**Campi Flegrei: Assessing the Eruptive Potential and Multifaceted Impacts of a Restless Supervolcano**

**I. Introduction to Campi Flegrei: A Restless Caldera**

**A. Geological Context and “Supervolcano” Status**

Campi Flegrei, situated west of Naples, Italy, is a vast volcanic depression known as a caldera, spanning approximately 12 to 15 kilometers (8 miles) in diameter.1 It stands as the largest active caldera in Europe and forms part of the Campanian volcanic arc.5 The volcanic system’s activity commenced over 60,000 to 80,000 years ago.6 The term “supervolcano” is frequently applied to Campi Flegrei, reflecting its substantial dimensions and a history marked by colossal eruptions, most notably the Volcanic Explosivity Index (VEI) 7 Campanian Ignimbrite event.3 However, it is important to note that the strictest definition of a supervolcano typically refers to systems capable of VEI 8 eruptions, a threshold not met by all of Campi Flegrei’s past events.5 The caldera system is a complex geological feature, comprising 24 distinct craters and various volcanic edifices, a significant portion of which are submerged beneath the Gulf of Naples.5

The geological setting of Campi Flegrei as a large, active, and historically explosive caldera is fundamental to appreciating the scale of the potential hazards it poses. Its geographical position, embedded within the densely populated metropolitan area of Naples, significantly elevates its risk profile, making it a subject of intense scientific scrutiny and public concern.3

The caldera’s structure is not a simple depression but rather a “nested” complex, formed by at least two major collapse events associated with the Campanian Ignimbrite and the later Neapolitan Yellow Tuff eruptions.6 This intricate geological architecture has profound implications for the volcano’s behavior. The faults, fractures, and varying rock strengths resulting from these past collapses create a heterogeneous subsurface. Such heterogeneity influences the pathways of ascending magma and hydrothermal fluids, potentially channeling them along zones of weakness or impeding their progress where more competent rock exists. Consequently, ground deformation patterns during periods of unrest may not be symmetrical, and the locations of future eruptive vents can be varied and difficult to predict with certainty.11 This complexity makes forecasting specific eruption characteristics more challenging than for volcanoes with simpler structures.

While the “supervolcano” designation accurately reflects Campi Flegrei’s capacity for exceptionally large eruptions, the primary concern for contemporary risk management is not necessarily a cataclysmic VEI 8 event. Instead, focus is often placed on the higher probability of smaller, yet still potentially devastating, eruptions in the VEI 4 to VEI 6 range, particularly given the extreme population density within and around the caldera.11 Italy’s National Institute of Geophysics and Volcanology (INGV) emphasizes that the probability of another eruption on the scale of the Campanian Ignimbrite or Neapolitan Yellow Tuff occurring without massive, easily detectable precursor signals is very low.20 Thus, while the potential for super-eruptions defines its geological character, the operational risk assessment is more nuanced, concentrating on events that are less extreme in magnitude but far more probable within human timescales. The dramatic “supervolcano” label, while useful for public awareness of the ultimate potential, can sometimes overshadow the more immediate and statistically more likely threat posed by these smaller, but still highly impactful, eruptions.

**B. Brief Overview of Eruptive History and Significance**

The eruptive history of Campi Flegrei is dominated by two cataclysmic caldera-forming events. The first was the Campanian Ignimbrite (CI) eruption, which occurred approximately 39,000 to 40,000 years Before Present (BP) and is classified as a VEI 7 event.2 This was followed by the Neapolitan Yellow Tuff (NYT) eruption, around 15,000 years BP, a VEI 6 event.2 In the 15,000 years following the NYT eruption, the caldera has witnessed over 70 smaller eruptions.6 These events were not randomly distributed in time but rather clustered into three distinct epochs of heightened activity, separated by prolonged periods of quiescence. The most recent eruptive event in this historical sequence was the formation of the Monte Nuovo cinder cone in 1538 AD, an eruption assessed as VEI 2-3.2

This eruptive history is the primary foundation for forecasting potential future scenarios, establishing Campi Flegrei’s capacity for a wide spectrum of eruption styles and magnitudes. The pattern of episodic activity, with eruptive epochs interspersed with long quiet intervals (e.g., an approximately 3,000-year hiatus before the 1538 AD eruption 2), is particularly significant. The current period of quiescence, approaching 500 years since the Monte Nuovo event, falls within the range of historically observed inter-eruptive periods. This historical context suggests that a return to eruptive activity is a realistic, albeit unpredictable, possibility.3 The volcano’s known behavior includes these long “rests” followed by renewed activity, meaning the current prolonged phase of unrest, while not guaranteeing an imminent large eruption, is consistent with its historical pattern of reawakening.

Furthermore, the phenomenon of resurgence – the slow uplift of the caldera floor – has played a critical role in shaping volcanic activity since the NYT eruption. The central part of the caldera has been uplifted by approximately 90 meters in the last 10,000 years.6 This resurgence has profoundly conditioned subsequent volcanism, with many post-NYT eruptions occurring at the margins of the uplifted area.39 This structural control, where the more rigid, uplifted central block may force magma to exploit weaknesses at its periphery, is a crucial factor in developing probability maps for future vent openings and is a key input for quantitative hazard models.11

**II. Current State of Unrest and Monitoring**

**A. Bradyseism, Seismicity, Ground Deformation, and Gas Emissions: Recent Trends**

Campi Flegrei is currently under a “Yellow” (Attention or *Attenzione*) alert level, a status maintained since 2012 due to persistent signs of unrest.6 This unrest is primarily characterized by bradyseism, a phenomenon of slow ground uplift and subsidence. Since 2005, a notable phase of uplift has been observed, accumulating to approximately 145 cm at the RITE GNSS station by April 2025\.2 The rate of this uplift has been variable; for instance, it was recorded at approximately 10±3 mm/month in August 2024, accelerated to around 30±5 mm/month between February and March 2025, and subsequently decreased to approximately 15±5 mm/month in early April 2025\.29

This ground deformation has been accompanied by a significant increase in seismic activity. Numerous earthquake swarms have been recorded, with some events being strong enough to be felt by the local population. A particularly notable event was the magnitude (M) 4.4 earthquake on May 20, 2024, which represents the highest magnitude earthquake instrumentally recorded at Campi Flegrei.6 Since 2021, peculiar “burst-like” seismic swarms, characterized by very short time intervals between individual earthquakes, have been observed, primarily located in an area that includes the main hydrothermal field of Solfatara-Pisciarelli and is associated with a geodetic anomaly (a region uplifting less rapidly) near Monte Olibano.35

Geochemical monitoring also reveals significant activity, with a persistently high flux of carbon dioxide (CO2​) from the Solfatara area, estimated at approximately 5000 tons per day (t/d). This emission rate is comparable to that observed at persistently degassing active volcanoes.29 The INGV and its Vesuvius Observatory continuously monitor these diverse parameters—seismicity, ground deformation, and gas emissions—to assess the evolving state of the volcano.2 Historical episodes of bradyseism, such as those in 1969-1972 and 1982-1984 which involved substantial uplift and led to evacuations, provide crucial context for understanding the current unrest.1

