Some of you will have watched 'San Andreas' in the cinemas recently - based on the possibility of the 'big one' hitting there (overdone to the point of absurdity, but I suppose that's the 'fun' of this type of disaster movie). Well, the highest probable magnitude for the long-awaited 'big one' on that fault line is 8.5ish - big enough, but not necessarily completely devastating in a region which is quite well earthquake-proofed.
Go north, however, to the Cascade subduction zone off the coast of northern California, Oregon, Washington, and British Columbia, and something much bigger and much more horrifying is over 70 years overdue (on average - it could hit next week, or it might not hit in our lifetimes). An earthquake of up to 9.2 in magnitude, exceeding the one which caused the Fukushima disaster, is possible, if the whole zone gives way (if the southern half only gives way, it will be up to an 8.6)
An article published today in the New Yorker magazine makes it clear...
Quote:
When the next very big earthquake hits, the northwest edge of the continent, from California to Canada and the continental shelf to the Cascades, will drop by as much as six feet and rebound thirty to a hundred feet to the west—losing, within minutes, all the elevation and compression it has gained over centuries. Some of that shift will take place beneath the ocean, displacing a colossal quantity of seawater. The water will surge upward into a huge hill, then promptly collapse. One side will rush west, toward Japan. The other side will rush east, in a seven-hundred-mile liquid wall that will reach the Northwest coast, on average, fifteen minutes after the earthquake begins. By the time the shaking has ceased and the tsunami has receded, the region will be unrecognizable. Kenneth Murphy, who directs FEMA’s Region X, the division responsible for Oregon, Washington, Idaho, and Alaska, says, “Our operating assumption is that everything west of Interstate 5 will be toast.”
In the Pacific Northwest, everything west of Interstate 5 covers some hundred and forty thousand square miles, including Seattle, Tacoma, Portland, Eugene, Salem (the capital city of Oregon), Olympia (the capital of Washington), and some seven million people. When the next full-margin rupture happens, that region will suffer the worst natural disaster in the history of North America. Roughly three thousand people died in San Francisco’s 1906 earthquake. Almost two thousand died in Hurricane Katrina. Almost three hundred died in Hurricane Sandy. FEMA projects that nearly thirteen thousand people will die in the Cascadia earthquake and tsunami. Another twenty-seven thousand will be injured, and the agency expects that it will need to provide shelter for a million displaced people, and food and water for another two and a half million. “This is one time that I’m hoping all the science is wrong, and it won’t happen for another thousand years,” Murphy says.
In fact, the science is robust, and one of the chief scientists behind it is Chris Goldfinger. Thanks to work done by him and his colleagues, we now know that the odds of the big Cascadia earthquake happening in the next fifty years are roughly one in three. The odds of the very big one are roughly one in ten. Even those numbers do not fully reflect the danger—or, more to the point, how unprepared the Pacific Northwest is to face it. The truly worrisome figures in this story are these: Thirty years ago, no one knew that the Cascadia subduction zone had ever produced a major earthquake. Forty-five years ago, no one even knew it existed.
The Earthquake That Will Devastate Seattle - The New Yorker
What's particularly frightening is that, because knowledge of this zone is so recent the area has no early warning system, and its buildings are not earthquake-proofed. And the figure given for deaths - 13,000 - could be dwarfed if it hits in summer and people are near the coast when the subsequent tsunami hits. The native Americans of the area had oral history of tribes in the Vancouver area being wiped out in an instant by the earth sinking and drowning them all - this was ignored by the post-Lewis and Clark settlers until geologists worked out that there must have been a sudden violent shift in 1699/1700 - now they realise that the stories were true.
As someone with a lot of friends and family in Seattle, Tacoma, and across the Pacific North-West, this is scary stuff. It's only a question of how much gives way, and when. Either way, it will be bad...