Neotectonic faults in the Southern Chile intra-arc (38°S–40.5°S): Insights about their seismic potential and the link with the megathrust earthquake cycle

Abstract

Megathrust earthquakes are a primary geological hazard in subduction zones. In oblique subduction margins, however, seismic threat must be also considered concerning crustal strike-slip faults driving the parallel-to-the-trench component of the convergent vector. These faults have proven their capacity to produce moderate-to-large shallow earthquakes, causing severe damage for surrounding areas. The Southern Chile Subduction Zone (SCSZ) is characterized by the oblique convergence between the Nazca and the South America plates. Between 37°S and 46°S, slip partitioning is significantly led by intra-arc faults like the Liquiñe-Ofqui Fault System (LOFS) and subsidiary NW to NE striking structures. Despite it has been demonstrated that the behavior of these faults is seismogenic, the inland evidence of Holocene deformation along them is scarce. This has concealed to accurately define individual active faults, discussing their seismic potential, and addressing their link with the megathrust earthquake cycle. In this contribution, we first present field evidence for Holocene deformation on intra-arc faults in the area. Based on this, we define 5 active faults and discuss their seismic capacity. To assess the link with the megathrust earthquake cycle, we calculate the Coulomb stress changes induced on these faults by the Mw 9.5 Valdivia Earthquake slip distribution and an interseismic period of 300 years. We conclude that 1) the identified neotectonic faults have the potential for producing moderate-to-large earthquakes (Mw 5.5 to 6.8) and 2) their occurrence would be enhanced by the coseismic and the interseismic stages of the subduction cycle. With this, we propose that the intra-arc faults addressed in this paper, and certainly others in the SCSZ, must be considered as significant sources of seismic hazard.