Deformation of a weak subducted slab and variation of seismicity with depth

Abstract

OCEANIC lithosphere is assumed to possess platelike properties whether it is lying at the Earth's surface or descending deep into the mantle; yet the geometries of subducting slabs indicate significant deformation as they descend through the mantle1–4, suggesting that lithospheric plates might be so weakened during subduction as to act not as a rigid solid, but as a viscous fluid. Numerical and laboratory experiments have shown that fluid 'slabs' can indeed take on realistic profiles5–7; however, it has not been clear whether a fluid or 'weak' model of slab dynamics can account for deep earthquakes, which are usually ascribed to the deformation of a rigid or 'strong' slab. Here we show, using numerical simulations of slab evolution, that a weak slab model is in fact consistent with seismic observations. Assuming that earthquakes occur at a rate proportional to deformation rate, we reproduce the observed variation with depth of seismicity rate and focal mechanisms, and the cessation of seismicity at 670 km depth. Provided that sinking material encounters resistance at depth (here modelled as a viscosity jump at 670 km), the pattern of seismicity can be explained by any mechanism for deep earthquakes in which the rate of seismicity is proportional to strain rate in the slab.