Ballistic hazard assessment: so far, so good?

Developments in volcanic hazard assessments for ballistic projectiles suffer from several limitations across a range of key concepts in volcanology. Firstly, they are often associated with short, impulsive events within long-lasting eruptive sequences originating from complex conduit heterogeneities. Fragmentation in such context typically results in unsteady, nonsustained plumes that are challenging to model. Secondly, our understanding of processes generating ballistic particles suffers from the lack of characterization of the total ballistic components of pyroclastic deposits. Most physical and hazard models are developed and calibrated upon a small fraction of the total ballistic population inferred either from direct field measurement or indirectly (e.g. the geometry of impact craters, remote sensing), which limits our empirical understanding of ejection conditions at the scale of a total deposit. Finally, although ballistic particles are unanimously recognized as a critical hazard in proximal areas, little debate has occurred regarding the purpose(s) of ballistic hazard assessment and few attempts explore alternatives approaches to concentric hazard zones. Here, we first start by presenting a new approach developed to probabilistically assess the hazard and physical impact of ballistics associated with Vulcanian explosions at La Fossa volcano (Vulcano, Italy). Limited field observations were inverted to define an eruption scenario, and various approaches to quantify probabilities of impacts are explored and discussed. The vulnerability of the built environment to roof perforation is assessed based on field studies and the existing literature to allow the first quantitative pre-event impact assessment for ballistics. Second, we take advantage of the ballistic field formed in Episode IV of the 1912 eruption of Novarupta (Alaska, USA), which is constrained with an unprecedented degree of detail, to discuss the shortcomings of current approaches to probabilistic hazard assessments for ballistic impacts and identify future research needs.