The most conspicuous and spectacular volcanic features on Earth are the graceful volcanic arcs that parallel the deep oceanic trenches where cold and strong surface plates descend into the Earth's mantle. More than three-quarters of the volcanos that erupt above sea level are found at these regions, called subduction zones. Inclined bands of earthquakes trace the descent of plates into trenches, beneath the arc volcanos, and ultimately down to depths as great as 690 km. Earthquakes at depths below about 40–60 km represent seismic failure within the sinking plate rather than the slipping motion between plates. Some of those earthquakes have been large and very damaging, and future quakes of this type pose a significant seismic hazard in northwest Washington state, Central America, and elsewhere. Most earlier studies have emphasized the role of the sinking force that accompanies the cold thermal structure, as well as the resultant higher density of descending plates in producing stresses that lead to seismic failure of the plate.
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