Nahanni earthquakes

In 1985, two major earthquakes shook the Nahanni region of the Mackenzie Mountains in the NWT, followed by a series of aftershocks. On October 5, 1985 a magnitude 6.6 earthquake shook the region, followed by a magnitude 6.9 quake in December of the same year. Prior to October, 1985, the Nahanni Range was thought to be a relatively quiet earthquake zone. Two more significant events (magnitude 6.2 and 5.5) occurred in 1988.

People in the NWT, Yukon, Alberta, Saskatchewan , BC and Alaska felt the vibrations from the 1985 quakes. While no community was closer than 100 km from the epicentres, the earthquakes caused widespread alarm in Wrigley, Fort Simpson, Nahanni Butte and Fort Liard, the four communities closest to the October quake. In Wrigley, residents reported that the ground rolled and vehicles bounced on the road and trees and power lines whipped back and forth.

Sections of the bank of the Mackenzie River slumped into the river. Scientists found that there had been strong ground movements associated with the quakes, resulting in large landslides and rockfalls in the central area. The biggest slide was a rock avalanche that was triggered by the October quake. This Nahanni rock avalanche was one of the largest ever to be recorded in Canada, and the first known to have been caused by an earthquake. It resulted in a 70 metre vertical scarp on the side of a mountain, with an estimated 5 to 7 million cubic metres of rock crashing 1.6 km down from the crest to the toe of the slide.

After the October, 1985 shock, seismologists moved instruments into the region to collect seismological data. These instruments recorded the ground acceleration of the December 1985 quake - documenting the strongest earthquake induced ground acceleration ever recorded anywhere -- in excess of 2 g!

A recent review of seismic hazard in northwestern Canada concluded that the earthquake hazard in the Mackenzie Mountains is high.

The authors also note that there may be significant secondary effects from earthquakes in the region including landslides, slope failures and sediment liquefaction. This does not bode well for the security of the Prairie Creek mine site, either during operations, or after the mining operations are finished.

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