The Fraser Valley's other quake threat

You’ve probably heard about The Big One: a potential earthquake caused by shifting tectonic plates just off the coast of Vancouver Island.

But in the Fraser Valley, there’s another potential earthquake threat lurking in the mountains just south of Sumas Prairie.

While it’s less likely to strike than the Big One, if it does, it could deliver an even more powerful jolt locally, with potential impacts on dikes, the Nooksack River, and homes on the Sumas and Chilliwack floodplains.

In the early morning hours of a Sunday in July 2015, some Abbotsford residents were woken by the sound of loud banging. Some felt their homes shifting beneath them. Others did not.

The culprit was a relatively tiny earthquake located to the southeast of the city, just across the US border. Such quakes are not an exceptional rarity, but they are a sign of the shifting, ever-changing nature of the earth in this infinitely complex region—and the possibility of something much bigger and more damaging.

There’s Mt. Baker, of course, an active volcano with the potential to blow its top, scatter ash for hundreds of miles, and send lahars barreling down Nooksack River valley. And of course there’s the Nooksack itself, which drains the volcano and has recently shown its power.

The Boulder Creek Fault isn’t quite in the league of those two threats. But since first showing up on US Geological Service (USGS) maps 15 years ago, studies have suggested that it has a very real chance of significantly shaking the Fraser Valley.

The fault sits along the Nooksack River, just south of the US-Canada border and Cultus Lake. The USGS has estimated that the fault could trigger an earthquake measuring 6.8-magnitude on the Richter scale. Such a quake could cause significant damage.

The 2001 earthquake that shook the Seattle region was a 6.8 magnitude event (though of a different character—we’ll explain in a bit).

The Richter scale works on a logarithmic scale in which a 5.0-magnitude earthquake is 10 times larger than a 4.0 quake and 100 times larger than a 3.0 quake, and so on. A 6.8-magnitude quake would be 3,100 times bigger than that 3.3-magnitude event that rattled Abbotsford in 2015.

That’s the scary news.

The good news is that the fault is shallow, so its energy will dissipate quicker than a subduction-caused quake like that in Seattle. And the fault is only believed to have ruptured three times in the last 9,000 years. So the likelihood that a Boulder Creek quake will hit within any of our lifetimes is relatively low; a massive Cascadia Subduction earthquake—i.e. The Big One—is still more likely. Finally, it’s also possible that if the Boulder Creek fault did rupture, it might only do so partially, sparing the region from the worst.

But it’s still worth being prepared. Because the Boulder Creek Fault could deliver a very real punch to much of the Fraser Valley.

When this reporter first looked at the fault in 2015, it was believed that the quake would only deliver fairly light damage to the area because of the shallow nature of the fault.

But in 2017, the USGS conducted a new scenario that suggests the potential for more severe damage across a larger area of the Fraser Valley.

A resulting “shakemap” suggests that the southern part of Sumas Prairie could suffer damage in the range of 7 to 7.5 on the Mercalli magnitude scale, a scale that measures the potential for damage by an earthquake at different points in the earth (the Richter scale only mentions the degree of shaking at an earthquake’s epicentre so it provides less information the further one gets from that location). Pictures would fall off walls and chimneys could crack and fall. (By comparison, that 2001 Seattle quake had a maximum Mercalli intensity of 8).

Damage would be most pronounced in older buildings. Homes as far north as Yarrow and Greendale and as far west as Abbotsford International Airport and all across central and eastern Abbotsford could be afflicted by severe shaking.

“The older buildings would see damage,” Canadian Geological Service seismologist Camille Brillon told The Current. While newer buildings and those with seismic retrofits might have negligible damage, those that date back to the 60s would be impacted—especially those with architectural features like chimneys, gables and similar structures.

The increased potential threat to the Fraser Valley is likely the result of better modelling of the region’s large floodplains.

“Those softer soils will shake more and then a concern would be liquefaction,” Brillon said.

That would leave central Chilliwack vulnerable, despite its distance from the fault’s epicentre. Indeed, a Boulder Creek rupture could be more intense than a subduction quake in both Chilliwack and Abbotsford, according to shakemaps of two possible scenarios.

In Abbotsford, the fault could deliver a rank-seven intensity—compared to The Big One’s forecast potential of rank 6.5. In Chilliwack, it could deliver a 6.5 intensity, compared to a 6 from The Big One.

Since liquefaction depends on the amount of water in the ground, damage from the quake could also depend on the recent weather and the season. Sumas Prairie sits below sea level and its buildings would be particularly at risk of earthquake damage during the late fall and winter, when the water table is highest.

Different quakes, same advice

Spokespeople for Abbotsford and Chilliwack both say their staff are aware of the Boulder Creek Fault. Abbotsford’s disaster plan is able to respond to a variety of emergencies and can be “scaled up or down as needed,” a spokesperson said. Chilliwack, meanwhile, is aware of the most-recent study of the fault and has determined that it doesn’t require revising its plans.

The Boulder Creek Fault is a relatively new discovery, but because its potential damage is similar to that of The Big One, it shouldn’t really change the average person’s preparations.

Whatever earthquake hits, if it hits, will require residents to be prepared in mostly the same way.

“For any kind of earthquake, any natural hazard, the advice is going to be the same: you have to have food, water, and a plan,” Brillon said.

Still, the potential for even slightly more-intense shaking from the Boulder Creek does emphasize the vulnerability of a key piece of regional infrastructure: the valley’s hundreds of kilometres of dikes.

A 2015 assessment of the region’s dikes—the same one that correctly predicted the 2021 collapse of the Sumas Prairie dike—suggested that almost all the region’s flood protections are vulnerable to liquefaction and lateral spreading in the event of an earthquake. The Boulder Creek shakemap suggests dikes in the eastern Fraser Valley may need to be built to sustain shaking that exceeds that which might be delivered by a Cascadia subduction earthquake.

The Boulder Creek Fault also carries an additional potential flood and landslide threat.

Communities closest to the fault that sit on, or near hillsides, could be at risk of seismically triggered landslides. A 2022 study looked at the risk of landslides in the Nooksack Watershed in the Mt. Baker area.

While that area didn’t look at the threat in Canada, researchers found that as many as 10% of all bedrock landslides over the last four millennia in the Nooksack watershed were caused by a single event: the last Boulder Creek earthquake. While the impact on Canadian mountainsides may be less dramatic, the same forces and much of the same underlying geology exists north of the border. And Lindell Beach and Cultus Lake Provincial Park’s campgrounds are less than 10km away, as the crow flies, from the fault.

Landslides could have knock-on effects.

The researchers wrote that “these landslides have the potential to inundate rivers creating

temporary dams that could eventually breach, resulting in catastrophic outburst floods.

The associated floodwaters could rapidly flow towards Bellingham or smaller towns in

the Nooksack watershed, resulting in extensive property and economic damage.”

And the Nooksack, as Abbotsford well knows, can deliver a significant punch to Sumas Prairie.

American officials have studied Boulder Creek closely, going so far as to simulate its impacts on buildings across multiple counties. One scenario envisioned 15 injuries, 77 “extensively damaged” homes, and more than $100 million in losses.

All those figures relate only to the United States. The current Canadian officials have not conducted any similar analysis of the fault’s threat.

A Boulder Creek earthquake could deliver more of a punch than a Cascadia earthquake. But that could come with a mitigating side effect: whereas provincial and national emergency responses are likely to focus on the hardest hit areas in a Big One scenario, a more locally focused Boulder Creek quake is likely to generate more help for Fraser Valley communities.

They may need it.