Case of Volcano Vesuvius

Mount Vesuvius

Mount Vesuvius is an active volcano situated in the bay of Naples in southern Italy. The mountain is made up of two summits, known as a humpback volcano. The main summit (el gran cono) was last measured to be 1281m, although this value varies a great deal with eruptions. Monte somma is the other peak measured at 1149m, which is the highest point of a summit caldera formed by the collapse of an originally much higher peak during an eruption.

El gran cono was formed during the eruption of 79, and is partially encircled by the caldera that forms monte somma. [1] Vesuvius is the only European volcano to have erupted in the last century, and was formed after the collision of the African and Eurasian tectonic plates. The African oceanic plate is pushed below the continental Eurasian plate, partially melting the plate boundary forming magma. It is classified as a stratovolcano. [2]

Stratovolcanos, also known as composite volcanos, are formed of many layers of different types of volcanic rock formed from previous eruptions. They are characterised by a steep nature, unlike shield volcanoes. The lava from such volcanoes is viscous, formed from felsic magma (high in silica), which gives a small radius for the lava when compared with less viscous lava. The viscous nature however, results in higher pressure build up in the magma chambers, and thus a more explosive nature of the volcano. The flows are typically pyroclastic formed of molten rock and high temperature gases. The resulting high desity leads to flows that descend in air and flow down the side of the mountain. [3] High population density around the volcano can be attributed to highly fertile volcanic soil.

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Fertility of volcanic soil is explained by the high mineral content found in volcanic deposits. The nature of the soil changes after an eruption, as the high rock content in pyroclastic flows destroys any vegetation present. The ground deformation from eruptions is known as a secondary hazard of the volcano. Another secondary hazard that is common to steep stratovolcanos are lahars, which are violent mudflows that often occur soon after eruptions. Rainfall after an eruption can trigger the mudslide for example, forming a flow of water and volcanic rock with a concrete like texture. The high density and high flow speeds (typically upto 30kph) can extend the damage radius of eruptions, burying villages and forests in its path.

Methods for Detecting an Imminent Eruption

Due to the potential human and material impact of an eruption, there is a strong need to anticipate them. The methods to do so have evolved in the last few decades and, as put by Sparks [4], have been increasingly based on models of the underlying dynamics of volcanoes rather than empirical pattern recognition. Indeed, the latter offers long term prediction but is only valid for a small num- ber of active volcanoes and rely on their past behaviour, which can change in the future [5]. The more recent models are based on various geophysical events triggered by the interaction of the rising magma with its surrounding. These events can include degassing, earthquakes and others, and their understanding is crucial to accurately predict eruptions. The study of seismicity is of particular interest as nearly all eruptions are announced by a renewed seismic activity [6].

The monitoring of seismic activity in the vicinity of a volcano allows to deter- mine the state of the magma and its potential transport as well as the monitoring of rocks failures in the ground. It is established that the transport of magma and its thermodynamics can cause seismic waves as it leads to pressure fluctuations in the ground [7]. The wide variety of underground phenomenons can cause different types of seismic waves in terms of period or amplitude. Therefore, if a long period seismic wave is picked up by a probe and linked to a transport of magma to the surface, it is possible to identify an imminent eruption. Some other surveillance techniques rely on the measurement of ground deformation. Ground defor- mation is due to changes in pressure due to movements of magma in chambers and conduits, which affect the surrounding crustal rocks [4].

Ground deformation can be measured using a wide range of techniques such as electronic distance measurements (EDM) using reflected laser or infrared light, measurement of ground tilt, use of the global positioning system (GPS), precise levelling and borehole sensors including strainmeters and tiltmeters [4] [8] [9]. Ground deformation can also be assessed via Global Synthetic Aperture Radar (SAR) This technique allows to construct 2D/3D maps with an amplitude and phase component using a mounted radar (satellite/UAV/aircraft) which emits elec- tromagnetic waves and analyzes their echoes [10]. Comparison between images can yield a change in SAR amplitude and allow to detect the growth of a dome or changes to the surface of the volcano which could indicate a future eruption.

