When will the 'Big One' earthquake hit? Scientists weigh in

A tsunami evacuation site sign is shown on high ground near the House of Huuayaht in the village of Anacla in Pachena Bay, B.C. Wednesday, Jan. 14, 2015. THE CANADIAN PRESS/Jonathan Hayward

A "swarm" of earthquakes off the coast of British Columbia has forced many to remember something they'd probably rather forget: at any moment, the "Big One" could hit about 100 kilometres west of Vancouver Island.

In the context of Canada and the U.S. Pacific Northwest, the "Big One" refers to a megathrust earthquake along the Cascadia subduction zone, where the Juan de Fuca tectonic plate meets the North American tectonic plate. The Cascadia spans about 1,000 kilometres from Northern California to Vancouver Island and is capable of producing earthquakes as powerful as magnitude 9, especially toward its northern end.

Researchers say they know the "Big One" is coming, but when should we expect it? Scientists say predicting when a megathrust earthquake will hit is a game of probability.

It's like guessing exactly which breath will be the one to make a balloon pop, says Ed Nissen, University of Victoria earth and ocean science researcher. Past experience might give you a feeling for it, Nissen said, but no two breaths, or balloons, are the same. It's not an exact science.

"The tectonic strain is kind of gradually building up along the faults, and eventually the stresses will overcome friction and the fault will slip in an earthquake," Nissen told CTVNews.ca in an interview over Zoom.

Nissen said scientists can't pinpoint exactly when those stresses will be enough to cause a sudden slip, but they can produce rough estimates based on probability.

When is the big earthquake expected?

Camille Brillon is a seismologist at Natural Resources Canada (NRCAN) who says the probability of a megathrust quake off the coast of B.C. in the next 50 years is between 10 and 15 per cent, with the likelihood rising over time.

"So, sometime in the next 200-or-so years, there likely will be a (magnitude) 9+ Cascadia earthquake," Brillon said, adding that scientists are able to make estimates like this partly based on data from past megathrust quakes along the fault.

According to NRCAN, 13 megathrust earthquakes have occurred along the Cascadia in the last 6,000 years, at an average rate of one every 500 to 600 years. However, some have been as close together as 200 years and some have been as far apart as 800 years.

Nissen says the last one was more than 300 years ago.

"The next one might not be for another 100 years or even 200 years," Nissen said. "But it's also true to say it could happen tomorrow. It wouldn't be a huge surprise."

How bad was the last 'Big One'?

The details of the last megathrust quake are "actually remarkably well known," Nissen said, thanks to the oral histories of Indigenous people living on Vancouver Island as well as accurate recordkeeping in Japan.

The last megathrust earthquake along the Cascadia subduction zone occurred at 9 p.m. PT on Jan. 26, 1700. Estimated to have been a magnitude-9 quake, NRCAN says it was one of the world's largest.

To provide a sense of scale, Nissen said a magnitude-9 earthquake releases 1,000 times more energy than a magnitude 7, the latter of which NRCAN classifies as a "major" earthquake.

The 1700 quake collapsed the houses of the Cowichan people on Vancouver Island and caused numerous landslides, according to NRCAN. The shaking was so violent that people couldn't stand, and it lasted long enough to make them sick.

On the west coast of Vancouver Island, a tsunami generated by the quake completely destroyed the winter village of the Pachena Bay people, leaving no survivors. The tsunami swept across the Pacific, causing destruction as far away as Japan, where recordkeeping made it possible to know the exact time of the quake.

The earthquake also left unmistakeable geological signatures consistent with the 12 other megathrust earthquakes before it.

Will there be any warning signs?

Nissen said there probably won't be any telltale signs the "Big One" is about to hit, since earthquakes happen so abruptly.

"There's nothing consistent which you could reliably use as a signal that an earthquake is going to happen," he said.

Some researchers see promise in studying the foreshocks that occur before some large earthquakes, but Nissen said even those aren't reliable enough to count on.

"It turns out not every earthquake has a foreshock," he said. "Most big earthquakes actually don't."

Fortunately, a new technology has emerged that can buy people enough time, before the shaking starts, to try and get somewhere safe.

Earthquake Early Warning (EEW) systems use special sensors to detect the first wave of energy that radiates from an earthquake, the P-wave, Brillon explained.

"Knowing when the P-wave arrives, we can quickly locate where the earthquake was, how big it was, and then when the following shear waves, or S-waves would reach a certain point, and how strong the shaking would be,” she said.

While both types of seismic waves can cause shaking, S-waves are known to be more destructive.

Sensors transmit this information to data centres that use algorithms to detect earthquake's location and magnitude, and an alert warns people in the area sometimes "tens of seconds before" shaking begins, according to NRCAN.

Canada has developed an EEW system that Brillon said is due to launch imminently, and Nissen said ongoing efforts to bolster the technology over the Cascadia fault could give people even more time to prepare.

"We're talking about 30 seconds-ish before those seismic waves hit Vancouver Island … a little bit longer if you live in Vancouver, which is obviously further to the east and further away from the fault," he said. Such a warning wouldn't prevent damage to buildings and other infrastructure, Nissen added, but it could save lives.

"This is about giving people a few seconds to get under a table or run outside if they have long enough … trains to come to a safe halt, lifts coming to the closest floor and opening the doors, heart surgeons putting down their tools, that kind of thing."

'The most dangerous section'

Meanwhile, a group of 20 researchers has obtained what it says is the most detailed imaging of the Cascadia subduction zone yet, providing valuable insight into how a future megathrust earthquake might unfold.

Using a research vessel towing an array of geophysical instruments along almost the entire length of the zone, they produced what they say is the first comprehensive survey of the complex structure of the fault beneath the seafloor.

The team published its research in the scientific journal Science Advances in June.

"The models currently in use by public agencies were based on a limited set of old, low-quality 1980s-era data," Suzanne Carbotte, lead author and a marine geophysicist at Columbia University, said in a media release. "The megathrust has a much more complex geometry than previously assumed. The study provides a new framework for earthquake and tsunami hazard assessment."

One of the study's findings is that the northern part of the fault, close to Vancouver Island and Washington state, is the section most likely to produce a major earthquake.

While the topography of other segments is relatively rough, with features that might erode the upper plate and limit how far any quake may spread within the segment, the northern portion is smooth.

"This means that it may be more likely to rupture along its entire length at once, making it potentially the most dangerous section," reads the media release.