Gravity anomalies, crustal structure, and seismicity at subduction zones: 2. Interrelationships between fore‐arc structure and seismogenic behavior

Abstract An ensemble‐averaging technique is used to remove the long‐wavelength topography and gravity field associated with subduction zones. Short‐wavelength residual anomalies are attributed to the tectonic structure of subducting and overthrusting plates. A paired (positive‐negative) fore‐arc anomaly is observed consisting of a long (>1000 km), linear, trench‐parallel ridge landward of the deep‐sea‐terrace basin. Ridges have amplitudes of 1500–3000 m and 160–240 mGal, wavelengths of 150–200 km, and high gravity anomaly to topography ratios (50–75 mGal km −1 ). The ridge crests correlate with the downdip limit of coseismic slip and strong interplate coupling and in Cascadia, the updip limit of tremor epicenters. The ridge crest may be interpreted as defining the boundary between the velocity‐weakening and seismogenic region of the subduction interface and the downdip frictional transition zone. In Tonga‐Kermadec, the Kuril Islands and Chile landward ridges are associated with extinct volcanic arcs. Paired anomalies are attributed to the preferential subduction erosion of the outer fore arc and a spatially varying combination of (a) lower crustal underplating beneath the inner fore arc, (b) the transformation of interseismic strain into permanent geologic strain via faulting, folding, or buckling of the inner fore arc, and (c) the relative trenchward migration of extinct volcanic arcs in regions operating with a net crustal deficit. Along‐strike transitions in fore‐arc morphology and seismogenic behavior are related to preexisting crustal structure of subducting and overthrusting plates. Fore arcs have the added potential of recording the time‐integrated response of the upper plate to subduction processes, and fore‐arc structure should be considered in tandem with seismological observations.