Differential contraction of subducted lithosphere layers generates deep earthquakes

The origin of forces for generating deep earthquakes remains elusive. We propose a new mechanical model that involves constant release of shear strain accumulated between the crust and mantle lithosphere caused by differential volume changes during phase transformations as a slab sinks. This generates increasing down-dip compression inside the slab with depth, consistent with the global distribution of deep earthquake focal mechanisms. Using experimentally calibrated slab rheology, we show that the estimated distribution of slab internal stress agrees well with the depth-frequency relationship of deep earthquakes in both cold and warm slabs, with the peak in seismicity at ~600 km depth corresponding to an induced stress maximum. This mechanism of residual stress within a cold slab also provides a solution to deep earthquakes occurring beneath former subduction zones such as western Tonga and Spain. We further suggest that this model may reconcile existing geodynamic paradoxes concerning the absolute strength of slabs, discrepancies on mantle rock strength between laboratory and geophysical inversions, as well as the competition between earthquake-generating stresses and slab-pull forces.