Earthquakes in Vancouver amplified by Georgia Basin, study finds
Friday, January 24, 2014, 8:22 AM -
A recent study is changing everything experts thought they knew about what would happen during an earthquake in Vancouver.
The study showed that seismic activity in the area is amplified as it passes through the Georgia Basin, which is the deposit of softer sedimentary rock that lies beneath Vancouver. The basin could make the quake seem three to four times stronger.
University of British Columbia's Sheri Molnar is the lead author of the study. She contends that if the quake was a strong enough it could be felt by people in vehicles and even damage well-constructed buildings. While experts have known the effect of basins on seismic activity, this is the first time the Georgia Basin has been studied to this level.
"We see that the basin itself causes an increase in motion about three to four times, and the level of shaking in the end is about an intensity four on average in Greater Vancouver, if it’s a very deep earthquake," Molnar explains. "But what it really means, in terms of shaking, is if it’s a deep earthquake, it might have felt about an intensity two, so it might have been very lightly felt, and now it’s going to be maybe an intensity four, which is a little bit higher, but still not going to cause damage. But still, you will definitely feel it. And for a shallow earthquake, it could have been an intensity four, like that of a deep earthquake, and now, because of the basin, you’re going to be experiencing closer to an intensity seven."
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Molnar compares the Georgia Basin to gelatin surrounded by a hard block of cheese. The "gelatin" would be hit by the earthquake and start bouncing around. The authors of the study took a look at both deep earthquakes and shallow earthquakes, focusing on the 6.8 magnitude.
"If we have an earthquake at great depth, say at 40 km depth, that’s going to be occurring within a plate that’s sub-ducting beneath Vancouver. And the earthquake waves will travel up through this very hard, dense rock that we have, and the analogy is a homogeneous cheese layer. And then it’s going to hit material that’s been in-filled into a basin, and that softer material is sedimentary rock, more like a jello-like material. And as the waves start to enter the basin, they’re going to increase in motion and start to slosh back and forth and that’s what’s causing increased motions in Vancouver, because Vancouver is sitting on top of this basin," says Molnar.
To study these tremors, they used computer software to determine the extent of the damage that could be caused by a quake that powerful. Construction of buildings is usually done with the knowledge that softer ground could create stronger tremors, but having the basin under Vancouver would mean that not only stronger shaking is being produced but also longer-duration shaking.
With files from Canadian Press