Abstract
We have outlined the structure and mineralogy of the mantle and the phase transitions undergone by olivine, the majority constituent of the upper mantle, at depths of 400 km and 660 km, under the effect of increasing pressure and temperature. These transitions are detected seismically by the observation of " discontinuities " in the profile of seismic velocities as a function of depth. We then introduced the mineralogical and thermal structure of a lithospheric plate, from its formation in an oceanic ridge to its return to the deep mantle in a subduction zone.
We describe the fracture mechanism invoked to explain surface earthquakes, explain why another mechanism is needed to produce deep earthquakes, and review the mechanisms proposed to date. We focused on the hypothesis put forward of the presence of a metastable olivine tongue within the plunging plate to explain deep earthquakes, explaining why this hypothesis is not universally accepted. We discussed and illustrated other proposed mechanisms, including a rock dehydration mechanism to explain intermediate-depth earthquakes.
