When you think of major earthquakes, the Pacific Northwest probably isn't the first place that comes to mind. California, Alaska, Chile, Indonesia, Turkey, Japan- these might come to mind first, and for good historical reasons too. I tend to snicker when people worry about "the big one" hitting California, because they are unaware that the largest earthquakes in the United States can only happen in Alaska or Washington. The assumption is that because California gets the most earthquakes it will also get the largest, but that's not true because of plate tectonics. Here in Washington, I've only felt an earthquake every five years, and none of those quakes were large or even alarming. However, a big one is due, and seismologists are predicting it will be soon.
Strike-slip faults like the San Andreas in California and the North Anatolian Fault in Turkey are narrow and weaker than other faults seen around the world. This is because the plates are moving parallel to one another. The reason for them being weaker is that there is less stress and strain on the plates, even though they may be extremely active. California and Turkey are renowned hotspots for earthquake activity, but you will rarely see a 7.5 magnitude or greater earthquake in these regions. That's not to say 7.5 isn't devastating; it certainly is, but it's nowhere near the kind of destructive potential seen in the 9.1 Sumatra earthquake in 2004, which killed over 200,000 (mostly by tsunami). Faults like the one in Indonesia are called reverse faults, and they can only be seen in subduction zones. Subduction zones aptly mean that one plate is subducting another, creating enormous stress on the ground above.
In the Pacific Northwest we have what's known by seismologists as the Cascadia Subduction Zone. It's not a very active fault system, but that only means that the stress has been building up over many years. When the fault finally releases the stress, it will do so in a big way. Scientists believe the Cascadia fault ruptures every 300 to 500 years, producing some of the most powerful quakes the world has ever seen. The last time it broke was in the year 1700, so it's due to rupture any moment now. It's estimated that the entire Seattle skyline would be destroyed and tens of thousands of lives would be lost.
There's also the possibility that Mt. Rainier could erupt in a big way. Vulcanologists list it as a Decade Volcano, meaning that it's one of the 16 most hazardous volcanos in the world. It's kind of far from here- about 60 miles- but the Rainier lahars could easily reach half that distance and penetrate the metropolitan area.
I couldn't bear to see this beautiful city crumble to pieces. I don't know if the M9 quake will happen soon, or even in my lifetime, but when it does, I pray that it's deep enough to prevent widespread damage. It won't matter how far away the epicenter is, in fact there won't even be an epicenter since megathrust quakes effect an entire faultline. I'm writing this because it's important for us in Seattle to be aware that we are at a major risk for natural disaster. Lest we fall asleep during the calm before the storm and get too cozy drinking coffee while listening to the soothing rainfall. A great thunder may come, not from above, but from the Earth below.
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