Another characteristic of active volcanoes which can be used to anticipate eruptions is the emission of gases. When magma rises, the pressure decreases and gases, which were initially dissolved in the magma, separate from the liquid phase, leading to fumes from the surface of the volcano [11]. Mon- itoring of those gasses can give indication on the current depth, composition and amount of magma stored in the chambers. The presence of gases, mainly carbon dioxide and sulfur dioxide can be ver- ified using analysis of rocks and minerals with Fourier Transform Infrared spectrometry (FTIR) and Secondary Ion Mass spectrometry (SIMS) [12]. Additionally, direct sampling can be used to capture, by hand, the gases as they escape in the atmosphere, allowing for a complete characterization of the chemical composition [13]. Due to the dangerousness of this method, a correlation spectrometer (COSPEC) or Infrared Carbon Dioxide Analyzer (LI-COR) can be used.

The first device allows to measure the concentration of sulfur dioxide in the plume emitted by the volcano, while the second analyzes carbon dioxide, from an airplane, allowing to calculate an emission rate [14]. Finally, it is interesting to note that eruption prediction is still a hot topic and innovations in this sector are frequently made. One example of the ongoing research to improve volcanoes monitoring is the use of Unmanned Aerial Vehicles loaded with sensors and cameras. The main advantage of this platform is its ability to fly over dangerous areas (craters) or even during eruptions for a long time, with low visibility. This allows to reduce physical risks to volcanologists while receiving real time data at a remote location [15].

Measures Already taken to Minimize the Impact of an Eruption

Simulations

The first measure that can be taken to minimise impact is to make the most accurate possible prediction on the size of the potential eruption and the areas affected. Predictions for eruptions are often simulated with a stochastic Bayesian approach, with the help of measurements taken around the volcanic area [16] which is the case for Vesuvius. There has been considerable work carried out to identify the probabilities of the next vesuvius eruption type. The classification of the volcano is important in the quantification of the risk and greatly changes the outcome of certain outcome porbability.

In the work documented by Marocchi et al. the classes of volcano range from A4-F4, which depend on the repose time of the volcano [17]. The evacuation plan is based on the assumption a plinian eruption is unlikely and the modelling is for a C4 category Vesuvius. C4 estimates give the least destructive outcomes for the eruption possibilities. These results lead to the modelling of damage areas for a sub-plinian eruption, specifically, similar to that of 1631.

This leads to a key problem with the damage prevention plans: the possibility of a plinian eruption. The consequences would be more severe, due to higher column of eruption, higher rock density in the eruption and wider overall dispersal [18]. The name plinian comes from the 79AD eruption and a famous witness account in a Pliny letter. This eruption was not however the most violent of its kind. As much as 3m ash deposits that can be found in the city of Naples from previous eruptions, yet the red zone does not include the coastal town [19].

If the F4 classification had been used for damage control for example, the repose time is assumed to have no upper limit, and plinian outcomes become more likely [17] Another important element of the simulations is the wind directions and strength. There is high uncertainty as of course it is completely impossible to predict and statistics are the only tool. It is not even guaranteed that wind direction would be consistent throughout an eruption. What is clear is that the pyroclastic fall out from certain wind directions would vastly change the overall damage including roof and building damage which are not fully accounted for in yellow zone modelling [20].

Evacuation

The most important step after simulations are evaluated is to mark a zone that would be directly affected by a maximum possible eruption. In this case direct danger from the volcano implies the area lies within the path of the volcanic ash and inhabitants should be evacuated before the eruption. This zone is known as the red-zone. The evacuation program established in 2001 and later updated in 2007, aims to take place over 72 hours before an eruption. The basis of the damage prevention comes from the sub-plinian eruption in 1631.

