Back-analysis of the seismic activity of the M Zone, Sigma Mine, Val-d'Or, Quebec
September 1994 - August 1995
by M. Côté

ABSTRACT

Group Leader in Mining Seismology, Experimental Mine, Mining Research Laboratories, CANMET, Natural Resources Canada, Val-d'Or, Quebec
M. Plouffe Seismic analyst, Experimental Mine, Mining Research Laboratories, CANMET, Natural Resources Canada, Val-d'Or, Quebec

In 1994, a seismic system was commissioned at the Sigma Mine. During the first year, 699 seismic events were recorded. The largest event had a magnitude value of 3.5 mN, and occurred in the 2502 EA stope, M Zone, on June 16, 1995, at 15:59 E.S.T. Being the largest seismic event ever at Sigma Mine, a report is thus specially dedicated to the seismic activity of the M Zone.

Monthly, 30 seismic events originate from the M Zone, about half of the seismic activity from the Sigma Mine. This is consistent with the fact that half of the ore produced between September 1994 and August 1995 comes from the 8 stopes in the M Zone. Five of these stopes show a real affinity between their seismic activity and their extraction. The 2802 E2A stope, with 129 seismic events, was the most seismically active stope during this period.

Of the 364 events located in the M Zone, 25% had a magnitude value below -1.5 mN, while 127 events had a magnitude above 0.0 mN. The liberation of the seismic energy was evenly distributed during the year, with important jumps on September 16, 1994, and June 16-21, 1995, except for April and May 1995, where an inflection is noted. Half of the largest seismic events show a pillar burst as the focal mecanism. The other half shows a fault-slip mecanism. The orientation of these focal planes shows a good trend between them and the direction of the main stresses.

Four important seismic sequences occurred during this period and always corresponded with major seismic events (> 1.5 mN). Three periods showed no activity at all; one of them occurred only 8 days before the June 16 events. The June 16 to 21 sequence, which started with the 3.5 mN event, seems to have been preceded by two precursors: the break-off of loading points pillars on the 24 level and a period of seismic inactivity of 13 days. The 3.5 mN event occurred immediately after the first long-hole blast in a sub-level of the 2502 EA stope. Durig the following 48 hours, a intense seismic activity occurred in the whole M Zone to finish on June 18. On June 21, 12 events, of which one had a magnitude of 2.2 mN, occurred on the other side of the cross-cut compared to the June 16 events, between the levels 24 and 25.

The seismic activity in the M Zone does not seem to follow the same trends. Sometimes, for a stope, the seismic activity is closely linked to the mining, like the 2802 E2A stope. Elsewhere, the type of rock and the structural geology seem to govern the stress regime. Some stopes seem to relax these stresses more regularly and more often (e.g. the 2802 E2A stope), while a stope like 2502 E2A, seems to accumulate these stresses and relax them more violently.

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