Braia, G.

The Averno 2 eruption (3,700 ± 50 a B.P.) was an explosive low-magnitude event characterized by magmatic and phreatomagmatic explosions, generating mainly fall and surge beds, respectively. It occurred in the Western sector of the Campi Flegrei caldera (Campanian Region, South Italy) at the intersection of two active fault systems, oriented NE and NW. The morphologically complex crater area, largely filled by the Averno lake, resulted from vent activation and migration along the NE-trending fault system. The eruption generated a complex sequence of pyroclastic deposits, including pumice fall deposits in the lower portion, and prevailing surge beds in the intermediate-upper portion. The pyroclastic sequence has been studied through stratigraphical, morphostructural and petrological investigations, and sub- divided into three members named A through C. Member A was emplaced during the first phase of the eruption mainly by magmatic explosions which generated columns reaching a maximum height of 10 km. During this phase the eruption reached its climax with a mass discharge rate of 3.2 106 kg/s. Intense fracturing and fault activation favored entry of a significant amount of water into the system, which produced explosions driven by variably efficient water-magma inter- action. These explosions generated wet to dry surge deposits that emplaced Member B and C, respectively. Isopachs and isopleths maps, as well as areal distribution of ballistic fragments and facies variation of surge deposits allow definition of four vents that opened along a NE oriented, 2 km long fissure. The total volume of magma extruded during the eruption has been estimated at about 0.07 km3 (DRE). The erupted products range in composition from initial, weakly peralkaline alkali-trachyte, to last-emplaced alkali-trachyte. Isotopic data and modeling suggest that mixing occurred during the Averno 2 eruption between a more evolved, less radiogenic stored magma, and a less

Pyroclastic density currents of the recent eruptions at Campi Flegrei Caldera (CFC — Southern Italy) have been studiedwith the aimof assessing the potential impact of similar events in the future. Eruptions of different scales have been investigated by means of the combined use of facies architecture, laboratory analysis and physical modeling. Both in the small (Averno 2) and intermediate (Astroni) scales, facies analysis indicates that deposits result from the emplacement of pyroclastic density currents like base-surge, formed by multiple closely-timed impulses of phreatomagmatic origin. In the large-scale event (Agnano-Monte Spina), the facies architecture suggests that the currents started as concentrated flows near the vent, as originating from the collapse of a dense eruptive column, and evolved laterally into expanded flows by the propagation of the basal shear current. Laboratory analyses on samples from the main layers of deposits allowed obtaining the input data for the PYFLOW code, which was used for reconstructing the flow dynamic characteristics of the currents. The expected damage is discussed in terms of the probability density function of dynamic pressure and particle volumetric concentration. In this way, the range of potential impact that similar pyroclastic density currents could cause to buildings, infrastructures and population is defined. In the large-scale event, the dynamic pressure ranges from9.38 to 1.00 kPa (integrating the basal 10mof the current) at distances of 1.5 and 4.0 km from the vent, respectively. The values are highly influenced by the local topography. In the intermediate-scale event, the dynamic pressure ranges from 2.43 to 0.26 kPa at distances of 1.1 and 1.4 km from the vent, respectively. In the small scale event, the dynamic pressure ranges from 1.49 to 0.39 kPa at distances of 0.5 and 1.1 km from the vent, respectively. The particle volumetric concentration at a height of 2mwithin the current is always lower than 0.01, but typically it is higher than 0.001 inside the caldera, and could have severe effects on the unsheltered population, also at locations (topographic highs) where velocity and dynamic pressures are low. In distal reaches outside the caldera the widespread fine ash originated by buoyancy from the currents, whichwas transported by lower atmosphere winds, is to be considered as a “fallout load” slowly accumulating on the ground surface (and building roofs) during the waning stage of the current.