– Thousands of tiny tremors over the past few months have moved parts of Washington state and Vancouver Island westward. It's a near-annual event that backs expectations by some scientists that a big earthquake may hit the Seattle area harder than their previous models suggested.

The recent wave of activity began in May and appears to be dying off, according to University of Washington earth sciences Prof. Ken Creager. It's a process, known as episodic tremor and slip, thought to increase stress on locked faults — areas where tectonic plates cannot move past each other.

Earthquakes occur when the pressure on locked zones reaches the breaking point and the plates snap past each other. Scientists believe an episode of tremors could someday trigger a so-called megaquake on the offshore fault called the Cascadia Subduction Zone. The 700-mile-long fault runs from Vancouver Island to northern California, and could unleash earthquakes of up to magnitude 9.0. It's one of the biggest fault lines in the United States.

While older models suggest that the locked zone is mostly shallow and offshore, the location of these tremors indicates that in a big earthquake, layers of rock jerking past each other may take place closer to Seattle than previously thought.

"That increases the risk that it could be closer to the population centers, which means stronger shaking in the Puget Sound cities," Creager said.

Of course, the timing of the next big earthquake is not known. The Pacific Northwest last had a megaquake about 300 years ago. Scientists widely expect the region to experience a similar event every 500 years on average.

"It could happen anytime in the next few hundred years, but it could be tomorrow," ­Creager said.

Seattle Times