Note that if you’re not concerned about the Cascadia subduction zone cranking out a magnitude 9, you can think about this for other hazards such as a large quake on the San Andreas fault or a monstrous eruption of Yellowstone.
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If you haven’t read it yet, any discussion of earthquakes in the Pacific NW should probably start with this amazing New Yorker article by Kathryn Schulz, from 2015. It’s a wonderful, and sometimes terrifying read. https://www.newyorker.com/magazine/2015/07/20/the-really-big-one
In this article, Schulz writes of M9 quakes in the Pacific NW “we are now three hundred and fifteen years into a two-hundred-and-forty-three-year cycle.”
This line got a ton of attention in the press and in social media, and for good reason! It sounds like we are truly overdue for a massive quake, which is a terrifying thought.
But Schulz follows this line with a really important caveat: “It is possible to quibble with that number. Recurrence intervals are averages, and averages are tricky: ten is the average of nine and eleven, but also of eighteen and two.”
Earthquakes occur because stress accumulates on faults, which are weak zones in Earth’s crust. Basically, the crust is being squeezed and stretched and sheared, and it’s only by the grace of friction that the ground doesn’t slide. This stress causes the ground to deform in between quakes.
When the squeezing and stretching and shearing becomes too much for friction to withstand, the plates slip, and voila: earthquake. (We can go into the gory details of this process later, if you're interested). The quake actually allows the ground to un-deform, and release all of the stored energy.
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https://www.newyorker.com/magazine/2015/07/20/the-really-big-one
Whoa. That sounds bad.
Let’s unpack what this means.