Those who live in Seattle and Portland may have heard inklings of this for many years, but increasingly the geological and seismic science community are sounding alarm bells about a potentially massive and thoroughly catastrophic earthquake that is likely to strike the Pacific Northwest coast, possibly within the next 50 years. Back in October, as we in the Bay Area recognized the 25th anniversary of the Loma Prieta quake, the New York Times produced a documentary short highlighting the grave and not often talked about seismic danger for the Pacific Northwest. Now, in the new issue of the New Yorker, writer Kathryn Schulz delves further into explaining the reasons for worry, and quotes a FEMA director as saying that if and when this expected big one hits the Washington and Oregon coast, "Our operating assumption is that everything west of Interstate 5 will be toast."
The cause is the increasing pressure along the Cascadia subduction zone, a place under the Pacific where the North American tectonic plate and the Juan de Fuca plate meet, and where the Juan de Fuca plate is slowly trying to slip underneath the North American plate.
The earthquake danger for the region was not fully understood until after the late 1980's when two scientists, Brian Atwater and David Yamaguchi, began examining a "ghost forest" of cedar trees in a salt marsh on the banks of the Copalis River. Though people understood the trees had all died because of saltwater intrusion, the assumption was it had happened slowly. That is, until Atwater and Yamaguchi took samples and found that via their rings, all the trees had died simultaneously between 1699 and 1700. Though Europeans would not make it to the region for a hundred years, and begin Western-style recorded history, it did not take long before scientists looked to Japan, where records of tsunamis and seismic events date back to 599 A.D. Because the Japanese hadn't felt any shaking prior to a devastating tsunami that hit in January of 1700, they referred to the event, which reportedly brought a six-hundred-mile-long wave their coast, as an "orphan tsunami."
But, of course, it wasn't. Scientists now know that a massive, 9.0-magnitude quake hit the northern Pacific coast at nine o’ clock at night on January 26, 1700. A subsequent tsunami wave the length of the continent struck our coast fifteen minutes later, and another wave took ten hours to cross the Pacific and devastate the Japanese coast, hitting there on January 27, 1700.
Parts of the coastline dropped many feet and became inundated with saltwater. Vancouver Island’s Pachena Bay people were, as seven generations of legend had it, all drowned at once that night.
The Cascadia subduction zone was to blame then, and history says it should repeat such an event every 240 years or so, meaning it is already 75 years overdue.
If only the southern part of the Cascadia subduction zone gives way it could cause an earthquake in the realm of 8.0 to 8.6, and the chance of that happening in the next fifty years is one in three. There's a one in ten chance of a full-margin rupture causing another potential 9.0 earthquake, in which it's estimated thirteen thousand people will die, and it will go down as the worst natural disaster in US history. By comparison, three thousand people died in SF's 1906 earthquake and fire.
The risks, and the potential loss of life, far exceed what a "big one" along the San Andreas fault would cause which, in case you missed the news, will never be like that stupid movie. Sorry to ruin your Monday.
Here's the earlier NYT piece: