Table of alert levels for the activity status of the Phlegraean Fields volcano (2025)

Expected variations in the main monitoring parameters

Monitoring parameters within baseline values: 

▪  No medium- to long-term uplift or subsidence
▪  Seismicity mainly absent
▪  Fumarolic gases are mainly of hydrothermal nature; the P and T of the hydrothermal system are stable over time

Probability of evolving into an eruption:

▪  VERY LOW

Expected level duration:

▪  NOT DEFINABLE

Possible hazardous phenomena:

▪  Occasional CO₂ concentrations at levels potentially harmful to human health, near emission sites, downwind areas, morphologically depressed areas, or enclosed spaces.

Expected variations in the main monitoring parameters

One or more monitoring parameters exceeding baseline values, slowly progressing and/or remaining prolonged over time:

▪  Uplift of the center of the caldera at a low rate and exhibiting radial geometry
▪  Sporadic volcanic-tectonic seismic activity of low energy
▪  Slight increase in the magmatic component of fumarolic gases; increase in CO₂ flows and degassing areas
▪  Minor variations in the equilibrium P and T values of the hydrothermal system

Probability of evolving into an eruption:

▪  LOW

Expected level duration:

▪  NOT DEFINABLE

Possible hazardous phenomena:

▪  CO₂ concentrations occasionally reaching levels potentially harmful to human health, near emission sites, downwind areas, morphologically depressed areas, or enclosed spaces
▪  Occasional perceptible seismic shaking

Expected variations in the main monitoring parameters

Monitoring parameters above baseline values, over time:

▪  Uplift of the center of the caldera at an average rate and exhibiting general radial geometry
▪  Frequent volcano-tectonic seismic activity with maximum Md sporadically between 4 and 4.5
▪  Moderate increase in the magmatic component of fumarolic gases; increase in CO₂ flows and extension of degassing areas.
▪  Further variations in the equilibrium P and T values of the hydrothermal system

Probability of evolving into an eruption:

▪  MEDIUM-LOW

Expected level duration:

▪  NOT DEFINABLE

Possible hazardous phenomena:

▪  Concentrations of CO₂ and other gases (H₂S) potentially harmful to human health near emission areas (including diffuse emissions), downwind areas, morphologically depressed areas, or enclosed spaces
▪  Mild to occasionally intense seismic shaking near epicentral areas
▪  Phreatic explosions localized in areas with intense hydrothermal activity
▪  Seismic-induced landslides near epicentral areas

Expected variations in the main monitoring parameters

Monitoring parameters significantly above baseline values, over time:

▪  Uplift of the center of the caldera at a medium-high rate and exhibiting general radial geometry
▪  Very frequent volcano-tectonic seismic activity with repeated events of maximum Md between 4 and 4.5
▪  Significant increase in the magmatic component of fumarolic gases and in the equilibrium P and T values of the hydrothermal system, and further expansion of the degassing areas

Probability of evolving into an eruption:

▪  MEDIUM

Expected level duration:

▪  WEEKS / MONTHS / YEARS (VERY HIGH)

Possible hazardous phenomena:

▪  High concentrations of CO₂ and/or H₂S, at levels potentially harmful to human health, in the downwind sectors of the areas inside the caldera and/or accumulations in morphologically depressed areas or enclosed spaces
▪  Seismic shaking, even intense, near the epicentral areas
▪  Phreatic explosions localized in areas with intense hydrothermal activity
▪  Seismic-induced landslides near the epicentral areas

Expected variations in the main monitoring parameters

Monitoring parameters significantly above baseline values, with further progression over time:

▪  Rapid ground uplift exhibiting general radial geometry, possibly not culminating in the center of the caldera. Possible opening of surface fractures
▪  Volcano-tectonic seismic activity with frequency and occurrence of events with maximum Md between 4.5 and 5. Possible occurrence of LP-type events
▪  Further variations in geochemical parameters, with enlargement of degassing areas and appearance of new emission points. Sudden variations in the level, temperature, and salinity of the aquifer. Possible appearance of SO₂ in fumarolic fluids.

Probability of evolving into an eruption:

▪  MEDIUM-HIGH

Expected level duration:

▪  WEEKS / MONTHS / YEARS (VERY HIGH)

Possible hazardous phenomena:

▪  High concentrations of CO₂, H₂S, and/or SO₂ at levels potentially harmful to human health, in areas downwind of the caldera and/or accumulations in morphologically depressed areas
▪  Heightened hydrothermal activity and expansion of emission areas with increased likelihood of phreatic explosions
▪  Frequent and intense seismic shaking near epicentral areas
▪  Seismic-induced landslides near epicentral areas
▪  Surface fracturing in areas with greater ground uplift

Expected variations in the main monitoring parameters

Monitoring parameters on very high and evolving values:

▪  Rapid ground uplift exhibiting general radial geometry, possibly not culminating in the center of the caldera. Possible opening of surface fractures
▪  Volcano-tectonic seismic activity with a very high frequency of occurrence and a general increase in the maximum Md up to 5. Possible occurrence of LP-type events
▪  Further variations in geochemical parameters, with enlargement of degassing areas and appearance of new emission points. Sudden variations in the level, T, and salinity of the aquifer. Possible appearance of SO₂ in fumarolic fluids

Probability of evolving into an eruption:

▪  VERY HIGH

Expected level duration:

▪  HOURS / DAYS (LOW)

Possible hazardous phenomena:

▪  Gas emissions (CO₂–SO₂) from areas of maximum uplift and ground fracturing
▪  Further intensification of hydrothermal activity with expansion of areas of intense emissions and increased likelihood of hydrothermal and phreatic explosions
▪  Very frequent and intense seismic shaking
▪  Seismic-induced landslides near the epicentral areas
▪  Rapid and localized ground uplift accompanied by surface fracturing/faulting
▪  Opening of new degassing areas, with probable phreatic explosions