While unrest has been a feature since 2005, the period from 2023 to 2024, in particular, has demonstrated an intensification of these phenomena, including an increase in the number and magnitude of seismic events and more frequent swarms, alongside fluctuating and at times rapid rates of ground uplift.9 This dynamic and accelerated behavior suggests that the volcanic system is not in a steady state and could be approaching new critical thresholds. The occurrence of “burst-like” swarms further points to evolving subsurface dynamics, possibly related to localized stress release or the migration of fluids.35

Interestingly, despite the significant ground deformation and heightened seismicity, some key geochemical indicators, such as maximum surface temperatures measured by infrared in the Pisciarelli and Solfatara areas, have shown relatively stable trends, although the CO2​ flux remains exceptionally high.29 However, analyses considering a broader range of parameters for assessing magmatic unrest indicate that while no geochemical anomalies specifically linked to *shallow magma movement* were detected during certain periods, parameters such as the number of volcano-tectonic (VT) earthquakes and maximum earthquake magnitudes *did* exhibit anomalous behavior.9 This apparent decoupling could imply that the current unrest is predominantly driven by physical processes within the crust, such as rock fracturing and the pressurization of fluids, rather than by a large-scale intrusion of fresh magma to very shallow levels. Alternatively, it’s possible that any magmatic gases being released at depth are significantly scrubbed or buffered by the extensive hydrothermal system before reaching the surface. The persistently high CO2​ emissions are a long-term characteristic and could originate from deeper magmatic degassing that feeds the overlying hydrothermal system, which in turn drives the more superficial manifestations of unrest.29 Indeed, some research suggests a substantial portion of the CO2​ could be non-magmatic, resulting from the decarbonation of hydrothermal calcite induced by the interaction with magmatic fluids.46

**B. The Debate: Drivers of Unrest – Magmatic vs. Hydrothermal/Geothermal Influences**

Understanding the primary drivers of the ongoing unrest at Campi Flegrei is a critical scientific endeavor with direct implications for eruption forecasting and risk assessment. Traditionally, significant ground uplift in volcanic areas has been attributed to the refilling of magma reservoirs at depth or the intrusion of new magma to shallower levels.1 However, recent research has introduced alternative or complementary hypotheses for the current activity at Campi Flegrei.

A notable contribution from Stanford University researchers challenges the purely magma-driven model for the recent unrest. Their studies propose that the observed seismicity and ground deformation are primarily caused by pressure buildup from water and vapor within a sealed geothermal reservoir, a phenomenon exacerbated by the “self-healing” properties of an overlying caprock that seals the system.1 This model suggests that managing groundwater levels could potentially mitigate the unrest.

Concurrently, research by INGV has identified a “weak layer” within the Earth’s crust beneath the caldera, situated at a depth of approximately 2.5 to 4 kilometers.52 This layer is characterized as being more porous and less resistant than surrounding rock, making it a preferential zone for the accumulation of magmatic fluids (gases and liquids) and potentially stalling the ascent of small magma intrusions. This weakened zone is linked to the current patterns of ground uplift and seismicity, with some interpretations suggesting that the present activity is primarily driven by gas accumulation within this shallow, structurally compromised layer.52

These perspectives—hydrothermal/geothermal pressurization versus magma-related processes within a weak crustal layer—are not necessarily mutually exclusive. A more integrated view suggests a hybrid system where the “weak layer” acts as a crucial interface. Deeper magmatic inputs, such as gases, heat, and possibly small batches of magma originating from a reservoir at 7-8 km depth 52, could ascend into this weak layer. Here, they interact with and pressurize the overlying hydrothermal/geothermal system, which could be the reservoir described in the Stanford model.1 The weak layer facilitates the accumulation of these deep-seated fluids, while the caprock provides the seal, leading to pressure buildup. The “self-healing” nature of the caprock could even be enhanced by mineral precipitation from these mixed magmatic-hydrothermal fluids.

This complex interplay between deep magmatic inputs and shallow hydrothermal responses has significant implications for interpreting precursory signals and anticipating eruption styles. If the current unrest is largely modulated by the hydrothermal system’s reaction to deeper inputs within this weak layer, then the precursors to a magmatic eruption might be intricate and potentially ambiguous. An eruption could be heralded by an intensification of hydrothermal signals, such as steam-driven (phreatic) explosions 1, before clear magmatic precursors become evident. Alternatively, a rapid transition to magmatic eruption could occur if the caprock fails catastrophically or if a new, more voluminous batch of magma bypasses or overwhelms this shallow system. The 1538 Monte Nuovo eruption, which was preceded by significant uplift and seismicity, is cited as potentially exhibiting features of a phreatic event, suggesting that pressure within a closed geothermal system played a role in its initiation.1

The suggestion of managing groundwater to mitigate unrest 1 offers an intriguing prospect for managing the current seismic hazard associated with the hydrothermal component. However, such measures would primarily address the symptoms driven by the geothermal system. While potentially reducing seismicity and deformation linked to fluid pressure, it would not prevent a larger magmatic intrusion from depth, although it might alter the shallow system’s response to such an intrusion by reducing the availability of water for explosive magma-water interaction. This highlights a distinction between mitigating current, possibly non-eruptive unrest symptoms, and mitigating the overarching risk of a future magmatic eruption.

**C. Current Volcanic Alert Level and Its Implications**

Since 2012, Campi Flegrei has been maintained at a YELLOW (Attention/Attenzione) volcanic alert level.6 This alert level signifies that the volcano is in a state of unrest, with monitored parameters consistently exceeding baseline or background levels. Such a state necessitates enhanced monitoring by scientific institutions like INGV and heightened preparedness by civil protection authorities. The Italian Civil Protection framework for volcanic risk employs four main alert levels: GREEN (Quiescence/Base Level), YELLOW (Attention), ORANGE (Pre-Alarm/Warning), and RED (Alarm/Eruption Imminent or Underway).55 The transition from one alert level to the next is determined by comprehensive evaluations of variations in monitored parameters (including seismicity, ground deformation, and gas emissions) and any other ongoing volcanic phenomena. The declaration of Orange and Red alert levels is a high-level decision made by the Prime Minister.56

Recognizing the complexities of interpreting prolonged unrest, recent discussions and potential revisions to the alert system (as of March 2025\) have introduced the concept of sub-levels within the Yellow and Orange categories.59 For the Yellow alert, these are “weak disequilibrium” and “medium disequilibrium” (with Campi Flegrei reportedly in “medium disequilibrium”). For the Orange alert, sub-levels of “strong disequilibrium” and “very strong disequilibrium” are proposed. These sub-levels aim to provide more granular indications of the volcano’s state and allow for more nuanced and timely responses from civil protection agencies. The Yellow alert level triggers specific operational phases, including intensified monitoring, targeted information campaigns for the public, and verification of emergency plan readiness.40

The maintenance of a Yellow alert level for over a decade (since 2012\) presents considerable challenges for risk communication and sustaining public vigilance.31 A prolonged “attention” status, without escalation to a more acute crisis level (Orange or Red), can inadvertently lead to public complacency or, conversely, foster chronic anxiety among the population living in the volcanic area. The introduction of sub-levels within the Yellow and Orange alerts may be an attempt to address this by allowing authorities to communicate more subtle yet significant changes in the state of unrest without immediately triggering the major societal and economic implications associated with an Orange alert, such as preparations for large-scale evacuation.

