The Ground Deformation History of the Neapolitan Volcanic Area (Campi Flegrei Caldera, Somma–Vesuvius Volcano, and Ischia Island) from 20 Years of Continuous GPS Observations (2000–2019)

: The Neapolitan volcanic area includes three active and high-risk volcanoes: Campi Flegrei caldera, Somma–Vesuvius, and Ischia island. The Campi Flegrei volcanic area is a typical exam-ple of a resurgent caldera, characterized by intense uplift periods followed by subsidence phases (bradyseism). After about 21 years of subsidence following the 1982–1984 unrest, a new inﬂation period started in 2005 and, with increasing rates over time, is ongoing. The overall uplift from 2005 to December 2019 is about 65 cm. This paper provides the history of the recent Campi Flegrei caldera unrest and an overview of the ground deformation patterns of the Somma–Vesuvius and Ischia volcanoes from continuous GPS observations. In the 2000–2019 time span, the GPS time series allowed the continuous and accurate tracking of ground and seaﬂoor deformation of the whole volcanic area. With the aim of improving the research on volcano dynamics and hazard assessment, the full dataset of the GPS time series from the Neapolitan volcanic area from January 2000 to December 2019 is presented and made available to the scientiﬁc community.


Introduction
Two of the most famous and high-risk volcanoes in the world, the Somma-Vesuvius and the Campi Flegrei caldera, overlook the Gulfs of Naples and Pozzuoli. Ischia island, the third active volcano of the Neapolitan volcanic area, is located at the north of the Gulf of Naples and a short distance from the Procida island.
The Campi Flegrei caldera (CFc) is an active volcanic system located to the west of the city of Naples. This whole area, with about 1.5 million inhabitants, is considered to have a very high volcanic risk [1].
CFc is known for the occurrence of slow vertical ground movements (bradyseism). Subsidence has been the dominant process in the last 2000 years, but a fast uplift preceded the Mt. Nuovo eruption (1538), and episodes of uplift are also documented by the wellknown evidence on the columns of the Roman Temple of Serapis [10][11][12][13][14] located near the Pozzuoli harbor. A new uplift phase started in late 2005 [16] and is ongoing. It is accompanied by increased seismicity, degassing activity, and compositional changes in the emitted fluids [17][18][19][20][21].
The island of Ischia has undergone a large resurgence process since about 56 ka, after a caldera forming eruption (Green Tuff eruption) [22]. The maximum uplift was of the order of 900 m with a rate of a few centimeters per year [23], forming the structural uplifted block of Mt. Epomeo. Effusive and explosive eruptions and avalanche processes accompanied the resurgence processes, and destructive earthquakes have characterized the most recent history of Ischia island [24][25][26][27]. The last eruption took place in 1302 AD. The southwestern part of the island is characterized by the presence of a high-temperature hydrothermal system, with geothermal gradients >150 °C [28]. The largest earthquakes occurred on 4 March 1881 and on 28 July 1883. The event of 1883 was characterized by an intensity Imax of XI MCS degree and a magnitude of 4.8 ≤ M ≤ 5.2 [26][27][28]. On 21 August 2017, an earthquake occurred in the area of Casamicciola Terme with a 2 km deep hypocenter and a magnitude of Md = 4.0 (Mw = 3.9) [29][30][31].
Somma-Vesuvius is a medium-sized strato-volcano located in the center of Campanian Plain. It consists of Mount Somma, an older volcano, whose summit part sank, generating a caldera, and the more recent Vesuvius, which grew inside this caldera. It is practically a symmetrical cone, whose maximum height is 1281 m a.s.l. In summary, the eruptive history of Somma-Vesuvius can be described as follows. About 37,000 years ago, the Phlegraean eruption of Ignimbrite Campana covered most of Campania with a layer of tuff. The stratigraphy of the Trecase 1 geothermal well shows [32] that on these deposits, due to effusive and