Evidence of shallow large earthquake associated with fault linkage

Tectonic earthquakes are usually thought to be the result of stick-slip frictional instability along a pre-existing fault. As a result, large earthquakes rarely nucleate at shallow depths. In the present study, we relocated the 8 September 2019 Ms 5.4 earthquake in Weiyuan, Sichuan, China, and 943 aftershocks. The results suggest rather surprisingly that these seismic events occurred at a depth of less than 3.5 km, rupturing the relay zone between two active thrust faults. Prior to the Ms 5.4 earthquake, the ruptured relay ramp zone was subjected to abnormally high shear stress, as demonstrated by our stress field modeling. This indicates that the two active faults are strongly interacted and prone to connect. The increased shear stress caused strong elastic accumulation and thus induced the Ms 5.4 earthquake. We believe this mechanism should garner widespread attention, particularly for shallow earthquake assessment.