Evolution of the rate-and-state frictional parameters of carbonate bearing faults at the brittle-ductile transition

The majority of the seismic events in the Mediterranean region are hosted in carbonate-bearing rocks at depths representative of the semi-brittle regime. Within this regime, a complex interplay between brittle (localized) and ductile (distributed) deformation mechanisms co-exists. Despite its relevance for seismic hazard, the influence of this interplay on the nucleation and propagation of seismic events is poorly studied. Up to now, most experimental work has been conducted far from in-situ conditions, mostly at room temperature and low confining pressure. Here we constrain the frictional behavior of faults in carbonate rocks under conditions relevant for their brittle to ductile transition. Velocity-step experiments are performed through the HighSTEPS (High Strain Temperature Pressure Speed) biaxial apparatus installed at EPFL,investigating sliding velocities from 10 μm/s to 0.01 m/s. Experiments are conducted under different values of confining pressure (0 – 50.0 MPa) and normal stress (9.5 – 95.0 MPa),keeping constant the ratio between σ/P~ 2. Local strain field along the fault is measured with strain gauges. Experimental results are modelled with rate-and-state friction laws (RSFLs) todefi ne the rate and state parameters relate to the critical conditions for fault stability. Moreover,microstructural observations of the post mortem samples are conducted at the SEM, to investigate the deformation mechanisms active during the experiments. We show that with increasing the confining pressure, the rate-and-state parameter a and b decrease. Whereas the critical distance D and the value of (a-b) both increase with increasing the confining pressure and the sliding velocity. These results shed light on the evolution of rate-and-state frictional parameters with depth, as well as their dependence on the strain partitioning between on-fault slip and bulk-accommodated deformations (elastic and plastic) with increasing depth.