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Significant earthquake hazard results from subduction of an oceanic plate beneath the Cascadia-Vancouver Island region straddling the US-Canadian border. Only within the past decade has it been discovered that this collision between continental and oceanic plates is accompanied by surprisingly regular episodic tremor and slip. The deployment of networks of strain, seismic, and geodetic instruments in Cascadia as part of project EarthScope has enabled scientists to unravel the details of how this slow slip and tremor co-evolve. For example, the locations of seismically observed tremor from the January 2007 ETS event (circles colored by date) show a 10 km/day northward migration and coincide with geodetically inferred slip of a few centimeters (lighter area on the gray shaded plate interface). Most of the relative plate motion in the slow slip area is accommodated by similar slip events that repeat every 14 months. Plate boundary slip in the “locked zone” to the west of the contours of partial locking occurs during great earthquakes such as the M = 9 Cascadia megathrust earthquake in 1700. Analyses of strainmeter data are beginning to show a migration of slow slip that appears to track the tremor path. Most models of slow slip associate its occurrence with a region transitional between where the plate is locked and where it is sliding continuously at greater depth. This new picture of slow slip and tremor suggests that the locked zone of the plate interface, and probable region of strong ground motion during future earthquakes, extends significantly further inland than had been thought, closer to the large population centers of Cascadia. (Image courtesy of K. Creager.)
Date Taken: February 18, 2009 Photographer / Contributor: K. Creager