 a megathrust earthquake.
“The same amount of slip must also occur onshore along the Washington coast,” Creager said. “While megathrust earthquakes account for most of the plate motion offshore, and perhaps slightly onshore, episodic tremor and slip harmlessly accommodates much of the plate motion that is taking place on plate interface just west of the Puget Sound region’s major population centers.”
The paper’s lead author is Mario La Rocca of Italy’s National Institute of Geophysics and Volcanology Vesuvius Observatory. Other authors are Danilo Galluzzo, also of Italy’s geophysics and volcanology institute, and Steve Malone, John Vidale, Justin Sweet and Aaron Wech of the UW.
Slip events occur on the interface between tectonic plates, but previous research has suggested that nonvolcanic tremor occurs in a broad range of depths from the plate boundary to 15 miles above it. The new research indicates the tremor is at the plate boundary, in essentially the same place as the slip.
The researchers used seismometer arrays at Sequim and Lopez Island in Washington state and at Sooke on the southern edge of Canada’s Vancouver Island to record an episodic tremor and slip event in 2004. La Rocca devised a novel method using the different times that specific waves generated by the tremor were detected by seismometers, and the data helped the scientists pinpoint the depth of the tremor. At the same time, GPS measurements recorded the slow plate slippage.
Since they were discovered in the last decade, slow slip and tremor events in western Washington and British Columbia have been recorded on a regular basis about every 15 months. GPS signals indicate slip of about 1 inch during an average episode.
“We are quite confident that each episodic tremor and slip event will increase the stress on the megathrust fault,” Creager said. “If a megathrust earthquake were to begin off the Washington coast, one might expect it to occur during one of these slow slip events.”
But he said the findings demonstrate that much research remains to be done.
“We’re just scratching the surface in understanding how all of this works.”
The work was funded by Italy’s National Institute of Geophysics and Volcanology, the National Science Foundation and the U.S. Geological Survey.