The Volcanoes of Naples: how effectively mitigating the highest volcanic risk in the World?

The Naples (Southern Italy) area has the highest volcanic risk in the World, due to the coexistence of three highly explosive volcanoes (Vesuvius, Campi Flegrei and Ischia) with extremely dense urbanisation. More than three millions people live to within twenty kilometres from a possible eruptive vent. Mitigating such an extreme risk is made difficult because volcanic eruptions forecast is today an empirical procedure with very uncertain outcome. This paper starts recalling the state of the art of eruption forecast, and then describes the main hazards in the Neapolitan area, shortly presenting the 10 activity and present state of its volcanoes. Then, it proceeds to suggest the most effective procedures to mitigate the extreme volcanic and associated risks. The problem is afforded in a highly multidisciplinary way, taking into account the main economic, sociological and urban issues. The proposed mitigation actions are then compared with the existing emergency plans, developed by Italian Civil Protection, by highlighting their numerous, very evident faults. Our study, besides regarding the most complex and extreme situation of volcanic risk in the World, gives guidelines to assessing and managing 15 volcanic risk in any densely urbanised area.


Mt. Vesuvius
The Somma-Vesuvius volcanic complex (SV) is the most known volcano of the Neapolitan area, and one of the most famous in the World, mainly because of its ancient eruption of 79 A.C., well described by Plinius the Young (Scandone et al., 2019).
SV is a renowned volcano also because it frequently erupted in the last centuries, so becoming popular as a natural attraction in Europe, and one of the classical stops of the XIX century Italian 'Grand Tour' (Astarita, 2013). towns around the volcano grew of about three times in population, becoming seamlessly connected to the city of Naples. It is important to note that towns around Vesuvius, which were primarily tourism-oriented before the II World War, are now very 135 busy outskirts of Naples hosting resident population. The present activity of this volcano, otherwise quiescent, only consists of a background seismicity seldom overcoming magnitude 2, with maximum magnitude M=3.6 reached by an earthquake occurred in October 9th 1999, during a period of increased seismicity rate and magnitude, lasted some months (between 1999 and2000;see De Natale et al., 2004). A convincing explanation for background seismicity, at this and similar volcanoes, was given by De Natale et al. (2000); however, except for the increased seismicity occurred in the mentioned 140 period, no other signals of anomalous activity have been ever recorded at this volcano (i.e. ground uplift, geochemical anomalies, consistent LP seismicity, seismic tremor, etc.).
The volcanic hazard at SV has been thoroughly described, in a probabilistic framework: firstly by Rossano et al. (1998) and then, in a more complete way, by De Natale et al. (2005). The main volcanic hazards are pyroclastic flows and ash/pumice fallout, but also associated hazards like earthquakes, lahars, lava flows and floods, need to be considered. In particular, large 145 floods caused by the re-mobilization, due to heavy rains, of the old, loose pyroclastic deposits on the topographic reliefs around Vesuvius, caused almost total destruction and 160 casualties in May 1998 (Mazzarella and Tranfaglia, 2002).
Somma-Vesuvius volcano was the first one for which a complex emergency plan had been issued, by Italian Civil Protection, in 1995. The features of this first plan were almost the same of the present ones (Dipartimento Protezione Civile, 2015), although some minor modifications have been made in the last decades. The eruptive scenario, used to define the red 150 zone (shown in fig.5), was a sub-plinian eruption, mainly because, at the time in which the first plan was issued, the idea that a shallow magma chamber was almost constantly fed by deeper magma was predominant in literature (e.g. Santacroce, 1983), and related computations of the volume of magma fed from 1944 were consistent with a sub-plinian eruption. Later on, such a volcanological hypothesis has been heavily questioned, mainly as a consequence of seismic tomography studies which failed to identify such a shallow chamber filled with molten magma (Zollo et al., 1996). In particular, Marzocchi et al. 155 (2004) pointed out that, from probabilistic estimates, after 60-200 years of quiescence a violent Strombolian eruption (VEI=3) is the most probable event, whereas a subplinian one (VEI=4) has a probability to occur much lower and a Plinian one (VEI=5) has still a probability >1%. However, in the emergency plan the sub-Plinian scenario has been maintained. The updated red zone of Somma-Vesuvius hosts today about 700.000 resident people, and totally or partially includes 25 municipalities (see Italian Civil Protection website: http://www.protezionecivile.gov.it/media-communication/dossier/detail/-160 /asset_publisher/default/content/aggiornamento-del-piano-nazionale-di-emergenza-per-il-vesuvio).

