What earthquake?

Starting in the mid-80s, geologists began looking into some odd clues around the Pacific Northwest. Clues like ghost forests, these swaths of dead cedar trees along the coast that died for no apparent reason.  Researchers from Oregon State starting taking sediment cores well out into the Pacific and found weird periodic layers of stuff.  Turns out, they were landslides from the continental shelf.  Big ones.

As recently as the '90s, Oregon thought of itself, weirdly, as being non-seismic.  I mean, the Ring of Fire is distinctly ring-ish.  And the Cascade Mountains have to come from somewhere, right?  Well, okay, about 50 miles off the coast, stretching from Cape Mendocino in California to well past Vancouver Island in Canada, lies the Cascadia Subduction Zone.  The CSZ is where the last remnants of the Juan de Fuca plate are being shoved under the (very slow-moving) bus that is the North American plate.  It's a hell of a thing.  Try this at home:  take a 10-mile thick slab of rock, all studded with boulders and whatnot, and push it under another slab of rock, maybe 50 miles thick and even less forgiving than your slab of rock.

No prizes for guessing:  this does not go well.  In other subduction zones, the average time between quakes is 100-200 years.  Here, we let the pressure build up a little bit.  Eventually, though, the North American plate overcomes the friction, snapping up along the edge (out in the ocean) and collapsing all through the center.  According to the offshore cores, the small ("small") earthquakes happen every 300 years or so:  these are in the 8.0-8.4 range.  The BIG earthquakes have an average return interval of around 500 years:  these can go up to 9.2.  The San Andreas is expected to max out around 8.0.

Uhh...

But wait.  There's more.  Those ghost trees....scientists cored them, too.  They think the trees were killed when the land under them suddenly dropped several feet and they were flooded with salt water.  It was sometime around the year 1700, probably in the winter.  You can learn a lot from coring, apparently.

Now, the punchline.  In 1996, a Japanese researcher published a paper about Japan's 'orphan tsunami,' an old report about one weird, 16-foot tall wave that came out of absolutely nowhere, with no warning and no associated earthquake.  It arrived in the middle of the night on January 26, 1700, about 10 hours after a massive earthquake rocked the Pacific Northwest.

So.  Oregon.  NOT non-seismic.  More about being "overdue" in the 'why now?' post.