One of the world’s most dangerous earthquake faults lies just 50 miles off the coast of the Pacific Northwest. To find out what might happen when the next big one hits, scientists simulated 50 different magnitude 9 earthquakes, and their work yielded a surprising result: Cities actually fared better when they were closer to the earthquake’s starting point.
The Cascadia Subduction Zone stretches more than 600 miles, from Cape Mendocino in northern California to Vancouver Island in Canada. There, along a giant rift in Earth’s crust, the Pacific’s sea floor sinks beneath the lighter North American tectonic plate.
Geological evidence shows that the fault has loosed a magnitude 9 earthquake at least 20 times over the last ten thousand years, along with another 20 or so magnitude 7 and 8 temblors. The last major Cascadia quake, likely a magnitude 8 or 9, jolted the region more than 300 years ago, unleashing a tsunami that inundated the Pacific Northwest and that was felt as far away as Japan.
That history suggests that it’s only a matter of time before the fault generates another devastating quake.
“We estimate the probability of having a magnitude 9 earthquake in the next 50 years is about 10 to 14 percent,” says Art Frankel, a seismologist with the U.S. Geological Survey.
In their simulations, Frankel and his colleagues modeled 50 combinations of three critical seismic effects: an earthquake’s origin, or epicenter; the length and direction of the rupture; and the intensity of the tremors along the fault.
The most damaging factor turned out to the direction in which a rupture spreads, study leader Erin Wirth at the University of Washington reported in 2017 at the Geological Society of America’s annual meeting in Seattle.
“If you have a large fault, the earthquake doesn't come from a point, it ruptures over a large area,” says Chris Goldfinger, a seismologist at Oregon State University who was not on the study team and who has uncovered much of the evidence for past earthquakes along the Cascadia Subduction Zone.
A rupture typically travels at more than 3,600 miles an hour. If it spreads along the entire length of the Cascadia Subduction Zone—as it would in the case of a magnitude 9 earthquake—tremors would shake the Northwest for two or three minutes. The simulations show that an offshore epicenter with a rupture heading inland would cause the most damage to the region’s cities.
“It’s like a train traveling toward you, a sort of Doppler effect where the waves pile up in front of the rupture as the rupture comes toward you,” Frankel says.
If, on the other hand, the epicenter occurs inland, say, in the northwestern corner of Washington state, the rupture propagates out to sea, dispersing the earthquake’s energy away from Seattle. But whether the earthquake starts off the coast or beneath the mainland, the Northwest won’t get off lightly.
“Shaking is going to be serious for any of these scenarios, that’s for sure,” Frankel says.
Those tremors will pose an enormous threat to Seattle and other cities in the Northwest, where many homes and buildings were constructed before local codes included any seismic standards.
“Cities that have fragile infrastructure will suffer a lot of damage from even relatively modest shaking,” says Goldfinger. Frankel and his team have shared their data with engineers, who will use it to evaluate how existing buildings will—or won’t—hold up to the rigors of a magnitude 9 earthquake. And there’s no doubt that a mega-quake will hit the region one day.
“The certainty is 100 percent,” says Goldfinger. “The geological information is so compelling, there’s no debate about that. We know it’s coming.”
