Great BC ShakeOut reminds us we're in an area that could suddenly shake violently

Hundreds of thousands of British Columbians joined millions worldwide in hunching under their desks at 10:17 our time yesterday morning.

It was the day to practise dropping, covering, and holding on in the event an earthquake rumbles through, which is a lot more of an inevitability than we perhaps would like to ponder.

If you think all you got from darting under your desk is a brief respite from work and possibly a small bruise on your noggin, you may be selling this Great British Columbia ShakeOut a little short.

“It helps people prepare psychologically for an earthquake so that it's not sort of some disastrous event out of the blue—people kind of know what's happening,” Taimi Mulder, an earthquake seismologist with Natural Resources Canada, tells Capital Daily.

“I think people can cope with things much better if they have an idea of what might happen and then what they could do.”

BC is the most seismically active area in the country, with thousands of shakers occurring every year, including an estimated 100 or so in the Victoria area each month—we just don’t feel the vast majority of them.

Mulder says small earthquakes ever so slightly relieve pressure within the earth’s crust but it’s a myth that they significantly lower the likelihood of a major quake.

There's a lot of math involved

The earthquake scale, Mulder says, is logarithmic, meaning it would take 10 magnitude-1 quakes to have the force of a magnitude 2 earthquake. One hundred magnitude-1 earthquakes would be needed to equal a magnitude-3 quake, and it would take one thousand mag-ones to equal a magnitude 4, and so on.

“So you're looking at millions of small magnitude-1 earthquakes to relieve the pressure from, let's say, the “Big One” offshore that we all talk about,” Mulder says from the Pacific Geoscience Centre in Sidney.

The last “Big One” occurred in January 1700, when 1,000km of the Cascadia Subduction Zone—where two major continental plates converge—slipped 22 metres under the Pacific Ocean. The sudden violent movement precipitated earthquakes that registered 8.7 and 9.5 on the old Richter scale—a quantitative measuring stick relegated to smaller quakes and often replaced by other magnitude scales considered more accurate. The resultant tsunami hit the Island and levelled Pachena Bay, killing scores of First Nations and was strongly felt more than 7,600km away in Japan.

Odds on a smaller quake happening first, but it's still a guess

Mulder says statistically, it’s more likely that a smaller magnitude-5 or six quake will be the next to hit BC as opposed to the “Big One” because history has shown a huge quake happens in repeat intervals every 250 to 800 years.

“That [a magnitude-6 or mag-7 quake] is much more likely to occur, statistics-wise, because they recur on tens of years or decadal timescales, compared to the ‘Big One’ offshore, which occurs on 250-to-800-year time scales.”

Still, Mulder says, “We now know we're in the time window for another large one—it could be today. It could be 500 years from now.”

As the Juan de Fuca plate subducts under the North American plate—which is below pretty much all of Canada—it crumples up the edge of the North American plate, causing quakes that so far have been up to magnitude-6 and magnitude-7 along the margins of the North American plate, which includes Vancouver Island.

The last time that happened below Canadian soil was in 1946 when a magnitude-7.3 earthquake—the biggest ever on Vancouver Island and the largest onshore quake recorded in Canada—tore through parts of the Island from its epicentre just west of Courtney. In 2001, the 6.8-magnitude Nisqually, Wash. earthquake shook BC, Oregon, and eastern Washington state from its epicentre in the state capital Olympia.

So we have that to look forward to.

How to measure depends on the size

On the subject of the Richter scale, you’ll probably notice that’s a term not used as often in media reports as it was 30 or 40 years ago. Mulder says the Richter scale basically uses the level of displacement at the ground surface to determine an earthquake’s magnitude.

“The misconception I often see is the difference between magnitude and intensity,” Mulder says.

Magnitude she says, is the amount of energy released at the source, and that's constant. It doesn't matter where on the planet you are. That earthquake, that particular earthquake, has that magnitude, and that magnitude doesn't change.

Intensity, on the other hand, describes the level of shaking, which is measured on the Modified Mercalli Intensity scale, and describes the physical impact of an earthquake at a given place, rather than its magnitude or force, which is related to the energy it releases.

Mulder says for larger quakes, seismologists use Moment Magnitude, which describes the amount of energy released at the source. This magnitude captures the longer wavelengths generated from larger earthquakes. Data from distant seismic stations is used.

Seismologists create isoseismal maps that show how the amount of shaking varies within the region in which the tremblor was felt. These intensity maps are based on damage reports submitted by the public.

Mulder encourages people to use Earthquake Canada’s Did you feel it? Link, where citizens can report the extent of damage to buildings and their surroundings.

“That’s a huge resource for us when people fill that out,” Mulder said.

We were not alone for the ShakeOut

Last year, 725,000 BC residents participated in the Great Shakeout, joining some 56 million around the world.

Mulder says Shakeout raises awareness about earthquakes and it forces people to physically get out of their chairs and go someplace.

“If you practise that, even if it's only yearly, you'll have this physical memory, and it will embed itself better in your brain of what you need to do during a damaging event, and that's going to save lives and prevent injuries.”

That brief, constrained period under our desks yesterday also provided a reminder to review and update emergency preparedness plans and to plot where to go to stay safe.

Not only are there possible aftershocks to brace for, but post-quake fires, breaks in gas lines, falling debris, shattered glass, and power lines also will present dangers.

Governments and rescue organizations agree that anyone living in the region should have an earthquake kit prepared, including first-aid supplies, medication, clothing, a phone charger, cash, and copies of important documents. (Don’t forget a manual can opener if you’re packing pet food).