B.C. enters window of increased quake risk
CTV.ca News Staff
Published Friday, February 2, 2007 11:20PM EST
Experts warn that British Columbia's south coast has entered a two-week window of increased probability in which a major earthquake could occur.
While the chances of that happening are still very slim, such an event could trigger a tsunami equal to the size and the strength of what hit the Sumatra-Andaman Islands in 2004.
Seismologists with B.C.'s Geological Survey of Canada made the announcement after measuring a series of minor tremors deep below the Earth's surface and along the Cascadia Subduction Zone -- a long, sloping fault that stretches from northern California to Vancouver Island.
The zone, which separates the Juan de Fuca Plate and the North American Plate, has a very large fault area which can produce large earthquakes with a magnitude of 9.0 or greater if a rupture were to occur.
"Where the Juan de Fuca plate is subducting beneath the North American plate, and where those plates are touching, it's locked," earthquake seismologist Alison Bird told CTV.ca.
"To the east of that, every 14 months we see low frequency tremor activity ... sliding along that zone, which is almost directly beneath us," Bird told CTV.ca in a phone interview from the Geological Survey of Canada headquarters in Sidney, B.C., near Victoria.
"When that happens, we believe what it's doing is loading additional stress onto the locked zone. So if that locked zone happens to be close to critical, it could theoretically trigger a mega earthquake."
Bird and her colleagues noticed a phenomenon, called Episodic Tremor and Slip (ETS), occurs in the area about every 14 months -- each time adding stress to the fault line and increasing the likelihood of a major earthquake.
The seismologists started detecting strong ETS movement last Friday.
If a major quake should occur, said seismologist Garry Rogers: "You're looking at a big tsunami happening at the same time. The west coast of Vancouver Island, the Washington and Oregon coasts, are all vulnerable to a force powerful enough to certainly bring down buildings.
Rogers told CTV.ca that the area of the Cascadia Subduction Zone is comparable in size to the rupture zone of the Dec. 26, 2004 earthquake in the Pacific Ocean that triggered a devastating tsunami in the Sumatra-Andaman Islands.
Another comparable scenario, said Roberts, is the 1964 earthquake that struck Alaska.
"Most of the bigger, taller buildings and structures like bridges suffered some damage, whereas most of the small structures such as houses suffered hardly any damage. So there's an analogy which is probably what is going to happen in Vancouver, Seattle and Victoria."
While larger structures that were built a number of years ago are most vulnerable, continued Roberts, recently-built structures are better off, "because what we do is put this type of knowledge into building codes. Older, bigger structures are more vulnerable because they don't have the present day knowledge built into their design."
The last known great earthquake in the Pacific Northwest -- the Cascadia Earthquake -- struck in January, 1700. While such phenomena are infrequent, with a cycle of every few hundred years, the region is due for a big one.
The Geological Survey of Canada added for an average week, the chances of a mega thrust earthquake occurring is 0.0005 per cent (or one in 200,000); during a week of Episodic Tremor and Slip this increases to 0.026 per cent (or one in 4,000) -- a 50-fold increase.
"We are at a time when (a mega thrust quake) could occur, but it could also be 100 years from now," Bird told CTV.ca.
Rogers compared the heightened risk to driving in rush hour as opposed to driving in less crowded road conditions.
"We all go out and drive every day and we take the chance of getting into a traffic accident. It's a low probability but we know it probably could happen, so we take the step to put our seatbelt on," he told CTV.ca.
"Now we know that the probability is much higher in rush hour and we still drive. What we are doing in Vancouver Island right now, we're driving in rush hour. We have been over the last week and we probably will be for another week. And then we turn to a lower probability."
Bird added this period of higher earthquake risk is a time to get organized and take precautions.
"What I encourage people to do is whenever we're about to have an ETS, it's a good time to use that as a trigger to go over your emergency plan with your family to make sure that your earthquake kit is full stocked," Bird said.
"Sometimes things get taken from earthquake kits to use for other things. So replenish the food and water and other equipment. It's just a good practice, a good reminder, like when the clocks change you check the battery in your smoke detector. For the west coast, you check your earthquake kit every 14 months."
- Click here for advice from Natural Resources Canada on how to prepare for an earthquake
Should B.C. escape the big one this time, the next window of higher earthquake risk will occur around April 2008.
Rogers said seismologists haven't figured out why the ETS phenomenon happens every 14 months in the Pacific Northwest.
"And interestingly enough it's 14 months fairly regularly here for the last decade as we looked back at our old records," he said. "But in Japan, it seems to be about one year; in New Zealand it appears to be about every few years; and Alaska every 18 months. We don't know if it's because of different subduction zones, or it's about where we are in the earthquake cycle."