Laboratory Observations linking Fault Surface Characteristics to Preparatory Earthquake Processes and Fault Stability

Preparatory earthquake processes such as slow preparatory slip (preslip) are connected to variations in frictional strength linked to frictional instabilities and appear in various scales across the Earth’s crust. For dry and bare surfaces, the fault surface characteristics affect the contact conditions. These conditions are established through asperities, which are topographical heights where the normal stress concentrates, imposing variations in fault strength. The effect of surface conditions on preslip can be studied in the laboratory where fault surface characteristics can be identified. Developing a more refined understanding of features controlling preslip (e.g., roughness) will lead to more realistic models describing frictional stability. In this study, we performed a triaxial test at sequentially increasing confining pressure steps (P_c = 60, 80, 100 MPa) on a saw-cut sample of Carrara Marble in dry and unlubricated conditions. Two types of technologies were used to study this frictional response in space and time: (1) an array of acoustic emission sensors monitored localized precursory seismicity and (2) quasi-static deformation in the fault-parallel strain was monitored using distributed strain sensing (DSS) with fiber optics. The differential stress was also measured throughout and allowed us to study the onset of frictional weakening/strengthening. In the first confining pressure step (P_c = 60 MPa), a single stick-slip event was observed with an associated 43 MPa static stress drop. In the subsequent confining pressure steps of P_c = 80 and 100 MPa, even though the normal stress on the fault was increased, no stick-slip events were observed, and the fault smoothly transitioned to sliding with smaller magnitude stress drops of 3 and 4 MPa, respectively. That suggests that a change in the frictional nature of the interface was incurred during the first rupture at P_c = 60 MPa. The high-density DSS array displayed a significant heterogeneous distribution of fault-parallel strain in time and space and experienced sudden reorganization at various phases of the experiment. Due to the high spatial resolution, DSS allowed us to investigate local deviations from an elastic response attributed to inelastic processes. A larger amount of local strain accumulation was needed to produce a stick-slip instability. At higher normal stress on the pre-ruptured fault, this level of locking was not possible in the subsequent confining pressure steps. Dissipative inelastic deformation was attributed to local frictional weakening that resulted in non-uniform preslip. Furthermore, priori measurements of contact pressure heterogeneities were obtained using a pressure sensitive film. These results showed regions of lower normal stress along the fault that correlated with regions that incurred more anelastic response on the DSS array. Post-mortem contact pressure measurements showed clear changes in the normal stress distribution that correlated to visual damage and wear. We believe that this contributed to the fault's inability to lock as before and mitigate dynamic rupture. Our results provide more insight into potential mechanisms controlling preslip distribution leading to dynamic and quasi-static frictional weakening.