No, recent small BC earthquakes won’t reduce Big One risk

Three small earthquakes won’t do anything to mitigate the chances of a future “Big One” devastating British Columbia, a geologist says.

Over just a few weeks, three recent earthquakes have rattled residents in southwestern BC. They include a 4.1-magnitude quake just east of Victoria, a 4.7-magnitude tremor centred on the Sunshine Coast, and a 3.9-magitude quake on Washington’s Olympic Peninsula. The quakes were all felt in Canada, but did no real damage.

Some hoped the earthquakes might have released some of the subterranean stress that could trigger a massive, devastating quake. But such hopes are misplaced, Simon Fraser University geologist Brent Ward says.

The Big One, when it occurs, will originate in the tectonic subduction zone where the Pacific Plate is attempting to push beneath the North American plate. Right now, the two plates are stuck against one another as pressure builds. Eventually, it will be released, triggering the “Big One.” Such an earthquake could happen tomorrow—or in 500 years.

The recent quakes, however, occurred not in that subduction zone, but much closer to the surface on crustal faults. Those faults exist within the North American Plate on which BC and Washington State sit, rather than the tectonic subduction zone just off the Pacific Coast. Another way to think of it is like this: a Big One earthquake would be the result of friction between two tectonic plates, whereas crustal earthquakes are the result of friction between cracks that exist within one plate.

“The stresses that are built up along the locked subduction zone—none of those were released by any of these smaller earthquakes,” Ward said.

Although the tremors aren’t necessarily a precursor to a future devastating quake and can’t be used to predict a future Big one, they are evidence of the build-up of stress in the ground below our feet, Ward noted.

Right now, the Pacific and North American plates are stuck against one another, creating a buildup of pressure. Some of that pressure, Ward says, ends up being transferred to the North American plate.

“In certain areas, that stress builds up enough that it exceeds the strength of the rocks and they break,” Ward said. “It's that rupture or breakage along a surface that results in the production of earthquake waves.”

In other words, the recent crustal quakes can be seen as a potential symptom of some of the underlying pressures miles below the earth’s surface. The tremors can’t be used to predict the future, because such surface quakes are relatively common, even if it’s rare to have such a number so close to one another. But they are evidence of the general stresses in the rock beneath western British Columbia.

The recent quakes are tiny compared to the eventual Big One. Earthquake magnitudes are calculated on a logarithmic scale. A 5.0-magnitude earthquake creates vibrations 10 times the size of a 4.0 magnitude quake, and a 6.0-magnitude quake has vibrations 10 times as large as a 5.0 magnitude quake. Given that the Big One’s could have a magnitude exceeding 8.0, its intensity and energy would be many thousands of times greater than the recent small quakes.

But the recent tremors could also have been much larger and do actual significant damage, Ward says. And because they are much more frequent than massive subduction quakes, Ward says he is more worried about the impacts of a large crustal quake than the Big One. He pointed to a 7.3-magnitude crustal earthquake in 1946 beneath Vancouver Island that cracked chimneys, liquified farmland, and caused landslides.

“If we had a similar earthquake now, with the population densities on Vancouver Island, we'd be looking at hundreds, if not thousands, of casualties and billions of dollars in damage,” he said.

Because they are caused by earth movement closer to the ground, the energy of a crustal earthquake is concentrated more locally than a subduction quake. Whereas a Big One earthquake could cause destruction across all of southwestern BC, a crustal quake would be felt more locally, with heavy damage close to the epicentre but fewer impacts regionally.

A shallow 2001 earthquake in Washington State, south of Seattle, injured hundreds, did more than $1 billion worth od damage, and was felt all around the region.

As scientists have used new technology to hunt for crustal faults, they have found increasing numbers of them in British Columbia and the western United States. They have also been able to better analyze the faults to gauge the hazard they pose to nearby communities.

For the Fraser Valley’s residents, perhaps the most concerning crustal fault is one located just south of the US border in the foothills of Mt. Baker. Labelled the Boulder Creek fault by US geologists, analyses suggest the fault could trigger a 6.7 magnitude quake that could cause significant damage in Chilliwack and Abbotsford. You can read our 2023 story on the fault here.