We document a correlation between along-strike variation in interseismic coupling depth on subduction interfaces, and the upper plate tectonic stress state in New Zealand, Vanuatu, and southwest Japan. Deep interseismic coupling occurs where the upper plate stress regime is contractional to transpressional, whereas a shallowing of interseismic coupling occurs where there is an along-strike shift to back-arc or intra-arc extension. To explain this relationship, we draw on theoretical studies suggesting that the fluid pressure state within the upper plate and on the subduction interface has a strong control on the depth of the transition from frictional to viscous behavior. Lower fluid pressures (e.g., close to hydrostatic) are expected where the over-riding plate is undergoing tectonic extension, whereas higher fluid pressures (e.g., close to lithostatic) are expected where the over-riding plate experiences long-term (e.g., >105 yr) tectonic shortening. Low fluid pressures within the upper plate may lead to a shallow frictional to viscous transition compared to an upper plate that is highly overpressured. We hypothesize that the state of tectonic stress and structural permeability in the upper plate are yet other variables to consider when evaluating which physical mechanisms control interseismic coupling of subduction megathrusts.

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