Observations of heterogeneous stope convergence behavior and implications for induced seismicity

Abstract

Analysis of stope convergence measurements in a burst-prone lead-zinc-silver mine is being used by the U.S. Bureau of Mines to study rock burst processes. Three convergence gages were installed in a 3-m wide stope of the Galena Mine with an inter-gage distance of approximately 8 m. The two-month period immediately after installation, during which the subsequent cut of the pillar was made, is the subject of the analysis described in this paper. Microseismicity rates and locations (local magnitude −5 and above) were also recorded during this period. Coseismic convergence steps within the 10-minute sample interval are observed at various times on all three gages. The convergences of the three gages do not track one another, indicating the blocky nature of deformation in this stope at a 10 meter or smaller size scale. The behavior of the three gages is different with respect to coseismic and interseismic convergence during the study period. For one of the gages, 82% of total closure occurs coseismically. The other two gages show much smaller percentages of coseismic convergence (30% and 35%), and proportionately higher percentages of interseismic steady creep. This heterogeneous convergence behavior may be indicative of the relative stability of the faults in the vicinity of each gage. An examination of stability models shows that the mine-geometry-related spatial variation in normal stress, acting on an existing set of faults, may explain the observed behavior.