For 50+ years after the discovery of deep earthquakes in the 1920s,their causal mechanism was one of the major unsolved problems of geophysics.It was clear that the physics that explains shallow earthquakes could not apply to deep ones because the pressure is too high at ≥～50km to allow a fault to move,even if you could initiate one.The answer for the deepest earthquakes(350-680km)turned out to involve the olivine-spinel phase transformation that should occur quietly in subduction zones but most of them are so cold that olivine is carried out of its stability field into the Mantle Transition Zone and when it warms up to a temperature at which the reaction can begin,an instability occurs that produces earthquakes.