Benioff zones are dipping, roughly planar zones of increased earthquake activity produced by the interaction of a downgoing oceanic crustal plate with an overriding continental or oceanic plate. They occur at boundaries of crustal plates called subduction zones. The earthquakes can be produced by slip along the subduction thrust fault or by slip on faults within the downgoing plate, as a result of bending and extension as the plate is pulled into the mantle. The zones have dips typically ranging from 40 to 60 degrees. The zones are also known as the Wadati-Benioff zone.
During the past century, improvements in seismic acquisition and processing led to the observation that the world's earthquakes are not randomly distributed over the earth's surface. Rather, they tend to be concentrated in narrow zones along the boundaries of continental and oceanic crustal plates. According to the plate tectonic theory, the crust of the earth is broken into a mosaic of seven major rigid plates floating over a much less rigid mantle. The plates are not static but are in constant motion. Most of the tectonic activity, such as the formation of mountain belts, earthquakes, and volcanoes, occurs at the plate boundaries. There are four different types of these seismic zones corresponding to the four main types of plate boundary interactions: subduction zones as along the western coast of South America ; strike-slip (transform) zones like the San Andreas system along the west coast of North America ; zones of seismic activity along midocean ridges like the mid-Atlantic Ridge system; and continental-continental collision zones such as the Himalyan where the Indian subcontinent is ploughing into Asia .
Benioff zones are found in subduction zones that form by the collision of two crustal plates of dissimilar density and thickness, for example an oceanic and continental plate. The heavier (thinner) crust of the oceanic plate is thrust or subducted under the lighter and much thicker crust of the continental plate. A deep ocean trench is produced where these two plates meet. Along the Peru-Chile trench, the Pacific plate is being subducted under the South American plate, which responds by crumpling to form the Andes. The earthquake zones that parallel the great oceanic trenches are typically inclined from 40 to 60 degrees from the horizontal and extend several hundred kilometers into the mantle along trends that reach thousands of kilometers in length. These zones are sometimes called Wadati-Benioff zones after two of the seismologists who first recognized them, Kiyoo Wadati of Japan and Hugo Benioff of the United States.
Benioff zones are the seismic expression of the deformation produced by the subduction of one plate under another. The subduction or "destruction" of the oceanic crust compensates for the creation of new ocean crust at the ocean ridges. The compensating result of both processes explains why the earth may not have significantly increased in size since its formation 4.6 billion years ago.
See also Continental drift theory; Plate tectonics; Sea-floor spreading