The following mechanism for deep earthquakes is proposed. Due to the relative motion of manlet and plate, material crosses a phase stability line. The new phase, with a different molar volume is nucleated several tens of kilometers away from the equilibrium depth creating a stress disturbance which triggers further nucleation and reaction at a rapid rate (~102 s) even for a diffusional phase transformation. The transformation causes a decrease in yield strength and effective viscosity, allowing fast shear deformations to occur. This mechanism is associated with the sort of hypocentral region compressions and shear deformations recently observed seismically for two deep earthquakes.