SEATTLE - University of Washington seismologists are closely monitoring another slow-moving tremor that's been detected under the Olympic Peninsula.

You can't feel this kind of earthquake, despite the fact that they release considerable energy. The reason is that they take weeks to unfold.These so called silent quakes are also known as deep tremor or tremor-and-slip events and occur miles under ground.

What's so weird about them is that unlike other quakes, where scientists can only estimate a loose period of time when a quake might occur, the silent quakes happen on a predictable schedule, about every 14 to 15 months.

So-called tremor-and-slip events have occurred about every 15 months since they were first detected in 2002. The latest was found early Sunday north of Olympia and west of Tacoma, and is expected to travel north under the peninsula toward Vancouver Island. UW scientists say it can't be felt at the surface, but the concern among scientists, including John Vidale, director of the Seismic Network, is that the slow slip events may be adding stress to the fault zoneandbring the world's worst kind of earthquake to our part of the country. Those quakes are expected to reachinto the magnitude 9 range.

The University of Washington is home to the Pacific Northwest Seismic Network, an array of monitoring stations that tracks quakes under the Northwest.Eight arrays with 10 recording stationsare being deployedalong the north coast of the Olympic Peninsula from Port Angeles to Port Townsend.

One of the reaon's we're placing these arrays is to detect the precise location of the tremor source, says Steve Malone,UW seismologist and one of the nation's leading experts on deep tremor. We've been able with our seismic network to get an approximate epicenter (for past events) but the resolution for depth has been very poor.

The big question is why do they unfold so slowly? With regular earthquakes we understand why they happen as they do, said John Vidale, director of UW-based Pacific Northwest Seismic Network and a UW professor of Earth and space sciences. Here it takes weeks and things move miles per hour not miles per second. What's slowing them down?

Vidale said better understanding the changing slow-slip patterns may provide clues in advance of the next Cascadia mega-quake.

Temor-and-slip events have been associated with the Cascadia subduction fault zone that runs along the Northwest coast, as well as about a dozen other fault lines worldwide.

The last one along the Washington Coast happened in the year 1700, 310 years ago. Those quakes typically come hundreds of years apart based on studies of pete layers in the soils of the Washington Coast. The same type of quaketriggered themassive tsunami in theIndian oceanin December of 2004 that's blamedfor the deaths of more than a quarter million people.

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