explosive eruptions, the Somma volcano began to grow. This activity was interrupted around 22 ky BP by the Plinian eruption of the Pomici di Base [33]. After this event, the Somma caldera began to form [34]. Then, 3 ky later, there was the subplinian eruption of the Verdoline Pomici [35]. The following 15 ky were characterized by two Plinian eruptions, Mercato Pumice and Avellino Pumice, then at least eight explosive eruptions followed [36]. This period of activity ended in 217 BC [37], preceding the 79 AD Plinian eruption. In 1985, an almost continuous subsidence phase started, interrupted only by miniuplift in 1989, 1994, and 2000 ( Figure 1), with small displacements ranging between 1 and 7 cm [15].
The island of Ischia has undergone a large resurgence process since about 56 ka, after a caldera forming eruption (Green Tuff eruption) [22]. The maximum uplift was of the order of 900 m with a rate of a few centimeters per year [23], forming the structural uplifted block of Mt. Epomeo. Effusive and explosive eruptions and avalanche processes accompanied the resurgence processes, and destructive earthquakes have characterized the most recent history of Ischia island [24][25][26][27]. The last eruption took place in 1302 AD. The southwestern part of the island is characterized by the presence of a high-temperature hydrothermal system, with geothermal gradients >150 • C [28]. The largest earthquakes occurred on 4 March 1881 and on 28 July 1883. The event of 1883 was characterized by an intensity Imax of XI MCS degree and a magnitude of 4.8 ≤ M ≤ 5.2 [26][27][28]. On 21 August 2017, an earthquake occurred in the area of Casamicciola Terme with a 2 km deep hypocenter and a magnitude of Md = 4.0 (Mw = 3.9) [29][30][31].
Somma-Vesuvius is a medium-sized strato-volcano located in the center of Campanian Plain. It consists of Mount Somma, an older volcano, whose summit part sank, generating a caldera, and the more recent Vesuvius, which grew inside this caldera. It is practically a symmetrical cone, whose maximum height is 1281 m a.s.l. In summary, the eruptive history of Somma-Vesuvius can be described as follows. About 37,000 years ago, the Phlegraean eruption of Ignimbrite Campana covered most of Campania with a layer of tuff. The stratigraphy of the Trecase 1 geothermal well shows [32] that on these deposits, due to effusive and explosive eruptions, the Somma volcano began to grow. This activity was interrupted around 22 ky BP by the Plinian eruption of the Pomici di Base [33]. After this event, the Somma caldera began to form [34]. Then, 3 ky later, there was the subplinian eruption of the Verdoline Pomici [35]. The following 15 ky were characterized by two Plinian eruptions, Mercato Pumice and Avellino Pumice, then at least eight explosive eruptions followed [36]. This period of activity ended in 217 BC [37], preceding the 79 AD Plinian eruption.
After the Plinian eruption of 79 AD, which destroyed Herculaneum and Pompei, a subplinian eruption occurred (472 AD). It was followed by a persistent activity of about 700 years [38,39]. From the 12th century, a period of low activity began, which ended with the subplinian eruption of 1631. Medium and small eruptions characterized the activity of Somma-Vesuvius until 1944, when the last eruption took place. Over the past eight decades, Somma-Vesuvius activity has been characterized by subsidence, low-temperature fumaroles, and hundreds of low-energy earthquakes per year. The seismic event of greatest energy occurred in October 1999 [40][41][42].
This paper presents and makes available to the scientific community the GPS time series of the north, east, and vertical components of the 41 stations monitoring ground deformation in the whole Neapolitan volcanic area. The time series of the CFc cover the period 2000-2019, and those of Ischia and Procida cover the period 2001-2019. These data provide a continuous and accurate history of the recent CFc unrest (Figure 1) and are a useful tool for the study of ground deformation patterns of the Somma-Vesuvius and Ischia volcanoes.

