Cascadia subduction zone

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The Cascadia subduction zone is a very long sloping fault that stretches from northern Vancouver Island to northern California.

Contents 1 Geography 2 Earthquakes 3 Volcanoes 4 See also 5 References 6 External links

[edit] Geography

The zone separates the Juan de Fuca, Explorer, Gorda 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.

[edit] 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 (about 40 miles) 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 (0.8 inches) over a 50 kilometer by 300 kilometer (about 30 mile by 200 mile) 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 300 to 600 years. There is also evidence of accompanying tsunamis with every earthquake, and one line of evidence for these earthquakes is tsunami damage, and through Japanese records of 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.

[edit] Volcanoes

The volcanoes within the subduction zone include:

• Mount Silverthrone
• Mount Meager
• Mount Cayley
• Mount Garibaldi
• Mount Baker
• Glacier Peak
• Mount Rainier
• Mount St. Helens
• Mount Adams
• Mount Hood
• Mount Jefferson
• Three Sisters
• Newberry Volcano
• Mount Mazama
• Mount McLoughlin
• Medicine Lake Volcano
• Mount Shasta
• Lassen Peak

[edit] See also

• Cascadia
• Cascadia Earthquake
• Cascade Mountains
• Cascade Volcanic Arc
• Geology of the Pacific Northwest
• North Cascades National Park
• Plate tectonics
• Subduction zone

[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.

[edit] External links

• Giant Earthquakes Beneath Canada's West Coast. Geological Survey of Canada - Pacific Division. Retrieved on 2006-06-10.