The generation of large earthquakes

Despite decades of observational, laboratory and theoretical studies, the processes leading to large earthquake generation remain enigmatic. However, recent observations provide new promising perspectives that advance knowledge. Here, we review data on the initiation processes of large earthquakes and show that they are multiscale and diverse, involving localization of deformation, fault heterogeneities and variable local loading rate effects. Analyses of seismic and geodetic data reveal evidence for regional weakening by earthquake-induced rock damage and progressive localization of deformation around the eventual rupture zones a few years before some large earthquakes. The final phase of deformation localization includes, depending on conditions, a mixture of slow slip transients and foreshocks at multiple spatial and temporal scales. The evolution of slip on large, localized faults shows a step-like increase that might reflect stress loading by previous failures, which can produce larger dynamic slip, in contrast to the smooth acceleration expected for a growing aseismic nucleation phase. We propose an integrated model to explain the diversity of large earthquake generation from progressive volumetric deformation to localized slip, which motivates future near-fault seismic and geodetic studies with dense sensor networks and improved analysis techniques that can resolve multiscale processes. The processes leading to large earthquakes remain enigmatic. Using detailed seismic and geodetic data, this Review examines how tectonic deformation and evolving fault behaviour initiate large earthquakes, and proposes an integrated model accounting for the diversity of observations.