August 19, 2010
Slow-moving ‘earthquake’ under Olympic Peninsula will be well recorded
UW seismologists have begun recording a slow-moving and unfelt seismic event under the Olympic Peninsula, and it promises to be the best-documented such event in the eight years since the regularly occurring phenomena were first discovered.
“It appears to be right on time,” Steve Malone, a UW Earth and space sciences professor, said of the most recent of what are termed episodic tremor-and-slip, or slow-slip, events. “The first signals were mostly fairly weak, but they were easily detected.”
The first ground motion associated with the event was recorded very early Sunday morning in an area north of Olympia and west of Tacoma. By Monday afternoon the signals were substantially stronger. If the event behaves like past occurrences, the source of the rumbling will move north through the Olympic Peninsula during the next week before crossing the Strait of Juan de Fuca to Canada’s Vancouver Island.
Such slow-slip events have been documented on the Olympic Peninsula at an average frequency of every 14 1/2 to 15 months since 2002 (an event last year actually started three months earlier than expected). They typically last several weeks and can release as much energy as a magnitude 6 earthquake.
The UW-based Pacific Northwest Seismic Network already has eight arrays with 10 seismic recording stations, each deployed across the northern part of the Olympic Peninsula from Port Angeles to Port Townsend. In the next couple of days, using funding from the National Science Foundation’s EarthScope program, crews will double the number of stations in each array to document this three-week event with unprecedented precision.
“One of the reasons we’re placing these arrays is to confirm the three-dimensional depth,” Malone said. “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.”
If several stations are focused on the same tremor source, he said, a very precise depth can be triangulated to provide better understanding about the stresses building up within the Earth’s crust.
Episodic tremor-and-slip events have been associated with the entire Cascadia subduction fault zone, which runs along the Northwest coast, as well as a dozen other dangerous faultlines worldwide. The fault zone is created by the Juan de Fuca tectonic plate sliding beneath the section of the North American plate where western Washington is located.
It is believed the two plates are locked together by friction near the surface but that they slide past each other easily at greater depth, where heat has made the rock more pliable. Slow-slip events are likely to be occurring at a depth where the plates transition from being locked to being free-moving, Malone said.
“Models indicate these events are loading a little extra stress on the fault zone,” he said.
Interest in slow-slip events has been intense in recent years because they alter stresses in the subduction zone, which ruptures in magnitude 9 megathrust earthquakes on the order of every 500 years. The last one occurred in 1700. (The earthquake that caused the devastating 2004 Indian Ocean tsunami had a magnitude of 9.1.)
With better understanding, changing slow-slip patterns might provide hints in advance of the next Cascadia megathrust quake, said John Vidale, a UW professor of Earth and space sciences and director of the seismic network.
“Maddeningly, we have no understanding of why the episodic slow slip lasts a month, rather than the few seconds of a normal earthquake or the continuous motion of flow deeper in the Earth, and we aim to figure it out,” Vidale said.