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

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 over the course of several weeks can release as much energy as a magnitude 6 earthquake.

Researchers believe the tremors may be associated with stress building in the Cascadia subduction fault zone, about 50 miles off the Pacific coast. The zone ruptures in mega-earthquakes about every 500 years, with the last occurring in 1700.

“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?”

Crews with the UW-based Pacific Northwest Seismic Network are doubling the number of seismic recording stations placed along the northern part of the Olympic Peninsula to better document this three-week event.

The stations, from Port Angeles to Port Townsend, will help researchers more precisely measure the three-dimensional depth of the tremor source.

“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,” said Steve Malone, a UW Earth and space sciences professor.