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.

Large subduction earthquakes of magnitude about 9 have occurred on the Cascadia subduction zone beneath the west coast of Canada and the United States on average every 500 years or so, but with considerable variability in repeat times. The last one was on January 26, 1700. All along this margin, continuous Global Position System (GPS) installations show a slow landward movement towards the northeast, demonstrating long term stress accumulation leading up to the next great Cascadia earthquake.

A team of scientists at (GSC) has discovered that GPS sites in the southern Vancouver Island region occasionally reverse their motion, moving seaward for periods of about two weeks before resuming their longer-term landward motion.


Figure 1 - Comparison of slip and tremor activity for the Victoria area. Small blue circles show the daily position of a GPS site near Victoria with respect to the interior of the North American plate. The continuous green line is the long term eastward motion of the site. The red saw-tooth line shows that detailed motion includes reversals in motion about every 14 months. The bottom black graph shows the number of hours of tremor activity observed on southern Vancouver Island. The next ETS episode on southern Vancouver Island is expected in May, 2004.

These brief reversals can be explained by slip on the deep Cascadia subduction fault, 20 to 40 km below the surface of the Earth, in a zone immediately down-dip from the part of the fault that will rupture during the next great earthquake. These slip events occur at surprisingly regular intervals of about 14 months.

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


Figure 2 - Map of Geological Survey of Canada seismographs and approximate source region used for the initial correlation of tremor activity with slip. ETS activity has now been identified beneath most of Vancouver Island and beneath Puget Sound to the south.


Figure 3 - 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.

The region of ETS also appears to define the landward extent of the fault rupture during subduction earthquakes and hence the proximity of the seismic energy source to large population centres. Thus, mapping the region of ETS activity will improve our definition of shaking hazard from subduction earthquakes. Understanding the ETS phenomena opens the possibility of a new era of more detailed seismic hazard assessment in both space and time along the Cascadia subduction zone.

Based on a review of seismic and GPS data from the past seven years, GSC scientists successfully forecast an ETS event in February/March 2003 and have forecast the next one to occur in May, 2004 (with an uncertainty of about +/- 2 months). In order to increase observational data for the study of this forecast ETS event, GSC scientists, in cooperation with international colleagues, have augmented existing GSC seismic and GPS networks by deploying a temporary suite of additional instruments.

For more information 2003 Science Express paper announcing the discovery of ETS More about giant subduction earthquakes More about deep slip Tremor research at University of Washington