Scientists: Silent Tremors May Foretell 'Big One'

"We don't really understand these things well enough yet to use them as a predictor."

But that may be possible in the future as scientists learn more about the events, which have been detected in most of the world's subduction zones. Changes in the normal patterns, such as stronger shaking or more extensive slip, could be the clue to pending earthquakes, Dragert said.

Unfortunately, the best experiment will be to analyze the tremor and slip that precede a massive quake but that hasn't happened. The 2004 subduction-zone quake in Sumatra provided little insight because too few sensors to detect tremor and slip were in the area.

A dense network of stations, such as the UW's, will be better able to pick up the weak signals and pinpoint their origins. "You can use it like a telescope to focus in," Creager said.

Tremor location could be a key to understanding how destructive future megathrust earthquakes will be. Creager and his colleagues are trying to find the boundary between the shallow, locked portion of the subduction zone, which generates the big quakes, and the deep portion where rocks slide past each other in harmless tremor-and-slip events.

So far, it looks like that boundary might run under the Olympic Peninsula from Port Angeles to Shelton. If that's true, the locked zone extends much closer to Seattle, Portland and other cities than previously believed. And the closer the quake, the worse the damage.

"It's a huge difference if the earthquake is 70-plus kilometers closer to where most of the people live," Melbourne said.

Others, including Dragert, say there's not enough evidence to conclude the danger zone is any closer than the Pacific Coast.

One of the most puzzling aspects of the silent earthquakes is their regularity. Recurrence times vary around the world, from roughly six months to a couple of years.

Creager and his team installed most of their instruments this month so they could monitor the next event, due in August.

But the tremors started in April and petered out by the end of May.

So the scientists will have to leave the temporary network of sensors in the field longer than expected and power them down to conserve batteries.

"It's just going to make some more work for us," Vidale said. "We'll be waiting for a tremor episode, and when it comes we'll rush back out there and put the batteries in."