Stress Features Inferred From Induced Earthquakes in the Weiyuan Shale Gas Block in Southwestern China.

Stress features, particularly local stress field and earthquake stress drops, are important to understand mechanism of induced earthquakes. Since shale gas exploitations in 2015, the Weiyuan shale gas block has experienced frequent earthquakes. In this paper, we determine focal mechanisms of 257 events with ML > 1.5 by fitting three‐component waveforms, invert for direction of maximum horizontal stress for two dense earthquake clusters, and then calculate stress drops for 17 earthquakes with moment magnitudes between 2.2 and 2.75 through the spectral ratio method. The focal mechanisms of all earthquakes are reverse faulting. The orientation of local maximum horizontal compressive stress is in the ESE direction, consistent with crustal movement indicated by GPS measurements. Both the focal mechanisms and microseismicity locations suggest the existence of steeply dipping faults with dip angles of ∼70°, which are relatively rare and difficult to slip under usual conditions, unlike major induced earthquakes with strike‐slip or low‐angle thrusting faults in the midwestern United States and western Canada. However, the steeply dipping reverse events can be induced by large pore pressure from hydraulic fracturing. The stress drops range from 2.5 to 54.7 MPa, comparable to those of potentially induced earthquakes in the midwestern United States. Our results imply earthquakes in the Weiyuan area are controlled by local tectonic stress and induced by large pore pressure from hydraulic fracturing, which advances our knowledge of reactivation of steeply dipping reverse faults. Plain Language Summary: Earthquakes in the Weiyuan area in southwestern China are thought to be induced by hydraulic fracturing due to shale gas production. Local stress field and earthquake stress drops provide crucial information to understanding their physical mechanisms. Here we determine source parameters of 257 events with magnitude greater than 1.5, invert for direction of maximum horizontal stress, and calculate stress drops for 17 representative events. The results suggested that all the earthquakes occurred on reverse faults and the inferred maximum horizontal stress orientations agree with local GPS measurements. Surprisingly, we find some steeply dipping faults with dip angle of ∼70°, which are assumed to be stable under usual stress conditions. The stress drops range from 2.5 to 54.7 MPa, which are comparable to those of potentially induced earthquakes in the midwestern United States. We infer that the steeply dipping reverse events are induced by high pore pressure from hydraulic fracturing. Our results imply earthquakes in the Weiyuan area are controlled by local tectonic stress and induced by hydraulic fracturing with high‐pressure fluid injection. Key Points: Focal mechanisms of 257 ML > 1.5 earthquakes suggest consistent maximum horizontal stress with local tectonic settingsSteeply dipping reverse faults may be reactivated by high pore pressure fluid during hydraulic fracturingStress drops of the induced earthquakes range from 2.5 to 54.7 MPa [ABSTRACT FROM AUTHOR]