Definitions

subduction zone

subduction zone

[suhb-duhk-shuhn]
subduction zone, large-scaled narrow region in the earth's crust where, according to plate tectonics, masses of the spreading oceanic lithosphere bend downward into the earth along the leading edges of converging lithospheric plates where it slowly melts at about 400 mi (640 km) deep and becomes reabsorbed. Subduction zones are usually marked by deep ocean trenches that often exceed 6 mi (10 km) compared to the ocean's overall depth of 2 to 4 mi (3 to 5 km). A pattern of earthquakes of shallow, intermediate, and deep focus occurs along the same angle as the descending plate, which is steeply inclined (30°-60°) toward the continent behind the trench in a zone called the Benioff Zone, discovered by the U.S. seismologist Hugo Benioff. This earthquake pattern enables geophysicists to trace the descending plate to depths of 600 to 700 km (370-440 mi), where temperatures are thought to be between 1,000°C; and 2,000°C; (1,800°-3,600°F;). As the oceanic plate descends, friction between the two plates probably causes partial melting of the descending plate forming a magma of andesitic composition that rises along fractures. If the overlying crustal plate is oceanic, the magma may erupt to form volcanic island arcs, such as Japan or the Aleutians. If the overlying plate is continental, a line of batholiths and volcanoes may be created as in the Coast Ranges of Canada and the W United States. See continent; continental drift; seafloor spreading.

Oceanic trench area in which, according to the theory of plate tectonics, the seafloor underthrusts an adjacent plate, dragging the accumulated trench sediments downward into the Earth's upper mantle. Seealso deep-sea trench.

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The Cascadia subduction zone is a subduction zone, a type of convergent plate boundary that stretches from northern Vancouver Island to northern California.

Geology

The zone separates the Juan de Fuca, Explorer, Gorda and the North American Plates. Here, the oceanic crust of the Pacific Ocean sinks 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.

The Cascadia subduction zone presents a challenge to current tectonic theory which generally holds that subduction occurs as a plate becomes older, denser and thicker with distance from the ridge which contributes new material to it. In the case of Cascadia, its associated ridge is just a few hundred miles (and in places less) distant from the subduction zone. This puzzle is a matter of ongoing research and discussion. The current hypothesis which attempts to explain it is that once subduction begins, it continues by the process of slab-pull, i.e. the weight of the previously subducted segment exerts a force behind it on the remaining slab above the subduction zone, regardless of its density, pulling it downward.

The Cascadia subduction zone runs from triple junctions at its north and south ends. On the north just south of Queen Charlotte Island, it intersects the Queen Charlotte Fault and the Explorer Ridge. On the south, just off of Cape Mendocino in California, it intersects the San Andreas Fault and the Mendocino fault zone at the Mendocino Triple Junction.

Earthquakes

The Cascadia subduction zone can produce very large earthquakes, magnitude 9.0 or greater, if rupture occurs 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.

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. The motion did not trigger an earthquake and was only detectable as silent, non-earthquake seismic signatures.

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.

San Andreas Quake Connection

Studies of past earthquake traces on both the northern San Andreas Fault and the southern Cascadia Subduction Zone indicate a correlation in time which may be evidence that quakes on the Cascadia Subduction Zone may have triggered most of the major quakes on the northern San Andreas during at least the past 3,000 years or so. The evidence also shows the rupture direction going from north to south in each of these time-correlated events. The 1906 San Francisco Earthquake seems to have been a major exception to this correlation, however, as it was not preceded by a major Cascadia quake, and the rupture moved mostly from south to north.

Volcanoes

The volcanoes within the subduction zone include:

See also

References

External links

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