While it is generally understood that changes in seismicity, ground deformation, and gas emissions are the primary parameters monitored for decisions on alert level escalation 6, the specific quantitative thresholds or the precise combination of phenomena that would trigger a shift from Yellow to Orange, or Orange to Red, are not explicitly detailed in publicly available documents.56 This lack of explicit, predefined public thresholds is likely due to several factors: the inherent complexity of interpreting volcanic signals, where no single parameter tells the whole story; the need to avoid false alarms which can erode public trust and have severe economic costs; and the desire for decision-makers to retain flexibility based on the totality of evidence and expert judgment from bodies like the National Commission for the Forecast and Prevention of Major Risks (Commissione Grandi Rischi). However, this opacity regarding exact trigger points can sometimes fuel public speculation and anxiety. Internally, scientific models like the Bayesian Event Tree for Eruption Forecasting (BET\_EF) do use defined thresholds to assign a “degree of anomaly” to monitoring measurements 42, but these are part of a scientific assessment tool and not necessarily the direct operational triggers for Civil Protection alert level changes, which involve a broader, high-level decision-making process including the Major Risks Commission and the Prime Minister.56 The newly proposed sub-levels may eventually have more clearly (even if internally) defined criteria for transitions between them, reflecting an evolving approach to managing and communicating risk during prolonged periods of volcanic unrest.

**III. Likelihood of Future Eruptive Activity**

**A. Probabilistic Forecasting: Methodologies and Expert Elicitation**

Given the inherent uncertainties in volcanic systems, deterministic prediction of eruptions (specifying exact time, place, and scale) is not currently feasible. Instead, scientific efforts focus on probabilistic forecasting, which aims to quantify the likelihood of future eruptive activity based on current understanding of the volcano’s behavior, its history, and real-time monitoring data. For Campi Flegrei, a key tool in this endeavor is the Bayesian Event Tree for Eruption Forecasting (BET\_EF) model. This model has been specifically calibrated for the caldera through a structured process of expert elicitation, where the collective judgment of a panel of volcanologists is used to define probabilities and relationships within the forecasting framework.9

The BET\_EF model assesses, among other factors, the daily probability that the ongoing unrest is driven by the movement of shallow magma and, subsequently, the monthly probability of an eruption occurring. For example, on May 20, 2024, a period of heightened seismic activity, the model estimated that the probability of the unrest being due to shallow magma movement ranged from 0.59 (representing the 10th percentile of expert opinion) to 0.91 (90th percentile). Based on this, the derived monthly probability of an eruption ranged from 0.01 (1%) to 0.17 (17%).9 It is crucial to understand that these probabilities are dynamic and fluctuate in response to observed anomalies in monitored parameters. Key parameters influencing these probabilities include the rate of ground uplift, and various seismicity indicators such as the number of volcano-tectonic (VT) earthquakes, the occurrence of deep VTs, and the maximum magnitude of earthquakes within a given period.9

The results from such probabilistic models, like the BET\_EF, indicate that there have been periods (e.g., August/September 2023 and April/May 2024\) where the calculated probability of the unrest being driven by magmatic processes *surpassed* the probability of it being driven by other, non-magmatic mechanisms.9 This suggests that a purely hydrothermal explanation for all phases of unrest at Campi Flegrei might be insufficient, and that periods of increased likelihood of shallow magmatic involvement do occur. However, it is important to note that even when the model indicates a higher probability of magmatic unrest, this does not automatically equate to an impending eruption. For instance, the same study that reported these elevated probabilities also concluded that, during the period analyzed, there was “no evidence of anomalies characteristic of a pre-eruptive phase” based on the full suite of monitoring data and expert interpretation.9 This highlights the inherent uncertainty: the system can exhibit signs consistent with the *potential* presence or movement of shallow magma, thereby increasing its probability in forecasting models, but these signals may not yet be unambiguous or sustained enough for experts to confidently associate them with an imminent eruption. This could imply that magma is present at shallow levels and causing some of an VTs, deep VTs, and maximum earthquake magnitude are particularly influential on the calculated probability of magmatic unrest.9 The general degree of unrest (represented as Node 1 in the BET\_EF model, often driven by uplift rate) has consistently been high during the recent period. However, the specific indicators for shallow magma movement (Node 2 parameters) have shown more variability. The absence of significant long-period (LP) seismicity and sustained volcanic tremor during the study period covered by the BET\_EF analysis is also noteworthy.9 LP events and tremor are often considered stronger indicators of magma or magmatic fluid movement within volcanic conduits. Their general absence, despite high rates of VT seismicity and ground uplift, adds to the complexity of definitively attributing all current unrest solely to active, shallow magmatic processes.

**B. Precursory Signals and Challenges in Short-Term Prediction**

Historical accounts and geological studies of past eruptions provide some insight into potential precursory signals at Campi Flegrei. The 1538 AD Monte Nuovo eruption, the caldera’s most recent, was preceded by a remarkable sequence of events spanning decades to days. This included about a century of ground uplift, an increase in the frequency of earthquakes in the years leading up to the event, heightened gas emissions, and, critically, very rapid uplift amounting to several meters in the final days and hours before the eruption commenced.2 For eruptions of a much larger magnitude, such as the caldera-forming events, it is anticipated that the precursor signals would be even more “macroscopic” and widespread.20 Current monitoring systems at Campi Flegrei are designed to detect precisely these types of changes: uplift, seismicity, and variations in gas composition and flux.6

Despite these capabilities, providing reliable short-term (days to weeks) eruption forecasts remains a formidable scientific challenge, particularly for complex caldera systems like Campi Flegrei. These systems can exhibit prolonged periods of unrest, lasting for years or even decades, without necessarily culminating in an eruption.50 A study by Kilburn and colleagues, cited in several sources 3, suggests that the crust at Campi Flegrei has undergone significant structural changes since the unrest began in 1950\. This implies that the volcano’s mechanical response to subsurface stresses may have altered, making it more difficult to forecast future behavior based solely on analogies with past unrest episodes, as previous patterns of activity may not be repeated. Furthermore, the potential for phreatic (steam-blast) eruptions, driven by the explosive interaction of magma or hot rocks with groundwater, adds another layer of complexity, as these events can sometimes occur with very little warning.31

The history of Campi Flegrei itself, with multiple significant unrest episodes in recent decades (e.g., 1950s, 1969-1972, 1982-1984, and the current phase from 2005-present) that have not led to eruptions, creates a “crying wolf” dilemma.2 Each episode of unrest, with its associated ground uplift and seismicity, raises concerns, but the lack of eruptive culmination makes it difficult to define precisely what constitutes a “critical” precursor versus a variation within a long-term, non-eruptive unrest phase. The system’s “background” level of activity is now significantly elevated compared to periods of true quiescence, potentially masking or making it harder to distinguish genuine pre-eruptive signals. The structural changes to the crust, as proposed by Kilburn et al., exacerbate this issue, as the volcano’s response to pressurization may have fundamentally changed, meaning that thresholds for rock failure or magma ascent might be different from those in the past.

