RENEWAL OF EXPLOSIVE ACTIVITY AT VESUVIUS - MODELS FOR THE EXPECTED TEPHRA FALLOUT

One of the major problems concerning the assessment of volcanic hazard at Vesuvius is to determine the type and size of the eruptive event that will characterize the volcano when it becomes active once again. During its history, Somma-Vesuvius has exhibited different types of activity, ranging from quiet lava emission to moderate strombolian activity, to catastrophic plinian eruptions. Available data support a behavior model characterized by the increasing size and explosiveness of the eruptions with increasing repose time, as a consequence of a roughly constant periodic supply of deep basic magma to a shallow magma chamber and differentiation and mixing in the chamber. After the A.D. 79 eruption, a homogeneous HK (high potassium) nature of erupted products was reflected by a magma alimentation rate roughly estimated at 1.5-2.0 millions of cubic meters per year. Assuming no major changes have occurred in the feeding system of the volcano after its last eruption in 1944, a volume of 40-70 × 106 m3 magma could be considered presently available for a renewal of activity at Vesuvius. The emission of such a mass of magma during a single eruption would result into the largest event since the highly disruptive 1631 subplinian eruption. Presently, no possibility exists to forecast the eruptive character of such an eruption, and either a "ultrastrombolian" or a "subplinian" case appear equally possible. The latter possibility implies the highest potential hazard. This paper provides the numerical simulations of the main eruptive phenomenon that probably will occur during this "maximum expected event": the fallout of tephra from a high, sustained eruption column. After the initial explosive opening of the vent, the scenario consists of the formation of a high convective column with lee-side fallout of pumice and lithic fragments, accompanied and followed by column collapses generating pyroclastic flows and surges. The column behavior was numerically simulated by using the Wilson and Walker column model with the following input parameters: mass eruption rate between 0.4 and 2.0 × 107 kg s-1, gas content between 5 and 10 wt.%, lithic content 20 wt.%, basal radius of the column larger than the minimum value for subsonic initial velocity of the jet. The resulting maximum column height and maximum gas thrust height were respectively 11-16 km and below 1000 m. The tephra fallout was modeled by a continuity equation solved on a nonuniform three-dimensional grid. The input parameters to the model are: seasonal wind velocity profiles; eddy diffusion coefficients of 3000 m2 s-1 (horizontal) and 50 m2 s-1 (vertical); particles whose nature and grain size are inferred after data on pyroclastic deposits from ancient Vesuvius eruptions distributed along a vertical linear source. The hazard implications of the different modeled cases are briefly discussed. © 1990.