Geological Signatures of Megathrust Activity – GeoSigMA

The largest earthquakes on Earth occur at subduction zones, where a dense tectonic plate slides past another as it sinks into the mantle. The plate interface, or megathrust, is populated by asperities where the two plates transiently stick to each other. This compresses the upper plate and stresses asperities to their breaking point every few 100 yrs. Mapping locked asperities and understanding their nature are top Earth science priorities with crucial societal implications. To this end, seismo-geodesists exploit ongoing deformation of the upper plate, which they assume elastic but can only measure on decadal time scales. The GeoSigMA project seeks to unlock new constraints on asperity distributions by leveraging measurements and models of upper plate deformation on much longer times scales. The game changer is the recent observation of striking similarities between ongoing deformation above locked patches, and features of the subduction landscape formed over 100s of kyrs. This suggests that the locking pattern is persistent and leaves a footprint in the morphology of the plate margin. Exploiting it requires a physical understanding of how the repetition of (seemingly) elastic cycles of upper plate loading and unloading can build permanent deformation, and how geomorphic agents encode it in the landscape. To achieve this, I will assemble a world-class team at the interface of geodynamics, geomorphology and seismology. We will design methods that map long-term deformation using the concavity of rivers and submarine canyons. At the same time, we will develop and distribute cutting-edge numerical tools for forward- and inverse- tectonic modeling. We will use them to assess when, where and how seismic cycle deformation can become inelastic. The culmination of these efforts will be the joint inversion of geodetic, geomorphological, and structural observations which will provide unprecedented constraints into the extent and nature of dangerous megathrust asperities.