When the predicted big earthquake hits off the coast of southwestern British Columbia, communities near sea level will have one more potentially life-threatening hazard to worry about -- tsunamis, or tidal waves.
Exactly how big the tsunamis will be and how far inland they will reach depends on the intensity of the earthquake. Still, the best way to predict what might happen is to look back and see what's already happened.
That's why a group of scientists, including SFU geographer Ian Hutchinson and plant paleontologist Rolf Mathewes, have spent the last three years probing the gooey depths of several tiny lakes on the west coast of Vancouver Island.
Preliminary results of their study suggest that tsunamis on Canada's exposed west coast may be smaller than previously thought.
Tsunamis can occur when an earthquake causes the ocean floor to rise or sink, displacing water and creating a series of large waves. The major seismic threat off Canada’s west coast lies 100 kilometres off Vancouver Island at the junction of two huge crustal plates, one of which is sliding under the other. At present, the two plates are locked, but when that lock breaks, the much-heralded large quake will take place.
"When that happens, it will take about 15 minutes, at most, for a tsunami to reach the Vancouver Island coastline," says Hutchinson. "The geophycists tell us that, on the outer coast, the worst waves would be about five metres high. But they'll build as they move up the inlets, meaning that low-lying communities such as Port Alberni could be hit by waves as high as 15 metres. That's a significant hazard."
Funded by the U.S. Geological Survey, Hutchinson and colleagues set out to find field evidence of past tsunamis in the region. "Other researchers are working on computer models of what these earthquake events might look like all along the Pacific coast," says Hutchinson. "It's important that they get real evidence of tsunamis in this region."
The team used a 'corer' -- basically a hollow tube -- to bore three metres into the sediments of small coastal lakes less than five metres above sea level. Core samples were examined for debris that might have been deposited by a big wave, and plant material was used to date each layer. "In one lake we got a record going back 3,000 years," Hutchinson says. "Some of them can go even longer if we could get deep enough."
Of the eight lakes cored to date, three have yielded tsunami deposits. "The lake bottoms are like brown gelatin, rich in organic matter," explains Hutchinson. "But tsunami deposits consist of gravels, sands and lots of forest material, because when a tsunami wave rushes into a lake, it strips the forest floor."
The team hopes to take more lake samples this summer, with a longer coring system that can go deeper -- and further back in time. So far, the results suggest that west coast tsunamis may not be as severe as widely believed.
"Some of the earthquakes that we thought would generate tsunamis don't appear in the core record," says Hutchinson. "It may be that most tsunamis produced offshore only reach up to three metres in height on the outer coast."
That would be good news for communities nestled in inlets, he adds. "A smaller wave might make all the difference in terms of whether there's widespread devastation or not."
The above story is based on materials provided by Simon Fraser University. Note: Materials may be edited for content and length.
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