The “phreatic wildcard” represents a distinct and significant concern. The potential for phreatic or hydrothermal explosions to occur with minimal or ambiguous warning 3 is particularly relevant for areas like Solfatara-Pisciarelli, which are characterized by intense, ongoing hydrothermal activity.6 While such explosions might be relatively small in terms of erupted volume, their location within or near populated and touristic areas means they carry a high potential for local impact. Critically, phreatic events could also serve as a trigger or an initial phase of a larger magmatic eruption if they successfully de-roof the shallow volcanic system or significantly alter the pressure balance, allowing magma to ascend more easily. The 1538 Monte Nuovo eruption itself involved phreatomagmatic phases, indicating magma-water interaction.6 If the current unrest is indeed heavily influenced by the geothermal system, as suggested by some research 1, then a sudden failure of the caprock leading to a phreatic event is a plausible scenario that could potentially escalate.

**C. Long-Term Eruptive Potential and Recurrence Intervals for Major Events**

Campi Flegrei’s geological record unequivocally demonstrates its capacity for extremely large-magnitude eruptions. The caldera has been the source of at least two super-eruptions: the Campanian Ignimbrite (CI) around 40,000 years BP (VEI 7\) and the Neapolitan Yellow Tuff (NYT) around 15,000 years BP (VEI 6).2 More recent research has identified another very large explosive event, the Maddaloni/X-6 eruption, dated to approximately 109,000 years BP, which was also likely a VEI 7 event with an estimated erupted magma volume of around 150 km3 Dense Rock Equivalent (DRE).5 The occurrence of multiple such events suggests that the recurrence of large, caldera-forming eruptions at Campi Flegrei operates on timescales of tens of thousands of years.22

The INGV assesses the probability of another CI or NYT-scale eruption occurring in the near future as “very low”.20 This assessment is based on the understanding that such colossal events would require an enormous accumulation of eruptible magma in the subsurface, which would, in turn, generate clear, widespread, and “macroscopic” precursor signals detectable by current monitoring networks well in advance of any eruption. In contrast, the intervals between the more numerous smaller eruptions that have occurred within the caldera (over 70 in the last 15,000 years) are much shorter, varying from decades to centuries.3

The confirmation of multiple caldera-forming events (CI, NYT, and Maddaloni/X-6) 22 is significant because it underscores that Campi Flegrei is a magmatic system capable of repeatedly accumulating and erupting very large volumes of magma over geological timescales. This reinforces its “supervolcano” characterization and implies that the underlying magmatic plumbing system is long-lived and highly productive. This understanding is vital for appreciating the ultimate hazard posed by the caldera, even if the immediate probability of its largest conceivable events is low.

Furthermore, the study of the ancient Maddaloni/X-6 eruption has introduced an intriguing consideration for long-term hazard assessment. Some interpretations suggest this eruption might have sourced from Campi Flegrei itself or from nearby volcanic fractures within the broader Campanian Plain.22 This has led some volcanologists to propose that the entire Campanian Plain, not just the immediate Campi Flegrei caldera, should be considered a potential, albeit very rare, zone for large-scale eruptions.22 This perspective broadens the geographical scope of long-term volcanic hazard consideration and hints at the possibility of a large, interconnected regional magmatic system, potentially linking Campi Flegrei with other volcanic features in the region, such as Mount Vesuvius.8

**IV. Potential Eruption Scenarios and Scales**

**A. Understanding the Volcanic Explosivity Index (VEI) in the Context of Campi Flegrei**

The Volcanic Explosivity Index (VEI) is a critical tool for classifying the size and intensity of volcanic eruptions. It is a logarithmic scale ranging from 0 to 8, where each whole number increase represents a tenfold increase in the volume of erupted material (ejecta).3 The scale also considers factors like eruption column height and qualitative descriptors of the eruption’s nature (e.g., effusive, explosive). Campi Flegrei’s eruptive history demonstrates a remarkable range on this scale, from relatively small VEI 2-3 events like the 1538 AD Monte Nuovo eruption, to the colossal VEI 7 Campanian Ignimbrite super-eruption.6

While the VEI provides a standardized measure for comparing past eruptions and categorizing potential future scenarios, it is important to recognize its limitations as a comprehensive hazard descriptor. A single VEI number, primarily based on erupted volume, does not fully capture all aspects relevant to hazard assessment, such as the mobility and temperature of pyroclastic density currents (PDCs), the specific characteristics of volcanic ash (grain size, composition, abrasiveness), or the precise location of the eruptive vent. Nevertheless, for planning purposes, VEI is invaluable. It allows authorities and scientists to define reference eruption scenarios, estimate the potential scale of various hazardous phenomena, and delineate hazard zones accordingly. For Campi Flegrei, VEI 4-5 eruptions are often used as reference scenarios for detailed emergency planning due to their significant historical precedent and severe potential impacts.11 Understanding the VEI helps to contextualize the magnitude spectrum of possible future events at Campi Flegrei and informs the subsequent detailed discussion of specific eruption scenarios.

**Table 1: Volcanic Explosivity Index (VEI) Scale and Relevance to Campi Flegrei**

| VEI | Classification | Ejecta Volume (Bulk DRE) | Typical Eruption Column Height | Typical Campi Flegrei Eruption Style(s) | Historical/Prehistoric Campi Flegrei Analogue(s) | Broad Indication of Impact Zone |

| :—- | :—- | :—- | :—- | :—- | :—- | :—- |

| 0-1 | Effusive/Gentle | \<0.001 km3 | \<1 km | Minor degassing, small steam/ash emissions | Solfatara/Pisciarelli hydrothermal activity (non-eruptive but analogous scale) | Very Local |

| 2 | Explosive | 0.001–0.01 km3 | 1–5 km | Phreatic, Strombolian, small Vulcanian, Phreatomagmatic | Monte Nuovo (1538 AD) \- low end of VEI 2-3 | Local |

| 3 | Severe | 0.01–0.1 km3 | 3–15 km | Vulcanian, Sub-Plinian, Phreatomagmatic | Monte Nuovo (1538 AD) \- high end of VEI 2-3; Solfatara eruption (1158 AD, \~4ka BP) | Local to Regional |

| 4 | Catastrophic | 0.1–1 km3 | 10–25 km | Sub-Plinian, Plinian, Phreatomagmatic | Astroni (\~4.1-3.8 ka BP) | Regional |

| 5 | Cataclysmic | 1–10 km3 | \>25 km | Plinian, Phreatoplinian | Agnano-Monte Spina (\~4.55 ka BP) | Regional to Continental |

| 6 | Colossal | 10–100 km3 | \>25 km | Plinian, Phreatoplinian, Caldera-forming | Neapolitan Yellow Tuff (\~15 ka BP) | Continental |

| 7 | Super-colossal | 100–1000 km3 | \>25 km | Ultra-Plinian, Caldera-forming | Campanian Ignimbrite (\~39-40 ka BP); Maddaloni/X-6 (\~109 ka BP) | Continental to Global |

| 8 | Mega-colossal | \>1000 km3 | \>25 km | Ultra-Plinian, Caldera-forming (no known VEI 8 at Campi Flegrei) | (No Campi Flegrei analogue) | Global |

*Sources for Table 1: VEI scale data from.3 Campi Flegrei analogues and characteristics compiled from numerous sources including.2*

