InSAR constraints on spatiotemporal changes in plate coupling beneath Shikoku along the Nankai subduction zone

Abstract Estimating spatiotemporal changes in plate coupling is important for understanding the nucleation processes of megathrust earthquakes. Using Global Navigation Satellite System (GNSS) and Interferometric Synthetic Aperture Radar (InSAR) observations, we obtained crustal deformation around Shikoku, southwest Japan, from 2014 to 2025. We estimated slip deficit distributions along the Nankai subduction zone using kinematic fault slip inversions based on GNSS, InSAR, and GNSS-InSAR joint observations. Both the GNSS- and InSAR-based slip deficit models showed that slip deficit rates beneath central Shikoku decreased by up to 41 mm/year after 2019 relative to 2014–2018. We interpret this as a weakening of plate coupling rather than a long-term slow slip event, because the estimated slip deficit rates were not significantly negative even after 2019. Our results demonstrate that InSAR-derived velocities, which are sensitive to vertical displacements, can independently constrain broad-scale changes in plate coupling beneath inland regions, although they do not drastically improve the spatial resolution of plate coupling estimates. These findings suggest that InSAR observations provide valuable information on plate coupling changes, particularly in subduction zones where dense GNSS observations are unavailable.