Tracking the November 26, 2022, Casamicciola debris flow through seismic signals (Ischia, southern Italy)

Abstract The movement of large masses on the Earth's surface, including landslides or debris flows, transfers energy to the ground, inducing both permanent and transient deformation. This generates measurable motion and seismic radiation that can be detected by proximal seismic networks. On November 26, 2022, during heavy rainfall, several debris flows inundated the northern slope of the active volcanic island of Ischia in southern Italy, causing fatalities and extensive damage. The island of Ischia is one of the most landslide-prone localities in Italy. It has already suffered comparable, even larger, events over the past two centuries, and with a population of around 60,000 and more than 300,000 tourists per year, it is considered a high natural risk zone. In this study, we performed seismological analyses over an extended range of frequencies and scales, using broadband data recorded by the local permanent seismic network, to study the dynamics of the November 26, 2022, event and to quantitatively characterise its evolution over time. Unlike post-event static surveys, a major advantage of the seismological approach is its ability to track the progression of the debris flow motion from the instant of the initial detachment. Our analyses allowed us to estimate the spatial and temporal origin of the rock detachments, the mass of the flowing material, the size of the coarse boulders, the speed at which the landslide approached the nearest seismic station, and the impacting overpressure that caused the most severe damage. These results further demonstrate that specifically designed instrumental (seismic and tiltmetric) networks represent an essential tool for real-time monitoring and for activating early warning systems prior to potential damage to inhabited localities.