Volcanic hazard assessment at the Campi Flegrei caldera

Previous and new results from probabilistic approaches based on available volcanological data from real eruptions of Campi Flegrei, are assembled in a comprehensive assessment of volcanic hazards at the Campi Flegrei caldera, in order to compare the volcanic hazards related to the different types of events. Hazard maps based on a very wide set of numerical simulations, produced using field and laboratory data as input parameters relative to the whole range of fallout and pyroclastic-flow events and their relative occurrence, are presented. The results allow us to quantitatively evaluate and compare the hazard related to pyroclastic fallout and density currents (PDCs) in the Campi Flegrei area and its surroundings, including the city of Naples. Due to the dominant wind directions, the hazard from fallout mostly affects the area east of the caldera, and the caldera itself, with the level of probability and expected thickness decreasing with distance from the caldera and outside the eastern sectors. The hazard from PDCs decrease roughly radially with distance from the caldera centre and is strongly controlled by the topographic relief, which produces an effective barrier to propagation of PDCs to the east and northeast, areas which include metropolitan Naples. The main result is that the metropolitan area of Naples would be directly exposed to both fallout and PDCs. Moreover, the level of probability for critical tephra accumulation by fallout is relatively high, even for moderate-scale events, while, due to the presence of topographic barriers, the hazard from PDCs is only moderate and mostly associated with the largest events. Hazard evaluation for explosive volcanoes in densely populated areas is a major goal of research in volcanology. Tephra fallout and pyroclastic density currents coexist in most explosive eruptions, and their relevance and the way that they extend the area affected by volcanic hazards will depend on the eruption style and magnitude as well as environmental conditions (topography, wind pattern). Therefore, hazard evaluation at an active volcano requires the acquisition of as much data as possible relative to past eruptions and their related effects. Nevertheless, due to the high variability of the eruptive phenomena, the volcanological records are not exhaustive enough to be able to define the whole range of possible events for probabilistic evaluation. A more accurate approach requires the exploration of a wider repertoire of possible events which can be reproduced by numerical simulations, in order to produce probabilistic hazard maps. Campi Flegrei caldera, which has been active since at least c. 39 ka ago, produced a total of c. 60 highly explosive eruptions, with VEI ranging between 0 and 6. In most cases tephra fallout from sustained column and pyroclastic density currents has been produced. The mechanism of their formation, their intensity and the extension of the affected area, mainly depend on the eruption magnitude. However, volcanological studies (e.g. Mastrolorenzo 1994; Mastrolorenzo et al. 2001) and modelling (Rossano et al. 1998) have shown that tephra fallout distribution is strongly influenced by seasonal wind changes, while the spreading of pyroclastic density currents is mostly controlled by the topography of the caldera and its surroundings. Due to the widespread urbanization of the area around the caldera, and particularly of its surroundings, a quantitative evaluation of the hazards related to such phenomena is necessary to better define of the emergency planning. In this paper, previous and new results of integrated probabilistic and volcanological approaches to numerical simulations of fallout and PDCs events are presented in order to compare the hazards for different types of events. Explosive eruptions at Campi Flegri caldera The Campi Flegrei (CF) is a volcanic field dominated by a Quaternary 12-km-wide caldera depression formed during two high-magnitude eruptions: the 39 ka BP Campanian Ignimbrite (De Vivo et al. 2001) and the 14 ka BP Neapolitan Yellow Tuff (Deino et al. 2004) (Fig. 1). After 160 G. MASTROLORENZO ET AL. these two main events, c. 60 eruptions occurred in the last 14 000 years, from scattered vents located within the caldera. Explosive eruptions in volcanic history of Campi Flegrei have VEI values of 0 to 5, with erupted masses ranging between less than 108 and 1013 kg; column height from less than 1 to 45 km, magma eruption rate from hundreds of kg s−1 to 108 kg s−1; and wide variability in total grain-size distribution, fragmentation and dispersion (Table 1). However, extremely large events are rare, since VEI is generally lower than four. A few very large-scale surge-and-flow deposits are the prevalent eruptive mechanisms in large explosive eruptions, while the tens of intermediate and small-scale deposits occurred as subordinate episodes of more complex eruptive sequences. Effusive activity, with associated lava flows and domes, contributes only a few per cent to the total volume of the volcanic products. The post-NYT intracalderic activity, prevalently hydromagmatic, mainly formed pyroclastic cones, consisting of interbedded pyroclastic surge and fallout beds. The pure magmatic strombolian, sub-plinian and plinian events, only subordinate, consist of scoria and lapilli beds, with dispersion ranging between a few km2 and some thousands of km2. Smallscale strombolian deposits are mostly dispersed within the caldera rim, while the larger deposits spread outside the caldera in a large area, mostly towards the East. Probabilistic approach Probabilistic hazard maps have been developed for Campi Flegrei for pyroclastic falls and currents, using a generalization of the approaches proposed by Rossano et al. (2004) and De Natale et al. (2006).