Assessing minimum pyroclastic density current mass to impact critical infrastructures: example from Aso Caldera (Japan)

Abstract

Abstract. We describe a method for calculating the probability that a distal geographic location is impacted by a pyroclastic density current (PDC) of a given size, considering the key related uncertainties. Specifically, we evaluate the minimum volume and mass of a PDC generated at the Aso caldera (Japan) that might affect each of three distal infrastructure (target) sites, with model input parameter uncertainties derived from expert judgement. The three target sites are all located 130–145 km from the caldera, but in well-separated directions and thus, for each, we test the different topographic shielding effects. To inform our probabilistic analysis, we apply alternative kinetic energy assessment approaches, i.e. rock avalanche and density current dynamics. In the latter formulation, the minimum mass needed to reach the targets ranges between median values ~283×1012 kg and ~465×1012 kg (M7.5–7.7), depending on the site. Rock avalanche dynamics modelling indicates ~3-times greater mass would be required to reach the target sites with 50 % probability, while the hypothetical scenario of a relatively dilute distal ash-cloud would require ~3-times less mass. We compare our results with the two largest recorded Aso eruptions, showing that a catastrophic eruption, similar to Aso-4, ≈M8, would present a high conditional probability of PDCs reaching the target sites, i.e. 32 %–96 %, in the density current formulation and contingent on uncertainty in the erupted mass and on target site direction. This said, for Aso the current occurrence probability of such a colossal initiating eruption has been estimated <10-8 in the next 100 years.