Friction during earthquakes: 25 years of experimental studies

Dynamic fault strength τ (rock friction in the broad sense) and its evolution with seismic slip and slip rate are among the most relevant parameters in earthquake mechanics. Given the large slip rate (1 m s⁻¹ on average), displacement (up to tens of meters), effective stress (tens of MPa), typical of seismic faulting at depth, thermo-mechanical effects become outstanding: dynamic fault strength is severely affected by fluid and rock phase changes, extreme grain size reduction, and the production of amorphous and unstable materials in the slipping zone. Here, first we will summarize the most relevant findings about dynamic fault strength during seismic slip mainly obtained thanks to the exploitation of dedicated experimental machines (i.e., rotary shear apparatus). However, the interpretation of this experimental dataset remains debated because of technical limitations which impede us to measure fundamental parameters such as temperature, strain rate, pore fluid pressure and grain size in the slipping zone. Without a sound estimate of these physical parameters, any constitutive law proposed to describe the evolution of dynamic fault strength during simulated seismic slip remains speculative. Then, we will discuss the results of some recent experiments which exploit new technical approaches to overcome the main limitations of the previous studies. The experimental approach, together with field studies of the geometry and architecture of exhumed faults and modelling, remains our most powerful tool to investigate seismic-related deformation mechanisms in both natural and human-induced earthquakes.