Figure 2.
Illustration of northern 2007 episodic tremor and slip (ETS) event in Cascadia (from Gomberg et al., 2010). The oceanic Juan de Fuca plate subducts beneath the North America plate at ~4 cm/yr roughly perpendicular to the coast (white arrow). The plates are coupled for part of their interface (tan-colored surface) such that relative motion is inhibited or “locked” to a varying degree. Uncertain are the location and mechanism by which the locking changes to a freely slipping interface. The fraction of relative plate motion is portrayed as continuous aseismic slip that increases down-dip from 40% to 80% (dashed contours). Inland of the locked zone, tremor epicenters projected onto the plate interface (circles) overlie the area that experienced slow slip (gray area on plate interface) during the last two weeks of January 2007. Color shading of tremor epicenters shows its temporal migration.