Source Parameters of Three Moderate Size Earthquakes in Weiyuan, China, and Their Relations to Shale Gas Hydraulic Fracturing

On 7 September 2019, an M(L)5.4 earthquake struck the Weiyuan area in southwestern China. This is the largest and most damaging event in the Weiyuan shale gas field. In the next 4 months, two earthquakes with M(L)4.3 and M(L)5.2 rattled the same area. In this study, we obtain source parameters of these earthquakes (e.g., centroid location, focal mechanism, and rupture directivity) to investigate whether they are induced by hydraulic fracturing of shale gas production. We first invert for high-resolution centroid locations and focal mechanisms of two M similar to 3 earthquakes recorded by a temporary dense seismic array. We then adopt the M similar to 3 events as references to obtain reliable centroid locations and rupture directivity of those moderate-size earthquakes. The results show that all earthquakes have centroid depths of no more than 3 km and are within 2 km to injection wells of shale gas production, which suggest that these earthquakes may be induced by local hydraulic fracturing. The M(L)5.2 earthquake has the same rupture directivity to NE as the M(L)5.4 event and is located in the zone of positive Coulomb stress change caused by the latter. However, the long time lapse of the M(L)5.2 earthquake and the existence of an injection well within 2 km suggest that it is more likely induced by hydraulic fracturing than triggered statically by the M(L)5.4 earthquake.