Rapid Reduction in Subducting Plate Strength in the Kermadec Trench:
A New Application of Admittance Analysis Using Trench-Parallel Profiles

Magali I. Billen and Michael Gurnis

Wide-spread faulting apparent in seafloor bathymetry and seismic reflection profiles, as well as the occurrence of large lithosphere-scale normal faulting events in trenches, suggest that subducting plates suffer significant permanent deformation at shallow depths. Deformation of lithospheric plates in subduction zones may account for a substantial fraction of the total energy dissipation associated with plate motion, reducing plate strength through permanent deformation, and thus regulating plate speeds by limiting the coupling of slab-pull forces to plates at the surface. However, plate-bending analysis of trench-perpendicular profiles are not sensitive to variations in plate strength, and do not provide a unique constraint on even average plate strength as elastic, elastic-plastic and viscous plate models are all able to reproduce trench-forebulge topography. We use 1D admittance analysis employing multitaper spectral estimates of a series of adjacent trench-parallel bathymetry and gravity anomaly profiles to measure the change in flexural response of a subducting plate within a trench. This method provides a robust constraint on the magnitude and length-scale of the reduction in plate strength for profiles of 1000 km long, while overcoming the challenges posed by the large aspect ratio of the subduction environment and steep trench topography. Application to the Kermadec Trench, using four trench-parallel profiles, reveals a rapid reduction in plate strength, characterized by a drop in flexural rigidity by a factor of 1000 between the forebulge and trench-axis, equivalent to a 15–20 km decrease in effective elastic thickness. Our results indicate that an appropriate rheology for the subducting plate would allow for complete loss of elastic strength of the plate within 100 km of the trench axis, and therefore slab-plate coupling occurs through viscous stresses and trench-forebulge topography is a dynamic feature.

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