Lessons from recent and past activity
The large case history discussed above for the recent and past activity at some active Italian volcanoes emphasizes the wide range of hazardous phenomena associated to the very large range of different eruption styles associated with mid-intensity eruptions. Similarly, the impact these eruptions can have on the territory is largely variable as a function of the eruptive dynamics, intensity, magnitude and product dispersal.
Fallout of pyroclastic material
Ballistic ejecta are generally considered those block- or bomb-sized fragments that follow a parabolic trajectory after expulsion with only a very minor interaction with local winds. Ballistic ejecta pose an important hazard especially for those low-to mid-intensity eruptions characterized by a very small amount of material expelled from the vent during each explosive outburst. Large explosive outbursts can also be associated with the ejection of ballistic clasts, but they are generally interspersed with a larger quantity of other falling material, so that the general hazard is mainly posed by the thick accumulation of pyroclasts non following ballistic trajectories.
The possibility of vent opening in densely inhabited areas (for example at Phlegrean Fields and Etna), including tourist sites at some areas (as for example at Etna and Stromboli) mean that ballistic ejecta are a serious hazard from Italian volcanoes. Casualties have recently occurred at these two volcanoes.
Hazards posed by fallout of pyroclastic material falling down from a convective columns are generally high, including in mid-intensity eruptions if the eruption vent is close to inhabited areas, and serious consequences have been associated with recent and past activity from Italian volcanoes. Effects of tephra fallout span a large range of possibilities - including possible damage to agriculture, buildings, health, transportation, power and water supply and waste water systems (http://volcanoes.usgs.gov/ash/). All but the first of these are more typical for Italian volcanoes due to the very close proximity of eruptive vents to largely inhabited areas.
Downwind fallout of coarse-grained material over large areas is mainly related with violent Strombolian eruptions, while ash-dominated fallout material can be both associated with violent Strombolian and Vulcanian activity. The last three centuries of Vesuvius activity have recurrently presented this problem over the whole volcano and especially in the eastern and southeastern adjacent sectors - up to distances of about 30-40 km due to the prevalent easterly direction of tropospheric winds. As a consequence, several violent Strombolian eruptions of the last two centuries (1872, 1906, 1944 eruptions) caused important damage to buildings in some of the villages at the foot of the volcano and especially in its eastern sector.
As clearly shown by the reported examples, mid-intensity eruptions are often associated with the production of large amounts of ash. This occurs both in eruptions characterized by phases of magma-water interaction (Vesuvius AP3 or the first phase of AD 1538 Monte Nuovo eruption at Phlegrean Fields) or during clearly magmatic activity (Etna 2001). During these eruptions ash is generally advected by low-level convective columns up to considerable distances (i.e. ash fallout is described up to 300 km south-east of Phlegrean Fields during the AD 1538 Monte Nuovo Eruption; satellite images show that ash plumes from the 2001-2002 Etna eruption were still visible over North Africa), mainly provoking damage to transportation, agriculture and water supply systems (Blong and McKee, 1995; http://volcanoes.usgs.gov/ash/). The recent eruptions of Etna caused, for example, the prolonged closure of the Catania and Reggio Calabria airports, and in the case of a northerly dispersal, could have caused a much larger economic damage over the whole Mediterranean area.
Generation of pyroclastic density currents
Pyroclastic density currents (PDC) represent one of the main sources of hazard associated with explosive volcanic activity. This type of phenomenon is typically associated with Plinian (s.l.) activity and only minor PDCs are sometimes associated with mid-intensity events like cone-forming and Vulcanian eruptions. The area swept by a PDC and its maximum runout are a function of eruption intensity, the less intense generally being able to produce dense PDCs which are strongly controlled by topography, have a reduced mobility and rapidly stop where the slope reduces. This is the case, for example, of some PDCs associated with violent Strombolian activity at Vesuvius (the 1822 eruption, Arrighi et al. 2001), of the glowing avalanches which formed along the Vesuvius slopes during the 1944 activity (Hazlett et al. 1991; Cole and Scarpati, 2010) or along the northeastern flank of Stromboli volcano during the 1930 eruption (Rittmann, 1931), or at the end of the 5 April 2003 paroxysm of Stromboli, only confined to the summit area (see above) and of the two block and ash flows generated during the Vulcanian phase 2 of the AD 1538 Monte Nuovo eruption. A short runout is also associated with dilute PDCs which can be generated during cone-forming (AD 1538 Monte Nuovo) and Vulcanian activity (1888-1890 eruption of La Fossa of Vulcano, Eolian Island, Frazzetta et al. 1983). Hazards associated to this type of activity are reduced, but are still relevant when eruptive vents are close to urbanized areas. Main hazards of this type of PDCs are mostly related to particle concentration and the temperature of the gas cloud mixture which can provoke asphyxiation and burns to humans or set fires in vegetated and urbanized areas. Expected dynamic pressure is less relevant as a source of hazard because of the generally small mass transported and the reduced velocity of the current.
Seismic activity and ground deformations
Deformation and seismic activity precedes, accompanies and follows eruptions, and it is not clearly related with its intensity (Mc Nutt, 1996; Sandri at al. 2004). Magnitude of earthquakes associated to volcanic activity is generally low (on average lower than 4) and it mainly impacts the restricted area of few kilometers around the vent. The common occurrence of seismic activity in swarms of hundreds to thousands of earthquakes of similar magnitude occurring in a very short time (days to hours) over the course of a volcanic crisis can have an important cumulative effect on structures, which are progressively weakened by the continuous ground shaking (Zuccaro et al. 2008). Important effects on buildings are also increased in some areas by the concomitant overloading by fallout tephra or by the effects of the lateral pressures exerted by eventual PDCs.
Similarly, large deformation in the form of vertical movements or extended fracturing is often restricted to the area closer to the vent and may have a strong impact on buildings. As a consequence, the final impact of an eruption crisis can be largely increased by this type of activity, as for example in the case of the AD 1538 Monte Nuovo eruption when the town of Pozzuoli was destroyed, in large measure, by the effects of deformation and seismic activity rather than by the reduced load of ash.
An extended network of fractures often forms during summit and lateral eruptions at Etna, causing local problems to communication and power infrastructures or buildings. The famous AD 1669 eruption of Monti Rossi opened a fracture system inside the village of Nicolosi, building a 200 m high cone in its outskirts (Corsaro et al. 1996).
Violent Strombolian and Vulcanian eruptions are often associated with phases of lava effusion, whatever the composition of the magma involved. Paroxysmal explosive phases of these eruptions at Etna and Vesuvius are both preceded and/or accompanied by lava flows erupted from lateral vents or fissures. Vulcanian activity is generally associated with the slow extrusion of a lava plug (possibly occurred between the first and second phase of the AD 1538 Monte Nuovo eruption) or of a lava dome or coulée (as in the historical activity of La Fossa of Vulcano, Frazzetta et al. 1983). Also, eruptions characterized by continuous ash emission can be associated to lava effusion that, in the case of the AP3 eruption of Vesuvius, was possibly confined inside the caldera depression.
Impact of lava flow activity during mid-intensity, mixed explosive-effusive eruptions can be very large and, in some cases, can be comparable to damage (in terms for example of economical costs) related to the explosive part of the eruptions. During the recent activity of Vesuvius (1906 and 1944 eruptions) damage related to lava flow was huge, partially destroying the villages of Boscotrecase and San Sebastiano, respectively. Hazard from lava flow invasion should be so taken in account when dealing with the expected eruption scenario for his type of eruptions.