Work is proceeding at Garson Mine after an earthquake Wednesday that originated at the Vale site but was felt by people residing more than 10 kilometres away.
Vale spokesperson Angie Robson said the company experienced a 2.9 magnitude seismic event at the 5,200 level of the mine shortly after 5 p.m.
Fortunately, “no employees were injured as a result of this event,” she noted, and “work is continuing as normal, except in restricted areas of the mine.”
Robson said there was no damage to mobile equipment, “although there is some displaced rock that needs to be addressed, as well as some repair work to infrastructure in the affected area of the mine.”
The company does not expect the disruption “to have any material impact to production at Garson,” she said.
Glencore’s Nickel Rim mine, located just a few kilometres away, was not impacted at all by the event, said spokesperson Iyo Grenon.
Natural Resources Canada also confirmed a quake occurred on Wednesday, although curiously the coordinates it provides for the event — 46.46 N, 80.65 W — place the epicentre southeast of Wahnapitae, near Red Deer Lake.
Robson said Vale is aware of the NRCan report but is confident a quake occurred at the Garson Mine site.
“We have sophisticated equipment that monitors for this sort of thing and we definitely recorded a seismic event through ground control at Garson,” she said.
A blast was carried out at the mine just prior to the event, she said, and the company believes “it was associated with the blast.”
The Garson quake happened at 5:08 p.m., according to the monitoring equipment at the mine site.
That is the same time Natural Resources Canada specifies for the earthquake, of an identical scale, that it places closer to Red Deer Lake.
David Wood, a Sudbury rock engineering consultant, said he felt the tremor at his home near Ramsey Lake and checked the Earthquakes Canada webpage maintained by NRCan for details.
He also completed a “Did you feel it?” questionnaire at the site, which allows the federal department to gather data on the intensity and duration of earthquakes, as experienced by citizens.
“I filled that out to say, ‘yes, I did feel it,’ and it gave a Mercalli estimate of II,” he said.
The modified Mercalli scale differs from the Richter scale, Wood explained, by using “more subjective” information, such as whether the shaking was enough to awaken the resident of a home, to determine the “felt intensity” of a seismic disturbance.
A level II event on the Mercalli scale is considered “weak,” felt “only by a few persons at rest, especially on upper floors of buildings,” according to the United States Geological Survey.
Wood said he was curious to see whether Wednesday’s event had originated in a mine or was a natural event, as he has been studying the frequency and distribution of earthquakes in the Sudbury area, along with their causes.
By poring through the Earthquakes Canada database, Wood was able to confirm 850 seismic events in a 100-km radius of Sudbury, nearly all of which were mining-related.
Fourteen, however, were natural quakes, and almost half of those occurred in recent years along a north-south fault that runs roughly from Garson to Minnow Lake.
The spot NRCan pinpointed for Wednesday’s earthquake did not line up with the Garson-Minnow Lake fault, he said, nor was it a location where an earthquake had previously occurred.
It was also, at 2.9 on the Richter scale and II on the Mercalli, relatively minor, as are most earthquakes — mining-induced, or natural — in the Sudbury area.
“Seismic activity in the mines around here might be a magnitude 1 (Richter) event, and a rockburst might be up to a magnitude 3.5,” he said. “If you look at the records for this area, as I have done, there are hundreds of events, but most are small, and mining-related or mining-induced. This is a very stable area we live in and we really don’t get that many random events around here.”
Most of the bigger quakes in the region, he noted, have happened in the Abitibi-Temiscamingue region near the Ontario-Quebec border, such as the 6.1 quake that rocked Timiskaming in 1935, or the 5.2 convulsion at Kipawa in 2000.
While mining might seem like a destabilizing activity that could cause bigger seismic problems, Wood said it can actually have the opposite effect.
“Part of what mining does is disturb things, and by disturbing things you hopefully redistribute the stresses that would normally otherwise build up,” he said. “So you prevent earthquakes from occurring by creating smaller, little ones, instead of waiting until it all builds up and one big one lets go.”
While each mine in the area has its own microseismic system to monitor for tremors, Natural Resources Canada has just the one local seismograph station at Creighton Mine, which it uses in conjunction with stations near Kipawa and Orillia to measure and locate earthquakes.
“NRCan may, on the basis of the evidence coming from the mines, adjust the location,” Wood said. “This would be a preliminary location, based on those three recording stations, which pick up the signal at different times.”
The times relate to “their distance from the epicentre,” he said. “So you take the time of arrival at those three locations and back-calculate to where the one place had to be, more or less, that the vibration came from.”
Increased data from mine monitoring may lead the department to “refine their evaluation and move the location of the earthquake,” he said.