Crustal Deformation Associated with the Seismic Cycle in the Central Andes from InSAR and GNSS Geodetic Time Series

Abstract

The Central Andes subduction has been the theater of numerous large earthquakes since the beginning of the 21th Century, notably the 2001 Mw8.4 Arequipa, 2007 Mw8.0 Pisco, and 2014 Mw8.1 Iquique earthquakes. A better knowledge of the interplate coupling distribution and seismic cycle in this area is thereby fundamental for improving our understanding of large earthquakes segmentation, and ultimately improving our knowledge of the seismic potential in the area. Interseismic models from inversions of 80 GNSS velocities in Central and South Peru (12–19°S) on a 3-D slab geometry indicate that the locking level is relatively high and concentrated between 20 and 40-km depth. Locking distributions indicate a high spatial variability of the coupling along the trench, with the presence of many locked patches that spatially correlate with the seismotectonic segmentation. Our study confirms the presence of a creeping segment where the Nazca Ridge is subducting, we also observe a lighter apparent decrease of coupling related to the Nazca Fracture Zone (NFZ). However, since the Nazca Ridge appears to behave as a strong barrier, the NFZ is less efficient to arrest seismic rupture propagation. Considering various uncertainty factors, we discuss the implication of our coupling estimates with size and timing of large megathrust earthquakes considering both deterministic and probabilistic approaches. We estimate that the South Peru segment, from the Nazca Ridge to the Arica bend, could have a Mw=8.4-9.0 earthquake potential depending principally on the considered seismic catalog and the seismic/aseismic slip ratio (Lovery et al., 2024). We use large-scale InSAR Sentinel-1 time series, processed in the frame of the FLATSIM-Andes project (Thollard et al., 2021), encompassing the Central Andes (7–26°S) on the 2015-2021 period. These InSAR data provide a useful complementarity to the GNSS data, with a higher spatial resolution in exchange for a lower temporal resolution. Subsequently, it allows to better define the contours of the asperities, or the maximum locking depth. We modelled the effects of non-tectonic processes such as solid earth tides (SET), ocean tide loading (OTL), and ionospheric electronic content (TEC) on the ramps in range and azimuth, in order to measure ground deformation in a stable reference frame, with sufficient accuracy for large-scale tectonic applications, allowing vertical and horizontal decomposition. In order to perform joint inversions of GNSS and InSAR interseismic velocities, we also develop finite element models of the subduction zone with more complex viscoelastic rheology (Maxwell and Burger laws). Viscoelastic models are expected to produce a broader displacement, with horizontal displacements extending further inland. Higher magnitudes of deformation in the late-stage of the interseismic period and a shallower optimal locking depth have also been reported for viscoelastic models. These features are key factors to make the link between short-term and long-term deformation, and to discriminate the slip on the slab interface from internal deformation. We investigate the viscoelastic effects associated with the great 2001 Mw8.4 Arequipa earthquake, in order to assess its impact on the interseismic loading estimate on the subduction megathrust.