Abstract 

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The terrorist threat is a complex phenomenon that affects all the modern cities. Differently from the natural event, terrorist ones include the human will of perpetrators, changing their perspective of urban areas. Starting from the identification of a well-accepted definition, terrorism was analysed organizing all the inherent characters and exploring previous European experiences and the main scientific literature. 

The relevance of the terrorism phenomenon was achieved investigating the sample of European events during the last 20 years based on the Global Terrorism Database (GTD) through the Probabilistic Risk Assessment (PRA) approach. Here, the main features are highlighted, simplifying the parameters involved in the GTD classification. In detail, the PRA focuses on the predominant attack types (AT) and a reduced number of Environmental urban Classes (EC), firstly, and on Open Areas, then. The latter are referred to the real open urban areas (identified for squares and streets) and to the external areas (identified as Spaces of Relevance - SoR) related to critical classes of buildings. 
Finally, a parametric deconstruction of the phenomenon was identified. Here, all the indexes involved in the Hazard, Vulnerability and Exposure variations were identified and validated to determine a full parametric algorithm useful in quantifying the Matrix of Terrorism Risk for real open BE.

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