Proceedings of the 12th International INQUA meeting on paleoseismology, active tectonic and archaeoseismology

218 PATA Days 2024 M E T H O D O L O G Y We selected eight specimens of fault gouges, mainly hosted in igneous protoliths, from different fault zones of the AFS. We chose five fault gouges from faults with evidence of Quaternary reactivations, two without such evidence, and one from an exhumed fault zone for comparison. X-ray diffraction analysis was conducted to characterize the bulk mineralogy and clay composition of fault zones. Frictional properties were investigated at subseismic velocities (1 - 300 µm/s), and at seismic velocities (up to 1 m/s) under a wide range of normal stress between 10 and 125 MPa. We used two complementary machines at the INGV HP-HT laboratory: i) A biaxial apparatus in a double- direct shear configuration (Collettini et al., 2014), to evaluate the frictional strength by low-velocity frictional tests at sliding velocity of 10 µm/s. Here we also assessed the fault frictional stability at sliding velocities between 1-300 µm/s by velocity dependence of friction experiments (Ruina, 1983). ii) A rotary shear apparatus (Di Toro et al., 2010) was used to conduct a novel experiment in which we induced fault instability by reducing normal stress achieving sliding velocities up to 0.5 m/s. We designed this experiment to evaluate the slip and friction evolution during a fault reactivation process triggered by the post-seismic phase of subduction earthquakes. Finally, to explore the dynamic weakening of friction during seismic reactivations, we perform high-velocity frictional tests (1 m/s). We perform a microstructural analysis on the recovered specimens after frictional tests to explore the deformation mechanisms. This analysis was based on Backscattered Electrons images obtained using the JSM-6500F Scanning Electron Microscope located at INGV. R E S U LT S & D I S C U S S I O N Mineralogical analysis reveals that fault gouges are notably enriched in phyllosilicates and clays (i.e. chlorite, illite, and illite/ smectite mixed-layers) compared to the protoliths. In smaller proportions, quartz, feldspars, amphiboles inherited from the protoliths, and salts, sulfates, and carbonates originating from supergene processes. The frictional properties show a strong dependence on phyllosilicate content. Low-velocity friction tests reveal a low effective friction coefficient (average of 0.28 ± 0.10; Fig. 2A) compared to Byerlee's law (0.6 < µ <0.85), attributed to the enrichment of weak minerals as phyllosilicates on the sliding surface due to fault- related fluid flow processes. The velocity dependence of friction experiment revealed that all fault gouges exhibited velocity-strengthening behavior (0.001 < (a- b) stability parameter < 0.008), indicating stable aseismic creep deformation at low velocity (Fig. 2B). Our measurements indicate that higher chlorite content leads to a notable increase in the critical slip distance, thereby enhancing frictional stability and lowering the probability of seismic slip events. In the fault reactivation experiment, we assessed how friction and slip evolve during the unclamping of upper plate faults triggered by coseismic and post- seismic relaxation of subduction earthquakes. We simulate this process to induce fault instability