DEEP FOCUS earthquakes or “intra plate” earthquakes, occur within the sub-ducting oceanic plates as they move beneath the continental plates. Appearing along fault lines, these are earthquakes with focus much deeper within the earth. A deep focus earthquake occurs when two tectonic plates slide towards one another followed by subduction, or when the mineral olivine is in a transitional phase.These are typical of the subduction zone of the earth which are seismically active zones, often existing in patterns as in Wadati-Benioff zones. They happen as huge quakes with larger magnitudes, as a great deal of energy is released with the forceful collision of the plates.

The mechanics of these earthquakes have puzzled geophysicists, as brittle fracture and frictional sliding at depths exceeding 100-200 km would require great rock strengths which does not seem possible because of the high temperatures in such zones. They are believed to be associated with a catastrophic phase change as large portions of slab or slab-entrained material convert from spinal structure to a more compact perovskite structure

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