Cascadia fault zone
5:11 pm
Thu August 29, 2013

Research Cruise Reveals New Findings about Megaquake Fault

It's been a busy summer on the high seas for researchers trying to figure out the inner workings of an ominous earthquake fault.

The Cascadia subduction zone runs offshore from Vancouver Island to Northern California. When it rips, we could have a magnitude-9.0 catastrophe. 

University of Washington geophysicist Paul Johnson led a nearly month-long research cruise to the likely epicenter for the Big One. His ship carried an unmanned mini-sub to probe the sea floor directly over the still somewhat-mysterious Cascadia earthquake fault.

"The Cascadia subduction zone is unusual in a global sense in that it is very quiet. The subduction zone is locked very tightly. So it is building up a lot of stress, but it doesn't creak and groan,” Johnson said.

Johnson's team tried to tease out the earthquake fault's secrets another way—namely by sending down sensors and probes to measure heat and fluid flow.

"We went out there and found the plate is very hot. Beneath the toe of the subduction zone, it is about 230 degrees (Celsius),” he said.

If the plate boundary is very hot, the rocks soften and slide past each other more smoothly. That means the tension builds up somewhere else. Johnson's team kept probing farther and farther out to discover where that is. They found the colliding plates seize up off Washington "substantially farther offshore" than previously thought.

That matters, says Johnson, because “the lock zone is actually where the epicenters are. That's the place where all the energy is stored like a spring."

So if that spring pops far offshore, its energy releases farther away from Northwest's population centers along Puget Sound and around Portland.

"That may or may not be good,” Johnson said. “That means that when it breaks you are likely to get a very large tsunami. If it is far inshore, you don't get a lot of tsunamis. You get a lot of onshore damage."

Johnson says where he measured off Grays Harbor, the lock zone extends from roughly 50 to 150 miles offshore. As you move south, the tension buildup comes closer to shore, and maybe even goes under the coastline in Oregon. 

Other research cruises this summer involving university scientists from Oregon, Washington, and Canada have tried to suss this out by retrieving and redeploying ocean bottom seismometers. Technicians have also laid cables from the Oregon coast out to the fault zone.

Upcoming expeditions will hook up undersea sensors to allow continuous, real time monitoring from the scene. The National Science Foundation is the main funder of the effort.

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