**B. Scenario 1: Small-Scale Eruptions (e.g., Phreatic, VEI 2-3 Strombolian – similar to Monte Nuovo 1538\)**

Small-scale eruptions, with a Volcanic Explosivity Index (VEI) of 2 to 3, are considered the most probable type of event should Campi Flegrei reawaken in the near future.20 These eruptions can manifest as phreatic (steam-blast) explosions, which may involve little to no fresh magma, or as Strombolian and Vulcanian activity, characterized by the ejection of incandescent lava fragments, ash, and volcanic bombs.3

The 1538 AD eruption that formed the Monte Nuovo cone serves as the primary historical analogue for this scenario. This event was relatively small, with an estimated VEI of 2-3, erupting approximately 0.02 to 0.04 km3 (Dense Rock Equivalent) of magma over the course of about a week.3 The eruption was dominated by phreatomagmatic explosions—resulting from the interaction of magma with external water—which generated pyroclastic surges and flows. These currents, while destructive, had a limited runout, extending a few hundred meters to approximately 1 kilometer from the vent.19 Ash fall from the Monte Nuovo eruption was primarily local but was reported as far as Apulia and Calabria in southern Italy.32 Precursory phenomena to the 1538 event were significant and prolonged, including ground uplift that began about a century earlier, an increase in earthquake frequency in the preceding years, and very rapid uplift (several meters) in the immediate days and hours before the eruption.2

The impacts of such an eruption today would be primarily local but severe. The area immediately surrounding the new vent would face destruction from pyroclastic flows, surges, and ballistic projectiles. The village of Tripergole was completely destroyed during the formation of Monte Nuovo.6 Localized ash fall would cause building damage, disrupt transportation and communication, and contaminate water sources. While casualties might be limited with timely evacuation, the 1538 eruption resulted in 24 deaths when individuals ventured too close to the active vent during a lull in activity 32, underscoring the inherent dangers.

Even a “small” Monte Nuovo-sized eruption occurring within the highly urbanized Campi Flegrei caldera today would have catastrophic local consequences. The \~1 km runout of pyroclastic flows observed in 1538 19 would directly impact densely populated areas of Pozzuoli or other towns, depending on the precise vent location. The Red Zone, designated for the highest risk and requiring evacuation, currently houses approximately 500,000 people.3 A vent opening anywhere within this zone for even a “small” eruption would necessitate a large-scale evacuation and inevitably lead to significant destruction and displacement. This highlights a critical point: in such a densely populated volcanic setting, a low VEI does not equate to low impact.

Furthermore, the potential for phreatic or hydrothermal explosions to initiate an eruptive sequence, or occur as standalone events, is a significant consideration.1 These steam-driven explosions can occur with little or ambiguous warning 31 and could be particularly hazardous in areas with intense hydrothermal activity like Solfatara-Pisciarelli. Such events might be self-limiting or, more critically, could act as a prelude to a larger magmatic eruption if they de-pressurize the shallow system or create pathways for magma to ascend more easily. The 1538 Monte Nuovo eruption itself involved significant magma-water interaction (phreatomagmatic phases).6 If the current unrest is indeed heavily influenced by the geothermal system, as some research suggests 1, then a sudden failure of the caprock leading to a phreatic event is a plausible scenario that could potentially escalate if magma is readily available at shallow depths.

**C. Scenario 2: Moderate-Scale Explosive Eruptions (VEI 4-5)**

Moderate-scale explosive eruptions, classified as VEI 4 or VEI 5, represent a significant escalation in hazard compared to smaller events. Historical analogues within Campi Flegrei for such eruptions include the Astroni event (considered VEI 4\) and the Agnano-Monte Spina eruption (VEI 5), both of which occurred approximately 4,100 to 4,550 years ago.11 A VEI 4 eruption typically involves the ejection of 0.1 to 1 km3 of magma (DRE), while a VEI 5 event involves 1 to 10 km3 DRE.3 Such eruptions would generate powerful eruption columns reaching high into the atmosphere, leading to widespread ash fall and the formation of extensive and highly dangerous pyroclastic density currents (PDCs). These VEI 4-5 scenarios are often used as reference events for emergency planning by authorities due to their documented occurrence in the caldera’s recent geological past (at least two VEI 5 events in the last 10,000 years) and their severe potential impacts.11

Local Impacts (Campania Region):  

The local impacts of a VEI 4-5 eruption would be devastating.

* **Pyroclastic Density Currents (PDCs):** PDCs from a VEI 4 eruption could be partially confined by the caldera’s topography but are still likely to travel 3-5 km from the vent, potentially extending over 10 km if channeled by valleys. Such flows would directly threaten Pozzuoli and the western suburbs of Naples, including areas like Bagnoli, Fuorigrotta, and Posillipo.11 In a VEI 5 scenario, PDCs would be far less constrained by local topography and could propagate more than 25 km from the vent, extending well beyond the caldera rims. These larger flows would inundate the entire caldera, the western parts of Naples, and potentially reach into Naples city center, with the exact extent depending on vent location and specific flow dynamics (e.g., dilute, highly mobile PDCs can overtop barriers more easily).11 The hills of Camaldoli and Posillipo might offer some protection but could be overtopped by the most energetic flows.  

* **Ash Fall:** Heavy ash fall would blanket the region. For a medium-sized eruption (potentially VEI 4-5), ash accumulation could reach 30 cm in the designated Yellow Zone.36 Such loads would cause widespread roof collapses, particularly of older or poorly constructed buildings.3 Critical infrastructure, including power grids, water supply systems, transportation networks (roads, railways, ports), and communication systems, would suffer severe damage and disruption.3 Agricultural lands would be rendered unusable, and public health would be threatened by respiratory problems, eye irritation, and contaminated water sources.  

* **Evacuation:** The scale of these hazards would necessitate the full evacuation of the Red Zone (approximately 500,000 people) and potentially large parts of the Yellow Zone (over 800,000 residents).55

The topographic features of the Campanian region, such as the caldera walls and the hills of Camaldoli and Posillipo, can play a role in partially confining or diverting PDCs, especially for VEI 4 events.11 However, for larger VEI 5 eruptions, these natural barriers become significantly less effective. Highly mobile, voluminous, and dilute PDCs associated with VEI 5 events possess enough energy to overtop these topographic highs, thereby threatening much wider areas, including the city of Naples itself.11 This illustrates a critical non-linear increase in the hazard footprint with increasing eruption size, where the protective capacity of the landscape diminishes rapidly.

The impacts of ash fall extend far beyond the immediate physical damage of roof collapse. The clogging of drainage systems by ash can lead to severe flooding. Contamination of surface water sources and disruption to water treatment plants would create a critical shortage of potable water.7 Ash accumulation on power lines and substations can cause short circuits and widespread power outages. The abrasive and conductive nature of volcanic ash can also damage communication equipment and disrupt essential services. These effects are interconnected, leading to cascading failures across critical infrastructure networks. For example, power loss impacts water pumping stations, telecommunications, and hospital operations, severely hampering emergency response and long-term recovery efforts. The societal impact of extensive ash fall is therefore far greater than the sum of individual structural damages, leading to a systemic crisis.

Regional Impacts (Europe):  

A VEI 4-5 eruption at Campi Flegrei would have consequences extending across the European continent.

