A deep-focus earthquake in seismology (also called a plutonic earthquake) is an earthquake with a hypocenter depth exceeding 300 km. They occur almost exclusively at 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.
The following four subsections outline proposals which could explain the physical mechanism allowing deep focus earthquakes to occur. With the exception of solid-solid phase transitions, the proposed theories for the focal mechanism of deep earthquakes hold equal footing in current scientific literature.
Solid-solid phase transitions
The earliest proposed mechanism for the generation of deep-focus earthquakes is an implosion due to a phase transition of material to a higher density, lower volume phase. The olivine-spinel phase transition is thought to occur at a depth of 410 km in the interior of the earth. This hypothesis proposes that metastable olivine in oceanic lithosphere subducted to depths greater than 410 km undergoes a sudden phase transition to spinel structure. The increase in density due to the reaction would cause an implosion giving rise to the earthquake. This mechanism has been largely discredited due to the lack of a significant isotropic signature in the moment tensor solution of deep-focus earthquakes.
Dehydration reactions of mineral phases with high weight percent water would increase the pore pressure in a subducted oceanic lithosphere slab. This effect reduces the effective normal stress in the slab and allow slip to occur on pre-existing fault planes at significantly greater depths that would normally be possible. Several workers[who?] suggest that this mechanism does not play a significant role in seismic activity beyond 350 km depth due to the fact that most dehydration reactions will have reached completion by a pressure corresponding to 150 to 300 km depth (5-10 GPa).
Transformational faulting or anticrack faulting
Transformational faulting, also known as anticrack faulting, is the result of the phase transition of a mineral to a higher density phase occurring in response to shear stress in a fine-grained shear zone. The transformation occurs along the plane of maximal shear stress. Rapid shearing can then occur along these planes of weakness, giving rise to an earthquake in a mechanism similar to a shallow-focus earthquake. Metastable olivine subducted past the olivine-wadsleyite transition at 320--410 km depth (depending on temperature) is a potential candidate for such instabilities. Arguments against this hypothesis include the requirements that the faulting region should be very cold, and contain very little mineral-bound hydroxyl. Higher temperatures or higher hydroxyl contents preclude the metastable preservation of olivine to the depths of the deepest earthquakes.
Shear instability / thermal runaway
A shear instability arises when heat is produced by plastic deformation faster than it can be conducted away. The result is thermal runaway, a positive feedback loop of heating, material weakening and strain-localisation within the shear zone. Continued weakening may result in partial melting along zones of maximal shear stress. Plastic shear instabilities leading to earthquakes have not been documented in nature, nor have they been observed in natural materials in the laboratory. Their relevance to deep earthquakes therefore lies in mathematical models which use simplified material properties and rheologies to simulate natural conditions.
Notable deep-focus earthquakes
The strongest deep-focus earthquake in seismic record was the magnitude 8.3 Okhotsk Sea earthquake that occurred at a depth of 609 km in 2013. The deepest earthquake ever recorded was a small 4.2 earthquake in Vanuatu at a depth of 735.8 km in 2004.
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- Frohlich, Cliff (2006). Deep Earthquakes. Cambridge University Press. ISBN 978-0-521-82869-7.
- Kearey, Philip; Keith A. Klepeis; Frederick J. Vine (2013). Global Tectonics (3 ed.). John Wiley & Sons. ISBN 1-118-68808-2.
- "M8.3 - Sea of Okhotsk". USGS. 2013-05-25. Retrieved 2013-05-25.
- "M 4.2 - Vanuatu region". earthquake.usgs.gov. Retrieved 2018-01-22.