For 30 large-earthquake generating subduction zones, we quantify forearc basin size and subducting seafloor roughness to see if the size and shape of geomorphologic features determine earthquake magnitude. The subduction of geomorphologic features, such as ridges and seamounts, not only influences seismicity, but increases basal erosion and subsidence of the accretionary wedge, resulting in the formation of forearc basins. Many subduction zone ruptures have been associated with these basins, where a great portion of the ruptures' asperities collocate with the basins. First, we attempt to discern a relationship between forearc and subducting geomorphology by quantifying along-strike variations in the bathymetry associated with 30 different rupture zones and the sections of subducting seafloor adjacent to those rupture zones. Two parameters, determined from theoretical models fit to empirical semivariograms, characterize areas of bathymetry: the sill, which estimates the degree of relief in the terrain, and the range, which is the horizontal scale associated with the sill. The ratio of sill to range provides us with a measure of seafloor roughness at a scale relevant to the mechanics of subduction. We investigate the control that forearc basins and subducting bathymetric highs may have on the occurrence of large thrust earthquakes by separately comparing our rupture zone and subducting seafloor roughness measurements to the moment magnitudes of the 30 events. The trench-parallel dimension of bathymetrically high features on the subducting seafloor appears to correlate with moment magnitude. Also, we observe in the ratio of sill to range measurements that geomorphology on the subducting seafloor and forearc constrain the earthquake size at a margin.
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