Deep-focus earthquake

A deep-focus earthquake in seismology is an earthquake with a hypocenter depth exceeding 300 km. They occur almost exclusively at oceanic-continental convergent boundaries in association with subducted oceanic lithosphere. They occur along a dipping tabular zone beneath the subduction zone known as the Wadati–Benioff zone.

Preliminary evidence for the existence of deep-focus earthquakes was first brought to the attention of the scientific community in 1922 by Herbert Hall Turner. In 1928, Kiyoo Wadati proved the existence of earthquakes occurring well beneath the lithosphere, dispelling the notion that earthquakes occur only with shallow focal depths.

Deep-focus earthquakes give rise to minimal surface waves. Their focal depth causes the earthquakes to be less likely to produce seismic wave motion with energy concentrated at the surface. The path of deep-focus earthquake seismic waves from focus to recording station goes through the heterogeneous upper mantle and highly variable crust only once. Therefore, the body waves undergo less attenuation and reverberation than seismic waves from shallow earthquakes, resulting in sharp body wave peaks.

The pattern of energy radiation of an earthquake is represented by the moment tensor solution, which is graphically represented by beachball diagrams. An explosive or implosive mechanism produces an isotropic seismic source . Slip on a planar fault surface results in what is known as a double-couple source. Uniform outward motion in a single plane due to normal shortening gives rise is known as a compensated linear vector dipole source. Deep-focus earthquakes have been shown to contain a combination of these sources.

Shallow-focus earthquakes are the result of the sudden release of strain energy built up over time in rock by brittle fracture and frictional slip over planar surfaces. However, the physical mechanism of deep focus earthquakes is poorly understood. Subducted lithosphere subject to the pressure and temperature regime at depths greater than 300 km should not exhibit brittle behavior, but should rather respond to stress by plastic deformation. Several physical mechanisms have been proposed for the nucleation and propagation of deep-focus earthquakes; however, the exact process remains an outstanding problem in the field of deep earth seismology.

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