ABSTRACT

An ML 5.7 earthquake struck Xingwen County, Sichuan Province, China (28.239° N, 104.922° E) on 16 December 2018. More than two weeks later on 3 January 2019, an ML 5.3 earthquake occurred 8 km to the west. These were the largest and most damaging events in the Changning shale gas block and resulted in extensive damage to nearby farmhouses and other structures. Although the official hypocentral depths for both events are large than 5 km, the centroid moment tensor solutions based on the generated cut‐and‐paste method were very shallow (3 and 1.8 km for ML 5.7 and 5.3, respectively). In addition, both events were very close to hydraulic‐fracturing (HF) zones of horizontal wells, in which HF was ongoing. A convening chain of evidence, including spatiotemporal correlation between earthquakes and HF zones, statistic parameters of seismicity, and estimated overpressure required to activate the unfavorably oriented source faults of the largest events, suggests that a series of earthquakes were induced by HF at a depth of 2.53  km. In the Changning block, thus far, 11 ML4.0 events (including 3 ML5.0) were observed since the systematic HF operation began in 2014. The Omori‐type aftershock productivity is very low as compared to normal tectonic seismicity. In addition, estimated overpressure ranged from 0.3 to 5.8 MPa for Mw>3.5 events. The poroelastic stress changes are less than 1 MPa out of the treatment zones. Hence, overpressure‐driven reactivation of pre‐existing faults (most of them are unmapped and unfavorably oriented for slip under the present tectonic stress field) is considered to be the cause of these abnormal moderate‐size earthquakes.

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