15
Aug
10

Central Canada gets a lesson in seismology from Mother Nature herself

An earthquake in China, Haiti, or Chile, though tragic, seems far from home and difficult to imagine, even given the flood of multimedia that came from each of these disaster areas earlier this year.  On June 24, it all became frighteningly clear for residents of Quebec and Ontario.

“I was in the kitchen.  I was having a late breakfast,” says Albin Dzurnak, resident of Ottawa who experienced his first-ever seismic event in June when a magnitude 5.0 earthquake hit the region, with an epicentre near the Ontario-Quebec border about 60 km north of Ottawa.

“It was kind of strange, things were shaking, so I figured, well, this is not normal so it must be an earthquake,” says Dzurnak.

While media reports often focus on the magnitude figure of an earthquake to communicate its strength, the shaking is the first thing one feels.  And this is also central for engineers.  “From the engineering point of view, the magnitude of the earthquake is not the controlling parameter and becomes almost irrelevant information,” says Carlos Ventura, professor of civil engineering at the University of British Columbia.

“What is important is how far from the epicentre or from the rupture zone a building is located and the severity of the shaking at that location,” says Ventura.  “For example, the level of shaking experienced by the [8.8 magnitude] earthquake in Santiago, Chile [this year] is very similar to that experienced [there] in 1985, which was 7.8-magnitude.

Earthquake magnitude is a measure of the energy released by the seismic event, but the amount of shaking this creates in a given location depends on many factors, says Professor John Clague, Canada Research Chair in Natural Hazards Research at Simon Fraser University.

Shake map and location map for Ontario-Quebec earthquake

The ShakeMap and location map for the June 24, 2010, 5.2-magnitude, central Canada earthquake (depth: 16 km)

The severity of shaking depends on the distance from the epicentre, its depth, and other factors. Around the recent earthquake, “The crust is not as highly faulted and broken up, it’s less heterogeneous,” says Clague. “You’re right in the middle of a plate, the North America plate.”

“You’re not in an area where the crust is overly thickened, where it’s [not] particularly hot, and that allows seismic waves to propagate with less attenuation, less loss of energy over larger distances,” says Clague, which is why the central Canada quake was felt as far as New York City. “On the west coast, an earthquake like [that] would not be terribly remarkable.”

Most people have heard of the Richter scale for measuring the size of an earthquake, but in fact, the popularized scale, which originally applied mainly to California’s seismic landscape, has long been supplanted by the more accurate moment magnitude scale.

“If you’re to think of a fault as being the slipping of two surfaces, one against the other, the moment magnitude is the product of the area of the whole fault, times how far it slips, times how strong the rock was that it broke,” says professor and seismologist Olivia Jensen of McGill University. “The stronger the rock was that it broke, the stronger the earthquake.”

“For big events the numbers would be pretty much the same as Richter numbers,” says Jensen.

Summing up his feelings on his first earthquake experience with tongue slightly in cheek, Dzurnak says,  “I was happy about the earthquake and I wish there were more of those so that people are reminded that, ‘hey this is Mother Nature talking to you,’ and you have to deal with it.  You either deal with it or you die, that’s how it works.”

Sound: Listen to Professor John Clague to find out how a train can end up beside its track instead of on it and how man-made reservoirs and oil exploration can make earthquakes more likely:

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