On the still unpredictable but recurrent lahars: the November 26, 2022 case study at Ischia island (Italy)

Lahars, landslides and debris flows are rapid natural phenomena that can heavily impact on and modify the environment, not only that from which they are triggered but also the one in which they propagate or leave deposits. In particular, lahars can reach significant runout distances from source areas (e.g., several km) and this can mainly depend, among other factors, on the morphology experienced by such propagation. There are cases in the recent history of natural occurrences in which lahars impacted catastrophically on rural and urban settings, such as for example at Nevado del Ruiz volcano (Colombia) in 1985 causing the death of thousands of people living around there. A more recent event occurred on November 26, 2022 at Ischia island (Italy), which is an active volcano particularly subjected to the recurrence of these phenomena. In this case, the emplacement of some lahars caused the death of a few tens of people and the damaging of tens of building, besides the direct impact on local agriculture and tourism. In the nearby Neapolitan volcanic area, several other lahar events occurred in the historical past, not only during but also after or well after explosive eruptions, as the evidence that these phenomena are still to be considered as complex and often unpredictable extreme natural events, also exacerbated by the climate changes, but also that they have some recurrence that cannot be neglected. Such kind of recurrence is mainly related to the local weather, which can even affect the intrinsic behavior of the flows that detach from the source areas and invade the territory. On the other hand, this is not a strictly statistical issue, as there are instrumental measurements that support the fact that heavy rains can exacerbate a landscape already prone to sliding, avalanching, and other catastrophic phenomena. For this, the November 26, 2022 Ischia case study was chosen with the goal of reconstructing the physical features that led to the lahar generation and invasion, which is something that might occur in the future but that should be experienced with a dedicated scientific and territorial consciousness. What was done is an integration of multidisciplinary approaches, corroborated by data from the INGV-OV monitoring network installed on the volcano, capable of detecting the otherwise lost flow timing and dynamical behavior. In particular, the seismic evidence that accompanied the Ischia lahar events, along with the consideration of some lithological features leading to an estimation of flow velocity and dynamic pressure, allow to discriminate multiple lahar pulses over the early morning of November 26, 2022. The main findings of this contribution are that the potential of the Ischia lahars had a sort of recharge timespan which depended on the local weather and lithological features, while the threshold of the lahar trigger depended on the hydrogeological conditions. The seismic reconstruction of the entire event allowed to quantify the first of these two critical issues at Ischia island.