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

222 PATA Days 2024 C O N C L U S I O N This work contributes to understanding the complex dynamics between subduction earthquakes and the processes that precede earthquake propagationonupper plate faults using frictional experiments on gouges from the AFS as a proxy. Our analysis indicates a clear influence of mineralogy and the frictional properties of the fault gouges. The low frictional strength (0.15 < µ <0.41) is attributed to the enrichment of phyllosilicates. This implies that the faults easily initiate ductile deformation under viscous- frictional flow, evidenced by the microstructural analysis, attenuating the stress build-up. The velocity- strengthening velocity-neutral behavior (0.001 < (a−b) < 0.008) promotes mainly stable-sliding conditions. These frictional properties contribute to maintaining these faults aseismic during the interseismic phase and restricting but not excluding seismic reactivations. The low-to-fast transition in sliding velocity under normal stress reduction indicates that the reactivation process of upper-plate faults begins with stable sliding and that high normal stress reduction by post-seismic relaxation promotes unstable sliding. This is reflected in the microstructures, which show distributed (ductile) deformation superimposed by localized (brittle) deformation. Granular mineral- rich gouges and chlorite-rich gouges were more prone R E F E R E N C E S Aron, F., Cembrano, J., Astudillo, F., Allmendinger, R. W., & Arancibia, G. (2014). Constructing forearc architecture over megathrust seismic cycles: Geological snapshots from the Maule earthquake region, Chile. Bulletin of the Geological Society of America, 127(3–4), 464–479. https://doi.org/10.1130/B31125.1 Collettini, C., Di Stefano, G., Carpenter, B., Scarlato, P., Tesei, T., Mollo, S., Trippetta, F., Marone, C., Romeo, G., & Chiaraluce, L. (2014). A novel and versatile apparatus for brittle rock deformation. International Journal of Rock Mechanics and Mining Sciences, 66, 114–123. https://doi. org/10.1016/j.ijrmms.2013.12.005 Cortés, J., González L., G., Binnie, S. A., Robinson, R., Freeman, to reach unstable sliding than the illite+muscovite-rich gouges. Finally, high-velocity frictional tests showed an abrupt drop in friction during the seismic pulse at 1 m/s, reaching low residual friction coefficient values (0.12 < µ r < 0.21), indicating significant dynamic fault weakening during earthquake propagation, possibly triggered by thermal deformation mechanisms such as fluid pressurization or mechanical pressurization. Due to the low geothermal gradient of the Chilean forearc (23°S), the seismo-mechanical behavior could extend to depths near the base of the upper plate. Considering the complex mineralogy, we propose a model of stable, conditionally stable, and unstable patches, controlled by the competition in the amount of phyllosilicates such as illite-muscovite and chlorite and granular minerals such as feldspars, quartz, and amphiboles. A C K N O W L E D G E M E N T S This work was supported by the FONDECYT project 1200170. We extend our gratitude to the X-ray laboratory of the GFZ, Potsdam, Germany for their assistance with XRD analysis, and to the working group of the HPHT laboratory at the INGV, Rome, Italy, for providing access to the frictional experiments. S. P. H. T., & Vargas E., G. (2012). Paleoseismology of the mejillones fault, northern Chile: Insights from cosmogenic 10Be and optically stimulated luminescence determinations. Tectonics, 31(2), 1–21. https: //doi. org/10.1029/2011TC002877 Di Toro, G., Niemeijer, A., Tripoli, A., Nielsen, S., Di Felice, F., Scarlato, P., Spada, G., Alessandroni, R., Romeo, G., Di Stefano, G., Smith, S., Spagnuolo, E., & Mariano, S. (2010). From field geology to earthquake simulation: A new state- of-The-art tool to investigate rock friction during the seismic cycle (SHIVA). Rendiconti Lincei, 21(SUPPL. 1), 95–114. https: //doi.org/10.1007/ s12210-010-0097-x Ozawa, S., Nishimura, T., Munekane, H., Suito, H., Kobayashi, T., Tobita, M., & Imakiire, T. (2012). Preceding, coseismic, and postseismic slips of the 2011 Tohoku earthquake, Japan. Journal of Geophysical Research: Solid Earth, 117(7). https: //doi. org/10.1029/2011JB009120 Ruina, A. (1983). Slip instability and state variable friction laws. Journal of Geophysical Research, 88(B12), 10359–10370. https: //doi. org/10.1029/JB088iB12p10359 Sippl, C., Schurr, B., Münchmeyer, J., Barrientos, S., & Oncken, O. (2023). The Northern Chile forearc constrained by https: //doi.org/10.1016/j . jsames.2023.104326