Episodic tremor and slip is a new phenomenon discovered in the Cascadia subduction zone beneath Vancouver Island by scientists with the Geological Survey of Canada (GSC) . It consists of repeated slow slip events on the lower portion of the subduction fault, accompanied by unique non-earthquake tremor-like seismic signals that emanate from the same region.

GPS sites in the Western Canada Deformation Array (WCDA) on southern Vancouver Island region occasionally reverse their motion (slip), moving seaward for periods of about two weeks before resuming their longer-term landward motion.

Figure 1 GPS and seismic data showing the occurrence of episodic tremor and slip (ETS) at Victoria, British Columbia. The blue circles show daily changes in the east component of position of the continuous GPS site ALBH relative to Penticton which is considered fixed on the North America plate. The green line represents long-term eastward motion due to steady deformation caused by the locked portion of the plate interface on the Cascadia subduction zone. The red line segments show distinct periods of 14 to 15 months with elevated eastward trends, each punctuated by a 1 to 2-week reversal of motion that has been modelled by slow slip on the deeper plate interface. The black graph at the bottom shows the number of hours within a sliding 10-day window that show distinct, low-frequency, non-impulsive seismic activity that has been called "tremor". The correlation of the periods of slow slip with periods of extended tremor prompted the naming of this phenomena as Episodic Tremor and Slip.

GSC scientists have also discovered non-earthquake tremor-like seismic signals have been found to be associated with these events.

Figure 2 Example of seismic records of tremor activity at selected sites. It is the similarity of the envelope of the seismic signal on many seismographs that identifies ETS activity.

The tremor signals occur at the same time and come from the same region as the slip events. This associated tremor and slip phenomena has been named "Episodic Tremor and Slip" or ETS activity. These ETS events are not felt by people and do not cause any damage.

ETS events have important implications for improving earthquake hazard assessment on the Cascadia subduction zone. If the interpretation as slip on the lower part of the subduction fault is correct, ETS events generate a significant increase of stress on the bottom of the currently locked portion of the fault, where great earthquakes are thought to initiate. This is the first recognition of a time-varying component to earthquake hazard assessment along the Cascadia subduction zone. Compared to the steady year-round stress accumulation, this more rapid stress increase implies that a large subduction earthquake is more likely to happen during the time of an ETS event.

Based on a review of seismic and GPS data between 1996 and 2004, GSC scientists have successfully forecast the ETS events in February/March 2003, July, 2004, and September, 2005 (with an uncertainty of about +/- 2 months). In order to increase observational data for the study of these forecast ETS events, GSC scientists, in cooperation with international colleagues, have augmented existing GSC seismic and GPS networks by deploying temporary suites of additional instruments.

• Episodic tremor and slip on the Cascadia Subduction Zone: The chatter of silent slip (2003 Science Express paper announcing the discovery of ETS)
• A silent slip event on the Cascadia subduction interface
• A silent slip event on the deeper Cascadia subduction interface

See also:

• Tremor research at University of Washington