Identification of subducting plate structure within seismogenic zones and relationships with seismicity

Thursday, 18 December 2014: 10:50 AM
Dan Bassett and Anthony Brian Watts, University of Oxford, Oxford, 0X1, United Kingdom
The roughness of subducting plates is one of the most important parameters controlling the seismogenic behavior of subduction megathrusts, but in most regions the distribution of subducting relief is inferred from structure seaward of trenches. Spectral averaging techniques developed to remove the steep topographic gradients across forearcs are shown to improve resolution of local trench-slope uplift, that may be diagnostic of subducting relief. This interpretation is locally calibrated where the Louisville Ridge subducts at the Tonga trench. From a global extension of these techniques, >200 residual bathymetric anomalies are identified, enabling links between subducting relief, slip behavior and seismicity to be reconsidered. We interpret >150 potential subducting seamounts, 36 of which have height ≥1 km and area ≥500 km2. These anomalies are similar in wavelength, amplitude and morphology with unsubducted seamounts, are associated with aseismic regions in Tonga and Mariana, and prevented along strike rupture propagation in large recent earthquakes in Java (2006) and Japan (2011). Subducting aseismic ridges in Peru, Ecuador and Costa-Rica are associated with uplift and steepening of the outer-forearc and a local increase in the width and elevation of the volcanic-arc. Associations with complex large earthquakes, higher frequencies of small events, and creep suggest aseismic ridges may also subduct via the development and evolution of an adjacent fracture network. Megathrust complexity is expected to be greatest on subducting ridge flanks. The bathymetric expression of subducting relief is strongest near the trench (<70 km) and above shallow slab depths (<~17 km). Dip-parallel transitions in the surface expression of aseismic ridges may reflect physical transitions in megathrust slip-behavior and/or material properties in the overthrusting wedge. Aseismic ridges and seamounts represent seafloor roughness over different wavelengths and are associated with contrasting modes of isostatic compensation. Consideration of these differences, coupled with improved knowledge of where subducting bathymetric features are likely within seismogenic zones, may reconcile some debated aspects of the mechanics and seismological consequences of bathymetric relief subduction.