Multiple earthquakes this week highlight need for readiness
Friday, February 24, 2017, 12:09 PM - The so-called "slow slip" earthquake season is upon us; a 14-month alarm clock reminding British Columbians to be prepared.
Earthquakes Canada reported a magnitude 4.7 quake off the coast of northern Vancouver Island early Friday morning, less than 85 km away from a reported 4.4 magnitude tremor on Wednesday.
While neither quake was close enough to land or strong enough to be felt, they do highlight the divide between the Juan de Fuca and and North American plates, a seismic zone that has the eye of both researchers and emergency preparedness groups.
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An Episodic Tremor and Slip event (ETS, or 'slow slip') differs from a regular earthquake in a number of ways, mainly in how fast the shift in tectonic plates occurs. Where an earthquake is sudden and lasts only up to a few minutes, a slow slip event can last for weeks, and generates very deep quakes that can't be felt without sensitive instrumentation.
The events are interesting to seismologists because studying ETS behaviour sheds light on the position of the eastern limit of the rupture expected in the next "great earthquake" - defined as a magnitude 8 or 9 - and how close that might be to major west coast cities. According to Earthquakes Canada, each ETS event adds stress to the portion of the plates that are locked together, meaning each event increases the likelihood of the so-called "Big One."
This map shows the position of ETS earthquakes during the last event, Dec 22 2015 to Jan 16 2016. Image courtesy Pacific Northwest Seismic Network.
Slow slip events affecting southern British Columbia and the US Pacific Northwest have been occurring approximately every 14 months since at least the early 1990s.
Speaking to CBC News, Geological Survey seismologist Alison Bird said this week's quakes weren't related to the slow-slip phenomenon, but that the cycle serves as a good reminder for people to, "check on their kit [and] review their plan. It's a good 'prompt' to make sure you're ready for an earthquake when it happens."
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A study released last week said nearly 40 per cent of Victoria's buildings would be severely damaged or destroyed if a major earthquake struck the city; nearly 5,000 buildings would suffer extensive damage, along with city infrastructure such as sewers and power transmission, in the event of a magnitude 9 quake.
The study concluded the likelihood of such an event in the next 50 years in Victoria is 5 per cent. Other coastal cities have not yet undertaken such a survey.