Observations of the depth distribution of seismicity suggest that several mechanisms cause deep earthquakes. Cold subducting slabs exhibit a broad peak in seismicity between 300 and 530 km depth that is best explained by transformational faulting of metastable olivine. Cumulative seismic moment release increases sharply at ~530 km depth in both cold and warm slabs, implying that events deeper than this are controlled by an equilibrium rather than a metastable phase change, which would be temperature-dependent. This, together with the observation that deep seismicity predominates near the garnet-rich upper boundary of the subducting plate (former oceanic crust), suggests that the transformation of garnet to perovskite, which may release considerable water, is integral in the cause of earthquakes deeper than 530 km, where most deep earthquakes occur.