In her excellent article the New Yorker's Kathryn Schulz looks at the likelihood and the disastrous ramifications when we were struck by a large subduction zone earthquake here in the Pacific Northwest.
The Really Big One
An earthquake will destroy a sizable portion of the coastal Northwest. The question is when.
By Kathryn Schulz
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....
...When the next full-margin rupture happens, that region will suffer the worst natural disaster in the history of North America.
FEMA expects that over 12,000 deaths, 27,000 injured, and a million people left homeless.
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.
By taking seabed floor core samples geologists can see a record the Pacific Northwest geological history going back 10,000 years. Because each time there is a large subduction zone earthquake there is a vast amount of sediment that flows off of the North American continent and is deposited on the ocean floor in visible layers almost like tree rings.
Thanks to that work, we now know that the Pacific Northwest has experienced forty-one subduction-zone earthquakes in the past ten thousand years. If you divide ten thousand by forty-one, you get two hundred and forty-three, which is Cascadia’s recurrence interval: the average amount of time that elapses between earthquakes. That timespan is dangerous both because it is too long—long enough for us to unwittingly build an entire civilization on top of our continent’s worst fault line—and because it is not long enough. Counting from the earthquake of 1700, we are now three hundred and fifteen years into a two-hundred-and-forty-three-year cycle.
How would a disaster of this magnitude unfold?
Soon after that shaking begins, the electrical grid will fail, likely everywhere west of the Cascades and possibly well beyond. If it happens at night, the ensuing catastrophe will unfold in darkness. In theory, those who are at home when it hits should be safest; it is easy and relatively inexpensive to seismically safeguard a private dwelling. But, lulled into nonchalance by their seemingly benign environment, most people in the Pacific Northwest have not done so. That nonchalance will shatter instantly. So will everything made of glass. Anything indoors and unsecured will lurch across the floor or come crashing down:...
Houses that are not bolted to their foundations will slide off—or, rather, they will stay put, obeying inertia, while the foundations, together with the rest of the Northwest, jolt westward. Unmoored on the undulating ground, the homes will begin to collapse.
We here in the Pacific Northwest are woefully under-prepared for the inevitable. Tsunami Zones continue to be developed by builders without any restraints. We lack the automatic shutoff features to shut down hazardous infrastructure in the event of an earthquake that have been adopted in Japan.
FEMA calculates that, across the region, something on the order of a million buildings—more than three thousand of them schools—will collapse or be compromised in the earthquake. So will half of all highway bridges, fifteen of the seventeen bridges spanning Portland’s two rivers, and two-thirds of railways and airports; also, one-third of all fire stations, half of all police stations, and two-thirds of all hospitals.
The Pacific Northwest's rugged geological features would unleash thousands of landslides.
It will also induce a process called liquefaction, whereby seemingly solid ground starts behaving like a liquid, to the detriment of anything on top of it. Fifteen per cent of Seattle is built on liquefiable land,
Among natural disasters, tsunamis may be the closest to being completely unsurvivable. The only likely way to outlive one is not to be there when it happens: to steer clear of the vulnerable area in the first place, or get yourself to high ground as fast as possible. For the seventy-one thousand people who live in Cascadia’s inundation zone, that will mean evacuating in the narrow window after one disaster ends and before another begins.
A large Subduction Zone Earthquake off the coast could produce a Tsunami that would raise seawater levels by 20 to over 100 feet.
In the aftermath of a large Subduction Zone Earthquake the local economy would be likely to collapse. Oregon's OSSPAC says that it could be followed by a "mass-displacement event" as traumatized survivors move away from the region. The overwhelming needs following a large Subduction Zone Earthquake could put strains on our very unity as a nation.
This article scares the hell out of me. I live in the Tsunami Zone about 16 to 18 feet above Sea Level, 20 miles to the West of Interstate 5. I have done little in the way of disaster preparation. I do have my water heater strapped to the wall studs, and have five gallons of drinking water stored away. I live at the base of a steep hill with an 11% grade, but the geology here is prone to liquefaction with a layer of clay 2 feet below the surface.
We here in the Pacific Northwest need to start taking the inevitable seriously.
TSUNAMI INUNDATION MAPS for Washington pdf
Oregon Tsunami Clearinghouse
