RELATIONSHIP OF DEEP SEISMICITY TO THE THERMAL STRUCTURE OF SUBDUCTED LITHOSPHERE

RECENT experimental work on silicate olivine polymorphs 1 has confirmed earlier observations of mechanical failure in analogous compounds 2,3 as a result of phase transformations when nonhydrostatic stresses were applied to a metastable phase. This 'transformational faulting' 4 mechanism is considered to be a leading candidate 5, among others involving olivine transformation 6,7, for the cause of deep earthquakes. Evidence consistent with this mechanism comes from the observation 2-4 that, worldwide, earthquakes become more numerous with increasing depth after a seismicity minimum at about 350 km depth 8. This depth lies within the range anticipated for mineralogical transformations in the subducted lithosphere 9. But individual subduction zones differ in their thermal structures, so if transformational faulting indeed contributes to deep seismicity, this should be reflected in the location of the seismicity minimum for each zone. The depth at which the minimum occurs should depend on temperature in the same way as do the polymorphic transformations of olivine, and should always lie deeper than the depth at which the equilibrium transformation takes place. Here we test these predictions for eight subduction zones worldwide. We find that, with one exception (the North Japan zone), the depth of the seismicity minimum decreases linearly with increasing thermal age of the slab (a measure of its temperature profile). These results support the proposal that deep earthquakes are a consequence of phase transformations in olivine.