1. Institute of Tectonics and Earth Sciences Department, University of California, Santa Cruz, California 95064, USA

Correspondence to: Susan L. Bilek¹ Correspondence and requests for materials should be addressed to S.B. (e-mail: Email: sbilek@es.ucsc.edu).

The world's largest earthquakes occur along the contact between subducting and overriding tectonic plates in subduction zones¹. Rock and sediment properties near this plate interface exert important controls on the frictional behaviour of faults and earthquake rupture dynamics². An important material property to define along the plate interface is the rigidity (the resistance to shear deformation). Rigidity affects the degree of earthquake shaking generated by a given fault displacement through its influences on seismic wave speed and earthquake rupture velocity. Here we present an investigation of the relationship between the duration of earthquake rupture and source depth, which yields estimates of rigidity variation along plate interfaces in six subduction zones in the circum-Pacific region. If stress drop is assumed constant, rigidity appears to increase with depth in each seismogenic zone by a factor of 5 between depths of 5 and 20 km. This result is consistent with the hypothesis that 'tsunami' earthquakes (characterized by large slip for a given seismic moment and slow rupture velocity) occur in regions of low rigidity at shallow depths^{3, 4, 5}. These rigidity trends should provide an important constraint for future fault-zone and earthquake-modelling efforts.