Bare surface friction from low to high velocities and from low to high normal stresses

The recent development of high-velocity rotary shear apparatuses has significantly improved our understanding of fault friction evolution during seismic slip. Thanks to these apparatuses, it has been shown that fault strength significantly decreases with increasing slip velocity, revealing various dynamic fault weakening mechanisms that are likely active during earthquakes, including mechanisms that were unknown before conducting these experiments. However, these rotary shear apparatuses are limited in the maximum normal stress that can be applied to the experimental fault. Specifically, with few exceptions, most of the high-velocity shear experiments have been conducted at normal stress σ _N < 20 MPa. Therefore, our understanding of dynamic fault weakening mechanisms at high normal stresses is still limited.

Taking advantage of a new biaxial apparatus installed at EPFL, the HighSTEPS (High Strain Temperature Pressure Speed) apparatus, capable of shearing fault at high velocity (up to 0.2 m/s) and high normal stress (up to 100 MPa) we overcome this limitation. Here, we present a systematic study of bare surface friction on standard lithology (gabbro and granite) mapping fault mechanical behavior at boundary conditions from intermediate to high velocities (0.001 - 0.2 m/s) and from low to high normal stresses (10 - 80 MPa). These conditions are partially overlapping with previous data in the literature, allowing us to assess the reproducibility of high velocity experiments under a different geometrical configuration (biaxial versus rotary). Moreover, unique boundary conditions, velocity of 0.2 m/s and normal stress σ_N > 50 MPa, provide new insights on the mechanics of earthquake propagation.