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Introduction
In subduction zones where a thinner, dense oceanic plate descends beneath a thicker, colder, less-dense continental plate, it is common for the leading edge to lock under high friction.
As the plates attempt to slide past each other, frictional stress builds up along the fault boundaries accompanied by an increase in strain in the adjacent rocks. When the frictional stress exceeds a critical value, a sudden failure occurs along the fault plane that can result in a violent displacement of the Earth's crust. When this happens, the ensuing earthquake releases elastic strain energy and seismic waves are radiated. The process of strain, stress, and failure is referred to as the elastic-rebound theory. More on Subduction Zones
Elastic Rebound in a Subduction Zone
Oblique view of a highly generalized animation of a subduction zone where an oceanic plate is subducting beneath a continental plate. (See sketch below for parts.) This scenario can happen repeatedly on a 100-500 year cycle. The process which produces a mega-thrust earthquake would generate a tsunami, not depicted here.
Direct Link to Elastic Rebound in a Subduction Zone (Small 524kb)
Direct Link to Elastic Rebound in a Subduction Zone (Larger 1.5mb)
Direct Link to Elastic Rebound in a Subduction Zone No Text (Larger 1.5mb)
Relative Motion of the Plates in a Subduction Zone Assuming No Friction
Relative motion of the subducting plate assuming no friction between the plates.
Direct Link to No Friction (Small 172kb)
Direct Link to No Friction (Larger 488kb)
Below: Sketch depicting the components of a subduction zone. There is often confusion about what a tectonic plate is. The plate consists of the capping crust and a bit of the solid upper mantle. The plates ride over the asthenosphere, a semi-molten part of the mantle. Because oceanic crust is heavier than continental crust, the oceanic plate subducts beneath continental plate when they collide.
More About Subduction-zone Earthquakes
Earthquakes generated in this setting are called Great Subduction Zone earthquakes. They are the largest earthquakes in the world and can exceed magnitude 9.0. The devastating Sumatra-Andaman earthquake of December 26, 2004 was a magnitude 9.1. Earthquake size is porportional to fault area. The earthquake occurred on the interface between the India and Burma tectonic plates where the India plate subducts beneath the overriding Burma plate; the fault area was was about 1200 km long with maximum vertical displacement of 20 meters in places. That uplift caused by the elastic rebound of the overlying plate is what caused the deadly tsunami that killed over 225, 000 people.
A similar configuration of plates can be found along the Cascadia Subduction Zone. This is a very long sloping fault that separates the Juan de Fuca and North America plates and stretches from mid- Vancouver Island to Northern California. Because of the very large fault area, the Cascadia Subduction Zone could produce a very large earthqauke, magnitude 9.0 or greater, if rupture occurred over its whole area. It last ruptured in 1700.
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Animations By Jenda Johnson
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