January 21, 2014 by Vivian Luk, THE CANADIAN PRESS
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 that 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 quake 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,” says lead author Sheri Molnar, who’s in the University of British Columbia Civil Engineering department.
She says 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 says seismologists have known that 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, like 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 that 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 that softer ground would create stronger tremors during an earthquake.
But Natural Resources Canada researcher John Cassidy says soft materials beneath the surface – such as a basin – could also control the amount of rippling felt above.
“Essentially what the basin is doing is producing stronger shaking and producing longer-duration shaking,” said Cassidy, who supervised Molnar’s study.
“Instead of perhaps feeling strong shaking for 10 seconds, you might feel strong shaking for 20 or 30 seconds.”
Cassidy says the model showed that the area beneath the Georgia Strait shook the most.
When earthquake waves hit the southeast part of the Georgia Basin before reaching Metro Vancouver, southwestern areas such as Delta, Ladner and Richmond experienced tremors three or four times what they would be if the Georgia Basin were not there.
However, if the waves hit the northwestern or northeastern part of the basin, they took a different path and either did not affect the Vancouver area or did not cause a significant increase in motion.
Cassidy says the findings will help seismologists determine where to place more instruments to record future earthquakes. He also says the findings can be used to upgrade codes for buildings, bridges and other infrastructure to make them more structurally sound.
“If somebody is planning to put cables in the sea floor or any sort of infrastructure on the sea floor, this is really important information,” he says.
“The best defence against earthquakes is through good, modern building codes.”