Impact of explosive eruption scenarios at Vesuvius

In the paper the first attempt at the definition of a model to assess the impact of a range of different volcanic hazards on the building structures is presented. This theoretical approach has been achieved within the activities of the EXPLORIS Project supported by the EU. A time history for Sub-Plinian I eruptive scenario of the Vesuvius is assumed by taking advantage of interpretation of historical reports of volcanic crises of the past [Carafa, G. 1632. In opusculum de novissima Vesuvij conflagratione, epistola isagogica, 2a ed. Napoli, Naples; Mascolo, G.B., 1634. De incendio Vesuvii excitato xvij. Kal. Ianuar. anno trigesimo primo sæculi Decimiseptimi libri X. Cum Chronologia superiorum incendiorum; & Ephemeride ultimi. Napoli; Varrone, S., 1634. Vesuviani incendii historiae libri tres. Napoli], numerical simulations [Neri, A., Esposti Ongaro, T., Macedonio, G., Gidaspow, D., 2003. Multiparticle simulation of collapsing volcanic columns and pyroclastic flows. J. Geophys. Res. Lett. 108, 2202. doi:10.1029/2001 JB000508; Macedonio, G., Costa, A., Longo, A., 2005. HAZMAP: a computer model for volcanic ash fallout and assessment of subsequent hazard. Comput. Geosci. 31,837–845; Costa, A., Macedonio, G., Folch, A., 2006. A three-dimensional Eulerian model for transport and deposition of volcanic ashes. Earth Planet. Sci. Lett. 241, 634–647] and experts' elicitations [Aspinall, W.P., 2006. Structured elicitation of expert judgment for probabilistic hazard and risk assessment in volcanic eruptions. In: Mader, H.M. Coles, S.G. Connor, C.B. Connor, L.J. (Eds), Statistics in Volcanology. Geological Society of London on behalf of IAVCEI, pp.15–30; Woo, G., 1999. The Mathematics of Natural Catastrophes. Imperial College Press, London] from which the impact on the building structures is derived. This is achieved by an original definition of vulnerability functions for multi-hazard input and a dynamic cumulative damage model. Factors affecting the variability of the final scenario are highlighted. The results show the high sensitivity of hazard combinations in time and space distribution and address how to mitigate building vulnerability to subsequent eruptive phenomena [Baxter, P., Spence, R., Zuccaro, G., 2008-this issue. Risk mitigation and emergency measures at Vesuvius]. The first part of the work describes the numerical modelling and the methodology adopted to evaluate the resistance of buildings under the combined action of volcanic phenomena. Those considered here for this multi-hazard approach are limited to the following: earthquakes, pyroclastic flows and ash falls. Because of the lack of a systematic and extensive database of building damages observed after eruptions of such intensity of the past, approaches to this work must take a hybrid form of stochastic and deterministic analyses, taking into account written histories of volcanic eruptions and expertise from field geologists to build up a semi-deterministic model of the possible combinations of the above hazards that are situated both in time and space. Once a range of possible scenarios has been determined, a full stochastic method can be applied to find a sub-set of permutations and combinations of possible effects. This preliminary study of identification of the possible combination of the phenomena, subdividing them into those which are discrete and those which are continuous in time and space, enables consideration the vulnerability functions of the combinations to be feasible. In previous works [Spence, R., Brichieri-Colombi, N., Holdsworth, F., Baxter, P., Zuccaro, G., 2004a. Vesuvius: building vulnerability and human casualty estimation for a pyroclastic flow (25 pages). J. Volcanol. Geotherm. Res. 133, 321–343. ISSN 0377-0273; Spence, R., Zuccaro, G., Petrazzuoli, S., Baxter, P.J., 2004b. The resistance of buildings to pyroclastic flows: theoretical and experimental studies in relation to Vesuvius, ASCE Nat. Hazards Rev. 5, 48–50. ISSN 1527–6988; Spence, R., Kelman, I., Petrazzuoli, S., Zuccaro, G., 2005. Residential Buildings and Occupant Vulnerability to Tephra Fall. Nat. Hazards Earth Syst. Sci. vol. 5. European Geosciences Union, pp.1–18; Baxter, P.J., Cole, P.D., Spence, R., Zuccaro, G., Boyd, R., Neri, A., 2005. The impacts of pyroclastic density currents on buildings during the eruption of the Soufrière hills volcano, Montserrat. Bull. Volcanol. vol. 67,292–313] the authors investigated, by means of experimental and analytical methods, the limiting resistance of masonry and reinforced concrete buildings assuming each action separately. In this work the first attempt to estimate the response of the buildings to the volcanic seismic action or to the lateral dynamic pressure due to pyroclastic flow combined with an extra vertical load on the roof due to ash fall is performed. The results show that up to a certain limit of ash fall deposit, the increment of structure weight increases the resistance of a building to pyroclastic flow action while it reduces its seismic resistance. In particular the collapse of the top storey of R.C. buildings having large roofs could occur by accumulation of ash and a strong earthquake. Seismic and pyroclastic flow vulnerability of tall R.C. and masonry buildings with rigid floors is less sensitive to ash fall load combination. The model allows any sequence of events (earthquake, ash fall, pyroclastic flow) to be assumed and evaluates the spatial distribution of the cumulative impact at a given time. Single impactscenarios have been derived and mapped on a suitable grid into which the territory around Vesuvius has been subdivided. The buildings have been classified according to the constructional characteristics that mostly affect their response under the action of the phenomena; hence the vulnerability distribution of the buildings are assigned to each cell of the grid and by taking advantage from the combined vulnerability functions the impact is derived at time t. In the paper the following impact simulations are presented: - single cases of selected seismic sequence during the unrest phase (Sub-Plinian I) - ash fall damage distribution compatible to a Sub-Plinian I eruption - pyroclastic flow cumulative damage scenarios for selected cases (Sub-Plinian I). The model also allows either Monte Carlo simulation to evaluate the most probable final scenario or maximisation of some parameter sensitive to Civil Protection preparedness. The analysis of the results derived for a Sub-Plinian I-like eruption has shown the importance of the seismic intensities released during the unrest phase that could interfere with the evacuation of the area and the huge number of partial collapses (roofs) due to ash fall.