Shear zone stress release heterogeneity associated with two mining-induced events of M 1.7 and 2.2

We analyse space-time variations in stress release estimates for microseismic events (M < 0) and two larger seismic events of M 1.7 and 2.2 associated with a schist shear zone at Lockerby mine, Sudbury (Ontario). These events occurred following sequential sill pillar extractions between October 1992 and January 1994 and were recorded with a local underground array. Also, we calculate shear and normal stress variations along the schist using 3-D numerical models of the excavation. The distribution of post-event microseismicity suggests that the two larger events were related to the failure of local adjacent asperities. They, however, exhibit different source mechanisms as a result of local variations in the orientation of both the schist and principal stresses. Similar heterogeneous patterns along the schist zone are found in static stress drop, apparent stress, and dynamic stress drop distributions. Numerical modelling results show that the stress release patterns are not only influenced by the shear stress, but are also strongly dependent on the resisting stress level. We conclude that high stress release estimates are not necessarily indicative of higher slip potential, and that differences between the conditions at failure of the two larger events are likely due to variations in the normal relative to sheer stress levels. (C) 1998 Elsevier Science B.V. All right reserved.