Ground instability in the old town of Agrigento (Italy) depicted by on-site investigations and Persistent Scatterers data

Abstract. We combine on-site investigations with the interpretation of satellite Persistent Scatterers (PS) to analyse ground instability in the historic town of Agrigento, Italy. Geological and geomorphologic surveys, together with geostructural and kinematic analyses, depict the deformational patterns of the northwestern sector of the town, previously documented by extensive literature available for the neighbouring Valley of the Temples. The geological and geomorphologic maps are reconstructed by combining bibliographic studies, field surveys and aerial stereo-interpretation. ERS-1/2 PS data reveal deformation velocities up to 18–20 mm yr⁻¹ in 1992–2000 over the Addolorata landslide, and a sudden motion of 1.6 cm over the Bishop's Seminary in 1999. RADARSAT-1 PS data highlight velocities of 3.0 mm yr⁻¹ for St. Gerlando's Cathedral and reveals worsening of its structural instability since 2006. Ground instability of the town is controlled by low quality and high fracturing of the Agrigento formation rock masses, and the remarkable contrast between different mechanical behaviours of its calcarenite (brittle), silt and clay (plastic) facies. Slow landslides and widespread erosion are also recognised in the clays of the underlying Monte Narbone formation. Coexistence of these factors induces progressive retrogression of the edge of the Girgenti hill and damages the overlying historic buildings, whose stability and safe accessibility are nowadays almost compromised.