For the most part, the plate boundaries that are defined by very narrow bands of seismicity are also those that have only shallow earthquakes. These correspond to the mid ocean ridges. Shallow earthquakes are also characteristic of transform boundaries, although these are often marked by a broader zone of earthquakes than the mid ocean ridges. The broadest belts of seismicity are those that represent convergent plate boundaries: either subduction zones, or zones of continental collision.  The areas where the deepest earthquakes are located--the west coast of South America, around the Pacific Rim through Japan, the Philippines, Indonesia and the Tonga-Kermadec arc--are the subduction zones, where an oceanic plate converges with either continental or oceanic lithosphere.  There is also a very broad zone of seismicity stretching from southern Europe through central Asia that is marked by a smaller number of deep earthquakes.  This is a continent-continent collision zone, where the African and Indian plates are moving northward, converging with Eurasia.

SUBDUCTION ZONES: Recycling the Earth's Lithosphere
The map of the age of the ocean floor is strong evidence in support of the sea floor spreading hypothesis. The age distribution shows how new ocean floor is formed at the mid ocean ridges and spreads away on either side. If new ocean floor is forming at the ridges, then old ocean floor must be destroyed--or recycled--elsewhere.  If this were not the case then we would soon have too much extra lithosphere at the Earth's surface. If students have correctly set up the request described above for earthquakes at different depths, the filmstrip should clearly show that the belts of seismicity at greater depths have a slightly different position across the plate boundary. It should be evident that earthquakes become progressively deeper from one side of the boundary to the other. For example, along the west coast of South America, the earthquakes deepen from west to east. In Tonga, north of New Zealand, they deepen from east to west. Similarly, they increase in depth from east to west across the islands of Japan. A team of seismologists first related this pattern of earthquake depth distribution to Plate Tectonics in 1968 (Isacks, Oliver and Sykes, 1968). They demonstrated that this band of deepening seismicity in fact delineated the oceanic lithosphere as it descended into the mantle. Thus lithosphere is created at the mid ocean ridges, rides across the ocean basin for as much as 175 million years, and then is returned to the mantle in the Earth's subduction zones. These data also explain the very young age of the ocean floor relative to rocks found on the continents: oceanic lithosphere is constantly being recycled but continental lithosphere, because of its lower density and greater thickness, appears to be much more persistent at the Earth's surface. Rocks in Isua, Greenland, have been dated at 3.8 billion years--more than 20 times older than the oldest ocean rocks (Moorbath et al, 1978).

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