SHEAR ZONE STRESS RELEASE HETEROGENEITY ASSOCIATED WITH 2 MINING-INDUCED EVENTS OF M 1.7 AND 2.2

We analyse space-time variations in stress release estimates for micro seismic 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 u nderground array. Also, we calculate shear and normal stress variation s along the schist using 3-D numerical models of the excavation. The d istribution 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 va riations in the orientation of both the schist and principal stresses. Similar heterogeneous patterns along the schist zone are found in sta tic stress drop, apparent stress, and dynamic stress drop distribution s. Numerical modelling results show that the stress release patterns a re not only influenced by the shear stress, but are also strongly depe ndent on the resisting stress level. We conclude that high stress rele ase estimates are not necessarily indicative of higher slip potential, and that differences between the conditions at failure of the two lar ger events are likely due to variations in the normal relative to shee r stress levels. (C) 1998 Elsevier Science B.V. All right reserved.