Ash fallout scenarios at Vesuvius: Numerical simulations and implications for hazard assessment

Volcanic ash fallout subsequent to a possible renewal of the Vesuvius activity represents a serious threat to
the highly urbanized area around the volcano. In order to assess the relative hazard we consider three
different possible scenarios such as those following Plinian, Sub-Plinian, and violent Strombolian eruptions.
Reference eruptions for each scenario are similar to the 79 AD (Pompeii), the 1631 AD (or 472 AD) and the
1944 AD Vesuvius events, respectively. Fallout deposits for the first two scenarios are modeled using
HAZMAP, a model based on a semi-analytical solution of the 2D advection–diffusion–sedimentation
equation. In contrast, fallout following a violent Strombolian event is modeled by means of FALL3D, a
numerical model based on the solution of the full 3D advection–diffusion–sedimentation equation which is
valid also within the atmospheric boundary layer. Inputs for models are total erupted mass, eruption column
height, bulk grain-size, bulk component distribution, and a statistical set of wind profiles obtained by the
NCEP/NCAR re-analysis. We computed ground load probability maps for different ash loadings. In the case of
a Sub-Plinian scenario, the most representative tephra loading maps in 16 cardinal directions were also
calculated. The probability maps obtained for the different scenarios are aimed to give support to the risk
mitigation strategies.

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