Fault line under Strait of Georgia could cause 6.0 earthquake off Sunshine Coast, research finds

A B.C. researcher says he's identified a major fault line under the Strait of Georgia that could one day rupture and trigger a 6.0 magnitude earthquake off the coast of Vancouver.

Reid Merrill, a PhD student at the University of British Columbia, says it could be similar to a 2001 earthquake in the Seattle area that caused up to $4 billion worth of damage. 

The fault's location near the Sunshine Coast municipality of Sechelt — around 50 kilometres northwest of Vancouver — was pinpointed by analyzing 30 years of data tracking a cluster of quakes to the southwest of Texada Island. 

Merrill and his colleague Michael Bostock determined that the quakes make up an "earthquake nest," which spans 10 to 20 kilometres of the Juan de Fuca tectonic plate.

"A 20-kilometre long fault, if it were to rupture in its entirety, would be the equivalent of a greater than magnitude 6.0 earthquake, where the size of the fault is directly related to the magnitude of the event," said Merrill, who studies in the department of Earth, Ocean, and Atmospheric Sciences.

"The epicentre would be much closer to these metropolitan areas like Nanaimo and Vancouver and Sechelt. In some ways these earthquakes [...] can be more dangerous than the megathrust or the Big One."

According to Natural Resources Canada, an earthquake with a magnitude between 6.1 and 6.9 can cause damage to poorly constructed buildings and other structures in areas up to about 100 kilometres from the epicentre.

Alison Bird, a seismologist with Natural Resources Canada, says a major earthquake from this fault would damage older brick buildings and buildings constructed before seismic codes were in place.

She says Merrill's findings are a "good reminder that we live in earthquake country."

The Juan de Fuca tectonic plate, which is off the coast of British Columbia, Washington and Oregon, is being forced underneath the North American Plate. The energy from that movement is being stored up inside the rocks, waiting for the moment it will be released as a catastrophic megathrust earthquake — otherwise known as "The Big One."

The Texada Island earthquake nest, on the other hand, is located 65 kilometres below the surface within a part of the Juan de Fuca Plate that lies beneath the North American Plate.

Although between four and 10 seismic events occur every year in the location, the depth of their epicentre means much of their energy is lost by the time the vibrations reach the surface.

"Most of the earthquakes that we've caught are occurring on faults that we have been unable to map simply because they're so deep," Bird said.

The 6.8 magnitude Nisqually earthquake, centred near Olympia, Wash., on Feb. 28, 2001, was measured at a depth of more than 50 kilometres. Hundreds of people were injured as a result of the quake, which was estimated to have caused between $1 billion and $4 billion in damage across the heavily built-up Seattle metropolitan area.

Hard to predict next major quake

It's unclear what's causing the tremors southwest of Texada Island, Merrill said. One of his theories is that as water escapes the Juan de Fuca plate, it creates instability in the rocks around the fault. Another theory he finds less likely is that the tremors are aftershocks associated with large earthquakes that could have occurred over a century ago. 

Earthquake nests have also been identified in Romania, Afghanistan and Colombia. The most significant earthquake caused by one of the nests was a magnitude 7.7 in Vrancea, Romania almost 80 years ago.

Merrill says without knowing when the Texada fault's last major event occurred, it's difficult to predict when the next one might happen.

"There are some historical records that indicate that maybe Nanaimo was struck by a magnitude 5.0 earthquake in the early 1900s," he said. "So this could have originated from the fault that we've identified. But we can't say for sure." 

In the meantime, he says city planners in Nanaimo and Sechelt can use the findings about the Texada earthquake nest when selecting new building sites.

In CBC's Fault Lines podcast, CBC Vancouver seismologist Johanna Wagstaffe guides listeners through two earthquake scenarios so you can prepare yourself, your family and your neighbours. Download the podcast here.