Campi Flegrei Caldera
Campi Flegrei caldera, located in the Southwestern part of the Campanian Volcanic Zone, contains the Western District of the city of Naples (see fig.3). It is a collapse caldera, formed by the huge eruption of Neapolitan Yellow Tuff, occurred about 15.000 years BP (Rolandi et al., 2003;2019a;De Natale et al., 2016). Neapolitan Yellow Tuff, a VEI=6 event with an 165 erupted volume of about 40 km 3 , generated pyroclastic flows which represent the main eruptive products found in the Naples province, that have been the main building material in this area for more than 2.000 years, before the concrete became diffused. One of its facies, named 'Pozzolana' (from the name of the town, Pozzuoli, located at the caldera center) has been the main element composing the famous 'Roman cement' which allowed ancient buildings of the Roman age to be so resistant and long lived (Vanorio and Kanitpanyacharoen, 2015). Campi Flegrei caldera has been long thought to be firstly 170 formed by the larger European eruption ever known, namely the Campanian Ignimbrite, a VEI=7 event whose erupted volume estimates range between 150-300 km 3 (Rosi and Sbrana, 1987;Orsi et al., 1996). However, Rolandi et al. (2019b) recently demonstrated that Campanian Ignimbrite main products (Grey Tuffs) were erupted from the Campanian Plain, North of Campi Flegrei, and did not cause any caldera collapse. Campi Flegrei caldera eruptive activity has been dominated by phreato-magmatic eruptions, whose explosivity is due to the contact of rising magma with the large geothermal system 175 located beneath the caldera, down to about 2.5-3.0 km (Rosi and Sbrana, 1987;Piochi et al., 2014). The collapsed area, as recognized by geophysical data (Cassano et al., 1987;Capuano et al., 2003) has a radius of about 3 km, with center approximately located at the Pozzuoli town harbour. All the eruptions younger than 10.000 years are located within such an area, more frequently occurring from its borders, marked by buried caldera ring faults (De Natale and Pingue, 1993;De Natale et al., 1997). The eruptive history of Campi Flegrei is sketched in fig.6. The only eruption in historical times occurred 180 in 1538, and this is the reason why this volcanic area is by far less renowned than Vesuvius. However, this area is also characterized by unrest episodes with large uplift and subsidence of the ground level. In the last 2.000 years, subsidence, at an almost constant rate between 1.5 and 2.0 cm/year, has been generally dominant, except during about one century preceding the 1538 eruption, when fast uplift occurred, at an average rate about one order of magnitude larger than the secular subsidence (see Fig.7). Another episode of fast uplift, but without a subsequent eruption, has been inferred by 185 Morhange et al. (2006) between the VII and the VIII century. Starting since the 50's of the last century, the ground movements in the area again reversed to uplift, which, since 1950 to 1984 totalled about 4.5 m with peak rates, in 1983-1984, of about 1 m/year. After about 20 years of relatively fast subsidence following the 1984 peak of vertical ground displacement, uplift started again around year 2005 (Fig.7), at rates comparable to those of subsidence (on average 9 cm/year), but much lower than previous uplifts (Moretti et al., 2017;Troise et al., 2018). Both the post-1984 subsidence 190 and the subsequent and still ongoing uplift phase, showed minor, short-lived peaks of uplift followed by a fast recovery of the whole uplift (the so-called 'mini-uplift' episodes: see Gaeta et al., 2003;Troise et al., 2007). The intermittent uplift phases, started in 1950 and still ongoing, show a cumulative uplift, in about 60 years, consistent with an average rate of 0.075 m/year. The average uplift rate computed from the total uplift observed since about one century and half before the 1538 eruption, although in the very rough approximation of secular inferences, is about 0.1 m/year, which is the same order 195 of magnitude and not clearly distinguishable considering the large uncertainties. The 1950-2019 unrest, with intermittent ground movements and seismicity, gives a very precise idea about the large uncertainty to identify true eruption precursors in this area. In 1970, during the first, fast uplift episode clearly identified, the urban area of Pozzuoli closest to the harbour, namely the 'Rione Terra', was completely evacuated, and never inhabited anymore. In 1984, the whole town of Pozzuoli was evacuated (in the newly built town of Monteruscello), and spontaneously re-occupied some months after when 200 seismicity stopped and ground started to subside (Barberi and Carapezza, 1996) It is worthy to note that the emergency plan for Campi Flegrei, actually on the way to be completed, at the end of 2012 did not exist yet, not even the red zone was defined (it was officially released in 2015, see Department of Civil Protection website). This probably occurred because, differently from the first edition of Vesuvius emergency plan of 1995, there was 210 no idea of a given eruptive scenario for this area, not even the