Deep-focus earthquake
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.
Uniform outward motion in a single plane due to normal shortening is known as a compensated linear vector dipole source.
[4][5] 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.
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.
[3] 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.
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.
This hypothesis proposes that metastable olivine in oceanic lithosphere subducted to depths greater than 410 km undergoes a sudden phase transition to spinel structure.
This mechanism has been largely discredited due to the lack of a significant isotropic signature in the moment tensor solution of deep-focus earthquakes.
[1] Dehydration reactions of mineral phases with high water content would increase the pore pressure in a subducted oceanic lithosphere.
Metastable olivine subducted past the olivine-wadsleyite transition at 320–410 km depth (depending on temperature) is a potential candidate for such instabilities.
The result is thermal runaway, a positive feedback loop of heating, material weakening, and strain localisation within the shear zone.
Their relevance to deep earthquakes therefore lies in mathematical models which use simplified material properties and rheologies to simulate natural conditions.
[15] The Tyrrhenian Sea west of Italy is host to a large number of deep-focus earthquakes as deep as 520 kilometres (320 mi) below the surface.
In northeastern Afghanistan, a number of medium-intensity deep focus earthquakes of depths of up to 400 kilometres (250 mi) occasionally occur.
