Yes, you thought right: there hasn’t been a major earthquake in the Pacific Northwest since it was discovered by Western civilization. There have only been seven recorded in the region, averaging 6.3 in magnitude and killing 14 people total. And Seattle is the least likely metropolitan area in the US to have a natural disaster of any kind.
In fact, until 50 years ago, no one thought that earthquakes were much of an issue at all, in that area. But then, tectonic plate theory became mainstream and scientists noted that earthquakes and volcanoes were prevalent all around the so-called Ring of Fire: New Zealand, Indonesia, Japan, Alaska, California, Mexico and Chile. Notice how we skipped right over the Pacific Northwest: the scientists noticed that too.
The world’s tectonic plates. In the Pacific Northwest, the Juan de Fuca plate goes under the North American plate.
A very interesting article in The New Yorker tells us that these scientists then started drilling into the ocean floor off the coast of Washington state and found out that they could tell, from the stratification of the earth, when and how much land from the continent rushed into the sea — meaning, an earthquake had occurred, and how big it was. They went back 10,000 years and counted 41 major earthquakes, which means one happens every 243 years, on average. We know it’s been at least 210 years since there’s been one in that region, since Lewis and Clark went there in 1805. But how long has it been really?
Besides the ocean floor evidence for the year, we actually have some much cooler, and much more accurate data. There’s a “ghost forest” near the beach in Washington State, by the Copalis River. It’s called that because it consists of a bunch of dead trees standing in sea water. The theory had been that sea water got into the forest and killed off the trees but, in the late 1980s, two scientists figured out that they actually all died at the same time, in the winter of 1699-1700. So, due to that sudden onset, they theorized that an earthquake actually plunged the forest about six feet into the sea.
Copalis ghost forest
That’s pretty cool on its own, but then in 1996, they got historical confirmation: the Japanese have been keeping track of tsunamis for over 1400 years, and they knew that earthquakes caused them. But there was one “orphan” tsunami for which they felt no preceding earthquake: it happened on January 27th, 1700, and we now know that the reason they didn’t feel the parent is because the epicenter was so far away, off the edge of the Pacific Northwest. Ten hours after it shook, the tsunami it created had crossed the Pacific and hit Japan. It also turns out that the Native Americans of the region also have stories about entire tribes being wiped out long ago by the earth sinking into the sea, and canoes being flung into trees. It would’ve been the seventh strongest earthquake known to history.
So there you have it: the last earthquake to hit Seattle happened about a hundred years before Lewis and Clark, and 315 years before now. Subtract the average of 243 years from that, and you get an uncomfortable 72 years of the region being overdue for a big one. The most recent deadly quake the US had, was a 6.9 magnitude one near San Francisco, in 1989: it killed 63 people. The one that destroyed San Francisco in 1906 was 7.8 and killed 3,000 people — the most of any earthquake to hit the country. The one that hit Japan in 2011 was 9.0 and killed 16,000 people. It was the strongest one that country, which gets weekly earthquakes, had ever seen and the fourth strongest known to man. The one coming to Seattle could register 9.2.
Earthquake magnitudes are logarithmic, so a 7.8 earthquake is 8x stronger than a 6.9 one, and a 9.0 earthquake is 16x stronger than the 7.8 one. To find out how much stronger one earthquake is than another, take 10 to the power of the difference between them; for example: 10**(9.2-6.9) = 199.53. This means that the one coming to Seattle could be 200x stronger than the 1989 San Francisco earthquake. (By the way, did you know the Richter scale has been obsolete since the 1970s? The above, and generally all earthquake measurements, are actually stated in the Moment Magnitude scale.)
And to be clear, this is not fringe science: FEMA officially believes that there’s a 37% chance of an earthquake with magnitude 8.0 to 8.6 hitting the Pacific Northwest in the next 50 years; a 10-15% chance it will be in the 8.7 to 9.2 range. Because the area is horrendously ill-prepared for an earthquake, it will kill 13,000 people, destroy the local economy (including Amazon and Microsoft headquarters) and take almost two years to repair the infrastructure.
However, all of this might actually not happen until after the year 2160: before the earthquake in 1700, the previous two were in 1310 and 810, which makes 390 and 500 years between them. In fact, the average period between the last five earthquakes in the area was 460 years: almost twice the length of the 10,000 year average. Maybe they’ve just slowed down over the past couple of millenia, or maybe we’re overdue for quicker ones. In any case, move over San Andreas Fault: the Cascadia Subduction Zone is the new thing to fear.
via The New Yorker