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Basin would cause whole lot more shaking in Vancouver, studies find

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VANCOUVER -- New research is shaking up the entire notion of what could happen to the Vancouver area during an earthquake, indicating bridges and tall buildings would rattle and sway a whole lot more than previously thought.

Two studies published Monday in the Bulletin of the Seismological Society of America show seismic waves are amplified as they pass through the Georgia Basin, the deposit of softer sedimentary rock that lies partly beneath Metro Vancouver.

If a quake occurred within 100 kilometres of the city, such amplification could make the ground shake three to four times more than it would if the basin were not there.

The authors say people driving in vehicles would notice the shaking and suggest it could damage even well-constructed buildings.

"The shaking in (Metro) Vancouver would be greater because of the presence of the Georgia Basin, especially when the earthquake occurred to the south or southwest," said lead author Sheri Molnar, who's in the University of British Columbia Civil Engineering department.

She said the waves would spread outward from the earthquake and would have to cross the deep southeast portion of the basin before hitting Vancouver.

That would tend to cause the greatest increase in motion.

Molnar said seismologists have known sedimentary basins can increase shaking. But the influence of a basin in Canada has not been studied until now.

The Georgia Basin is shaped like an elongated bowl and lies beneath the Georgia Strait, between the Lower Mainland and Vancouver Island. It is one in a series of basins along the Pacific coast of North America and is filled with layers of silt, sand and glacial deposits.

She compares the Georgia Basin to gelatin surrounded by a hard block of cheese.

"We're bringing the earthquake up through the cheese, and then it's suddenly hitting the Jell-O mould and starting to slosh around and bounce around within that Jell-O."

British Columbia sits on what's known as the Cascadia subduction zone, where earthquakes tend to occur either within the Juan de Fuca plate or the overriding North America plate. Big subduction earthquakes, such as the one that struck Japan in 2011, also occur in the Juan de Fuca plate.

Molnar's studies examined the potential impact of deep earthquakes, with a magnitude of 6.8, that occur 40 to 50 kilometres beneath the surface, as well as shallow earthquakes of the same magnitude.

Molnar and her colleagues used computers to look at the impact on tall buildings or long structures.

Using three-dimensional simulations of different scenarios, the team found both deep and shallow earthquakes led to greater shaking if the seismic energy moved through the Georgia Basin.

Up until now, construction of buildings was based on the knowledge softer ground would create stronger tremors during an earthquake.


-- The Canadian Press

Republished from the Winnipeg Free Press print edition January 21, 2014 A8

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