Episodic Tremor and Slip (ETS) occurs at some Subduction zones (places where a tectonic plate descends beneath a lighter continental plate). At the Cascadia subduction zone, which extends from southern British Columbia to northern California, the Juan de Fuca plate begins to descend beneath the overriding North American plate just off the coast. ETS occurs about 25 to 40 kilometers (15 to 25 miles) below the surface along the boundary between these two plates.
GPS instruments near the edge of the North American plate occasionally move back towards the subduction zone instead of towards North America as would be expected as stress built up between the two plates. This gradual release of the built-up stress is now referred to as a slow slip episode.
Analysis of EarthScope data over several years shows that the slow slip is episodic, sometimes with a remarkably consistent frequency, such as the approximately 14 month recurrence interval seen between Seattle and Vancouver.