Luc Illien , Jens Martin Turowski, Christoph Sens-Schönfelder, Clément Berenfeld, Niels Hovius

Earthquakes introduce transient mechanical damage in the subsurface, which causes postseismic hazards and can take years to recover. This observation has been linked to relaxation, a phenomenon observed in a wide class of materials after straining perturbations, but its duration after earthquake ground shaking has not been constrained. Here, we analyse the effects of two successive large earthquakes and their aftershocks on ground properties using estimates of seismic velocity from ambient noise interferometry. We show that the relaxation time scale is a function of the state of the substrate at the time of seismic perturbation, rather than the intensity of ground shaking. Our study highlights the predictability of earthquake damage dynamics in the shallow subsurface and also in other materials. This finding may be reconciled with existing state variable frameworks by considering the superposition of different populations of damaged contacts.