The sound of silence – episodic tremor and slip on the Cascadia subduction zone

The boundaries between subducting oceanic plates and the plates that override them are characterized by complex and variable relationships between stress and friction.  In the upper parts of these boundaries, where rocks are cool, and friction is high, the plates are typically locked together for decades or centuries between major earthquakes.  In the deeper parts, where the rocks are much warmer and friction is correspondingly lower, the plates can slip easily, slowly and continuously past each other.  Recent analysis of high-sensitivity GPS data from British Columbia and Washington State has shown that the intermediate-depth part of the Cascadia subduction zone is characterized by episodic slow slip that is not associated with earthquakes (Dragert et al., 2001, Miller et al., 2002).

This type of motion was initially thought to be silent or aseismic (ie. producing no seismic record), but following a careful analysis of about 10 years of seismic data from numerous stations on Vancouver Island, Rogers and Dragert (2003) have discovered that the slip events are characterized by long-term low-frequency seismic tremor.

Seismic records from 9 stations on Vancouver Island showing evidence of coincident tremor related to episodic slip (from Rogers and Dragert, 2003)

The slip and associated tremor typically last for 1 to 3 weeks, and the slip events have a return interval of 13 to 16 months (see figure below). The slip is focussed just above the subduction zone interface at depths ranging from 20 to 40 km.

Easterly displacement (blue dots) over the period from January 1996 to March 2003 for GPS site ALBH (Victoria) 

Hours of tremor activity over 10-day intervals (black lines) for the same time period

The source of this tremor seismicity is not clearly understood, although tremor of this type is typically attributed to flow of magma or other fluids.  Tremor, interpreted to represent the flow of water released from the dehydrating oceanic crust, has been recorded on the subducting Phillipine Plate underneath Japan – but it is not correlated with slip (Obara, 2002).  Rogers and Dragert (2003) contend that the Cascadia tremor is actually related to shearing of rock, although they suggest that water derived from the subducting slab may play a role in reducing friction on the fault zone.

Understanding the mechanism and timing of mid-depth episodic slip is critically important to understanding the damaging earthquakes that take place at shallower depths.  Mid-depth episodic slip does release stress on the subduction zone as a whole, but some of that stress is just transferred up-dip, resulting in an increase in the stress on the locked part of the boundary. 

Cross section showing elastic stress changes due to silent slip as inferred by Dragert et al. (2001) on fault segment shown in black. Positive stress changes (warm colors) bring earthquake faults like that shown by the white line closer to failure.  (From Thatcher, 2001)

Rogers and Dragert (2003) suggest that one of these slips could be the event that triggers a large earthquake, and that by understanding the timing and nature of the slip events we might be able to recognize times when there is an enhanced probability for the occurrence of a damaging earthquake.

References

Dragert H, Wang K, and James T, 2001, A silent slip event on the deeper Cascadia subduction interface, Science, V. 292, p. 1525-1528.

Obara K, 2002, Nonvolcanic deep tremor associated with subduction in southwest Japan, Science, V. 296, p. 1679-81.

Rogers G and Dragert H, 2003, Episodic tremor and slip on the Cascadia subduction zone: the chatter of silent slip, Science, V. 300, p. 1942-43.

Thatcher, W, 2001, Silent slip on the Cascadia subduction interface, http://ajdubre.tripod.com/Physics/CascadiaSubduction-Sci-52501.html (October 2003)

Steven Earle, 2003. Return to Earth Science News