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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 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 depths3,4,5. These rigidity trends should provide an important constraint for future fault-zone and earthquake-modelling efforts.

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Figure 1: Normalized source duration as a function of source depth (depth below ocean bottom) for each of the regions analysed.
Figure 2: Data from the Alaska–Aleutian islands region.
Figure 3: Two end-member models for our data.
Figure 4: Generalized diagram of a subduction zone, with an expanded-scale view of the seismogenic-zone interface.

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Data were obtained through the IRIS datacenter. This work was supported by the US NSF. We thank S. Schwartz and E. Silver for comments on early drafts.

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Correspondence to Susan L. Bilek.

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Bilek, S., Lay, T. Rigidity variations with depth along interplate megathrust faults in subduction zones. Nature 400, 443–446 (1999).

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