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Patterns of induced microearthquakes at the Sullivan Mine, Kimberley, B.C. Coenraads, Robert Raymond


In June and July of 1980 a 12 station microseismic detection network was established over a 40 km² area about the Sullivan Mine, Kimberley, B.C. for the purpose of using hypocentre locations of mining induced earthquakes to delineate the position of fault planes in 3-dimensions. During the experiment 1551 microseismic events were recorded by 2 or more digital seismographs, but only 366 events were large enough to be recorded by 4 or more stations. A regional velocity of 5.30 kms⁻¹ was determined from calibration explosions and a 3-dimensional least squares type location program was used. Based on explosions of known location within the mine a location accuracy of 180 m in epicentral position and 500 m in focal depth was attained. Recorded microseismic activity occurred entirely within the mine bounds, centered on the active working areas. An extremely close relationship between level of microseismic activity and mining cycles was observed. The seismic array at the Sullivan Mine was found to be unsuitable for use as a geologic mapping tool, partly because of the lack of resolution. However several interesting features of the induced microseismicity have become apparent: (i) Influence of mining activity at the Sullivan is extremely localized, with microearthquake activity confined to the working areas. (ii) The correlation between mine activity and recorded microseismic activity was extremely high; the majority of microearthquakes, in particular the smaller events, occurred in the first ten minutes after large mine blasts. These appear to be cavity relaxation events. (iii) There appears to be an association between the epicentral locations and the edges of cave areas. The only activity which may be associated with faults occurs when they are quite proximal to the cave edges; that is where tensional stresses in the hanging wall are localized along these pre-existing planes of weakness. (iv) It is believed that regional stress conditions are unsuitable to allow fault reactivation below the mine. Geologic evidence suggests' that the regional stress in the vicinity of the Sullivan is either neutral or extensional, in which case fault reactivation below the mine will be suppressed by the load removal caused by mining.

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