Effects of metamorphic crustal densification on earthquake size in warm slabs

Abstract

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. Copyright 2004 by the American Geophysical Union.