Deposit-derived pyroclastic density currents at Stromboli: from the 1930 event reconstruction to probabilistic hazard assessment

Stromboli volcano, Italy, is characterized by persistent explosive activity occasionally punctuated by more energetic explosions, called paroxysms, during which deposit-derived pyroclastic density currents (PDCs) may be generated by the gravitational instability of hot, unstable pyroclastic deposits. Although typically confined within the Sciara del Fuoco, a prominent depression on the volcano’s NW flank, historical events such as the 1930 and 1944 paroxysms demonstrate that these flows can propagate beyond this depression, posing a significant hazard to inhabited areas and climbers.

This study combines the reconstruction of a well-documented historical event with a probabilistic hazard assessment to evaluate the potential impact of deposit-derived PDCs over the entire island. The September 11, 1930 paroxysm is reanalyzed by integrating new field observations, historical records, and numerical modeling, providing a test case for model calibration and a first probabilistic reconstruction of the phenomenon. Recent erosive floods exposed previously unrecognized PDC deposits in the San Bartolo valley, complementing those identified in the Vallonazzo basin. These new data, together with eyewitness accounts, were used to constrain maximum flow thicknesses along the valleys. A shallow-water dense granular flow model coupled with an inversion algorithm indicates that the PDC propagated mainly within these valleys, with limited secondary flows in adjacent basins. Consistently with field evidence, the Vallonazzo flow reached the sea, whereas the San Bartolo flow stopped near the local church, with an estimated total remobilized volume between 34,000 and 59,000 m³. Results also highlight the strong dependence of invaded areas on the location of the source material.

Building on this calibration, a new probabilistic framework based on random circular sector source models is applied to assess PDC hazard at the scale of the island. Six main drainage basins with significant hazard potential were identified. Among these, San Bartolo, Scalo dei Balordi, and Ginostra “A” show the highest conditional invasion probabilities, while other inhabited valleys exhibit lower but still non-negligible values. By coupling spatial invasion probabilities with a temporal occurrence model linking paroxysm frequency to PDC generation, we estimate a substantial probability of future PDC invasion outside the Sciara del Fuoco over decadal to multi-decadal timescales, despite the large uncertainties associated with the limited historical record.