Fault Roughness Promotes Earthquake-Like Aftershock Clustering in the Lab

Earthquakes rarely occur in isolation but rather as complex sequences of fore, main and aftershocks. Assessing the associated seismic hazard requires a holistic view of event interactions. We conduct frictional sliding experiments on faulted Westerly Granite samples at mid-crustal stresses to investigate fault damage and roughness effects on aftershock generation. Abrupt laboratory fault slip is followed by periods of extended stress relaxation and aftershocks. Large roughness promotes less co-seismic slip and high aftershock activity whereas smooth faults promote high co-seismic slip with few aftershocks. Conditions close to slip instability generate lab-quake sequences that exhibit similar statistical distributions to natural earthquakes. Aftershock productivity in the lab is linearly related to the residual strain energy on the fault which, in turn, is controlled by the level of surface heterogeneity. We conclude that roughness and damage govern slip stability and seismic energy partitioning between fore, main and aftershocks in lab and nature.