The 2019 MW 5.7 Changning Earthquake, Sichuan Basin, China: A Shallow Doublet With Different Faulting Styles

The increased seismic activity of the last similar to 10 years in Changning county of Sichuan Province comprised just small (mostly M-L < 5.0) injection-induced earthquakes. The M-W 5.7 earthquake on June 17, 2019, is the largest event ever reported there. Moment tensor of the mainshock was remarkably dominated by a compensated linear vector dipole. We resolve its fine structure showing it was a doublet, allowing approximation by a thrust- and strike-slip subevent. The mainshock nucleated as thrust faulting, which (together with the largest aftershocks) can be linked with previously known reverse faults, favorably oriented to regional stress field. Contrarily, the strike-slip segment of the mainshock, less favorably oriented, was probably facilitated by elevated pore pressure due to previous injections. Shallow active strike-slip faulting, not yet mapped in the region, is a new feature, important for future hazard assessment. Plain Language Summary Changning county in Sichuan Basin belongs to major salt-producing resources, with water injection wells drilled to similar to 3-km depths. Enhanced occurrence of small earthquakes was observed in the last similar to 10 years. However, on June 17, 2019, a damaging earthquake struck the region. Seismic agencies reported diverse focal mechanisms and pointed to departures from shear faulting, typical for tectonic events. To understand these enigmatic features, we modeled records from 12 broadband seismic stations, adding over 100 first-motion polarities. We found that the earthquake was very shallow (similar to 4 km) and consisted of two shear events: the initial thrust faulting and the following strike-slip faulting along an SE-NW trending near-vertical plane, aligned with aftershocks. The initial episode was normal, expected on existing faults and supported by tectonic stresses of the region. Contrarily, the significant strike-slip faulting appears to be rather abnormal, probably facilitated by the initial shock and by elevated pressure of underground fluids. The 2019 earthquake provided a rare opportunity to illuminate interplay between long-lasting water injections, tectonic stress, complexity of existing faults, and their dynamic interactions. That is how seismology aims at making industrial activities sustainable and safe.