When “the big one” finally hits, will B.C. be prepared?
A new study has produced the clearest-ever look at what’s going on beneath the seafloor in an area capable of producing some of the planet’s most powerful earthquakes — and warns the risk is highest for southern B.C. and Washington state.
The new data is “really exciting,” said John Cassidy, an earthquake seismologist with Natural Resources Canada.
“It gives us some of the best images of what’s happening just off the coast of Vancouver Island, Washington, Oregon.”
B.C.’s South Coast is in an area known as the Cascadia Subduction Zone, a 1,000-kilometre fault system spanning from the coast of Vancouver Island to northern California. In this area, tectonic plates under the ocean floor are slowly sliding under the North American plate.
Scientists have long known the built-up pressure from that process is eventually released in sudden, powerful earthquakes. But their knowledge of the specific undersea conditions in the Cascadia zone has, until now, been “fuzzy,” explained Columbia University marine geophysicist and study co-author Suzanne Carbotte.
Researchers spent 41 days in 2021 towing waterproof cables with 1,200 specialized microphones attached above 5,500 kilometres of seafloor, scanning detailed maps of the geologically-active area.
The data showed that not all segments of the fault system were created equal, with some less prone to the threat of big quakes.
The segment off the coast of Vancouver Island and Washington state’s Olympic Peninsula, by contrast, “is markedly flatter and smoother than segments to the south,” explained Carbotte.
“And those conditions, flat-smooth fault interface, are the conditions that are associated with the most destructive earthquakes.”
The region, researchers concluded, has the biggest potential for the largest earthquakes along the entire Cascadia Subduction Zone.
Cassidy said that’s because when the plates do eventually slip, they’re likely to slide farther in one shot than elsewhere.
“Some areas you might see 20 metres of slip within a few minutes,” he said.
What the study doesn’t say is when the next mega quake will come.
Historical records suggest the last one was in January 1700, and Cassidy said scientists have identified 19 such quakes in the 9.0 magnitude range in the last 10,000 years.
Such massive earthquakes, he said, are expected about every 500 years, but evidence has shown some gaps of only about 250 years and some as long as 850.
But while the research can’t say when the next quake will come, it can help us understand where it will hit and where the effects will be the worst, Cassidy said.
“This study is providing a lot more detail,” he said.
“That allows us to estimate ground shaking in a much, much better way, ground shaking tsunami hazard.”
The province has already said the information will help it prepare for the future.
“The understanding of these risk factors continues to evolve as research becomes available,” Emergency Management Minister Bowinn Ma said.
“Research is available to ensure our standards are aligned to keep people safe.”
Cassidy said the data will be used to update things like B.C.’s building and bridge codes to ensure the public is safe whenever the big quake comes.
And he said it will inform the next wave of research, allowing scientists to better understand the local threat area.
“If you don’t know something,” he said, “you can’t prepare for it.”
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