Bridging inner fault shear localisation and the propagation of earthquake rupture

Earthquakes leave different types of records that help decipher their dynamics. At the large scale monitored by remote sensing and seismic data, earthquakes arise from the propagation of rapid slip along tectonic faults, exhibiting rupture dynamics reminiscent of those driving shear fracture or slip fronts in stick-slip experiments. At the scale of the fault core, fieldwork revealed how the zone actively deformed during an earthquake is often extremely thin and that shear strains are highly localised.

In this work, we numerically simulate shear ruptures using a dual-scale approach, allowing us to couple a sub-millimetre description of inner fault processes and kilometre‑scale elastodynamics. Our results demonstrate how rapid strain localisation across a layer of fault gouge creates a sudden drop in the shear stress bearing capacity, producing earthquake rupture that closely follows fracture mechanics description. We quantify how the fracture energy governing rupture propagation is substantially smaller than that predicted by models that do not account for strain localisation. We show the existence of a unique scaling law between the localised shearing width and the rupture speed. Our results bring new insights on the multiscale mechanics that produces seismic rupture and indicate that earthquakes are likely to be systematically associated to extreme strain localisation.