Possible link between long-term and short-term water injections and earthquakes in salt mine and shale gas site in Changning, south Sichuan Basin, China

Late at night on 17 June 2019, a magnitude 6.0 earthquake struck Shuanghe Town and its surrounding area in Changning County, Sichuan, China, becoming the largest earthquake recorded within the southern Sichuan Basin. A series of earthquakes with magnitudes up to 5.6 occurred during a short period after the mainshock, and we thus refer to these earthquakes as the Changning M6 earthquake sequence (or swarm). The mainshock was located very close to a salt mine, into which for ~3 decades fresh water had been extensively injected through several wells at a depth of 2.7–3 km. It was also near (within ~15 km) the epicenter of the 18 December 2018 M5.7 Xingwen earthquake, which is thought to have been induced by shale gas hydraulic fracturing (HF), prompting questions about the possible involvement of industrial activities in the M6 sequence. Following previous studies, this paper focuses on the relationship between injection and seismicity in the Shuanghe salt field and its adjacent Shangluo shale gas block. Except for a period of serious water loss after the start of cross-well injection in 2005–2006, the frequency of earthquakes shows a slightly increasing tendency. Overall, there is a good correlation between the event rate in the Shuanghe area and the loss of injected water. More than 400 M ≥ 3 earthquakes, including 40 M ≥ 4 and 5 M ≥ 5 events, had been observed by the end of August 2019. Meanwhile, in the Shangluo area, seismicity has increased during drilling and HF operations (mostly in vertical wells) since about 2009, and dramatically since the end of 2014, coincident with the start of systematic HF in the area. The event rate shows a progressively increasing background with some fluctuations, paralleling the increase in HF operations. More than 700 M ≥ 3 earthquakes, including 10 M ≥ 4 and 3 M ≥ 5 in spatially and temporally clustered seismic events, are correlated closely with active fracturing platforms. Well-resolved centroid moment tensor results for M ≥ 4 earthquakes were shown to occur at very shallow depths around shale formations with active HF, in agreement with some of the clusters, which occurred within the coverage area of temporary or new permanent monitoring stations and thus have been precisely located. After the Xingwen M5.7 earthquake, seismic activity in the salt well area increased significantly. The Xingwen earthquake may have created a unidirectional rupture to the NNW, with an end point close to the NW-trending fault of the Shuanghe earthquake. Thus, a fault in the Changning anticline might have terminated the fault rupture of the Xingwen earthquake, possibly giving the Xingwen earthquake a role in promoting the Changning M6 event.