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City, Science

A rumble below

Posted by Hannah / January 10, 2006

snow sign.jpg When the orange snow clearing signs are hooked on to the telephone and electrical poles of my street, I become jittery, anticipating the machines' arrival. For one, a street devoid of cars is rare in the city core. But really I just like the massive trucks they set to the task. (If you don't live in Montreal, you have to imagine a snow clearing machine three-times the size you're used to. By living here you begin to realize why some cities--Toronto, Washington, D.C.--send in the army after a snowfall.) So, when my bookshelves and stereo began to rumble yesterday morning, I chalked it up to the arrival of the snow clearing behemoths, and pressed my face against the window.

The street was empty. A neighbour came out of her house and looked up and down the street. Deserted. I almost expected a tumble weed to roll on by. (Well, not really.)

Later that afternoon, I heard about the earthquake that had struck south of Montreal, near the U.S. border. It was only 4.2, a whimper on the fear scale, but the biggest by far in the last month in this region.

Apart from the portion of British Columbia that clings to the country, we rarely think of Canada of being particularly prone to earthquake activity, let alone Quebec. A map of recent earth quakes compiled by the Natural Resources Canada might have you think otherwise.

In fact, Montrealers live atop the second most active earthquake zone in Canada. Seismographs register about 300 earthquakes in the region annually. Most are small and may not even be felt by the hipsters hanging out at Mile End bar. Montreal, which falls into the Western Quebec Seismic Zone, was hit by an earthquake measuring 5.2 on the Richter Scale in 1732, causing chimneys to crack and spawning a fire that destroyed 300 houses. One girl was killed.

These earthquakes in Eastern Canada remain a bit of a mystery to scientists. Most earthquakes, like those on the west coast, occur when the edges of the earth's plates rub up against each other and begin to deform, building up energy. Should one of these plates break, an earthquake is released. We, par contre, are found on the stable North American plate. It's flat. It doesn't have any neighbouring plates to jostle it around. It means we're less prone to quakes, but it also means we're less able to predict an occurrence. So, why does it shake?

Earthquakes typically occur about 300 km below the grass, trees and sandboxes that sit atop the earth's crust. But those in eastern Canada tend to be much shallower-only 30 km below the surface. Scientists believe that our earthquake prone region may be due to a weak crust. The gash that is the St. Lawrence river contributes to it, as did the impact of an enormous meteorite 350 million years ago into the Charlevoix region.

Quake Quebec.jpg Yesterday's earthquake was the second I've felt in less than a year. The first one, a 5.4 on March 9, woke me up from a deep sleep at 2 in the morning. Being the nerd that I am, I started looking into it, and reported it to the Geological Survey of Canada. If you felt yesterday's, you can still add your experience to their records.

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