## VHAST

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#### Abstract

VHAST (Volcanic Hazard Analysis and Simulation Tool)is a probabilistic volcanic hazard assessment software. It has been developed in Visual Basic .NET and runs on Windows XP or later. VHAST supports hazard analysis of lava flows, pyroclastic flows and ash fall, using simplified models.

The software follows an innovative methodological approach for the probabilistic assessment of volcanic hazard. In this approach, based on the history of eruptions of the analysis volcano, and for a the desired volcanic product, eruption magnitude is defined by the user and magnitude annual excess rates are entered to the program. Excess rates account for the number of times, per year, that a magnitude value is equaled or exceeded in an eruption. For each eruption magnitude, a set of intensity parameters that define the final extent of volcanic products must be defined. Whithin VHAST, each intensity parameter is modeled as a random variable, allowing for the simulation of several stochastic eruptions for each magnitude value. Each simulation derives in a geographical distribution of an intensity measure associated to the modeled volcanic product. The geographical distribution of volcanic products intensity is then defined as a grid random variable, where each pixel is characterized by two probability moments computed from the set of simulated eruptions. Finally, an annual occurrence rate is assigned to each grid random variable as a function of the magnitude annual excess rate. This results in a representation of volcanic hazard that accounts for both spatial and temporal probabilities that a particular location is affected by volcanic products, taking into consideration all possible eruptions at the analysis volcano. This hazard representation is totally compatible with nowadays risk assessment methodologies.

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