## 2 Study Area
### Ground Deformation in Historical Times  
During the last 2100 years, the central sector of the Campi Flegrei caldera has undergone alternating phases of subsidence and uplift in the range of about 20 m, with maximum values centered in the Pozzuoli town central area, associated with fumarolic and hydrothermal processes, that are locally referred to as 'bradyseism' (e.g., [62; 63]). A long subsidence phase (about 18 m) continued at least since the Roman Republic Age (II-III centuries BC) to the Middle Age around the XV century, probably interrupted by an uplift during the V-VIII centuries. A strong uplift (reaching about 16 m) occurred for almost a century before the Monte Nuovo eruption (1538 AD) [54; 64; 65]. Figure 1: Geological map of the Campi Flegrei caldera (**b**) with study area location (**a**). References for the map elements: geological units [41], NYT caldera rim and inner-caldera resurgence [37; 38], volcanic and morphological elements ([42] and reference therein), volcano-tectonic faults and lineaments [57]. The quiescence after the 1538 phreato-magmatic eruption has been characterized by a progressive subsidence of more than 8 m in the central sector of the caldera area, lasting until 1950. Since that date, a new period of intermittent uplift has begun, with unrest episodes occurring in 1950-1951, 1969-1972, 1982-1984, during which the ground uplift at the port of Pozzuoli town reached about 4 m with respect to the ground level before 1950 (Figure 2). After the end of 1984, the ground subsided about 0.94 m, until the end of 2005. Since the last months of 2005, ground uplift started again and is still in progress (Figure 2); total uplift for the ongoing unrest has reached 1.39 m at the end of June 2024 [18]. The origin of this inflation-deflation process of the caldera (Figure 2) is still under debate and may be related to the magma intrusion, pressurization-depressurization of the hydrothermal system, or both [20; 21; 22; 54; 66; 67; 68; 69]. The ground uplift of the 1950-1951 was not accompanied by felt seismicity, and during the period 1969-1972, the seismicity was of very low magnitude, up to about M = 2.1. During the period 1983-1984, on the contrary, seismicity was very frequent (about 16,000 earthquakes were recorded by the analogue seismic network of the time) and with significant magnitude (M\({}_{\rm max}\) = 4.0) [43; 70; 71]; a total maximum ground uplift of 1.8 m was recorded (Figure 3) near the town center of Pozzuoli [72]. Seismic activity was characterized as Md = 4.0 during an event on 4 April 1983 and as Md = 3.8 on 8 December 1984, resulting in substantial damage to buildings and the evacuation of over 40,000 residents from the central town of Pozzuoli. Seismicity here is very shallow (in the first 3 km of the crust) so that even earthquakes of magnitude 4 or lower can be very damaging for the closest buildings in this very densely urbanized area. Figure 2: Ground deformation trend, located in the maximum deformation area close to the “Rione Terra” district in the Pozzuoli city center: (**a**) levelling measurements from 1905 to 2017 at the altimetric benchmark 25A [71; 73]; (**b**) GNNS measurements at the RITE station from June 2000 to June 2024 [18]. The location of the RITE station is shown in Figure 1; the RITE station is located in the same site as benchmark 25A. A subsidence of about 1 m occurred during 1985-2005. In the last two decades, the central portion of Campi Flegrei caldera has experienced ground uplift of about 1.4 m and an increase in magnitude and extent of seismicity, especially since 2021 [18; 74]. During the ongoing unrest, although the average uplift rate (70 mm/yr from June 2024 to 2025) is much lower than those in the '70s and 80s' (about 440 mm/yr in 1969-1972 and 590 mm/yr in 1982-1985 [71]) (Figure 2), seismicity reached the highest magnitude ever recorded before, during an earthquake of M = 4.2 which occurred on 27 September 2023 and a strongest magnitude of M = 4.4 which occurred on 20 May 2024.