Episodic Tremor and Slip (ETS) is the name given to a process that occurs deep below the Earth's surface, along faults that form the boundaries of tectonic plates. It involves repeated episodes of slow sliding, one plate over the other, of a few centimetres over a period of several weeks, accompanied by energetic seismic noise, called tremor. Tremor is distinctly different from the seismic signals generated by earthquakes.
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A map of the Cascadia Subduction Zone. The Cascadia Subduction Zone (CSZ) lies along the western margin of North America extending from northern California to northern Vancouver Island. This margin marks the plate boundary between the Juan de Fuca (JDF) oceanic plate located predominantly offshore to the west of the coastline, and the western edge of the North America (NA) plate. Over geological time scales, the JDF plate moves towards or converges with the NA plate in a northeast direction at a rate of 4 cm/yr. About 150 km offshore at the oceanic trench, the JDF plate begins diving down or subducting beneath the NA plate margin. At about 200 km inland from the coast, the subducting oceanic plate reaches depths of 100 km and remelts forming volatile magma which penetrates the overlying crust to form the active Cascadia Volcanic Belt. It is the convergence of the tectonic plates which make this the most active seismic region in Canada.
The occurrence of slow slip in combination with seismic tremors was first discovered in the Cascadia Subduction Zone (CSZ) by scientists of Natural Resources Canada in 2003. The CSZ, which marks the most active seismic region in Canada, comprises the plate boundary between the oceanic Juan de Fuca Plate and the continental North America Plate. Over millions of years, these plates converge at an average rate of 4 cm/yr. Along the western edge of North America, from northern Vancouver Island to northern California, the Juan de Fuca Plate is slipping or “subducting” beneath the North American Plate.
Instead of slipping continuously at the long-term average rate of 4 cm/yr, the two plates are currently stuck together along the upper portion of the subduction fault - this is referred to as the locked zone. Over time, tectonic stress builds up across the locked zone. Every 500 to 600 years, on average, the stress becomes too great, and the locked zone ruptures or becomes suddenly unstuck with 10 to 20 metres of fault slip, causing a magnitude eight or nine earthquake. An earthquake of this magnitude is referred to as a “great earthquake”.
A schematic cross-section for the Cascadia Subduction zone derived from observations of crustal deformation. The convergent motion of the subducting Juan de Fuca plate is not continuous but determined by the frictional strength on the plate interface. On the shallowest portion of the interface, from 0 to 15 km depths, friction is strong and the plates are locked together causing the overlying crust to deform for hundreds of years. At intermediate depths of the plate interface, 15 to 25 km, temperatures increase and, along with increases in released fluids, the frictional strength becomes gradually weaker over this depth range called the transition zone. The plate interface from 25 to 45 km depths which underlies Vancouver Island is characterized by frictional strength that appears to change with time. For about 15 months, plate motion is resisted by frictional strength which then disappears over a period of several weeks allowing the plates to slip a few centimetres. This region of the plate interface with this short-term stick-slip behavior is called the slip zone. Episodes of slip are invariably accompanied by seismic tremors on and above the slip zone.
A decade of monitoring crustal deformation in southwestern British Columbia using precise, continuous Global Positioning System (GPS) instruments, has revealed a similar “stick-slip” behaviour on a deeper portion of the subduction fault, but over a much shorter time scale, with very small, and slow slips. This deeper segment of the subduction fault is referred to as the ETS slip zone.
Geodetic and seismic observations used to identify Episodic Tremor and Slip (ETS).
Blue circles plot the day-to-day changes in the east-west component of the Victoria GPS station with respect to the NA plate from 1994 to 2007. The green line shows the average eastward linear motion (~ 5mm/yr) of the GPS station over 14 years. The shorter red-line segments show that over repeated cycles of about 15 months, the eastward linear motion is almost double (~ 9mm/yr) but each 15 month period is followed by a period of westward motion of about 4 mm over several weeks. These reversals of motion mark the occurrence of a slip episode on the deep plate interface. The "spikes" along the bottom of the graph show the number of hours containing tremors over a sliding window of 10 days. Note that there is tremor activity throughout the years but the tremor activity is greatly increased at the time of the slip events.
Seismic records of tremor activity recorded at 9 seismometers on Vancouver Island. The character of seismic recordings of tremor is significantly different from recordings of earthquakes. Tremor records appear very similar to background seismic noise that can be generated by windstorms. The key however is that this "noise" is correlated from station to station and travels across the network of seismic stations at s-wave velocities thereby pointing to a common tectonic not local atmospheric origin
Across the locked zone, extending down to depths of 15 to 20 km, the converging Juan de Fuca Plate has been squeezing the coastal edge of the North America plate landward for hundreds of years. At depths between 25 and 45 km, the subduction fault “sticks” for periods of about 15 months and then “slips” several centimetres over a two-week period. On the surface, the temporary deep sticking enhances the rate of landward motion for about 15 months, and the subsequent deep slip results in two weeks of reversed motion (i.e. seaward).
On Vancouver Island, the episodes of deep slip are accompanied by seismic tremors that are recorded by a network of seismographs previously established to measure earthquakes. Seismic data reveal that tremors persist, like background chatter, for the duration of the ETS events. Tremors therefore serve as clear messengers of slow plate slip - the two together signal that an ETS event is underway.
The Importance of ETS
Although ETS events are not felt by people and do not cause any damage, ETS episodes in the Cascadia Subduction Zone may lead to improved estimates of where and when the next great earthquake is likely to occur on Canada's west coast.
Where - ETS events define the eastern or landward boundary of the locked zone that will rupture during the next great earthquake and, as a consequence, how close that rupture will be to major west coast cities. This enables a more accurate estimate of the expected magnitude of ground shaking in those cities.
When - Although ETS alone will not help us predict an earthquake at this time, ETS may provide the basis for improved forecasting. Each ETS episode adds a small amount of stress on the locked portion of the subduction zone. This implies that as the stress level increases with each ETS event, one of them could trigger a great earthquake. Therefore, the likelihood of a great earthquake is enhanced during ETS. However, since ETS episodes of various sizes occur at different times all up and down the full length of the Cascadia subduction zone, we cannot yet say which ETS event will be the likely trigger.
Researchers are investigating possible relationships between the many other earthquakes that occur off the subduction fault in both the North America and Juan de Fuca plates, and the location and timing of ETS. Small changes in stress of the North America Plate are being measured using very sensitive strain meters recently placed at the bottom of 200m deep drill holes. This information, combined with the GPS and seismic data is being used to determine the exact physical processes involved in ETS.
Further reading on Episodic Tremor & Slip
- Episodic tremor and slip on the Cascadia Subduction Zone
- A silent slip event on the Cascadia subduction interface
- A silent slip event on the deeper Cascadia subduction interface