eruptive vent could be defined because it could be everywhere in the caldera area. In fact, Rossano et al. (2004) firstly suggested to use a probabilistic scenario made up of any possible kind of eruption from any possible vent spanning the caldera area; the probability of each eruption type was inferred from its frequency in the last 10.000 years. Rossano et al. (2004), using a rigorous Bayesian approach and a simplified modelling technique for pyroclastic flows on the actual topography, first obtained a probability hazard map which clearly indicated, as 215 the most probable zones experiencing pyroclastic flows, an area very similar to the presently defined red zone. More accurate results, using the same methodology, were further obtained by Mastrolorenzo et al. (2006b). However, their hazard maps were never considered by Civil Protection authorities, at the time, and only 11 years after they officialised the red zone, based on a paper by Neri et al. (2015), which used a very similar probabilistic approach (although with a much more approximated and less rigorous pyroclastic flow modelling technique). The only difference between the two methods 220 (besides the oversimplification of pyroclastic flow modelling) is that Neri et al. (2015) assumed a non uniform probability for vent opening, based on the assumptions and results of Bevilacqua et al. (2015). The final results are, regarding the definition of the red zone, very similar to the results obtained by Rossano et al. (2004) and Mastrolorenzo et al. (2006b). The red zone of Campi Flegrei hosts today about 600.000 resident people, and totally or partially includes 6 towns and several suburbs of Naples city (see Italian Civil Protection website: http://www.protezionecivile.gov.it/attivita-rischi/rischio-225 vulcanico/vulcani-italia/flegrei/piano-nazionale-di-protezione-civile).

Ischia island
Ischia island, located South-West of Campi Flegrei, is another volcanic field, characterized by both effusive and explosive eruptions. Eruptions here, in fact, range from lava flows to phreato-magmatic ones, thus being half-way between the Vesuvius and Campi Flegrei eruption styles. The Ischia volcanism developed between about 130-150 ka BP (Vezzoli, 1988) 230 and 1302 AD (de Vita et al., 2006;2010). Fig.8 shows the eruptive history of Ischia island. The volcanism in the island is strictly linked with the resurgence phenomena of the Mt. Epomeo, an horst which is thought to move up and down (with dominant uplift in the past, due to its prominently high topography). Resurgence has been ascribed to repeated injections of magma at depths of 2-3 km, where a laccolite magma chamber is hypothesized (Orsi et al., 1991;Cubellis and Luongo, 1998;Tibaldi and Vezzoli, 1998;Acocella and Funiciello, 1999;Molin et al., 2003;Carlino et al., 2006;Paoletti et al., 2009;235 Sbrana et al., 2009;). The last eruption occurred in 1302, so also Ischia is poorly recognized as a volcano, by common people. The movements of the Epomeo horst also cause slip on the bordering faults, which decouple it from the rest of island. For this reason, Ischia has been struck in the past by several catastrophic earthquakes which, although not so high in magnitude, are very shallow thus very destroying within a short distance (see Table 1). The most destructive earthquake at our knowledge has been the July 23 rd , 1883 event, which completely destroyed the town of Casamicciola and parts of 240 neighbouring towns, killing 2313 people (De Natale et al., 2019). This catastrophic event was preceded, two years before, by a slightly smaller event, killing anyway 126 people; these two larger events were the final ones of a sequence of 6 large What is also relevant, for the subsequent considerations, is that both the two existing Emergency Plans (for Vesuvius and Campi Flegrei), regarding the evacuation plan in case of red alarm, relies only on the use of land transportations, not considering the use of ships. This choice, that is probably driven by the concern for possible tsunamis accompanying 290 eruptions (anyway extremely rare in such areas, and surely not expected before the eruption), cannot be equally applied to an Emergency plan of the Ischia island that, however, is the only remaining volcanic area where there is not yet any Plan. Coming back to describe more details of the Emergency Plans, it is important to note that the first three steps of the alert: 300 Green, Yellow, Orange, are decided by the National Civil Protection, normally upon advice from the National Commission for High Risks, whereas the last step, from Orange to Red (implying complete evacuation of the red zone in 72 hours) is only decided by the Italian Premier (Dipartimento Protezione Civile, 2015). Regarding the evacuation plan, which starts once the Red alert is declared, evacuated people is meant to be distributed in all the Italian Regions, according to a correspondence between each municipality and a given Italian Region. There is however no other detail in the Plan, programming, for 305 instance, exactly where (i.e. in which houses, hotels, etc.) people will be re-allocated in each Region.

