Rigidity variations with depth along interplate megathrust faults in subduction zones

The world's largest earthquakes occur along the contact between subducting and overriding tectonic plates in subduction zones(1). Rock and sediment pr operties near this plate interface exert important controls on the friction al behaviour of faults and earthquake rupture dynamics(2). An important mat erial property to define along the plate interface is the rigidity (the res istance to shear deformation). Rigidity affects; the degree of earthquake s haking generated by a given fault displacement through its influences on se ismic wave speed and earthquake rupture velocity. Here we present an invest igation of the relationship between the duration of earthquake rupture and source depth, which yields estimates of rigidity variation along plate inte rfaces in six subduction zones in the circum-Pacific region. If stress drop is assumed constant, rigidity appears to increase with depth in each seism ogenic zone by a factor of similar to 5 between depths of 5 and 20 km. This result is consistent with the hypothesis that 'tsunami' earthquakes (chara cterized by large slip for a given seismic moment and slow rupture velocity ) occur in regions of low rigidity at shallow depths(3-5). These rigidity t rends should provide lan important constraint for future fault-zone and ear thquake-modelling efforts.