University of Aarhus, Denmark, Department of Earth Sciences

Tephra fallout associated with renewal of volcanism at the Campi Flegrei caldera is a serious threat to the Neapolitan area. In order to assess the hazards related with tephra loading, we have considered three different eruption scenarios representative of past activity: a high-magnitude event similar to the 4.1 ka Agnano-Monte Spina eruption, a medium-magnitude event, similar to the ∼3.8 ka Astroni 6 eruption, and a low-magnitude event similar to the Averno 2 eruption. The fallout deposits were reconstructed using the HAZMAP computational model, which is based on a semi-analytical solution of the two-dimensional advection– diffusion–sedimentation equation for volcanic tephra. The input parameters into the model, such as total erupted mass, eruption column height, and bulk grain-size and components distribution, were obtained by best-fitting field data. We carried out tens of thousands simulations using a statistical set of wind profiles, obtained from NOAA reanalysis. Probability maps, relative to the considered scenarios, were constructed for several tephra loads, such as 200, 300 and 400 kg/m2. These provide a hazard assessment for roof collapses due to tephra loading that can be used for risk mitigation plans in the area.

A simple semi-analytical model presented in Pfeiffer et al. (in press) (“Hazmap”-modified version) was applied to reconstruct the tephra deposit of a Plinian or sub-Plinian phase (called Astroni 6, after Isaia et al., 2004) of an eruption of the Astroni volcano (ca. 4100-3800 BP) within the Campi 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 two different types of particles (juvenile pumice and unspecified dense particles) 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 could be achieved, although the quality of the input data is poor because of the lack of a sufficient number of distal sample points. Therefore, best fitting input parameters could not be well constrained and the presented results must be seen as a fairly rough estimate on eruption conditions only. IN particular, the erupted mass and eruption column height predicted by the model are considerably smaller than those presented by other authors (Isaia et al., 2004). However, the discrepancy is large enough to raise the question about the precision of other estimates as well.

A simple semianalytical model to simulate ash dispersion and deposition produced by sustained Plinian and sub-Plinian eruption columns based on the 2D advection-dispersion equation was applied. The eruption column acts as a vertical line source with a given mass distribution and neglects the complex dynamics within the eruption column. Thus, the use of the model is limited to areas far from the vent where the dynamics of the eruption column play a minor role. Vertical wind and diffusion components are considered negligible with respect to the horizontal ones. The dispersion and deposition of particles in the model is only governed by gravitational settling, horizontal eddy diffusion, and wind advection.The model accounts for different types and size classes of a user-defined number of particle classes and changing settling velocity with altitude. In as much as wind profiles are considered constant on the entire domain, the model validity is limited to medium-range distances (about 30–200 km away from the source). The model was used to reconstruct the tephra fall deposit from the documented Plinian eruption of Mt. Vesuvius, Italy, in 79 A.D. In this case, the model was able to broadly reproduce the characteristic medium-range tephra deposit. The results support the validity of the model, which has the advantage of being simple and fast to compute. It has the potential to serve as a simple tool for predicting the distribution of ash fall of hypothetical or real eruptions of a given magnitude and a given wind profile. Using a statistical set of frequent wind profiles, it also was used to construct air fall hazard maps of the most likely affected areas around active volcanoes where a large eruption is expected to occur.

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.