Towards a spatial database for estimating the ash-fall pyroclastic deposit (APD) thickness in southern Italy: A tool for multi-hazard assessment in the landscapes impacted by active volcanoes

Explosive volcanoes can generate huge pyroclastic density currents (PDCs) and propel considerable quantities of ash into the atmosphere. The PDCs rapidly propagate along the ground and the ash plume is subject to wind-driven dispersion and may deposit over thousands of square kilometers, depending on the magnitude of the eruption. In south Italy, the observed PDCs and ash-fall pyroclastic deposits (APDs) in the stratigraphic records demonstrate the history of tephra-producing eruptive events, being important for assessing volcanic hazards. APDs also play a critical role in geo-hydrological hazards (e.g., landslide and erosion) because their geotechnical and hydraulic properties are usually different from the underlying bedrock (mainly carbonate rocks, flysch sequences, tuff and lava in southern Italy). For estimating the thickness of APDs and PDCs, the interaction of the ash deposited during eruptions (APD0) with denudational processes needs to be considered. The main aim of our work is to conduct a literature review to collate a complete database of APD0 thickness (i.e., the areal isopachs proposed by researchers for showing the ash dispersal pattern during a given eruption) associated with the major eruptive events in Campania. Then, the spatial distribution of cumulative APD0 thickness is computed in southern Italy, mainly comprising Campania, Molise, Basilicata, Puglia, and Calabria regions. After preparing a list of major tephra-producing eruptions of the Phlegrean Fields, Somma-Vesuvius and Roccamonfina volcanoes, the corresponding syn-eruptive ash dispersal patterns are retrieved from previous publications, digitized in ArcMap 10.8.2 and the cumulative APD0 is calculated. It is evident that the volcanic ash released into the atmosphere during eruptions is generally transported towards the east (except for the southward dispersion of the ash in a couple of Somma-Vesuvius eruptions) and mostly deposited in Campania region (cumulative APD0 thickness > 10 cm), being consistent with the distance from volcanoes. The 40-ka Campanian Ignimbrite and 26-ka Masseria del Monte large-magnitude eruptions are mostly responsible for APD0 outside Campania region. The spatial cumulative APD0 thickness is under 70 cm in south Italy, except for Campania and north-west Puglia. The results show approximately 2.5 m cumulative APD0 thickness at Somma-Vesuvius volcano which declines to about 0.95 m in Phlegrean Fields area. The central and northern sectors of Campania are characterized by cumulative APD0 thickness greater than 0.3-0.4 m. The present dataset of major tephra-producing eruptions facilitates developing techniques for APD thickness spatial modeling, depending on the available field measurements, to have an idea of erosion and landslide processes during heavy rainfall events and the related hazards.