Focal Mechanism of Mining-Induced Seismicity in Fault Zones: A Case Study of Yongshaba Mine in China

It is essential to investigate focal mechanisms of induced seismicity for understanding the rock fracturing, the failure mode, and the hazard evolution in underground mines. But the conventional methods using empiricism to infer the source mechanisms usually lead to ambiguous results for individual events. An optimized moment tensor inversion method using full waveforms was employed to quantitatively determine the rock fracturing orientation and the type of rupture process. Source parameters including the scalar moment, the moment magnitude, the full moment tensor, and the fault plane solutions were resolved of a seismic sequence in fault zones. Results show that the shear failure events in the fault F1 vicinities have similar focal mechanisms and suggest that the fault F1 is a reverse fault. The resolved strikes and dips are basically constant with the orientation of the fault. The events in the fault F2 vicinities are mainly dominated by shear-tensional failure. But the shear events experienced shear rupture and crack opening simultaneously, resulting in slippages not along the actual fault plane. There are more events in the fault F3 area which are characterized by complicated focal mechanisms. Three of the non-shear events are dominated by compressional failure and related to rock collapse, while the other non-shear events are dominated by tensional failure and related to crack opening. The shear dominated events experienced dual effects of shear failure and compression failure. The resolved fault plane solutions cannot reflect the actual geometry of the fault. It is proved that the moment tensor inversion is able to quantitatively analyze the focal mechanism of mining-induced seismicity in fault zones and it provides beneficial understandings of mining-induced fault slips.