Active tectonics of Central Andes from GNSS and InSAR time series

The Central Andes are a place of considerable interest for the study of the physical processes involved in subduction. Indeed, it hosts significant seismic events relatively frequently, with four earthquakes of magnitude greater than eight in the last three decades: the 1995 Mw8.0 Antofagasta, the 2001 Mw8.4 Arequipa, the 2007 Mw8.0 Pisco, and the 2014 Mw8.1 Iquique earthquakes. In addition, structural heterogeneities, such as the Nazca and Perdida oceanic ridges, appear to segment seismic ruptures along the Peru-Chile trench. Estimating the seismogenic potential of the Central Andes, particularly in Southern Peru, where the amount of geodetic data available has increased considerably in recent years, is therefore a key issue.

Using 200+ GNSS sites, and InSAR mean velocity maps (2015-2021) processed in the framework of the FLATSIM Andes project (FormaTerre, 2020), we measured the deformation of the overriding plate on the horizontal and vertical components. In order to model interseismic and postseismic processes with a realistic structure and rheology, we developed a finite element method model of the subduction, featuring Newtonian viscoelastic Burgers rheology in the asthenosphere and an elastic cold nose. Accounting for the postseismic displacements associated with great subduction earthquakes, we propose a viscoelastic interseismic coupling model with unprecedented resolution in the area. This model shows significant heterogeneity, with high coupling off the coast of South Peru and Chile, and weaker coupling where oceanic structures, notably the Nazca Ridge and the Nazca Fracture Zone, subduct beneath the South American continent.

The spatial resolution provided by InSAR, notably on the vertical component, is of great interest to investigate the partitioning of the deformation in the upper plate, which is a fundamental aspect in the perspective of a unified interseismic coupling model at the scale of Peru and Chile. For this purpose, we quantified the East and vertical displacements across the Cuzco fault system (up to 3 mm/yr and 2 mm/yr on the East and vertical components respectively) and the Cordillera Blanca (up to 1.5 mm/yr and 3 mm/yr on the East and vertical components respectively), which have been proposed by Villegas-Lanza et al., 2016 to delimitate a rigid block motion referred as the Peruvian Sliver. In addition to this partitioning at crustal structures, primary (3-4 mm/yr) and secondary (2 mm/yr) zones of uplift are observed in Peru and Chile, at about 130 and 250 km from the trench, respectively. The secondary zone of uplift is associated with high topography, suggesting partial interseismic plastic deformation of the upper plate. Also, the secondary uplift zone in Peru is primarily observed in the flat-slab region and tapers with the transition to dipping-slab. Both the primary and secondary uplift zones are collocated with trench-parallel stripes of intraslab seismicity, which could be linked to fluid migration processes or fracturing of the slab.