Dossier

Update of the National Emergency Plan for Vesuvius

 

Reference event and alert levels

The landmark event. The document "Scenarios and alert levels for the Vesuvius" delivered from the National Commission identifies an explosive sub-Plinian eruption as a reference event for the National emergency Plan for the Vesuvius, confirming what was already assumed in the 2001 Plan.

This scenario envisages:
• the formation of an eruptive column of several kilometers;
• the fall of volcanic bombs and blocks around the crater and smaller particles (ash and lapilli) up to several tens of kilometers away;
• the formation of pyroclastic flows that would flow down the volcano's slopes for a few kilometers.

Seismic activity may precede the eruption and accompany the different phases, causing particularly serious damage to buildings already burdened by the load of products launched in the first phase of the eruption.

The eruptive scenarios expected. For the definition of the reference scenario, we have evaluated the probability of occurrence of different scenarios, corresponding to three types of explosive eruptions (with Plinian volcanic explosivity index VEI = 5, sub-Plinian eruption with VEI = 4 and violent Strombolian VEI = 3).

On the basis of statistical studies, for the Vesuvius an event of less energy (VEI = 3) would be more likely (slightly above 70%), however, the experts considered that scenario to be taken as reference should be an explosive sub-Plinian eruption with VEI = 4 for the following reasons:
• has a conditional probability of occurrence that is quite high (slightly less than 30%);
• corresponds to a reasonable choice of "acceptable risk" given that the probability of this event will be exceeded by Plinian eruption with VEI = 5 is only 1%;
• geophysical data do not reveal the presence of a shallow magma chamber with sufficient volume to generate a Plinian eruption type.

Areas at risk planned for a sub-Plinian eruption, taken as the reference scenario for the new Vesuvius Plan, cover also those for Strombolian eruption, of less energy.

However, we underline that despite having been identified a sub-Plinian eruption as a key event, the present state of knowledge - should phenomena linked to a possible reactivation occur - it would not be possible to determine what type of possible eruption will be from the analysis of the precursors.

The hazard areas. On the basis of the reference scenario and of the different hazard areas two zone of the Plan (yellow and red) have been defined ,for which different operating measures are envisaged. New specific operational indications will be elaborated to manage any effect connected to flooding phenomena and invasion caused by mud slides (lahar)

The alert levels. The working group has decided to maintain the same alert levels provided in the 2001 Plan, which are a basic level (green), a level of attention (yellow), a pre-alarm level (orange) and an alarm level (red), in accordance with the provisions for most of the monitored volcanoes on our planet. The four levels of alert mark time before a possible resumption of eruptive activity.

Currently, the alert level of Vesuvius is green, which means that there has been no abnormal phenomenon compared to the ordinary activity that has characterized the volcano for decades. The transition from one level of alert to the next one corresponds to the variation of the parameters (seismicity, ground deformation, composition of the gas in the fumarole, etc.) routinely monitored, 24/7, managed by the monitoring system of the Vesuvius observatory of Ingv and it involves a gradual increase in the probability of eruption of a reactivation of the volcano. Consequently, the increasing response of the civil protection system is modulated in accordingly, which culminates with the evacuation of the population present in the red zone (alarm level).

Latest update: 31 August 2018

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