* **Ash Dispersal and Air Travel:** A significant volcanic ash plume would be injected into the atmosphere, dispersing over Southern and Central Europe, and potentially further afield depending on eruption duration and meteorological conditions.3 This would lead to severe and prolonged disruption to air travel, likely exceeding the chaos caused by the 2010 Eyjafjallajökull eruption in Iceland.3 Campi Flegrei’s location in the central Mediterranean means its ash cloud would directly affect some of the world’s busiest air corridors.  

* **Economic Repercussions:** Beyond the airline industry, the eruption would have widespread economic repercussions due to disruptions in tourism, commerce, and international supply chains.3  

* **Acid Rain:** The release of large quantities of sulfur dioxide (SO2​) and other volcanic gases would lead to the formation of acid rain over extensive areas of Europe.8 This would negatively impact agriculture by damaging crops and altering soil chemistry, and harm natural ecosystems, including forests and aquatic environments.

**D. Scenario 3: Large-Scale Caldera-Forming Eruptions (VEI 6-7, e.g., Neapolitan Yellow Tuff, Campanian Ignimbrite type – low probability, high consequence)**

The largest known eruptions from Campi Flegrei are the Neapolitan Yellow Tuff (NYT), which occurred approximately 15,000 years ago (VEI 6, erupting 40-50 km3 DRE and covering an area of about 1,000 km2 with its deposits), and the cataclysmic Campanian Ignimbrite (CI) eruption, dated to around 39,000-40,000 years ago (VEI 7, with an estimated DRE volume of 181-300 km3 and ash dispersed over an area of 3 to 3.7 million km2).2 Pyroclastic density currents from events of this magnitude would inundate vast swathes of the Campanian Plain to the north of the caldera and much of the city of Naples to the east.11

Local and Regional Impacts:  

An eruption of VEI 6-7 magnitude would lead to the complete devastation of extensive areas of the Campania region, burying the landscape under thick deposits of tuff and ignimbrite.6 Pyroclastic flows would travel for tens of kilometers, with the CI flows extending over 80 km from the source.11 Such an event would involve significant caldera collapse, fundamentally and permanently altering the regional geography.6 Widespread, thick ash deposits would cover much of Italy and large parts of the Mediterranean basin.  

Continental and Global Impacts:  

The continental and global consequences of a VEI 6-7 eruption at Campi Flegrei would be catastrophic.

* **Ash Dispersal:** Massive quantities of volcanic ash would be dispersed across Europe and potentially into Asia and North Africa. Ash from the CI eruption has been found in sediment cores from as far away as Greenland 88, attesting to its vast dispersal.  

* **Climatic Impacts (Volcanic Winter):** The injection of hundreds of millions of tonnes of sulfur dioxide into the stratosphere would lead to the formation of a dense aerosol veil, reflecting sunlight and causing a significant global cooling event known as a “volcanic winter”.16 For example, the CI eruption is estimated to have caused a temperature drop of 2 to 4°C in Western Europe for one to two years.23 The eruption of Toba in Indonesia (\~74,000 years ago, a VEI 8 event) led to several years of severe global cooling.23  

* **Agricultural Failure and Famine:** The global cooling, reduced sunlight, and widespread ash deposition would lead to catastrophic failures of agriculture worldwide, resulting in global food shortages and famine.16  

* **Ecosystem Disruption and Societal Collapse:** Such an event would cause widespread disruption to ecosystems, potentially leading to species extinctions. The CI eruption, for instance, has been implicated as a contributing factor in the decline of Neanderthal populations in Europe.3 The societal consequences would likely include mass displacement, resource conflicts, and potentially the collapse of complex societies.

While the INGV currently assesses the probability of such a “super-eruption” occurring in the near future as “very low,” emphasizing that it would be preceded by “enormous” and “macroscopic” precursor signals that would be readily detectable 20, these events serve as crucial benchmarks. They define the maximum destructive capacity of the Campanian volcanic system and inform our understanding of the ultimate scale of hazard the region could face, even if such threats materialize only on very long, geological timescales. Awareness of this VEI 6-7 potential is therefore necessary for any truly comprehensive, long-term strategic thinking about regional and global resilience, even if current emergency planning rightly focuses on more probable, smaller-magnitude scenarios like VEI 5\.11

A VEI 6-7 eruption at Campi Flegrei would transcend a purely Italian or European disaster, becoming a global crisis of unparalleled proportions. The climatic impacts 23, coupled with the devastation of agriculture and the disruption of global supply chains 16, would trigger cascading socio-economic and geopolitical consequences far beyond the regions directly affected by PDCs or ash fall. This underscores the profound systemic risk posed by such large-magnitude volcanic events in an increasingly interconnected world.

**Table 2: Summary of Major Historical/Prehistoric Eruptions at Campi Flegrei**

| Eruption Name | Approximate Date (Years BP or AD) | Estimated VEI | Estimated Erupted Volume (DRE km3) | Primary Eruptive Phenomena | Broad Extent of Impact |

| :—- | :—- | :—- | :—- | :—- | :—- |

| Monte Nuovo | 1538 AD | 2-3 | 0.02-0.04 | PDCs (local), ash fall (regional), cone formation | Local to Regional |

| Agnano-Monte Spina | \~4,550 BP | 5 | 1-10 (inferred from VEI) | PDCs (regional), ash fall (regional/continental), Plinian | Regional to Continental |

| Neapolitan Yellow Tuff | \~15,000 BP | 6 | 40-50 | PDCs (extensive regional), ash fall (continental), caldera collapse | Regional to Continental |

| Campanian Ignimbrite | \~39,000-40,000 BP | 7 | 181-300 | PDCs (vast regional), ash fall (continental/global), caldera collapse | Continental to Global |

| Maddaloni/X-6 | \~109,000 BP | 7 | \~150 | PDCs (vast regional), ash fall (continental), caldera collapse | Continental |

*Sources for Table 2: Compiled from.2*

**V. Local Impacts: The Neapolitan Region**

The proximity of Campi Flegrei to the densely populated Neapolitan region, including the city of Naples itself, means that any eruption, regardless of its scale, will have profound local consequences. The primary hazards are pyroclastic density currents (PDCs), ash fall, associated seismic activity, and potential tsunamis.

**A. Pyroclastic Density Currents (PDCs): Hazard Zones, Runout Models, and Affected Areas (Pozzuoli, Naples districts)**

Pyroclastic density currents are fast-moving, ground-hugging avalanches of hot gas, ash, and volcanic rock fragments. They are extremely dangerous due to their high temperatures (hundreds of degrees Celsius), high velocities (tens to hundreds of km/hour), and destructive force.69 The Italian Civil Protection has designated a “Red Zone” around Campi Flegrei, encompassing areas at high risk from PDCs. This zone currently includes approximately 500,000 residents, and preventive evacuation is considered the only viable protective measure in the event of an impending eruption.3

Numerical simulations and probabilistic hazard assessments, such as those by Mastrolorenzo et al. 11, provide crucial insights into potential PDC inundation areas for different eruption scenarios:

* **VEI \<4 eruptions:** PDCs from these smaller events are likely to be largely confined within the caldera boundaries, with runout distances typically of a few kilometers. However, local topography can channel flows, potentially extending their reach along valleys.11  

