On August 8, a peculiar seismic event began just west of Seattle.
It's nothing to worry about; seismologists reported feeling nothing even as their instruments recorded tremors sweeping beneath their motel in Sequim, Washington. But over the course of about three weeks the event — known as episodic tremor and
slip (ETS) or a "silent earthquake" — will release as much energy as a magnitude 7.0 quake.
What scientists learn from this event could help them — and the millions of people living in the shadow of the Cascadia megathrust fault — prepare for the magnitude 9.0 earthquake that will one day strike.
Cascadia is one of the most dangerous faults in the world, and unquestionably the scariest in North America. In 1700, it rattled the Pacific Northwest with a magnitude ~9.0 earthquake that sent a tsunami crashing into Japan. Studies suggest it repeats every 300-900 years, with an average of 500 years between spasms.
Every 15 months or so, the fault undergoes a period of ETS. The shimmies start down around 50 kilometers (31 miles) beneath Earth's surface, where the subducting Juan de Fuca tectonic plate grinds underneath the North America plate. Gradually, tremors migrate up and west along the Juan de Fuca to about 30 kilometers' (19 miles') depth, before they split and head north and south along the plate (a plot of tremor recorded in 2007 shows this behavior, above).
The key to gauging the danger to Seattle, Vancouver, Portland, and all of the people living near Cascadia lies in figuring out where the tremors stop. ETS events let Cascadia slip a couple of inches, relieving the stress built up along the fault in the preceding 15 months. But that only happens up to about 30 kilometers below the surface — the fault is locked at shallower depths, filled with all the stress built up since 1700.
Trouble is, no one knows where the slipping portion of the fault ends and the locked part begins — it could be at 30 kilometers' depth, or it could be 15.
"Where fault is locked could be a factor of two difference in distance to Seattle," Ken Creager of the University of Washington told Discovery News. That could have a huge bearing on how much strong shaking reaches heavily populated areas.
The team is also trying to figure out whether ETS events can trigger conventional earthquakes. In theory, Creager said each episode loads up the locked part of the fault with 15 months' worth of stress. So if the fault is nearing a critical state, it should pop during an ETS.
"We just don't know right now," he said. "Potentially one day we might be able to say when there will be a big earthquake, but we're not there yet."
Image: Aaron Wech, University of Washington