Gravity anomalies, crustal structure, and seismicity at subduction zones: 1. Seafloor roughness and subducting relief

An ensemble averaging technique is used to remove the long-wavelength topography and gravity field from subduction zones. >200 residual bathymetric and gravimetric anomalies are interpreted within fore arcs, many of which are attributed to the tectonic structure of the subducting plate. The residual-gravimetric expression of subducting fracture zones extends >200 km landward of the trench axis. The bathymetric expression of subducting seamounts with height 1 km and area 500 km(2) (N=36), and aseismic ridges (N>10), is largest near the trench (within 70 km) and above shallow subducting slab depths (SLAB1.0 <17 km). Subducting seamounts are similar in wavelength, amplitude, and morphology to unsubducted seamounts. Morphology, spatial distributions, and reduced levels of seismicity are considered inconsistent with mechanical models proposing wholesale decapitation, and the association of subducting seamounts with large-earthquakes. Subducting aseismic ridges are associated with uplift and steepening of the outer fore arc, a gradual reduction in residual bathymetric expression across the inner fore arc, and a local increase in the width and elevation of the volcanic-arc/orogen. These contrasting expressions reflect the influence of margin-normal variations in rigidity on where and how the upper plate deforms, both to accommodate subducting relief and in response to stresses transmitted across the plate interface. The outer fore arc and arc have lower rigidity due to fracturing and thermal weakening, respectively. Similar associations with complex earthquakes and fault creep suggest aseismic ridge subduction may also be accommodated by the development and evolution of a broad fracture network, the geometrical strength of which may exceed the locking strength of a smooth fault.