* **VEI 4-5 eruptions:** PDCs from these moderate-to-large eruptions can overcome some topographic barriers and extend significantly beyond the caldera rim. Extensive areas of Pozzuoli, Bacoli, Monte di Procida, and Quarto, as well as western suburbs of Naples such as Bagnoli, Fuorigrotta, and Posillipo, could be inundated. Depending on the vent location and flow characteristics (e.g., volume, concentration, mobility), PDCs from a VEI 5 event could even reach parts of Naples city center.11 The Camaldoli and Posillipo hills can act as partial barriers, but their effectiveness diminishes with increasing eruption size and PDC mobility. Measured runout lengths for historical PDCs at Campi Flegrei are typically on the order of kilometers, though rare events have reached up to 15 km.102 Some modeling studies show potential runouts ranging from 3 to 20 km depending on the input parameters used.102  

* **VEI 6 (NYT analogue):** PDCs from an eruption of this magnitude, similar to the Neapolitan Yellow Tuff, could travel approximately 30 km from the vent.11  

* **VEI 7 (CI analogue):** The colossal Campanian Ignimbrite eruption generated PDCs that traveled over 80 km.11

It is estimated that up to 3 million people in the broader Neapolitan area could be potentially exposed to volcanic hazards in the event of renewed eruptive activity at Campi Flegrei.11

A critical factor influencing PDC dispersal is the location of the eruptive vent.11 Given Campi Flegrei’s complex caldera structure with numerous past eruptive centers and potential for new vents to open across a wide area 5, predicting the exact source of a future eruption is highly uncertain. This inherent unpredictability necessitates probabilistic hazard mapping, considering multiple vent scenarios, as undertaken in the detailed studies by Mastrolorenzo and colleagues.

Beyond the mere extent of inundation, the dynamic pressure exerted by PDCs and their extreme temperatures are key factors determining their destructive potential and lethality.11 Even relatively “wet” surges, which might incorporate more environmental water, can retain temperatures exceeding 200°C in their distal reaches.15 Survival within any area affected by a PDC is virtually impossible, reinforcing the official policy that pre-event evacuation from the designated Red Zone is the only effective measure to safeguard the population.

**B. Ash Fall: Thickness Distribution, Roof Collapse Hazard, Infrastructure Disruption, Agricultural Damage, and Public Health**

Volcanic ash fall is a more widespread hazard than PDCs, capable of affecting much larger areas with varying degrees of severity. The Italian Civil Protection has defined a “Yellow Zone” surrounding the Red Zone, which is home to over 800,000 residents.55 This zone is identified as being exposed to significant volcanic ash fallout, which could necessitate temporary removal of the population from buildings rendered vulnerable by ash accumulation or made inaccessible.

The impacts of ash fall are manifold:

* **Roof Collapse:** The weight of accumulated ash is a primary concern, particularly for flat or low-pitched roofs and older or poorly maintained structures. Even a few centimeters of dense, wet ash can impose critical loads, leading to roof collapse.3 For a moderate-sized eruption at Campi Flegrei, ash deposits could reach 30 cm in parts of the Yellow Zone.36 A VEI 3 eruption scenario suggests that 2.5 million people could be exposed to at least 2 cm of ash, with 144,000 people exposed to 25 cm of ash.3  

* **Infrastructure Disruption:** Ash can severely disrupt critical infrastructure. Transportation networks are highly vulnerable: roads become slippery and visibility is reduced, airports are forced to close due to risks to jet engines, and railway systems can be disabled by ash clogging switches and affecting signals.3 Power systems can fail due to ash causing insulator flashover on transmission lines or damaging substations. Water supplies can be contaminated by ash, and water treatment facilities can be overwhelmed by increased turbidity and clogging of filters. Communication systems can also be affected by ash damaging equipment.  

* **Agricultural Damage:** Ash fall can smother crops, contaminate pastures, and affect livestock, leading to significant agricultural losses.3  

* **Public Health:** Volcanic ash poses significant health risks. Fine ash particles (PM10​ and PM2.5​) can be inhaled deep into the lungs, causing or exacerbating respiratory illnesses such as bronchitis and asthma. Ash can also cause eye irritation (conjunctivitis) and skin problems.7

The vulnerability of buildings in Naples to ash-induced roof collapse is a major concern, given the city’s diverse building stock, which includes many historical and potentially fragile structures.8 The specific load (measured in kilograms per square meter or centimeters of thickness for a given ash density) that will cause a roof to fail varies significantly depending on factors such as roof design (shape, pitch), construction materials, structural integrity, and age. Detailed vulnerability assessments, similar to those conducted for seismic risk in the bradyseismic area 82, are essential for accurately identifying buildings at highest risk within the Yellow Zone and for prioritizing mitigation efforts or temporary evacuations.

The impact of volcanic ash is not limited to the immediate period of deposition. Fine ash particles can remain suspended in the atmosphere for extended periods, affecting air quality. Once deposited, ash can be easily remobilized by wind, leading to recurrent air quality problems and further deposition, or by rainfall, which can transform ash deposits into destructive mudflows (lahars) that can inundate new areas or re-impact previously affected ones.43 The cleanup and disposal of vast quantities of volcanic ash is a monumental and costly long-term undertaking. Thus, the consequences of ash fall extend well beyond the eruptive phase itself, posing chronic health risks, ongoing challenges to infrastructure operation and maintenance (e.g., repeated cleaning of roads, railways, and water filtration systems), and the persistent threat of secondary hazards like lahars.

**C. Seismic Hazards Associated with Unrest and Eruptions**

Seismic activity is an intrinsic component of the unrest at Campi Flegrei and would be a significant hazard both preceding and during any eruption. The current phase of bradyseism is characterized by frequent earthquake swarms, many of which are felt by the local population. These have included events up to M4.4, causing public alarm and some instances of structural damage to buildings.1 Historical seismicity linked to past bradyseismic crises in the area is estimated to have reached a maximum intensity of VIII on the Modified Mercalli Scale, corresponding to earthquake magnitudes of approximately 4.5 to 5.0.40

Intensified seismic activity is a key precursor to volcanic eruptions. These pre-eruptive earthquakes can themselves cause significant damage to buildings and critical infrastructure, potentially weakening structures and hampering evacuation efforts even before the eruption begins.50

The persistent, relatively low-magnitude seismicity (typically M\<4.5) associated with the ongoing bradyseism can lead to cumulative fatigue damage in buildings.1 Over time, repeated shaking, even if not individually catastrophic, can progressively weaken structural elements, increase cracking, and reduce the overall resilience of buildings. This “softening up” of the building stock increases its vulnerability to collapse from any stronger pre-eruptive earthquakes that might occur, or from the subsequent imposition of heavy ash loads during an eruption. This cumulative effect is a critical consideration that might be underestimated if vulnerability assessments only focus on the impact of single, larger seismic events or ash loads in isolation.

Furthermore, earthquakes occurring immediately before or during the initial phases of an eruption pose a direct threat to evacuation procedures. Damage to roads, bridges, and railway lines, or the collapse of buildings onto critical escape routes, could severely compromise the ability to evacuate the population safely and efficiently.50 This underscores the necessity for proactive measures, such as the seismic retrofitting of key buildings along designated evacuation routes and critical transport infrastructure, to ensure their functionality during a crisis.

