Geodynamics of forearc topography and strain linked with giant earthquake rupture area

The forearc topography at several subduction zones on Earth is complex, involving a ridge and depression (e.g. Alaska, Chile, Cascadia) in between trench and magmatic arc. The ridge and depression show trench-normal extension and shortening, respectively. Here we propose that forearc topography and strain result from subduction interface suction forces that bend the overriding plate, with shortening in the depression accommodating lithospheric inner-bend deviatoric compression and extension in the ridge accommodating outer-bend deviatoric tension. We present a four-dimensional geodynamic subduction experiment that demonstrates the spatial and temporal correlation between forearc depression and shortening, and between forearc ridge and extension. Maximum topographic curvature spatially corresponds with maximum surface strain, substantiating our mechanical explanation for the anomalous topography and strain at several subduction zones. Our experiment provides new insight into the largest recorded earthquakes (1960 Valdivia, 1964 Alaska), characterised by rupture areas with an extreme trench-parallel extent, overlain by forearc domains with pronounced ridge-depression topography. Our experiment predicts high tensile deviatoric normal stresses on the subduction zone plate boundary interface that generated these earthquakes. We propose a conceptual model in which such stresses facilitate lateral rupture growth. Our global compilation of subduction zone rupture extent and proxy for interface stress corroborates this.