## 5 Discussion  
Our work carefully investigated the detailed features of the vertical ground displacement pattern at Campi Flegrei and its relationship with the main structural and seismo-tectonic features of the area. What is very clearly observed during uplift and subsidence episodes in this area, in fact, is the remarkably constant shape of the ground displacements (Figure 18), which appear radially symmetrical to a large extent. Furthermore, seismicity also appears remarkably concentrated within a limited area and mainly clustered in a few clearly evident sectors [7] (Figure 7). In order to obtain relevant information on the ground deformation shape, we compared recent normalized deformation patterns (Figure 18)--referring to uplift data from 2016 to 2021 obtained from both Sentinel-1 ortho-vertical DInSAR data and coeval GPS data--with those of the last 120 years, which were derived by levelling measurements between 1905 and 1992 and 2000-2019 GNNS data on uplift and subsidence gathered in different unrest periods [102]. Figure 18 shows a similar shape for the three plotted datasets. It is evident that unrest episodes at Campi Flegrei caldera, occurred in the last 120 years during both inflation and deflation episodes, involve similar ground deformation patterns, with a radial symmetry around a constant deformation center. The GPS curve shows slightly higher values than the Sentinel-1 curve at distances of 1 to 6 km. Minor differences can be observed at distances lower than 3000 m, where the curve by [PERSON] & [PERSON] [102] shows values slightly higher than the 2016-2021 curves, and between 3000 and 9000 m., where the curve shows values slightly lower than the Sentinel and GPS curves. Our results confirm that the overall first-order geometry of the vertical ground deformation pattern at Campi Flegrei is radial, 'bell-shaped', and very concentrated in a small area [7; 13; 37; 67], which likely corresponds to the resurgent block inside the caldera [54] (Figure 10). The vertical deformation we inferred in the analyzed period (2016-2021) is an uplift, but it has been observed from MT-InSAR, precision levelling, and GPS measurements during the period 1905-2019 that the subsidence in the area is also perfectly specular to the uplift [69; 111; 112; 113]. As shown by [PERSON] and [PERSON] [76] and [PERSON] et al. [77], the very constant shape of deformation pattern both for uplift and for subsidence as well as the large amount of deformation can be well explained by the presence of a central resurgent  
Figure 17: Linear correlation between GPS (x axis) and Sentinel-1 (y axis) data for each point. area bordered by ring faults. Such a structure is likely to be represented by the central resurgent block recently clearly evidenced by [PERSON] et al. [54] and already hypothesized and supported by the combined observations of seismicity and ring faults at sea by [PERSON] et al. [41; 42]. The main radially symmetrical pattern has been explained by different authors in terms of the main volcanic source, which could almost equally be direct magma pressure caused by sill intrusions [21; 22] or an increase in pressure in the geothermal system caused by hot gas rising from the main magma chamber [7; 23; 114]. However, our work identifies, for the first time, some peculiar deviations (anomalies in uplift) from such a 'bell-shaped' pattern, which can be interpreted in terms of structural heterogeneities in the area. The main radial pattern of vertical deformation data was parametrized by a polynomial function of the fourth order. A careful analysis was carried out regarding the order of the polynomial to be used to describe the first-order deformation geometry. Based on the following considerations, the fourth-order polynomial was deemed the best fitting.