**D. Potential for Tsunami in the Bay of Naples**

The geographical setting of Campi Flegrei, with a significant portion of the caldera submerged beneath the Bay of Pozzuoli and the greater Bay of Naples, introduces the potential for tsunami generation during a major eruption.3 While not the primary hazard associated with most eruption scenarios, a volcanically generated tsunami could have significant local impacts along the densely populated coastlines of the region.

Several mechanisms could trigger a tsunami in this context:

1. **Caldera Collapse:** A large-scale explosive eruption leading to significant and rapid subsidence of the caldera floor, particularly in the submerged sector, could displace a vast volume of seawater, generating tsunami waves. The formation of the Neapolitan Yellow Tuff caldera, for example, involved substantial collapse.6  

2. **Pyroclastic Density Currents Entering the Sea:** Large-volume, fast-moving PDCs flowing into the Bay of Naples would rapidly displace seawater, pushing it outwards and generating tsunamis. This is a well-documented mechanism for tsunami generation at coastal and island volcanoes worldwide.  

3. **Submarine Explosions:** Explosive activity originating from vents on the seafloor within the caldera could directly displace water and create tsunamigenic waves.  

4. **Eruption-Triggered Submarine Landslides:** The intense seismicity and ground deformation associated with an eruption could destabilize submarine slopes within the Bay of Naples or along the flanks of nearby volcanic structures like Ischia, leading to large submarine landslides. Such landslides are known to be efficient tsunami generators.18 The geological record around Ischia shows evidence of extensive lateral collapses and debris avalanches offshore, indicating that slope instability is a regional geohazard.18

A comprehensive tsunami hazard assessment for Campi Flegrei would need to consider these multiple potential sources and their varying characteristics in terms of wave height, run-up, and warning times.

**E. Impact on Population and Critical Infrastructure within Red and Yellow Zones**

The Campi Flegrei Red Zone is home to approximately 500,000 residents, while the Yellow Zone accommodates over 800,000 people.3 These zones encompass numerous municipalities, including Pozzuoli, Bacoli, Monte di Procida, Quarto, and significant portions of the western districts of the city of Naples itself. Critical infrastructure, such as hospitals, schools, major transportation hubs (ports, railway lines), and utility networks (power, water, gas, communications), is extensively developed within these designated hazard zones.

The Italian Civil Protection’s emergency plan involves complex logistics for the evacuation of these large populations. This includes the establishment of “waiting areas” within municipalities, from which residents opting for assisted transport would be taken by bus to larger “meeting areas” outside the Red Zone. From these meeting points, further transport via bus, train, or ship is planned to host regions and autonomous provinces across Italy, based on pre-established “twinning” arrangements.55

The vulnerability of “lifeline” infrastructure within the hazard zones is a paramount concern. Hospitals, emergency service centers (fire, police, medical), transportation networks, and utility systems located within the Red Zone would likely be rendered completely inoperable by the passage of PDCs. In the Yellow Zone, heavy ash fall could cripple these same facilities, leading to power outages, water shortages, communication breakdowns, and impassable roads. This means that the local capacity for emergency response and medical care would be severely diminished precisely when it is most desperately needed. The evacuation of particularly vulnerable populations, such as patients in hospitals and residents of nursing homes, presents an enormous logistical and ethical challenge.8 Self-sufficiency of the affected area in the aftermath of a significant eruption would be impossible; all aid and resources would need to come from outside the impacted zones, but access would be severely hampered by damaged infrastructure and ongoing hazards.

Beyond the immediate physical damage and casualties, the displacement of over a million people from the Red and Yellow Zones would precipitate a massive and prolonged humanitarian crisis. The loss of homes, businesses, livelihoods, and community structures would be immense. The aggregated economic property exposure across the volcanic areas of Campi Flegrei and nearby Vesuvius has been estimated at $85 billion (in 2010 U.S. dollars).3 Even if an eruption does not occur, prolonged unrest leading to evacuations (as happened on a smaller scale in Pozzuoli in 1982-1984 when 40,000 people were displaced 1) causes significant social and economic disruption. A full-scale evacuation followed by a destructive eruption would lead to permanent displacement for a large number of people, the loss of irreplaceable cultural heritage, and a profound economic shock to the Campania region and to Italy as a whole.

**Table 3: Population and Key Municipalities in Campi Flegrei Evacuation Zones**

| Zone | Approximate Population | Fully Included Municipalities | Partially Included Municipalities/Naples Districts | Primary Hazard Defining the Zone |

| :—- | :—- | :—- | :—- | :—- |

| Red | \~500,000 | Pozzuoli, Bacoli, Monte di Procida, Quarto | Giugliano in Campania (part), Marano di Napoli (part), Napoli (Municipalità 1: Chiaia, S. Ferdinando (part); Municipalità 9: Soccavo, Pianura; Municipalità 10: Bagnoli, Fuorigrotta) | Pyroclastic Density Currents |

| Yellow | \>800,000 | Villaricca, Calvizzano, Mugnano di Napoli, Melito di Napoli, Casavatore | Marano di Napoli (part), Napoli (Municipalità 1: Chiaia (part), S. Ferdinando (part); Municipalità 2: Avvocata, Montecalvario, Pendino, Porto, Mercato, S. Giuseppe; Municipalità 3: Stella, S. Carlo all’Arena; Municipalità 4: Vicaria, S. Lorenzo, Poggioreale, Zona Industriale; Municipalità 5: Vomero (part), Arenella (part); Municipalità 6: Ponticelli, Barra, S. Giovanni a Teduccio; Municipalità 7: Miano, Secondigliano, S. Pietro a Patierno; Municipalità 8: Piscinola, Marianella, Chiaiano (part), Scampia; plus other Naples districts) | Significant Volcanic Ash Fall |

*Sources for Table 3: Compiled from.55 Note: Specific Naples districts listed may vary slightly between sources or over time with planning updates; this table represents a general compilation.*

**VI. Continental Impacts: Europe**

A significant eruption at Campi Flegrei, particularly one of VEI 4 or higher, would have consequences extending far beyond the local Neapolitan region, impacting the European continent in several critical ways.

**A. Ash Dispersal Modelling: Affected Regions, Concentrations, and Duration**

The historical record and scientific modeling demonstrate the capacity of Campi Flegrei eruptions to disperse volcanic ash over vast distances. Ash from the colossal Campanian Ignimbrite eruption (\~40,000 years BP) has been identified in sedimentary layers across Eastern Europe and as far as Greenland, covering an estimated area of 3 to 3.7 million km2.25 Even eruptions of moderate magnitude (VEI 4-5) would generate substantial ash plumes capable of affecting large parts of Southern and Central Europe.3 The direction and extent of ash dispersal are primarily controlled by wind patterns at various altitudes in the atmosphere at the time of eruption, as well as by the height of the eruption column and the duration of ash emission.

Volcanological institutions like INGV utilize sophisticated ash dispersal models, such as FALL3D, to forecast the trajectories and concentrations of volcanic ash clouds in the event of an eruption.106 These models are continuously updated with meteorological data. Complementing these forecasting tools, tephrochronology—the study of volcanic ash layers preserved in geological and archaeological records—helps reconstruct the dispersal patterns of past eruptions, providing valuable data for validating models and understanding long-term hazard \[109, S\_S

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