Normal upper plate fault reactivation in northern Chile and the subduction earthquake cycle: From geological observations and static Coulomb Failure Stress (CFS) change

The northern Chile subduction zone constitutes a seismic gap where no great interplate earthquakes (Mw > 8.8) have nucleated since 1877. Active normal faulting occurs conspicuously in the Coastal Forearc at the southern end of this gap, between 22.7°S and 24°S. There, upper crustal extension is present right above a suggested subduction segment boundary of the Nazca-South America interplate contact. In this work, by using Coulomb Failure Stress Change (CFS) models, we explore the effects of coseismic and interseismic stages of the subduction earthquake cycle on the reactivation of pre-existing upper plate normal faults in this area. We attempt to unravel the first-order interplate conditions required to explain this extensional pattern, putting specific focus on the influence of the megathrust segmentation defined by the Mejillones Peninsula. It is suggested that the restricted occurrence of active normal faulting in the study area is tightly related to the structure of the interplate seismogenic zone. Positive CFS values are induced in the study area by the coseismic and interseismic stages of the subduction earthquake cycle, in a region characterized by lower coupling beneath the Mejillones Peninsula. We propose that fault reactivation in this zone is particularly favored during the coseismic stage by earthquakes occurring offshore of the Mejillones Peninsula and surrounding areas. In the adjacent subduction segments, coseismic and interseismic CFS values tend to counterbalance each other, resulting in no tensional conditions capable to cause normal fault reactivation. We suggest that the coupling variation in the seismogenic zone is long term controlled by the geological structure of the upper plate.