Review of manuscript se-2021-148

Abstract

What controls the location and segmentation of mega-earthquakes in subduction zones is a long-standing problem in earth sciences. Prediction of earthquake ruptures mostly relies on interplate coupling models based on Global Navigation Satellite Systems providing patterns of slip deficit between tectonic plates. We here investigate if and how the seismic and aseismic patches revealed by these models relate to the distribution of deformation along the plate interface, i.e. basal erosion and/or underplating. From a mechanical analysis of the topography applied along the Chilean subduction zone, we show that extensive plate interface deformation takes place along most of the margin. We show that basal erosion occurs preferentially at 15 km depth while underplating does at 35 ± 10 and 60 ± 5 km depth, in agreement with P-T conditions of recovered underplated material, expected pore pressures, and spatial distribution of marine terraces and uplift rates. Along southern Chile, large sediment input favors shallow accretion and underplating of subducted sediments, while along northern Chile, extensive basal erosion provides material for the underplating. We then show that all major earthquakes of southern Chile are limited along their down-dip end by underplating while, along northern Chile, they are surrounded by both basal erosion and underplating. Segments with heterogeneously distributed deformation largely coincide with lateral earthquake terminations. We therefore propose that long-lived plate interface deformation promotes stress build-up and leads to earthquake nucleation. Earthquakes then propagate along fault planes shielded from this long-lived permanent deformation, and are finally stopped by segments of heterogeneously distributed deformation. Slip deficit patterns and earthquake segmentation therefore reflect the along-dip and along-strike distribution of the plate interface deformation. Topography acts as a mirror of distributed plate interface deformation and should be studied systematically to improve the prediction of earthquake ruptures.