Source Characterization of Some Collapse Earthquakes due to Mining Activities in Shandong and Beijing, North China

With the large-scale underground mining, several collapse earthquakes occurred in recent years, causing serious casualties and huge property losses. The source parameters can provide key information for analyzing causes of collapse earthquakes in very shallow crust. However, the heterogeneity of shallow velocity structure is stronger than the deeper crust. In this study, we tested the source inversion robustness of shallow non-double-couple (non-DC) events based on the general cut-and-paste (gCAP) method. The results show that, when the uncertainty of the velocity model is about 10%, the uncertainty of non-DC components is less than 15% with high signal-to-noise ratio (similar to 10) data. We studied the focal mechanisms of several earthquakes in Pingyi, Shandong Province, Fangshan, Beijing, China, and also in Utah, United States, as a comparison. For the Pingyi events of December 2015, focal depths are very shallow and the sources contain about 65% closing crack components. For the Fangshan earthquake on 28 August 2017, results show that the focal depth is about 1 km. The source contains about 60% closing crack component. The focal depths and mechanisms of events in Pingyi and Fangshan are similar to the Crandall Canyon Mine event on 6 August 2007 in Utah, with a very shallow focal depth and 72% closing crack component. The primary source of collapse events in the three regions could be caused by impact force with direction to mine cavity due to the collapses of roof strata. The secondary DC source may be caused by the shear movements of mine roof and floor.