![]() ![]() Exceptions to this rule include Romania and the Hindu Kush mountain system. Benioff zones coincide with tectonically active island arcs such as Japan, Vanuatu, Tonga, and the Aleutians, and they are normally but not always associated with deep ocean trenches such as those along the South American Andes. Dip angles of these slabs average about 45°, with some shallower and others nearly vertical. The deeper-focus earthquakes commonly occur in patterns called Benioff zones that dip into the Earth, indicating the presence of a subducting slab. Below intermediate depth the distribution is fairly uniform until the greatest focal depths, of about 700 km (430 miles), are approached. ![]() The frequency of occurrence falls off rapidly with increasing focal depth in the intermediate range. ![]() ![]() About 3 percent of total energy comes from deeper earthquakes. Of the total energy released in earthquakes, 12 percent comes from intermediate earthquakes-that is, quakes with a focal depth ranging from about 60 to 300 km. It should be noted, however, that the geographic distribution of smaller earthquakes is less completely determined than more severe quakes, partly because the availability of relevant data is dependent on the distribution of observatories. In fact, the great majority of earthquake foci are shallow. Most parts of the world experience at least occasional shallow earthquakes-those that originate within 60 km (40 miles) of the Earth’s outer surface.
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