Vancouver Island sits near dangerous section of Cascadia fault line

Image: Study / Science Advances

A report released last week is furthering seismologists’ understanding of the Cascadia Subduction Zone (CSZ)—and is, unfortunately, clarifying just how screwed Victoria will be in the event of the “Big One.”

The new research is a major advancement in monitoring the Earth’s fault lines, particularly those with little action to monitor, like the CSZ which sits approximately 160 km from the Island. The CSZ is “unusual,” according to Edwin Nissen, a UVic earth science professor and lead scientist on earthquake research at the university. Unlike other fault lines, like the San Andreas Fault, we don’t experience frequent earthquakes with CSZ—this makes it harder to gauge the fault structure. This new study is considered groundbreaking—excuse the pun—in clarifying future movements with these faults.

Scientists have known for some time that the CSZ—which runs from BC down to Northern California—has a risk of slipping and causing an earthquake with a magnitude of 9 or higher. They’ve also determined that we’re due for one of these quakes in the near future (there’s about a 50% chance it could hit in the next 80 years). How that quake will appear in different regions is becoming more clear.

The new data shows that the texture of the northern section of the CSZ—running below the South Island and Washington—is much smoother and flatter than its southern counterpart. These textures are important for understanding the magnitude of a future earthquake. A rougher texture essentially has more “speed bumps” in the way to slow the movement of tectonic plates, while the smooth sections will slide without impediment.

This means, as the plates move and slip along these boundaries—which they inevitably will—the northern section, where Victoria is located, will experience a dramatic slip resulting in one very large earthquake. The southern section, running from Oregan to Northern California, will experience multiple medium-magnitude quakes.

In the south section, earthquakes would last around 30 seconds. Here in the north, the one large quake could last between two to three minutes.

Which is the better scenario? That’s a question Nissen, and his team have been debating for some time.

Smaller quakes less damaging—but likelier

The smaller earthquakes are more likely to occur in our lifetime than the “Big One,” which Nissen says has a 15-30% chance of occurring. But the damage from a larger quake would be far more catastrophic.

“From a long timescale perspective, I think I'd rather have lots of smaller earthquakes,” said Nissen. “But, given that the likelihood of one of these huge earthquakes in our lifetimes is a shade under 50/50, maybe it's better that we just don't—the best thing would be that we don't have any earthquakes.”

Another important discovery from the report showed that the CSZ is likely much more shallow than once thought. This means that the plates won’t hit deep enough in the earth to melt down from heat, remaining brittle and hard. This will lead to more pronounced ground shaking—and more damage to urban centres as a result.

How this will affect Victoria

If a mega-thrust earthquake were to hit BC’s coast tomorrow, the damage could be the worst natural disaster in Canada’s history.

In addition to its close proximity to CSZ, Victoria is nestled between two smaller fault lines that run close to the city’s centre.

The Leech River Fault has been known to scientists for some time, but last year a study released by Nissen’s team, with lead author Nicolas Harrichhausen, proved a second fault line runs below Victoria, known as the XEOLXELEK fault—the W̱SÁNEĆ name for Elk Lake, which the fault runs under.

Their research found that a 6-7.6 magnitude earthquake occurred approximately 2,500-4,500 years ago along this fault, offering a glimpse into the potential strength of a future earthquake.

A risk assessment of an earthquake from the Leech River fault—XEOLXELEK would be similar—predicts that nearly 1K people could die, 7K buildings could be damaged, and it would cost the province $20B.

The mega-thrust CSZ would lead to even more damage, with a prediction of roughly 3.5K people dead, 18K buildings damaged, and $40B in costs.

Victoria’s downtown brick masonry buildings will see the most damage—most of these buildings were built before construction codes were updated with seismic reinforcements in mind. These buildings are brittle, unreinforced, and most likely to crumble.

Nissen says that if an earthquake were to take place at night, when shops and restaurants in these buildings are closed, casualties won’t be as high. But if the quake happens during the day, when tourists and locals are out shopping, the impact could be devastating.

Quake could bring museum, pool, and Parliament down—on everyone inside

Nissen also points to the buildings that could lead to mass casualties: The Crystal Pool is likely to collapse in an earthquake. On Thursday, council voted on a referendum to decide whether Victoria should borrow $170M to rebuild it, with staff saying it is not safe to continue operating.

