Earthquake Source Parameters in Southwestern China and Their Rheological Implications

Earthquake depth distribution provides key information on rheological behavior of the crust, which usually shows a brittle-ductile transition at a depth of about 10 km. In this study, we use the generalized cut-and-paste method to obtain source parameters of 571 earthquakes in the Sichuan-Yunnan region of China between 2009 and 2017. We were able to successfully determine focal mechanisms, moment magnitudes, and centroid depths of 536 earthquakes with a minimum moment magnitude of 3.2. Our moment magnitudes and centroid depths are systematically smaller than the magnitudes (M-s and m(b)) and hypocenter depths from the China Earthquake Network Center and International Seismological Centre catalogs for M >= 4.0 earthquakes. The earthquake depths in the Sichuan-Yunnan region are mostly in a 5-9 km range, with an average at 7.6 km. About 23% earthquakes have centroid depths <5 km and are concentrated in the southern Sichuan basin. Only very few earthquakes are deeper than 19 km. Compared with the earthquake depth distribution in southern California, the SichuanYunnan region has many shallower earthquakes. The depth distribution suggests that the brittle-ductile transition in the Sichuan-Yunnan region is shallower than the transition beneath southern California, which is probably due to the existence of newborn faults in the Sichuan-Yunnan region.