Cascadia subduction zone

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Structure of the Cascadia subduction zone

Area of the Cascadia subduction zone

The Cascadia subduction zone is a very long sloping fault that stretches from mid-Vancouver Island to northern California. It separates the Juan de Fuca Plate and the North American Plate. Here, the oceanic crust of the Pacific Ocean is pushed toward and beneath the continent at a rate of 40 mm/yr.

The width of the Cascadia subduction zone fault varies along its length, depending on the temperature of the subducted oceanic plate, which heats up as it is pushed deeper beneath the continent. As it becomes hotter and more molten, it eventually loses the ability to store mechanical stress and generates earthquakes.

Because of the very large fault area, the Cascadia subduction zone can produce very large earthquakes, magnitude 9.0 or greater, if rupture occurred over its whole area. When the "locked" zone stores up energy for an earthquake, the "transition" zone, although somewhat plastic, can rupture. Thermal and deformation studies indicate that the locked zone is fully locked for 60 kilometers downdip from the deformation front. Further downdip, there is a transition from fully locked to aseismic sliding. (Nedimovic, et al., 2003)

In 1999, a group of Continuous Global Positioning System sites registered a brief reversal of motion of approximately 2 centimeters over a 50 kilometer by 300 kilometer area. The movement was the equivalent of a 6.7 magnitude earthquake. (Dragert, et al., 2001) The motion did not trigger an earthquake and was only detectable as silent, non-earthquake seismic signatures. (Rogers & Dragert, 2003)

The last known great earthquake in the northwest was in January of 1700, the Cascadia Earthquake. Geological evidence indicates that great earthquakes may have occurred at least seven times in the last 3,500 years, suggesting a return time of 400 to 600 years. There is also evidence of accompanying tsunamis.

A future rupture of the Cascadia Subduction Zone would cause widespread destruction throughout the Pacific Northwest.

Other similar subduction zones in the world usually have such earthquakes every 100–200 years; the longer interval here may indicate unusually large stress buildup and subsequent unusually large earthquake slip.

Famous volcanoes associated with this zone are Mt. St. Helens and Mt. Rainier.

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[edit] References

Atwater, BF (1987). "Evidence for great Holocene earthquakes along the outer coast of Washington State". Science 236 (4804): 942-44.
Nedimovic MR, Hyndman RD, Ramachandran K, Spence GD (2003). "Reflection signature of seismic and aseismic slip on the northern Cascadia subduction interface". Nature 424 (6947): 416-20. PMID 12879067.
Dragert G, Wang K, James TS (2001). "A silent slip event on the deeper Cascadia subduction interface". Science 292 (5521): 1525-8. PMID 11313500.
Rogers G, Dragert H (2003). "Episodic tremor and slip on the Cascadia subduction zone: the chatter of silent slip". Science 300 (5627): 1942-3. PMID 12738870.