Episodic Tremor and Slip (ETS) is the process of slow slip events on the lower portion of the Juan de Fuca plate as it subducts below the North American plate, accompanied by non-earthquake tremor-like seismic signals, which emanate from the same region. These tremor events do not cause damage and are only detectable by seismic instruments, but they provide estimates to scientists about where and when the next great earthquake might occur.

A cross-section of the Cascadia Subduction Zone, looking north. The green area is the locked zone where the two plates are currently stuck together. The pink area is the slip zone, and the yellow stars indicate tremors.

Slip:
GPS sites on southern Vancouver Island (WCDA) occasionally reverse their usual eastward direction of motion, or slip, moving backwards for about two weeks before resuming the usual, longer term, eastward motion.

Tremor:
Tremor-like seismic signals have been found to occur at the same time and place as slip events. The evidence for this was found by examining the distances measured by the GPS network (WCDA) and seismic signals.

Click here to see an animation of ETS movement - pay close attention to the movement of the GPS receiver!

This work is based on materials provided by the UNAVCO Education and Outreach Program with support from the National Science Foundation and NASA under NSF Cooperative Agreement No. EAR-0735156.