Three-plate Dynamics of the Andean Earthquake Cycle

The South-Central Andes topography results from a three-plate framework, where the Andean block is compressed between the Nazca plate to the west and the South American craton to the east. Interseismic GNSS observations consistently show that basal décollements beneath the eastern fold-and-thrust belts accommodate permanent shortening through aseismic thrust creep. However, their behavior during great megathrust earthquakes has remained poorly understood.

We combine constraints from the 2010 Mw 8.8 Maule earthquake with previous evidence from the 2015 Mw 8.3 Illapel and the 1995 Mw 8.0 Antofagasta earthquakes and demonstrate that basal décollements systematically creep in a normal sense during the coseismic phase. This backsliding occurs as a mechanical response to abrupt stress changes from megathrust rupture: the direction of décollement slip during earthquakes is opposite to their interseismic motion.

By integrating these coseismic observations with independent three-plate interseismic models, we present a unified framework for Andean orogenic-wedge dynamics that reconciles forearc-to-backarc deformation. This framework provides the first comprehensive explanation for the long-observed obliqueness deficiency in Andean megathrust slip distributions. Our results demonstrate that three-plate models are essential for accurately capturing both long-term orogenesis and the complete earthquake cycle, representing a paradigm shift from conventional two-plate approaches with broad implications for other subduction boundary zones and seismic hazard assessment worldwide.