Mt Somma-Vesuvius is a composite volcano on the southern margin of the Campanian Plain which has been active during the last 39 ka BP and which poses a hazard and risk for the people living around its base. The volcano last erupted in 1944, and since this date has been in repose. As the level of volcanic risk perception is very high in the scientific community, in 1995 a hazard and risk evaluation, and evacuation plan was published by the Italian Department of Civil Protection (Dipartimento della Protezione Civile). The plan considered the response to a worst-case scenario, taken to be a subplinian eruption on the scale of 1631, and based on volcanological reconstruction of this eruption, assumes that a future eruption will be preceded by about two weeks of ground uplift at the volcano’s summit, and about one week of locally perceptible seismic activity. Moreover, by analogy with the 1631 events, the plan assumes that ash fall and pyroclastic flow have been recognized as the primary volcanic hazard. To design the response to this subplinian eruption, the emergency plan divided the Somma-Vesuvius region into three hazard zones affected by pyroclastic flows (Red Zone), tephra fall (Yellow and Green Zone), and floods (Blue Zone). The plan at present is the subject of lot controversy, and, in our opinion, several assumptions need to be modified. The May 1998 landslide, caused in the Apennine region east of the volcano by continuous rain fall, led to the definition of a zone affected by re-mobilisation of tephra during a future eruption of Somma-Vesuvius (Blue Zone). The May 1998 rainfall period in the Apennine area east of the Somma-Vesuvius highlight, however, the relationships between strong meteoric events and re-mobilisation of tephra lying on the flank of the Apennine limestone mountain. This form of hazard needs to be extended.over the Apenninic areas, from the Somma-Vesuvius toward Avellino (NE), as well as Salerno (E-SE), where several towns are located near Apenninic river beds, and where there are good reasons for believing that a high flood risk occurs in conjunction with havy rain fall periods and during a probable future eruption at Somma-Vesuvius.
In conclusion, there are several indications that the Somma-Vesuvius region is affected by a primary volcanic hazard, when the volcano erupt, and by a secondary volcanic hazard when the volcano is in a repose period as volcanic debris flows and floods can be mobilized during periods of heavy rains.