Probabilistic short‐term volcanic hazard in phases of unrest: A case study for tephra fallout

Abstract

During volcanic crises, volcanologists estimate the impact of possible imminent eruptions usually through deterministic modeling of the effects of one or a few preestablished scenarios. Despite such an approach may bring an important information to the decision makers, the sole use of deterministic scenarios does not allow scientists to properly take into consideration all uncertainties, and it cannot be used to assess quantitatively the risk because the latter unavoidably requires a probabilistic approach. We present a model based on the concept of Bayesian event tree (hereinafter named BET_VH_ST, standing for Bayesian event tree for short‐term volcanic hazard), for short‐term near‐real‐time probabilistic volcanic hazard analysis formulated for any potential hazardous phenomenon accompanying an eruption. The specific goal of BET_VH_ST is to produce a quantitative assessment of the probability of exceedance of any potential level of intensity for a given volcanic hazard due to eruptions within restricted time windows (hours to days) in any area surrounding the volcano, accounting for all natural and epistemic uncertainties. BET_VH_ST properly assesses the conditional probability at each level of the event tree accounting for any relevant information derived from the monitoring system, theoretical models, and the past history of the volcano, propagating any relevant epistemic uncertainty underlying these assessments. As an application example of the model, we apply BET_VH_ST to assess short‐term volcanic hazard related to tephra loading during Major Emergency Simulation Exercise, a major exercise at Mount Vesuvius that took place from 19 to 23 October 2006, consisting in a blind simulation of Vesuvius reactivation, from the early warning phase up to the final eruption, including the evacuation of a sample of about 2000 people from the area at risk. The results show that BET_VH_ST is able to produce short‐term forecasts of the impact of tephra fall during a rapidly evolving crisis, accurately accounting for and propagating all uncertainties and enabling rational decision making under uncertainty.

Rational volcanic hazard forecasts and the use of volcanic alert levels

“…Bayesian approaches are often used in conjunction with Event Trees (e.g., Newhall and Hoblitt 2002;Marzocchi et al 2004Marzocchi et al , 2008Newhall and Pallister 2015, and references therein), that represent the complex ramification of possible outcomes, each one quantified as a probability distribution which is allowed to evolve as long as new information is added (e.g., when new observations are available). To-date, Bayesian approaches have been employed in a large number of situations in volcanology, including forecasts of volcanic hazards over the shortterm (Aspinall et al 2003(Aspinall et al , 2006Marzocchi et al 2008; Lindsay et al 2010;Brancato et al 2011Brancato et al , 2012Bell and Kilburn 2012;Marzocchi and Bebbington 2012;Sandri et al 2009Sandri et al , 2012Selva et al 2012Selva et al , 2014Garcia-Aristizabal et al 2013;Anderson and Segall 2013;Rouwet et al 2014;Aspinall and Woo 2014;Sobradelo et al 2015;Boue et al 2015;Tonini et al 2016;Bartolini et al 2016) as well as over the long-term (Martin et al 2004;Baxter et al 2008;Neri et al 2008;Orsi et al 2009;Marzocchi et al 2008Marzocchi et al , 2010Sobradelo and Martì 2010;Passarelli et al 2010a, b;Selva et al 2012;Marzocchi and Bebbington 2012;Sandri et al 2012Sandri et al , 201...…”

Assessing future vent opening locations at the Somma‐Vesuvio volcanic complex: 2. Probability maps of the caldera for a future Plinian/sub‐Plinian event with uncertainty quantification

“…Elsewhere, vent opening probability maps, sometimes including information on associated uncertainty, have been produced to assess potential future activity in monogenetic volcanic fields [ Connor et al , ], on volcanic islands [ Alberico et al , ; Marti and Felpeto , ; Becerril et al , ], at composite stratovolcanoes [ Cappello et al , ], and at calderas [ Alberico et al , ; Selva et al , ; Bevilacqua et al , ]. The only approach which considered vent opening variability at SV is that proposed by Selva et al [], who used a vent opening area solely for the evaluation of tephra fall hazard. In this latter case, the authors defined a vent opening area consisting of a circle of 6 km radius subdivided into five distinct areas, each with constant probability of vent opening.…”

The Volcanic Hazards Assessment Support System for the Online Hazard Assessment and Risk Mitigation of Quaternary Volcanoes in the World

“…Recently, the probabilistic volcanic hazard assessment (PVHA) has been developed using multiple methods such as event tree (e.g., Newhall and Hoblitt, 2002;Neri et al, 2008), BET_VH (Bayesian Event Tree for Volcanic Hazard) long-term (e.g., Marzocchi et al, 2010;Selva et al, 2010) and BET_VH short-term (e.g., Selva et al, 2014). Other probabilistic tools to evaluate volcanic scenarios using the Bayesian event tree method (HASSET; Sobradelo et al, 2014) have been developed.…”