Earlier this week, the New Yorker published an article on “The Big One” that the PacNW is overdue for
Under pressure from Juan de Fuca, the stuck edge of North America is bulging upward and compressing eastward, at the rate of, respectively, three to four millimetres and thirty to forty millimetres a year. It can do so for quite some time, because, as continent stuff goes, it is young, made of rock that is still relatively elastic. (Rocks, like us, get stiffer as they age.) But it cannot do so indefinitely. There is a backstop—the craton, that ancient unbudgeable mass at the center of the continent—and, sooner or later, North America will rebound like a spring. If, on that occasion, only the southern part of the Cascadia subduction zone gives way—your first two fingers, say—the magnitude of the resulting quake will be somewhere between 8.0 and 8.6. That’s the big one. If the entire zone gives way at once, an event that seismologists call a full-margin rupture, the magnitude will be somewhere between 8.7 and 9.2. That’s the very big one.
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. (Watch what your fingertips do when you flatten your hand.) 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.”
Now, Blogstation Tacoma is not all that far from I-5, but it is West of it. I’m not at all worried about a tsunami in my backyard though, for two reasons: First, I have the Olympic Mountain range, topping out at nearly 8000 feet in height, blocking the Pacific from meeting me at my doorstep, and secondly, I’m very well above sea-level.
The six foot elevation drop makes me want to exchange all of the wood in my house with steel, but that is not going to happen, so I’m just going to hope I’ve got everything strapped down tight.
The Wife found this article before I did and wanted to discuss it. I told her all of the above and then some other precautions I’ve taken “just in case” of some situation like the earthquake were to happen. I told her that the part of I-5 the author was likely speaking of was the section from around Olympia south to Sacramento and that even then, I wasn’t’ sure that a 200ft wave would reach that far inland. But that lots of stuff would fall down (overpasses, bridges, houses, apartments and possibly a high-rise building or two).
Also that Portland, OR would get the lions share of the death toll.
She seemed relieved that I had already worked this out years ago.
And then she showed me this post over at clickbait central
Could a catastrophic earthquake really destroy Seattle
Wherein another geologist said basically what I said about the tsunami and the failing of infrastructure and some buildings, but he added that while we are overdue for this quake to hit, there is only a 15% change it will happen in the next 50 years.
So, one opinion is made for Hollywood, the other has eaten a hash brownie and isn’t worried. I leave it to y’all to read and plan accordingly.