VOLCANO MADNESS: [Geology2] Geodynamics Episodic Tremor and Slip (ETS)

Friday, June 24, 2011

[Geology2] Geodynamics Episodic Tremor and Slip (ETS)

From the Geological Survey of Canada:

Geodynamics

Episodic Tremor and Slip (ETS)

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.

Plate Tectonic Setting for ETS

Figure 1. Map of the Cascadia Subduction Zone which extends along the west coast of North America from northern Vancouver Island to northern California. The big arrows show the direction of overall plate motion, the smaller arrows show relative motion across plate boundaries. The line with the triangular teeth marks where the oceanic Juan de Fuca Plate dips beneath the North America Plate.

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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".

Details of ETS Observations on Canada's West Coast

Figure 2. A Cross-section, looking north, of the Cascadia Subduction Zone showing the locked (bright green), transition, and slip (bright pink) zones on the interface between the Juan de Fuca and North America plates. Note the tremors (in yellow) on and above the slip zone.

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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.

Figure 3 This figure shows both the horizontal movement of a single GPS station (Victoria) in the saw-toothed line and corresponding tremor activity (the blue spikes along the bottom of the figure) for the same 13+ year time interval.

The saw-toothed line: The blue circles represent daily observations of location of the Victoria GPS station. The slope of the green line indicates that, on average, the site is moving east at 4.8 mm per year. The red lines mark eastward motions of the site for about 15 months followed by two weeks of westward motion, creating the saw-toothed shape of the graph. Those two week breaks mark ETS events.

Tremor activity: The spikes at the bottom of the graph show the number of hours containing tremor activity over 10-day periods. Note how each ETS break in the motion of the Victoria station is also characterized by the greatest tremor activity (the highest spikes).

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Figure 4. Seismic tremor records, characteristic of an ETS event, as observed at nine different seismograph stations on Vancouver Island.

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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.

Research Continues

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.

Source

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