A numerical reconstruction of fall deposits from the Agnano-Monte Spina (4100 BP) Plinian eruption in the Campi Flegrei area, Italy

Abstract

A simple semianalytical model was applied to simulate the tephra deposits produced by two Plinian phases called B1 and D1 of the Agnano-Monte Spina eruption (4100BP) within the Campli Flegrei volcanic area in Italy. In this model, the eruption column is assumed to act as a line source in order to neglect complex near/vent interactions. Therefore, the validity of the model is limited to the medium and far areas from the vent (beyond 10-20km), where the assumption of a line source can be justified. The distribution of the particles in the atmosphere is assumed to be only controlled by gravity, wind and eddy diffusion. The model accounts for particles of different types and (juvenile pumice or ash particles, lithic fragments and crystals) within a used-defined range of granulometric classes. The numerically calculated deposit was confronted with the observed deposit. Applying a least/squares method it was tried to optimize input variables such as distribution of particles and mass within the eruption column, wind and diffusion parameters by fitting the computed deposit with the observed one. A good correlation between the numerically calculated and the measured deposit as well as a good agreement between the fitted variables with independently found parameters of the eruption could be achieved. The results allowed to re-estimate eruption parameters such as minimum erupted mass (2-3x1011 kg each), eruption column height (16-23 km for B1, ca. 30 km for D1), grain-size spectrum of erupted tephra, and the wind field at the time of the eruption.