Abstract
The Phlegraean Fields (in Italian “Campi Flegrei”) are an active volcanic area located west of Naples, which includes the municipalities of Bacoli, Monte di Procida, Pozzuoli, Quarto, Giugliano in Campania and part of the city of Naples. The Phlegraean Field extend over an area of 16km and are not characterized by a single main volcanic edifice, but are rather a volcanic system active for more than 80,000 years, with several volcanic centers located within and close to an area depressed called “caldera”. Starting from September 2023, a new bradyseismic crisis began in the Campi Flegrei area and 1106 earthquakes were recorded with a maximum magnitude = 4.2 with depths concentrated between 3 and 4 km deep.
The population living in the areas affected by the bradyseism phenomenon is used to living with this force which releases energy from the ground, modifying its heights and morphology. In the city of Pozzuoli it is very easy to meet people who – clinging to popular knowledge – maintain that bradyseism is nothing other than the breathing of the volcano that sleeps underground.
This series focuses on the visible changes in the geography of the territory and the monitoring activities that volcanologists have been carrying out in the area since the beginning of the crisis.
The "breath" of the Phlegraean Fields
Campi Flegrei are a volcanic area of the “caldera”, different from the classical vulcans in that there is no single building, which does stand on the Ovest of the hill of Posillipo, in the city of Napoli. The caldera was built deep into a vast area, because of the flow of the magmatic chamber below in rapid succession. This area experiences volcanic activity since at least 47,000 years ago. The two major eruptive episodes occurred about 39,000 and 14,900 years BP, while the last eruption occurred in 1538 AD. From the middle of the last century the caldera has experienced several episodes of uplift and subsidence. The area is also characterized by shallow seismicity, which is generally low in magnitude. Two important unrest phases occurred in 1969–1972 and 1982–1984. During the 1982–1984 bradyseismic crisis, the maximum measured uplift was of almost 1.8 m and it was accompanied by >16,000 earthquakes. After 1984, the caldera experienced a 20 years long phase of subsidence occasionally interrupted by small and short uplift episodes. In 2000 the uplift rate slightly reversed and in 2005 a new uplift phase clearly began. Since 2000, also the seismicity rate increased with time, especially onshore below the area of the Solfatara crater.
The movement of the ground in these areas is called “bradyseism” (a word of Greek etymology which means “slow earthquake”); the lifting depends on an increase in internal pressure, between the surface and about 3-4 km of depth, which may be due either to the intrusion of magma, coming from the main reservoir which is located at about 8 km, in very superficial levels ( about 3 km deep), or by heating rocks soaked in water (the Phlegraean caldera has a large geothermal system), which expands and increases in pressure like a pressure cooker heated on the stove.
The phenomenon consisting in a periodic lowering (negative bradyseism) or raising (positive bradyseism) of the ground level, relatively slow on the human time scale but very faster than geological times. It is not perceptible in itself, but visually recognizable along the sea shore, showing the progressive emergence or submergence of buildings, coasts and territories. The lowering phases, which currently represent the normal condition, are aseismic and are characterized by low speed. The uplift phases instead present greater speed of ground motion and are accompanied by intense local seismic activity.
In the two-year period 1970-72 and 1982-84 the Phlegraean area was affected by bradyseismic crises in which the ground, in the town of Pozzuoli in particular, underwent a lifting of approximately 3.5 m. The first crisis caused the forced abandonment of the Rione Terra area; the second crisis in particular was characterized by intense seismicity with serious damage to buildings. After the crises there was a period of general subsidence, interrupted starting from 2005 by a reversal of the phenomenon which led to a constant rising of the ground, which is currently still ongoing. Compared to the 1984 crisis, the ground has risen by more than 2 meters (source INGV: National Institute of Geophysics and Volcanology).
The main suspect for the ground uplift and associated earthquakes that are affecting the Campi Flegrei could be a cylinder of rock located 2 kilometers deep just under the caldera, which is expanding causing the deformation of the surrounding space. This is suggested by a recent study [1] published in the Journal of Volcanology and Geothermal Research, carried out by the University of Bologna and the National Institute of Geophysics and Volcanology. This mechanism also played an important role in the uplift phase which occurred between 1982 and 1984 and does not involve the ascent of magma, whose movement would therefore be secondary. «Although the magmatic contribution cannot be excluded the results obtained with this source of deformation, linked to the arrival of hot and high-pressure fluids, allow us to explain effectively both the rate of uplift and the trend of seismicity, without the need to invoke the rise of magma in the superficial layers of the Campi Flegrei caldera».
In Pozzuoli there is a neighborhood near the port area called Rione Terra. In 1970 and 1984, fearing an imminent eruption, 3,000 inhabitants of the Rione Terra and 40,000 inhabitants of the entire town of Pozzuoli, in the center of the caldera, were evacuated respectively. Rione Terra is a sort of village dug into a tuff hill overlooking the sea which was cleared in 1970 due to bradyseism, as the rising of the land due to underground volcanic activity is defined. It has now been renovated, largely with European funds and the rest with contributions from the government and the region. The seventeenth-century buildings have been rearranged and painted pale yellow or Pompeian red, and the accommodations have been transformed into comfortable apartments to rent to tourists. The cramped rooms where up to five people slept in a single bed, and where there were no bathrooms or drinking water, will be converted into artisan workshops and art galleries. The staircase leading to the Duomo has been rebuilt and a glass façade protects what remains of the seventeenth-century cathedral, which burned in 1964, and the Roman temple on which it was built.
Starting in September 2023, a new wave of earthquake swarms began with over 1000 earthquakes recorded in the area: on 9/27/2023 a magnitude 4.2 earthquake occurred, and on 10/2/2023 a magnitude 4.0 earthquake occurred. These earthquakes occur within the first 3 km of the crust; therefore, they are very close to homes located near the epicentre, and damage buildings. This damage accumulates over time, because the next earthquake finds a structure already damaged by previous earthquakes. For this reason the earthquake of 2/10/2023 did more damage than the previous one, and four buildings had to be evacuated due to serious injuries. «If the ground lifting continues, the buildings located near the epicentral area are at very high risk of extensive damage, up to collapse» explains volcanologist Giuseppe Di Natale, former director of the Vesuvian Observatory of INGV and one of the main scholars of the caldera of the Phlegraean Fields. Furthermore, «if the pressure in the subsoil increased beyond the resistance limit of the overlying rocks, these could fracture completely, giving rise to a phreatic eruption, less dangerous than a magmatic eruption, but in a densely populated area like this it would still be catastrophic» [2].
