Rupture access to hydrous minerals controls aftershocks in subduction zones.

Aftershock productivity varies widely among subduction-zone earthquakes of similar magnitude. We investigate how slab hydration and rupture geometry modulate access to hydrous minerals and the generation of pressurized fluids from co-seismic frictional heating along the interface between subducting slabs and overriding plates. We describe ten large and major earthquakes (Mw > 6.8) that generated thousands of aftershocks (Mw > 4), and eleven nearby earthquakes of similar magnitude that generated few, if any, aftershocks. Kinematic and petrological constraints reveal that earthquakes producing rich aftershock sequences ruptured along slab interfaces containing serpentinized peridotite and hydrated oceanic crust. By contrast, earthquakes with few aftershocks occurred in flat-slab regions where rupture planes were oblique to the hydrated interface (intraslab events). Oblique rupture reduces the volume of volatile-bearing minerals accessed per unit rupture area, diminishing the pressurized fluid production needed to drive aftershock sequences. Globally, we propose that slab geometry and rupture orientation regulate access to fluid-producing hydrated rocks and thereby control aftershock productivity through co-seismically generated pressurized fluids in subduction zones.