Evacuation Plans: strength and weakeness
We do not aim to discuss in detail the Emergency Plans in their intermediate steps. We only focus on what should occur after the declaration of the Red alert, which implies the rapid (within 72 hours), complete evacuation of the red zone. As already mentioned, the red zones of Campi Flegrei and Vesuvius presently contain, respectively, about 600,000 and about 700,000 310 people. The evacuation plans, in the present formulation, state they must be evacuated by roads, on-land. Most of residents in the red zones (and also out of them) are sceptic about the real possibility to successfully evacuate such a high number of people within three days (Solana e al. 2008;Carlino et al., 2008). They believe it would be not possible, both because of the likely massive panic that would spread across, and for the huge traffic which characterizes the few main roads to evacuate, even in normal days. In the turmoil that would likely accompany a massive evacuation, it is easy to imagine those roads 315 completely jammed. These are, however, just feelings of the people, and we will assume here the evacuation can be successfully organized. There are anyway two former, successful example of evacuation in the Neapolitan area, both of them in the Campi Flegrei area. The first evacuation occurred in 1970, at the beginning of the first recent (recognized) large Campi Flegrei unrest of the period 1969-1972. About 3000 people were forcedly evacuated, in just one day, from the Rione Terra, a district of Pozzuoli just behind the Port of Pozzuoli, which was at the time (and also in the following unrest 320 episodes) the area of maximum uplift. After that episode, in 1984, when the subsequent unrest was rapidly progressed and continuous earthquakes caused extreme concern, the whole town of Pozzuoli, about 40,000 people, was evacuated and transferred in a new town: Monteruscello, located about 3 km NW and built in few months to host the Pozzuoli citizens (Barberi and Carapezza, 1996). The evacuation of Pozzuoli town was, in the opinion of several experts, probably the most successful operation of Civil Protection in Italy. However, it involved more than an order of magnitude less people, with 325 respect to the present red zone of Campi Flegrei (or, equivalently, of Vesuvius). In addition, the main productive activities (factories) in the Pozzuoli area were not stopped, and evacuated people could go to work anyway in the 'red zone' of that time. Finally, while the Rione Terra was never let to be populated again, the complete evacuation of Pozzuoli lasted about one year or less, and after that period the town was fully populated again, because people almost spontaneously came back. https://doi.org/10.5194/nhess-2020-51 Preprint. Discussion started: 27 March 2020 c Author(s) 2020. CC BY 4.0 License.
The red zones defined in the present emergency plans are chosen in order to take into account the largest eruptions having 330 non negligible probability, so involving very large numbers of people; and some volcanologists ask for even higher precaution (e.g. Mastrolorenzo et al., 2017). Actually, however, defining very large red zones to be evacuated before an impending eruption, could seem a better caution, but it also makes the evacuation decision a much more heavy responsibility to assume; dramatically costly in case of false alarm.
