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The Evaluation of Volcanic Risk in the Vesuvian Area


Largely based on Scandone et al, 1993

The Risk is defined as the product (UNESCO, 1972, Fournier d'Albe, 1979):
 

Risk = (Value) x (Vulnerability) x (Hazard) (1)


Where the Value is the number of lives or the monetary value of goods at risk in a volcanic area; the Vulnerability is the percentage of lives or goods likely to be lost because of a given volcanic event; and the Hazard is the probability that a certain area be invested by a certain volcanic phenomenology.
The Hazard Evaluation We hypothesizes that the eruptions of Vesuvius of such a size to produce substantial damage (VEI=3,4,5) follow a Poisson distribution (Wickmann, 1966). We considered only three classes of VEI likely to produce risk to human life at Vesuvius, namely those with VEI3 3.
VEI = 3: also defined as violent strombolian, are those eruptions like the one of 1944 that often caused some casualties.
VEI = 4 : also defined as sub-plinian are those eruptions as the ones occurred in 472 AD and 1631. These eruptions are likely to produce extensive damages and casualties within the Vesuvian area.
VEI = 5: also defined as plinian are those eruptions like the one of 79 AD, likely to cause extensive damages and casualties also in areas outside the vesuvian area.
The associated probabilities of occurrence of at least one eruption, in ten years, are:

P3>(1,10) = 0.0989

P4>(1,10) = 0.0175

P5>(1,10) = 0.0030

The Volcanic Hazard

is given by the probability of occurrence of each class of eruption multiplied by the relative probability that an area be affected by a certain volcanic phenomenology. We evaluated separately the different hazards relative to the different phenomenologies.

Vulnerability


The vulnerability is the percent of Value (human life) at risk as a consequence of volcanic activity, and depends mostly on the different phenomenologies and their relevance during an eruption.

Results


The Value is based on the number of inhabitants living in each town around Vesuvius as of 1989. We evaluated the Risk according to equation 1.

We show in figure the list of the relevant towns of the vesuvian aerea classified according to their risk on a logarithmic scale.

Six towns fall within the group of very high Risk: Torre Annunziata, Torre del Greco, San Giorgio a Cremano, Portici, Ercolano and Napoli (eastern zone). They lay on the southern flank of the volcano along the coast and represent one of the highest concentration of inhabitants living in the proximity of an area susceptible to be destroyed by a volcanic eruption.

The figure shows the map of the Volcanic Risk for the entire area of Vesuvius subdivided into three zone (very high, high, medium). The highest risk area coincides with the densely inhabited, coastal zone. This area has been severely damaged by past eruptions of Vesuvius. Since the last eruption of 1944, it has been the area of highest urban development, as it represented the natural expansion line for the town of Naples.

The eastern sector of the town of Naples is within the very high risk zone because of the high number of inhabitants. It owes its coefficient to the effect of eruption with VEI=4 (60%) and VEI=5 (40%).

The probability of such events is low, but the coefficient of risk is high because of the number of inhabitants. The pie diagram shows the percentage of risk of the towns around Vesuvius. The towns of Torre Annunziata, Torre del Greco, San Giorgio a Cremano, Portici, Ercolano and Naples (eastern zone) cover more than 50% of the risk of Vesuvius.

Conclusion

We evaluated the hazard basing on the entire past life of the volcano and identified the events likely to cause human loss as those with VEI=3. The estimated coefficient of risk represents the expected life loss if no action is taken to reduce the effect of an impending eruption. We estimate a total number of casualties ranging among 15000 and 20000, should an eruption occur unexpectedly at Vesuvius with VEI ranging among 3 to 5.

The classification into four different classes of Risk permits to grade the measures that should be taken to reduce the hazard posed by an impending eruption. Urban planning should be aimed at reducing the density of inhabitants in the six towns graded at very high risk. These towns should also be the first to be evacuated in case of an impending eruption.

The towns graded at high risk should be evacuated only when a sufficiently high certainty exists about the true occurrence of an eruption. The towns graded at medium risk should take measures aimed at the reduction of risk such as the removal of ash from the roofs etc. and evacuation should be considered depending on the size of an eruption and location of the town. We want to emphasize that the coefficient of risk that we evaluated is only for human life, but can be easily upgraded to account for the monetary loss, should these data be available.

References


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