UW Earthquake Scientists Have Hi-Res Findings on Fault Tremors

Tremor map courtesy PNSN

From August 8 to September 8, 2010, University of Washington scientists were tracking episodic tremor and slip (ETS), an unfelt "slow-motion earthquake" that released about as much energy as a 6.5 magnitude quake. (For more background on ETS, read "The Northwest is Earthquake Country.")

This year was different, in that they were using an array of arrays--a much larger system of data-gathering stations--to get a much more high-resolution "look" at what was going on underground. Previously, they had followed what seemed like the ETS's slow migration northward, from western Washington to Canada's Vancouver Island. 

But the new data gleaned from eight arrays, each with 20 to 30 sensors, paint a much different picture. "The source of the tremor generates streaks that travel 60 miles per hour back and forth along a southwest-northeast track. Several hours of this activity produces what shows up as bands of tremor that steadily migrate northward at a much slower speed, about 6 miles per day," reports UW News.

Or, as they blogged from the field, on the Pacific Northwest Seismic Network, "After moving slightly to the northwest over the past few days the tremor seems to have moved back to near where it started or even slightly to the south. Uhmmm. What is going on here?" The "field" being the Olympic Peninsula and its spotty cellphone reception, servicing instrument sites without the aid of GPS to help find them (or signal to call to get picked up) required real outdoor skills. But no graduate students were lost--at least, not permanently--during the month-long tremor. 

Tremor map showing tremor migration over time, PNSN

Another finding is that the southwest-northeast angle of the tremor "matches almost exactly the angle, about 54 degrees, at which the Juan de Fuca plate meets the North American plate." Scientists are still trying to determine the depth from which the tremor emanates, and to better understand how the tremor loads and perhaps releases stresses at different parts of the Cascadia fault zone.