Effects of metamorphic crustal densification on earthquake size in warm slabs

Some recent damaging earthquakes occurred in the lower crust or mantle of warm subducting slabs. They are consistent with a theoretical prediction that larger events tend to be deeper inside the slab as a result of mechanical damage to the crust caused by metamorphic rock densification. The densification begins in a thin layer along the slab surface, inducing a stretching force in it. Fracture spacing scales with layer thickness, resulting in a "shattered" upper crust in which earthquake ruptures have limited propagation distance. In contrast, the more uniform untransformed substrata can host larger ruptures. Often, the lack of compression in warm-slab mantle is also consistent with a shattered crust.