On the stability of carbonate-bearing faults at the brittle-to-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, both brittle behavior (i.e. deformation is localized on the fractures and on the faults) and ductile one (i.e. deformation is distributed and accommodated in the rock core) coexist. 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 (Strain, TEmperature, Pressure, Speed) biaxial apparatus installed at EPFL, investigating sliding velocities from 10^-6 m/s to 10^-2 m/s. Experiments are conducted under different values of confining pressure (P_c 15 MPa and P_c 50 MPa) and normal stress (σ_n 29 MPa and σ_n 95 MPa) on the experimental faults, keeping the ratio between them constant (around 2). The local strain field along the fault was measured with strain gauges. The collected data were modeled with rate-and-state friction laws (RSFLs) to define the rate and state parameters relate to the critical condition for fault stability. Moreover, microstructural observations of the post mortem sample were conducted at the SEM, to investigate the deformation mechanisms active during the experiments.

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 deformation with increasing depth.