[1] In subduction zones, where older and colder oceanic crust sinks under another tectonic plate, deep-focus earthquakes may occur at much greater depths in the mantle, ranging from 300 km (190 mi) up to 700 km (430 mi).
[2][3] The cause of deep-focus earthquakes is still not entirely understood since subducted lithosphere at that pressure and temperature regime should not exhibit brittle behavior.
A possible mechanism for the generation of deep-focus earthquakes is faulting caused by olivine undergoing a phase transition into a spinel structure,[4] with which they are believed to be associated.
Earthquakes at this depth of focus typically occur at oceanic-continental convergent boundaries, along Wadati–Benioff zones.
For example, many earthquakes occurring in oceans are assigned fixed depth because of limited local seismic records.