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

Fig. 1: a) Main traces of the AFS (black lines). b) Study area showing the five study sites (red dots). c) Schematic profile of the Atacama Fault System. MF = Mejillones Fault, CHF = Caleta Herradura Fault, NG = Naguayan Fault, CCF = Caleta Coloso Fault, SCF = Salar del Carmen Fault, BF = Bolfin Fault, PF = Paposo Fault. with extensional focal mechanisms at shallow depths (<20 km), occurring even in dormant faults or unmapped faults, and in some cases with surface ruptures (Aron et al., 2014; Ozawa et al., 2012). These observations compound the cascading risks of subduction earthquakes and challenge us to understand the link between the seismic cycle of subduction earthquakes and the remote triggering of upper plate faults. This study aims to understand the frictional properties and deformation mechanisms that lead to shallow reactivation of upper-plate faults in subduction zones. We designed a suite of laboratory experiments to explore the frictional properties of natural fault gouges from the Atacama Fault System (AFS) located above the seismogenic subduction zone of northern Chile (Fig. 1). We conducted a microstructural analysis on the recovered specimens to investigate the deformation mechanisms. We selected segments of the AFS (23°S-25°S) because of the evidence of high-angle normal faults reactivations with surface ruptures, clearly exposed by metric fault scarps. According to paleoseismological studies, these reactivations have generated Mw ~7 – paleo-earthquakes during the Pleistocene-Holocene (Cortés et al., 2012). In addition to this long-term seismic behavior, fifteen years of continuous seismic monitoring indicate extremely low microseismic activity at 23°S (Sippl et al., 2023). These characteristics resemble the shallow reactivation of upper-plate faults during the post-seismic phase of subduction earthquakes.