Quake likely to rumble for weeks

A slow-moving earthquake has begun in the Olympia-Tacoma area and is expected to rumble north beneath the Olympic Peninsula for the next several weeks, according to University of Washington seismologists.

The so-called slow slip events have occurred every 15 months or so since 2002 and may be associated with a buildup of stress in the Cascadia subduction fault zone, about 50 miles off the Washington and Oregon coasts, home to mega-earthquakes every few hundred years.

“It appears to be right on time,” said Steve Malone, a University of Washington Earth and space sciences professor. The event was first recorded north of Olympia and west of Tacoma early Sunday. By Monday afternoon the signals were much stronger. The deep tremor is not felt without seismic monitoring equipment.

The UW-based Pacific Northwest Seismic Network has sent crews out this week to double the number of seismic stations on the Olympic Peninsula from 10 to 20 in a bid to pinpoint the depth of the slow-moving 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,” Malone said in a news release from the UW.

The fault zone, which runs along the Northwest coast, is created by the Juan de Fuca oceanic plate sliding beneath the North American continental plate. Earthquakes of magnitude 8 or larger can happen when the oceanic plate sticks as it’s pushed beneath the continent.

The last subduction zone earthquake in the Pacific Northwest occurred in 1700. A modern day subduction zone earthquake would cause major fatalities, injuries and damage to buildings and infrastructure, according to a 2005 working group assigned the task of predicting damage from a magnitude 8 to 9 earthquake.

“This particular type of earthquake is especially hazardous to tall buildings, which could lead to significant fatalities in downtown areas,” the Cascadia Region Earthquake Workgroup said in its 2005 report.

A better understanding of deep tremors like the one that just started might provide hints in advance of the next Cascadia megathrust quake, said John Vidale, director of the Pacific Northwest Seismic Network and UW professor of Earth and space sciences.

“Maddeningly, we have no understanding of why the episodic slow slip lasts a month, rather than a few seconds of a normal earthquake of the continuous motion of flow deeper in the Earth, and we aim to figure it out,” Vidale said.

John Dodge:360-754-5444 jdodge@theolympian.com