IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

3A2_4C-O1

Room A6Date/Time: July 23 10:45-11:00

New methods of probabilistic hazard assessment of tephra dispersal:
application to the Neapolitan area
Antonio Costa1 , Arnau Folch2 , Giovanni Macedonio3 , Laura Sandri1 , Jacopo Selva1
1Istituto Nazionale di Goefisica e Vulcanologia, Sezione di Bologna, Italy, 2 Barcelona Supercomputing
Center, Spain, 3 Istituto Nazionale di Goefisica e Vulcanologia, Sezione di Napoli, Italy
E-mail: antonio.costa@bo.ingv.it

The intrinsic uncertainty and variability associated to the size of next eruption strongly affects short to long term tephra hazard
assessment. Often, emergency plans are established accounting for the effects of one or a few selected reference scenarios.
Probabilistic hazard assessments for tephra dispersal need the definition of eruptive scenarios with associated probabilities,
a meteorological dataset covering a representative time period and a tephra dispersal model. Probabilistic assessment resultsfrom weighting a statistical number of simulations considering different volcanological and meteorological conditions.
Volcanological inputs such as erupted mass, eruption column height, eruption duration, bulk granulometry, fraction of
aggregates typically encompass a wide range of values. Because such a variability, it is clear that the eruptive scenario cannot
be adequately defined using single values for the volcanological inputs. Here we account for this variability using a range foreach eruptive parameter and assuming a uniform or beta Probability Density Function between the extreme values of therange. Once a PDF is defined, meteorological and volcanological inputs are chosen by using a stratified sampling methodthat consists of dividing the population into mutually exclusive, exhaustive and homogeneous sub–groups before sampling.
The ECMWF reanalysis dataset are used for exploring different possible meteorological conditions. This procedure allows
avoiding the bias introduced by selecting a single reference setting and thus neglecting most of the intrinsic sources oferuptive variability. Results from one or more dispersal models are then integrated through a Bayesian tool to quantify
epistemic uncertainties. Finally, this Bayesian tephra dispersal hazard assessment allows us to produce probabilistic hazard
curves for strategic structures and of standard hazard maps in analogy with the seismic case, representing the information
primarily used in risk mitigation actions. As examples, here we explore this issue by analyzing long term volcanic
hazard for tephra fall at Vesuvius and Campi Flegrei, Italy. We integrate the simulations by two tephra dispersal models,the analytical HAZMAP and the numerical FALL3D models respectively, into BET_VH.
The results obtained clearly show that volcanic hazard based on all possible eruptive settings is significantly different fromthe analysis based on a single reference setting, as used in volcanic hazard common practice. We highlight the differenceswith a previously published study in which three different scenarios were again considered, but each scenario had fixedvolcanological inputs. As examples, we also show the resulting probabilistic hazard curves for a few strategic points in Naples,and a standard hazard map displaying, at every point, the maximum tephra load with a probability equal to 10% expected
to be exceeded in 50 years.

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