Phase transformation and deep-focus earthquakes

Various types of energy can be stored in metastable materials at high pressure. If appropriate forms of energy released at a reasonable fast rate ( 102) become available, some of the released energy must radiate elastic wave in a solid body. The energy efficiency in earthquake production for an earthquake having a magnitude of 7 or 8 was found to be 0.1%. “Earthquakes” caused by metastable phase transformations occurring under high pressure and temperature conditions have recently been confirmed in the laboratory by studies of the α → γ phase transformation in olivine. It is proposed that most earthquakes occurring at depths greater than ca 50 km inside the Earth represent the loci of the kinetic boundaries of the various phase transformations known to take place in the mantle silicates. Even the peculiar wedge-shaped double seismic zone observed between 70 and 180 km in the intermediate-focus earthquakes and the deep double seismic zone observed between 300 and 450 km can be interpreted in terms of this model.