Another choice which could be surely debated, about the effectiveness of the present evacuation plan, is the lack of 335 evacuation by sea, with large ships which could rapidly move a lot of people without any traffic problem. This choice is probably due to the fact that the ports in Campi Flegrei and Vesuvius towns (except for the port of Naples) are not suited to host large ships; another obstacle is probably thought to be the possibility that tsunamis may accompany the eruptions. The evacuation by the sea, anyway, is the only one which can work for an evacuation plan of Ischia island, which is compelling and, sooner or later, must be done. Moreover, looking at historical eruptions of Neapolitan volcanoes, it turns out there is no 340 evidence for any tsunamis associated to their eruptions.
We proceed now to make clearly evident the main problems of the present day Emergency Plans. As already said, we will not discuss the steps from Green to Orange, nor we want to assess the details of the first evacuation phase, i.e. the way to move 600,000-700,000 people out of the red zone. Regarding the present choice to move people exclusively on-land, we just noted that evacuation by sea, using large ships, would be much more rapid and effective, avoiding the multiple problems 345 linked to the traffic and to the lack of appropriate roads.
We want, instead, to discuss here two problems, which are also in some way interrelated: the extremely high number of people to evacuate in case of an impending eruption, and the lack of plans, today, to reallocate such a high number of evacuated people taking into account realistic times people will have to spend out of their homes in the red zone.
Regarding the first problem, namely the high number of people to evacuate, it is clear that the decision-makers have to take a 350 very big responsibility to declare the Red alert, which will cause dramatic social problems and economic damages. The economic loss per each year the evacuation lasts can be reasonably estimated by considering that 600000 people are almost 1% of the total Italian population. So, by suddenly stopping the economy produced by 600000 people would represent a loss of 1% of the Italian PIL. Since the annual Italian PIL is around 2000 G€, 1% is about 20 G€. To such high cost it should be added the cost of assistance to the evacuated people (i.e. travel, hosting, subsistence, services, etc.), whose minimum 355 estimate (15-20 k€ per year per person) gives another 10-20 G€/year. A total cost in the range 30-40 G€/year (for Campi Flegrei; for Vesuvius it would be about 20% larger) represents the amount of one of largest annual financial package of Italian Government; so, it is likely unsustainable, even for just one-two years. But the real problem, fundamental also to evaluate the real amount of social disease and total economical loss, is the second one: how much time will such a large number of people spend out of the original towns? To answer this question, we can consider two possible cases: 360 1) the eruption occurs in short times after the alert

2) the eruption does not occur in short times
In the first case, it is clear that a considerable part of the evacuated area will be destroyed or anyway seriously affected, so that several years, probably decades, will be needed to restore conditions to make liveable again the area. But, anyway, the occurrence of an eruption volcanologically represents a new variable, which will make even more unpredictable what could 365 be the subsequent activity of the volcano. A clear example of such a long lasting eruptive phase, for a volcano which was quiescent since 400 years, is the case of Soufriere Hills in Monserrat, erupted for the first time in 1995, evacuated since then and still in alarm because experiencing consecutive eruptions (Smithsonian Institution website: https://volcano.si.edu/volcano.cfm?vn=360050 and references therein) Regarding Neapolitan volcanoes, volcanologists normally assume that, after a long non-eruptive period, a new eruption of 370 Mt. Vesuvius will open a new cycle of eruptive activity, which can become much more frequent like it was during the XVII to XX centuries period (Santacroce, 1983). For Campi Flegrei and Ischia, which are dormant since several centuries, the https://doi.org/10.5194/nhess-2020-51 Preprint. Discussion started: 27 March 2020 c Author(s) 2020. CC BY 4.0 License. occurrence of an eruption today would make equally much more unpredictable the future evolution of volcanic activity. All these considerations make very evident that, in the event 1, realistic times to repopulate the red zone would be extremely long or indefinite, on the basis of objective considerations. 375 What would happen if, on the contrary, the eruption would not occur in short times after the evacuation (event 2)? In this case, we have to consider that, if the precursory signals were so strong and evident to convince decision-makers to evacuate 600,000-700,000 people causing a real 'disaster', in an economical and social sense, they could certainly not decide to put again people at the same high risk situation, considering that times of preparation of an eruption, are mostly unknown but can certainly be very long in some cases. Also in this case, a typical example of what could happen is given by the unrest 380 episodes at Campi Flegrei. We know that intense uplift episodes started already in 1950, although apparently that unrest was not noted. However, after the unrest of 1969-1972, people thought the danger were over, even if the 'Rione Terra', the urban area very close to the Port, which was evacuated at that time was never re-populated again. After about 10 years, a new unrest episode started, with even higher rates of uplift and much more intense seismicity (De Natale et al., 1991). At the end of 1985, people again thought the danger was over, and Pozzuoli town was populated again after the evacuation. However, 385 after about 20 years, a new unrest episode has started, still on-going today, which again poses large concern in the volcanologists, authorities and population. In practice, the alert at Campi Flegrei lasted 70 years till now, and is still ongoing; the first evacuated zone, Rione Terra (3000 people evacuated), has not been repopulated; Pozzuoli was, but it must be noted that only 40,000 people were evacuated, that the main economic activities were not stopped, and that Monteruscello (the new town hosting evacuated people) was very close to Pozzuoli and 'inside' the Pozzuoli municipality. In case of 390 600,000-700,000 people, scattered all along Italy (as the present evacuation plan prescribes), it would be impossible, today, to consider they could come back to their homes in similar conditions.

