In my post a couple of days ago about the unstable geology of the Greek island of Thera, I stated that this is far from the only place in the world where lots of people live in harm's way from the vagaries of plate tectonics. I mentioned the Cascadia Subduction Zone, off the coast of the Pacific Northwest, and included a link to the rather desultory post I'd made a while back about what's going on over there. A loyal reader of Skeptophilia wondered if I might write a more thorough piece about the terrifying situation looming in that beautiful part of the world, so here I am to elaborate, and probably to scare the absolute shit out of anyone living in coastal British Columbia, Washington, Oregon, and northern California.
Even after the general acceptance of plate tectonics by geologists in the 1960s and 1970s, it took a long time for them to see what was happening in the northeastern Pacific. The presence of a ridge (divergent zone) meant that the seafloor was spreading on both sides; the fact that the small Explorer, Juan de Fuca, and Gorda Plates were being shoved eastward meant that there had to be a trench somewhere between the ridge and continental North America. But the earliest sounding techniques couldn't find one. It turned out that it was buried -- submerged under hundreds of meters of muck, silt and sand washed out of the region's numerous rivers.
This, and the fact that there hadn't been a big earthquake in the Northwest since settlement by people of European descent, led a lot of geologists to the conclusion that the trench was "aseismic." Either the small plates east of the ridge weren't moving, or they were slipping underneath the North American Plate so smoothly that there were no measurable earthquakes.
This wasn't just a little bit wrong. This was stunningly wrong. This was wrong with whipped cream and a cherry on top.
The Explorer, Juan de Fuca, and Gorda Ridges are very much active spreading centers, and the fact that there haven't been any recent big earthquakes along the trench -- the Cascadia Subduction Zone, denoted on the map by the line with black triangles -- is not good news. The entire coastline of the Pacific Northwest is compressing as the three small plates get shoved under North America, just like trying to slide something underneath a throw rug makes it rumple and hump up. In fact, surveys measuring the positions of the peaks in the Cascade Range and on Vancouver Island have found that the whole terrain is being squished west-to-east, so entire mountains are being pushed toward each other.
Imagine the power required to do that.
Further, the fact that the trench is filled with mud doesn't mean the subduction zone is aseismic; quite the opposite. It turns out that a large part of the mud deposits there are turbidites -- the result of colossal underwater landslides.
Such as might occur during an enormous earthquake.
More of the mechanism was elucidated in 2003, when researchers found that the whole region was experiencing a phenomenon called episodic tremor and slip, where deeper parts of the conjoined plates -- the bits that are hotter and more plastic -- slip against each other, causing barely a rumble. This slip/tremor happens like clockwork every fourteen months. While this may sound like a good thing, it's actually the opposite. Releasing stress that has built up in the deep parts of the fault merely passes that stress upward to the colder, shallower parts that are still locked together, each ETS episode dialing up the energy like the clicking of another tooth in a ratchet.
So along the subduction zone, the two opposing sides of the plates are stuck together, building up more and more tension -- tension that will one day be released as the faultline unzips, and the whole northwest coast of the continent springs back toward the west.
To say the result will be catastrophic is understatement of the year.
It's happened before. In fact, geologists taking cores of the aforementioned turbidite sediments off the coast of Washington found evidence that in the past ten thousand years it's happened nineteen times. The spacing between megathrust earthquakes -- as these are called -- varies between three hundred and nine hundred years, with the average being around five. And the last one happened a little over 323 years ago.
We actually know down to the hour when it happened -- about 9 PM local time, January 26, 1700. Indigenous tribes in the area have a long tradition that many years ago, there was a terrible earthquake one midwinter night, during which the seashore dropped and salt water flooded in, killing many people. Evidence from tree rings in "ghost forests" -- the trunks of hundreds of western red cedars that had all been killed simultaneously by an influx of salt -- showed that some time in the 1690s or early 1700s there had been a massive flood from the ocean as the coastline suddenly dropped by several meters. The exact date was determined from records across the Pacific, where Japanese scribes describe what they called an "orphan tsunami" (a huge wave that, from their perspective at least, was not preceded by an earthquake) striking coastal Japan. Knowing the speed with which such waves travel across the ocean, geologists were able to determine exactly when the fault last unzipped from end to end.
The earthquake that resulted is estimated to have been somewhere between 8.7 and 9.2 on the Richter Scale, and to have resulted in land movement averaging around twenty meters.
Not pleasant to consider how that would play out if it happened today.
The worst part, for coastal communities today, is how close the Cascadia Subduction Zone is to shore. At its closest approaches -- near the west coast of Vancouver Island, and from central Oregon south to Cape Mendocino -- it's estimated that the lag time between the ground shaking and the first of the tsunami waves striking the shore will be around eight minutes. That's eight minutes between being thrown all over the place by an enormous earthquake, and somehow getting yourself to high ground before you're hit by a giant wall of salt water.
I remember when I first heard in detail about the dangers of the Cascadia Subduction Zone -- in 2015, from Kathryn Schulz's brilliant analysis in The New Yorker called "The Really Big One." It impressed me so much I actually used the fault as a plot point in my novel In the Midst of Lions, where the story is bracketed by earthquakes (one of them massive). But when I was a Seattle resident in the 1980s, I had no idea. I still dearly love the Northwest; not only does it have the ideal climate for a fanatical gardener like myself, it has amazing spots for hiking and camping. During my time there I spent many happy days on the coast of the Olympic Peninsula -- never realizing that a monster lurked offshore.
So while I miss many things about the Northwest, I know I could never live there again.
It may be that the fault won't rupture for another two hundred years; on the other hand, it could happen tomorrow. While our ability to analyze plate tectonics is light years beyond what it was even thirty years ago, when the situation in the Northwest first began to come clear, we still don't have any way to determine when the earthquake will happen with any kind of precision. At the moment, all we know is that it will rupture, sooner or later.
And I don't want to be anywhere near it when it does.
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