Rockfall hazard assessment at Baia Castle cliff (Phlegrean Fields, Italy): implications for cultural heritage sites in a seismically active volcanic area

Coastal cliffs developed in volcanic tuffs are highly susceptible to instability due to the combined effects of structural discontinuities, marine erosion, and seismic activity. This study presents a multi-method assessment of rockfall hazard at the Baia Castle cliff (Phlegrean Fields, southern Italy), an area affected by ongoing bradyseism and with a significant cultural heritage exposure. UAV-based photogrammetry, field structural surveys, and geomechanical characterisation were integrated to reconstruct the geometry and discontinuity network of the cliff. Kinematic and pseudo-static stability analyses were performed to identify potential failure mechanisms, while multi-temporal digital terrain models (2022–2025) allowed quantification of recent cliff retreat. Results indicate that instability is primarily controlled by steeply dipping fracture sets, favouring wedge sliding and toppling mechanisms. The comparison of digital terrain models reveals a maximum retreat of 5.6 m and a mobilised volume of approximately 280 m3 associated with recent rockfall events. The analysis of seismic records indicates that the March 13, 2025, rockfall occurred under relatively low peak ground acceleration (~0.015 g), suggesting that failure was not directly triggered by a single event but rather resulted from cumulative damage induced by repeated low-magnitude seismicity, in addition to several predisposing factors. These findings highlight the critical role of progressive seismic weakening in volcanic coastal cliffs and provide a transferable framework for hazard assessment in similar geological settings.