Elements for a reliable Evacuation Plan and Emergency Management
The nature and size of volcanic hazard in the Neapolitan areas, as well as the experience of previous evacuation inside the Campi Flegrei area, give important suggestions on how to build a really working Emergency Plan. The previous experiences 395 of evacuation inside the Campi Flegrei area were successful (although no eruption occurred), but limited to 3000-40000 people. Increasing the number of evacuated people by 1 to 2 orders of magnitude, although it could seem to be more conservative with respect to the possible occurrence of larger eruptions, introduces additional, very huge problems. They are related, how we explained in the previous paragraph, to the extreme responsibility taken by the decision maker, in terms of economic and social costs, as compared to the high uncertainty about the evolution of volcanic phenomena. These problems 400 necessarily translate into very long times of permanence of evacuated people out of the red zone, in case of evacuation. Such times can be estimated, in the most optimistic way, in the order of many years or decades. This means that the evacuation plan cannot simply provide that all the people goes safely away from the red zone: it must provide a sort of 'second life' for the evacuated people, which must live in the new place for decades, perhaps forever. Obviously, in this case it is not realistic to assume (as the present plan implicitly does) that several hundred thousands people can live for decades as refugees, in 405 temporary accommodation like hotels, etc., assisted by the Government. Making some simple (and optimistic) calculations, besides the unbearable social unease, the economic costs of such a condition would be on the order of 30-40 billions euros per year. The amount of economic and social costs of an evacuation from one of the two main volcanic areas, operated as imagined till now, thus clearly demonstrate this is not only a problem for Italy, but surely of European scale.
It should be now clear that the problem of volcanic hazard in the Neapolitan area cannot be afforded in the way it has been 410 thought till now. In view of a rational approach to this incredibly hard problem, some basic conditions should be reached well before the starting of a volcanic crisis possibly leading to an eruption. The most basilar conditions are: 1) the number of residents in the red zones must be decreased; https://doi.org/10.5194/nhess-2020-51 Preprint. Discussion started: 27 March 2020 c Author(s) 2020. CC BY 4.0 License.
2) the urban areas in the red zones must be made less densely populated and chaotic, with large roads, escape routes and edifices resistant to seismicity which accompanies volcanic unrest; 415 3) the evacuation of the population must be completely organized well before the crisis: all people should be assigned a new home, a new working perspective, and all the services for living there for many years/decades, likely forever (schools, hospitals, medical care, leisure's, etc.).
The first two points are fundamental in order to make really feasible a massive evacuation in case of red alert, and to protect the population from the most common phenomena (mainly earthquakes) occurring during unrest and pre-eruptive phases. 420 The third point is, on the contrary, compelling to avoid that a possible evacuation would result in a social and economic disaster. However, a careful prior organization of a future evacuation, for all the population (point 3), may also help to afford the problem at point 1, and consequently the problem at point 2. In fact, a prior organization of the 'second life' of people in case of evacuation may convince several people, if incentivized in some way, to abandon in advance the red zone, well before any significant official alert. A significant decrease of residents in the red zone (point 1) will make easier to re-425 organise and re-planning the urban areas, making them more resistant and resilient (point 2). Associated to these measures, another important improvement of the Emergency Plans would be to introduce the concept of 'progressive evacuation'. At present, only massive, total evacuation of the whole red zone is considered in the Emergency procedures. As we already discussed, deciding to move several hundreds thousands people is a very huge responsibility for decision makers; in particular because, even in presence of strong anomalies which can be considered pre-eruptive signals, the probability of 430 false alarm is extremely high: probably anyway higher, even very close to the eruption time, than the probability of eruption.