Red zone

This area is under the biggest threat, and would be subject to pyroclastic flows from the slopes of the volcano. The pyroclastic flows of such a volcano often have a preferential direction (non symmetric nature) however this direction can not be predicted. Red zone policies do not wait for eruption to take effect due to the high speed and destrcutive nature of pyroclastic flows (100-500kmh [3]). This surround- ing area of the volcano is made up of 25 municipalities (until recently was only 18), of around 200km2 and a population of around 675,000.

Yellow zone

The yellow area is the area that is estimated to be prone to the falling of pyroclastic particles from the ash cloud. This would include damage to buildings as well as respiratory problems. It is noted that in 1631 only a small percentage of the yellow zone was affected, but due to uncertainty it covers surrounding provinces. Blue zone This is a sub-region of the yellow zone, which would also be prone to flooding due to the hyrdo-nature of the area.

There are also four classifications of alarm state which are detailed. The first two levels which correspond to very low and low probability of eruption are ’basic’ and ’attention’ phase. The volcano is currently in the base state, and has been for a long period of time, which indicates the overdue nature of an upcoming eruption, but does not suggest imminent danger. In an attention phase, whilst an eruption is not necessarily clear, municipality leaders in the red-zone would be advised to begin logistics of the evacuation of the towns. The third alarm state is the pre-alarm state, which is triggered by further increase in the physical and chemical parameters suggesting volcanic activity.

At this stage the area is controlled by police and rescuers and the population have the choice to evacuate the area with their house surveyed for theft. Health centres are gradually evacuated and the cultural heritage and property of the municipalities are relocated. The final stage of alert, that suggests that an eruption over the course of the following weeks is almost certain, is the alarm stage. This essentially consists of initially entirely evacuating the red zone to safer areas known as a primary evacuation. The 72 hour plan comprises on 12 hours of logistics planning, 48 hours of evacuation with 500 buses and 220 trains deployed from the region [21].

An additional 12 hours is allowed as a caution period. It is thought by local experts that the plan for evacuation does not go far enough, and in a documen- tary presented by channel4 the evacuation routine was described as ’insufficient’ [19]. The eruption similar to the one that took place in 1780 BC could bury the red zone, making the area uninhabitable for decades. In effect an eruption of similar scale which is possible, would lead to a refugee crisis. This is something that the evacuation plan does not properly account for.

After the primary evacu- ation a more permanent relocation would take place, in a process that has been labelled ’twinning’. Twinning is a process that pairs the individual 25 minicipalities with regions outside the yellow zone, which would receive the evacuees [22]. Where the critics say there is not sufficient planning is the lack of evacuation planning from within the yellow zone. In the case of pyroclastic fallout, not only would the population be forced out of the area the infrastructure would also be damaged.

Studies have been carried out to evaluate the roof resistance to loads in pyroclastic fallout in the area. As mentioned in the Simulations section the wind direction plays a huge role, however in envisaged scenarios presented by the university of Geneva, in the case of a south easterly wind directed towards Rome (probability 0.93%) up to 50,000 roofs could collapse affecting one of the main cities in Italy [23]. Not only do preventative measures for a pyroclastic fallout seem minimal, there does not seem to be much economic preparation for such an event.

A local volcanologist, Giuseppe Mastrolorenzo, stated ‘It is politically negative to talk about the cataclysmic event’ [19]. He goes on to confirm that preventative measures do not go nearly far enough. In 2012, italian geologists were sentenced to manslaughter for failing to correctly predict a 2009 earthquake killing over 300 people [24]. This potential eruption could have a significantly higher death toll if incorrectly assessed, making the under preparation more shocking.

Italian Government Grant

There is a total population within the red-zone of 600,000 people, of which a targeted 150,000 residents were hoped to be removed from the area. The government offer a grant of 30,000 euros to local businesses in the area in return for relocation out of the Vesuvius red-zone [25]. Unfortunately this has not been an effective policy for the simple reason that the residents do not view it as enough of a risk/incentive to relocate. The plains of Vesuvius are of course high in minerals from volcanic deposits. This issue highlights one of the biggest problems in the damage prevention – the locals do not trust the warnings of the severe nature of a possible eruption. This of course puts the authorities in a difficult position.

References

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