Quaternary sea-level change and slope instability in coastal areas: Insights from the Vasto Landslide (Adriatic coast, central Italy)

Abstract

This paper focuses on the possible role of sea-level change in triggering and controlling large coastal slope instabilities, and focuses on the representative case study of the Vasto Landslide. The town of Vasto is 143ma.s.l. and sits atop an uplifted Quaternary regressive sequence widely outcropping along the Adriatic coast of central Italy. The coastal slope is affected by large slope instabilities (e.g. “Vasto Landslide”) including evidence of present activity. Well-documented historical disruptive events affected the town and the coastal slope in 1816, 1942 and 1956. Field evidence suggests that sea cliff retreat must have removed considerable volumes of rock before the activation of the large slope failures. Thus, a geological-evolutionary model of the landslide is proposed here that considers the landforms, geological evidence and borehole stratigraphy, as well as the combined effect of Quaternary uplift and eustatic oscillations on the coastal slope. Significant evolutionary steps were identified, and a slope stability analysis was performed using a stress–strain numerical modeling solved by a Finite Difference Method (FDM) to analyze the following: 1) the landslide mechanism, 2) the type of activity, and 3) the cumulative deformations that occurred during the morpho-evolutionary steps. Numerical modeling was calibrated by considering the present landforms as well as the effects recorded during historical events. The results confirm that the Vasto Landslide was activated in the Middle Pleistocene (215ka BP) as a consequence of wave-cut erosion and progressive uplift of the coastal slope. Moreover, the landslide evolved as a retrogressive, single-styled landslide comprising two major blocks. Using this framework, the historical events are interpreted as local re-activations, due to meteorological factors, of the ancient rupture surfaces affecting the entire slope.