Slow fracture propagation on a fault surface with an embedded viscous layer

On the basis of observational evidence and theoretical models, we assess a possible mechanism for earthquake occurrence on plate margins. Since it is largely accepted that a condition of aseismic sliding may prevail at depth below active faults, we propose that the sequence: aseismic slip at depth rt arrow stress concentration on the upper brittle zone rt arrow seismic rupture may be a common mechanism on plate margins. In particular, we suggest that aseismic slip at depth could be responsible for the so-called 'silent earthquakes', those observed wavetrains which display a dominant low-frequency content, and propose a simple model which accounts for very low slip velocities on deep fault sections in terms of a viscous drag force.