramps in the basement. East of the Weiyuan anticline, extensive small‐displacement thrust faults developed between the Cambrian and Silurian shale detachments in the sedimentary cover. DEM simulation showed that the structural wedge formed by the underlying blind thrust ramp and back thrust (BT) controlled the Weiyuan anticline formation. With increased shortening, the fault slip continued to propagate east of the Weiyuan anticline along the Cambrian detachment, leading to the development of extensive thrust faults. 3‐D structural modeling showed that M > 4 induced earthquakes were primarily located on a BT in the structural wedge, while M < 4 events were distributed along the hydro‐fractured Silurian shale and Cambrian detachment. Densely developed thrust faults in the sedimentary cover connect the overpressured Silurian shale to the Cambrian detachment, promoting the downward diffusion of fluid pressure and favoring micro‐small M < 4 induced seismic events. This study provides new insights into the seismic hazard assessment of HF development in fold‐and‐thrust belts.