Active spreading at Maltese heritage sites: unravelling instability mechanisms via integrated InSAR, photogrammetry, and geotechnical analysis

This study presents the first integrated, site-scale assessment of active spreading processes threatening Malta's coastal heritage, establishing a novel framework for monitoring slow-moving hazards. We combine Multitemporal Interferometric Synthetic Aperture Radar (MT-InSAR) using a five-year archive (May 2011–August 2015) of COSMO-SkyMed satellite imagery provided by the Italian Space Agency, with Structure-from-Motion (SfM) photogrammetry and geotechnical investigations. A key methodological contribution of this work is demonstrating the reliability of combining asynchronous datasets: despite non-overlapping temporal coverage, the convergence of satellite and ground-based data validates the active spreading hypothesis. Our results quantify previously undocumented rates and spatial patterns of displacement, with Madonna tal-Aħrax experiencing localised cliff retreat of 3–10 mm/year, while Kappella tal Lunzjata exhibits significantly higher velocities of 7.6–17.6 mm/year and cumulative displacements reaching 72 mm. This work provides new mechanistic insight into spreading processes in the Maltese context, demonstrating how permeability contrasts between the Upper Coralline Limestone and the underlying Blue Clay Formation promote water accumulation, clay swelling, and progressive strength degradation. This feedback-driven mechanism establishes a self-sustaining cycle of plateau instability that is difficult to arrest. Beyond site-specific findings, this work establishes a transferable monitoring and interpretation framework for heritage sites threatened by slow, hard-to-detect deformation in layered coastal settings. By demonstrating how MT-InSAR can be operationally combined with targeted 3D reconstruction and geotechnical evidence, the study could represent an example for early-warning assessment, and long-term risk mitigation in geomorphologically complex environments. • Integrated remote sensing and geotechnics reveal ground failure at heritage sites. • First COSMO-SkyMed InSAR analysis of ground displacement in Malta. • Limestone-clay permeability contrast drives water accumulating at layer interfaces. • Clay swelling induces surface slippage across Maltese landscapes. • Coastal spreading in Malta triggers a self-reinforcing cycle of plateau degradation.