The Big-Shake, Big-Wave Theory
See how a megaquake would shake out in the Pacific Northwest
The CSZ is a different beast. Up in the Northwest, the plates don’t merely grind past each other. The heavier Juan de Fuca plate dives under (subducts) the lighter North American plate at a rate of 1.6 inches per year. Hence, a subduction zone. Transform faults like the San Andreas are capable of throwing off major quakes—up to 8.1—but not megaquakes. Rule of thumb: the longer the fault rupture, the bigger the quake. Only subduction zones have the length necessary to generate the mammoth 9.0’s.
The CSZ is especially deceptive because it’s been inactive for all of recorded history.3 “Seismically quiet as Kansas,” says Robert Yeats, the éminence grise of West Coast seismology and the author of Living with Earthquakes in the Pacific Northwest: A Survivor’s Guide. “Or so we thought.”
Back in the 1970s and ’80s, Yeats and others attributed the CSZ’s quiescence to a kind of hyper-lubrication. The subduction zone must be so slippery, they thought, that the Juan de Fuca plate is sliding under the North American plate as if on a bed of axle grease.
Then in 1979, John Adams, a New Zealand geologist working in Canada, noticed something funny. Going over data from the National Geodetic Survey, America’s surveying corps, Adams found that highways along the Washington and Oregon coast were gaining about one to two millimeters of elevation per year. His findings held all the ominous portent of a line from a Tommy Lee Jones disaster movie: Um, guys, why are all the roads rising?
Other evidence compounded the concern. In 1986, Brian Atwater, a researcher at the USGS, was canoeing along the shore of Willapa Bay, north of the Oregon-Washington state line, during a low tide. He noticed evidence of a “ghost forest,” old cedar stumps half-buried in the tidal marshes. The stumps sparked a memory; at a talk by USGS geologist Tom Ovenshine years earlier, Atwater had heard that spruce and willow thickets in Alaska’s Cook Inlet had dropped five feet during the 1964 Good Friday quake. Could the same thing have happened here? Tree-ring tests by colleagues confirmed that the Willapa Bay forest died in 1700. So did other buried estuary stumps along Washington’s southern coast. That date corresponded with historical accounts of a massive tsunami striking the islands of Japan in January 1700.
This startling evidence made seismologists sit up and take notice. Clearly, the Cascadia subduction zone had ruptured in a megaquake in 1700, down-dropping the Northwest coast several feet in elevation and unleashing a killer tsunami.
As for the rising roads, well, think of it this way. Take a fishing rod and jam the tip up against a low garden wall. Now hold the rod at the butt and slowly push the tip into the wall. As tension builds, the rod will bow upward under the strain. That, in a nutshell, is what the Northwest coast is doing.