The Royal BC Museum is also likely to be completely destroyed. Nissen says he understood the outrage felt when the province announced an $800M rebuild of the museum, amid a housing and cost of living crisis. However, he fears the outcome of it falling while hundreds of tourists and school groups are inside. The Parliament Building is also likely to be destroyed, which could lead to the death of several political leaders.

He’s not advocating to tear down historical buildings in the city. But as a seismologist, he would “certainly advocate putting money into retrofitting.”

“That's very expensive. We're talking billions and billions of dollars. I'm a seismologist, of course, I think it should be the first priority of the province,” says Nissen.

He says he understands that there are several concerns that are immediate, such as homelessness. Still, if a mega-thrust earthquake occurred, it’s estimated that 50% of Victoria’s population could end up homeless.

“So, you know, if you think that this housing crisis—if you think it's bad now, after a major earthquake, it will be absolutely catastrophic,” says Nissen.

“If the earthquake’s tomorrow, then we'll say we were too slow. It'll be a complete disaster.”

Additional factors could make the death toll higher

This may all sound like a worst-case scenario, but Nissen stresses these predictions are the most likely to happen.

They also don’t account for the domino effect of damage that can take place after an earthquake.

Fires can result after a quake, with broken gas mains and sparks from fallen electrical wires potentially leading to an explosive combination.

Landslides could occur, especially along the Malahat, cutting off aid from the North Island. Aid from off the Island could also be blocked if the docks at Swartz Bay are damaged or the airport shut down.

During the winter, the ground holds more water leading to concerns of infrastructure essentially melting.

“That waterlogging effect has a huge impact on liquefaction—liquefaction is where the ground, as it's shaking, liquefies,” says Nissen, recalling the images from Christchurch, New Zealand in 2011, when a 6.2 magnitude earthquake caused cars to sink into the streets.

The Christchurch earthquake is an example Nissen said we can look to in Victoria. The 6.2 magnitude quake occurred close to the city centre, with a similar proximity as the Leech River and XEOLXELEK faults are to Victoria. If those faults were triggered, we can expect a similar scene to play out here. The proximity of both the Leech River and XEOLXELEK faults to Victoria would also prevent earthquake early warning systems from working.

If a mega-thrust from CSZ occurs, it will be even worse.

What to expect throughout BC when the mega-thrust hits

When the mega-thrust earthquake—or, the “Big One,” as we’ve come to know it—happens, municipalities on BC’s coast will have different concerns to prepare for.

For those in the CRD, ground shaking will pose the biggest threat. For those on the west side of the Island, tsunamis will do the most damage.

Tofino’s less susceptible to the effects of ground shaking thanks to its smaller-sized, mostly wooden buildings, which have more flexibility to absorb movement, unlike the brick buildings in Victoria’s downtown.

Victorians don’t need to fret about tsunamis, says Nissen. Waves from a mega-thrust earthquake are expected to only reach 3-5 metres near the capital. The CRD has developed an interactive map to show which areas would be impacted by tsunamis. Since Victoria is already a few metres above sea level, the cliff at Dallas beach could be “more than enough” protection, according to Nissen.

But Tofino is right at sea level. The District of Tofino has created a guide for evacuation routes to higher ground, which should be at least 20 metres above sea level on the West Island.

Time is of the essence though: Victorians will have about 1-2 hours to get to high ground before a tsunami hits; those in Tofino would have between 10-20 minutes.

One solution that could save people in Tofino from attempting to run against the clock to reach higher ground is a tsunami vertical evacuation tower—essentially a reverse bomb shelter, built high above ground to withstand the wall of water. These are common in Japan and have slowly been added to Washington’s coastline, but are not yet part of Canada’s emergency response.

Vancouver could also experience as much damage—or more—as locations on the Island, despite being further away. Nissen says this is due to the sediments in the ground under Vancouver, which could act like “jello,” further amplifying the ground shake in the region.

None of these situations are ideal, of course, but this understanding helps with emergency planning, which Nissen says is essential. Victoria residents can review the city’s emergency planning guides, or join one of the emergency preparedness workshops held throughout the year. You can also revisit Capital Daily’s interview with Gregor Craigie on what to do when the “Big One” arrives.

“There's going to be a lot of panic, so it's important for people to remind themselves and even practice what to do in an earthquake,” says Nissen