How likely is a big earthquake in the Cowichan region?

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Yesterday’s snow day was a good reminder of the importance of being prepared for emergency situations. And so, today, I’m answering this question sent to us by Kim Schiavon: “The fault line runs under Herd Rd I believe. What are the possibilities of an earthquake? And what could be the damage on this area?”

Thanks for the question! To get answers, I spoke last week with Natural Resources Canada seismologist Alison Bird.

She says that because we live in earthquake country, earthquake awareness and preparedness are imperative. In the first half of the 20th century, Vancouver Island experienced several large, damaging earthquakes. The last major one was a magnitude-7.3 quake near Courtenay-Comox in 1946. But since then, it’s been a bit quiet.

“People have been lulled into a false sense of security,” she says.

What is the seismic situation in Cowichan Valley?

Bird says that there indeed is a fault line that runs under Herd Road. But she notes that it doesn’t appear to be active. In fact, while many ancient inactive faults have been mapped, most recent earthquakes in our region are happening on unmapped faults that are well below the surface. She also notes that a fault doesn’t have to be right underneath you to cause great damage.

Vancouver Island resides on the North American plate, and the offshore Juan de Fuca Plate pushes underneath the North American Plate, Bird explains. “The two plates are moving toward each other. The North American Plate is bending and buckling as a result of that stress.”

This is called subduction, and what’s called a megathrust earthquake can occur when the stress is released along the subduction zone where the two plates meet. Bird says that subduction zones, such as our Cascadia subduction zone, are where you get the world’s largest earthquakes, the ones of magnitude 9 and above. “Our subduction zone is the only one around the Pacific that has not ruptured in the last century,” she points out.

Bird notes that while a megathrust earthquake may be the most dramatic thing that can happen in our region, shallow earthquakes occurring within the North American or Juan de Fuca plates can also cause major damage.

Different earthquakes cause different types of shaking, she says. For example, a shallow coastal earthquake close to Duncan could potentially cause just as much damage here as a megathrust earthquake.

How likely are we to experience a damaging earthquake?

A damaging earthquake is generally around magnitude 7, Bird says. But she notes that it’s not so much the size of the earthquake but the size of the shaking that comes from it that matters.

The chances of the Cowichan Valley area experiencing a damaging earthquake is about one in three in the next 50 years, according to Bird. “It could be actually a bit higher than that,” she says. “We’re just going on what we know so far about the earthquakes in the area but we’ve only been monitoring for about 100 years or so. It’s actually a very short time window for this kind of research.”

Megathrust earthquakes have occurred in our region every 200 to 800 years, Bird says. The last one happened in 1700. “We are in a period when one could occur,” she explains.

What sort of damage might we expect in a large earthquake?

“It is very common for there to be service outages (electricity, gas, water, sewage, Internet) for some time, so people should assume they will be ‘camping’ for at least a few days,” Bird says. “As we live on the Island, it could be a week or two, so people should put their kit together with that in mind, to ensure they have a reasonable level of comfort.”

Single-family dwellings bolted to foundations tend to fare well in part because wood is somewhat flexible, Bird explains. More at risk of failing are chimneys, porches, garages and poorly constructed additions. She adds that older, masonry buildings that haven’t been retrofitted are prone to damage.

Outside of the home, roadways can be damaged and temporarily impassable, Bird says. In particular, on and off ramps as well as softer hills are prone to slumping, she explains. For an overall picture of what could happen, she recommends the 2015 New Yorker article “The Really Big One.”

“I would really encourage people to assume it’s going to happen,” Bird concludes. “We drive around with our seatbelts on, we have smoke detectors in our homes, we also need to think about earthquakes.”

What should you do during an earthquake?

Regardless of what type of building you’re in, Bird says that the recommended course of action is “Drop, Cover and Hold On.” She says it’s important to resist the temptation to try to run outside, which would put you at risk of being knocked down or injured by falling debris. 

“The outside edges of buildings are where damage tends to be concentrated. Most people assume that damage will be in the form of collapse, but this would be rare in the types of buildings we have in B.C.,” says Bird. She notes that In the 2003 earthquake in San Simeon, California, the only people who died were running out of a building instead of getting underneath a table. 

She recommends going to each room in a house and practicing where you can drop, cover, hold on. This creates an important muscle memory. “Brains don’t function very well under stress,” Bird says. If you practice, “your body will take over and keep you safe.”

The strongest waves in an earthquake are horizontal, Bird says. This can cause objects to fly around side to side instead of up and down. As a result, she says it’s important to go around your house and office, and look for things that could fall or fail in some way and secure or relocate them.

After the shaking has stopped

Most people will survive an earthquake, Bird says, which is why it’s so important to be prepared. 

“I want to be comfortable. And for me part of my comfort is food. I like spicy food, so in my kit, I’ve got about at least two week’s worth of food. I’ve got some chili peppers in there. I’ve got some Indian food. I’ve got a bar of really good chocolate for when things get desperate. … You don’t want to be eating food that you don’t like.”

She also recommends creating a plan ahead of time with your family for where you’ll meet after an earthquake. If you need to communicate, she says to send a text instead of making a call to make sure the communication lines are available to those who need emergency services.

Bird says she also keeps a pair of sturdy shoes and a flashlight under her bed. “Because if it happens at night, probably the power will be out, and there’s going to be some broken glass and other debris,” says Bird.“I don’t want to be walking on that in bare feet or slippers.”

Want to learn more?

• PreparedBC offers many resources including an earthquake guide and a neighbourhood preparedness guide.
• Natural Resources Canada tracks the latest earthquakes.
• The Great British Columbia ShakeOut happens every year in October to raise earthquake awareness and preparedness.
• Here is some emergency preparedness information from the CVRD, the Municipality of North Cowichan, and the City of Duncan.

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Correction: A previous version of this article failed to note that shallow earthquakes on the Juan de Fuca Plate can also cause damage to the region, and that a megathrust earthquake can occur when stress is released anywhere along the subduction zone, not just on the North American plate. We’ve updated the story to reflect those facts. (January 17, 2020)