Thousands of tremors have happened north of Seattle since last week.
This burst of underground activity is part of trend that started two weeks ago under Vancouver Island. It's called slow-slip, and occurs about every 14 months in the Puget Sound region.
Seismologist John Vidale, who directs the Pacific Northwest Seismic Network, says the phenomenon is moving south a few miles per day, but it's no cause for alarm.
Vidale: "We won't feel it; this motion is tiny, only our most sensitive instruments see it. And we expect this activity might continue to migrate south toward Tacoma over the next week or two, or it might quit. We really don't know when it's going to run out of steam."
His team did record a 4.8 magnitude earthquake last week off of San Juan Island that he thinks could have been prompted by the slow slip.
But he says usually the tremors don't result in earthquakes.
Vidale explains the two plates that meet under the Puget Sound region tend to move about an inch every year, and the movement happens over several weeks.