China quake likely worse than ours would be, seismologist says

By Jeff Nagel - Richmond Review - May 13, 2008

The earthquakes that are likeliest to hit Metro Vancouver aren't expected to be quite as severe as the one that devastated China's Sichuan province Sunday night.

Damage comparisons are difficult to make because of different geology and building construction.

But seismologist Alison Bird said the most severe quakes close to Vancouver would likely be magnitude 6.5 to 7.5 â less than the 7.9 earthquake that has killed at least 12,000 in southwestern China.

"We can have similar earthquakes, but not quite like what they've had," Bird said.

"It really depends on how shallow the earthquake is, what size it is and its location relative to places like Richmond, Vancouver or Victoria."

The Chinese quake was both very shallow and near several populated cities.

Similar close-to-the-surface quakes here could be very strongly felt and damaging and are also considered more probable than the "Big One" â a megathrust quake in which the entire Juan de Fuca plate makes a sudden major shift towards the continent.

Bird says a mega-quake would likely be magnitude 9.0, but it would be deeper and offshore â likely further away than a more localized quake.

That would likely translate into shaking for a longer period of time, but possibly with less intensity.

"It's more of a swaying rather than a high frequency shaking," she said.

A damaging close-to-surface earthquake is projected to hit Vancouver about once every 50 years.

Megathrust quakes, which can simultaneously rock the coast from southern B.C. to northern California, tend to strike every 300 to 500 years. The last of its kind happened in 1700, so either type of quake is considered possible now.

In either scenario, much depends on local building construction.

Wood frame structures tend to stand up well in quakes, and Metro Vancouver has more of them than the riskier masonry buildings in many other places.

Bird said B.C.'s new building code is more conservative and that's cause for optimism.

And work continues on public building and school seismic retrofits to reinforce older structures.

Another risk is low-laying areas built on sediment â like Richmond â where liquefaction could occur.

A quake's tremors temporarily turns the ground in such areas into something like quicksand, causing some buildings to tilt over.

"People have this misconception that Richmond is going to disappear, which is not the case," Bird said.

But she said leaning buildings can cause different damage as their contents shift.

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China quake likely worse than ours would be, seismologist says

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