The experience of the past, and in particular the two successful but limited evacuations in the Campi Flegrei area (1970 and 1984, respectively 3000 and 40000 people) suggests to operate a progressive evacuation, which starts in a limited area, where precursory signals (and/or prior data) indicate the highest probability of eruption and/or of phreatic explosions, and then proceeds in progressively larger areas if the pre-eruptive signals increase (or first eruption phases start/progress). Past 435 examples of successful evacuation (i.e. Pinatubo 1991, see Tayang et al., 1996) operated in a progressive way, by enlarging the evacuated area following the evolution of the eruptive activity. Such a procedure has the advantage to allow to evacuating the most hazardous areas without causing disastrous social and economic consequences and, in particular, without to be pushed to wait for macroscopic unrest signals (in the hope to absolutely avoid false alarms). When operating with progressive evacuation, in the first steps (with relatively few people evacuated) residents could be let free to choose if 440 definitively abandon the red zone, proceeding to the planned 'second life', or to wait for some time in temporary housing, likely not very far from the evacuated area.
The association of prior programmed 'second life' of evacuated people and of the progressive evacuation could hence work very well, in cases similar to the very long and variable 1950 to present Campi Flegrei unrest, to help decreasing the number of residents and to allow improving the urban resilience in the risky areas. 445

Conclusion
The Neapolitan volcanic area, with three explosive volcanoes and about three million people closely exposed, has the largest risk in the World. The volcanic risk is here associated with other risks, the main one being the seismic risk. Risk mitigation in this area is, for these reasons, a paradigm to manage all the situations of densely populated volcanic areas in the World. It is very clear that, given the present state of the art of volcanology, volcanic risk mitigation in densely populated areas cannot 450 rely only on eruption forecast, still based on empirical procedures largely uncertain and not even really quantifiable in a probabilistic way. For this reason, we suggest here that effective mitigation procedures must, in these cases, to be flexible enough to take into account economic, social and political considerations in addition to volcanological ones. In fact, in densely populated areas one is faced by the double problem of low reliability of forecast and no possibility to estimate the size of the eventual eruption. In the present emergency plans for Neapolitan volcanoes, the probability of missed alarm is practically neglected, and the 'red zones' (i.e. the areas to be quickly evacuated before the eruption) are assumed very large on a precautionary base, in order to manage the occurrence even of the largest eruptions, unless considered very unlikely.
These two assumptions are both very critical: the first one is demonstrated to be simply wrong, from all recent eruptions experience; the second one, in the light of volcanological considerations, is demonstrated to make the evacuation decision too heavy for decision makers, because potentially catastrophic in economic and social terms, mainly considering the high 460 probability of false alarm. Once we put in evidence the clear faults of the present emergency plans, we show what should be the guidelines for making them really effective. The first, essential requirement for Neapolitan area, is to decrease population in the most exposed urban zones, well before any volcanic emergency. There are economical considerations, only mentioned here and to be deepened with the help of economists and social scientists, which could make feasible such a difficult task in this region. Another imperative action is to improve the quality of buildings, reinforcing them to be resistant to earthquakes 465 which unavoidably precede and accompany both eruptions and unrest episodes. Once the closest urban areas are made more resistant and hence more resilient, the possible evacuation before an impending eruption must be thoroughly programmed in advance, so to minimize the economic and social impacts. A really feasible emergency plan, in addition, should consider a 'progressive' evacuation, which would start from the most risky area and then would progressively proceed to farther areas, if the precursory signals increase or eruption starts. 470 deformations in the Phlegrean Fields caldera, Italy, Geology, 34, 2, 93-96, 2006 Moretti, R., De Natale, G., Troise, C.: A geochemical and geophysical reappraisal to the significance of the recent unrest at   780 urbanization). Note than more than 3 million people live in the shown volcanic area, than making it the most risky in the World.