The Neapolitan Volcanoes Continuous GPS Network
The Neapolitan Volcanoes Continuous GPS (NeVoCGPS) network, operated by the Istituto Nazionale di Geofisica e Vulcanologia-Osservatorio Vesuviano (INGV-OV), was developed to monitor and quantify ground deformation due to volcanic and seismic activity at CFc, the Somma-Vesuvius volcano, and Ischia-Procida Islands [16,43,44]. The network comprises 37 continuous GPS (cGPS) stations (black dots in Figure 2a-c).
After the Plinian eruption of 79 AD, which destroyed Herculaneum and Pompei, a subplinian eruption occurred (472 AD). It was followed by a persistent activity of about 700 years [38,39]. From the 12th century, a period of low activity began, which ended with the subplinian eruption of 1631. Medium and small eruptions characterized the activity of Somma-Vesuvius until 1944, when the last eruption took place. Over the past eight decades, Somma-Vesuvius activity has been characterized by subsidence, low-temperature fumaroles, and hundreds of low-energy earthquakes per year. The seismic event of greatest energy occurred in October 1999 [40][41][42].
This paper presents and makes available to the scientific community the GPS time series of the north, east, and vertical components of the 41 stations monitoring ground deformation in the whole Neapolitan volcanic area. The time series of the CFc cover the period 2000-2019, and those of Ischia and Procida cover the period 2001-2019. These data provide a continuous and accurate history of the recent CFc unrest (Figure 1) and are a useful tool for the study of ground deformation patterns of the Somma-Vesuvius and Ischia volcanoes.

The Neapolitan Volcanoes Continuous GPS Network
The Neapolitan Volcanoes Continuous GPS (NeVoCGPS) network, operated by the Istituto Nazionale di Geofisica e Vulcanologia-Osservatorio Vesuviano (INGV-OV), was developed to monitor and quantify ground deformation due to volcanic and seismic activity at CFc, the Somma-Vesuvius volcano, and Ischia-Procida Islands [16,43,44]. The network comprises 37 continuous GPS (cGPS) stations (black dots in Figure 2a-c).
For further details on NeVoCGPS network design, instrumentation, and data management, the reader is referred to [16,43].  For further details on NeVoCGPS network design, instrumentation, and data management, the reader is referred to [16,43].
At present, 21 stations of the NeVoCGPS network are operating on land in the Campi Flegrei area, seven stations on Ischia and Procida islands, and nine stations on the Somma-  Table 1). Some stations started operating as early as 2000. The seafloor sector of the caldera is monitored by the MEDUSA (Multiparametric Elastic-beacon Devices and Underwater Sensor Acquisition system) marine infrastructure consisting of four instrumented buoys deployed in early 2016 in the Gulf of Pozzuoli [45]. A cGPS station was installed on top (blue dots in Figure 2a and Table 1) of each of the four elastic-beacon buoys, which in turn are rigidly connected by a steel cable or pole to the ballast on the sea bottom. This methodology is suitable for seafloor deformation measurements in shallow water (less than 100 m in depth) [45][46][47]. For more details on MEDUSA infrastructure, buoy design, and seafloor module equipment the reader is referred to [45,47].

Data Processing
The cGPS data of the NeVocGPS network were processed using the Bernese GPS software v. 5.0 [48] on a daily basis with the IGS final orbits and Earth rotation parameters (ERPs) [49].
The principal parameters and models used in GPS data processing strategy are reported in Table 2. Since 2000, important updates in the processing strategies have been adopted by the IGS Analysis Centers and different reference frames have been realized [53][54][55]. Due to these changes, the resulting time series parameters and the combined IGS products are highly inhomogeneous and inconsistent over time [56].
To obtain high-precision results, all the cGPS data collected by the NeVoCGPS network during the full time period (2000-2019) were processed using the same processing strategies, the updated products, and the most recent models.
Station discontinuities due to antenna calibration file updates or associated with equipment changes were estimated and corrected in the time series. The geodetic datum was realized by three No-Net Translation conditions imposed on a set of eight IGb14 reference stations (Minimum Constraint Solution), which were included in the processing.
To highlight the volcanic deformation and remove the background regional tectonic pattern, the processing results (time series and velocity fields) were transformed into a local reference frame defined by six stations of the INGV RING (Rete Integrata Nazionale GNSS) network [58] located outside the Neapolitan volcanic area (purple triangles in Figure 2a) and also included in the cGPS data processing. The velocities of the selected RING stations reflect the tectonic motion of the area. The velocities, uncertainties, and noise properties of the six RING station were estimated using the Hector software package [59], based on the maximum likelihood estimation algorithm (MLE). A power law plus white noise model was assumed to take into account the temporal correlated noise in the time series and to compute more realistic uncertainties for the velocities [60,61]. An annual and semiannual seasonal signal were estimated and removed. The results of the RING time series analysis are presented in Table 3. The local reference frame was realized by subtracting a mean horizontal velocity (17.1 mm/y and 21.5 mm/y, in the north and east component, respectively) of the six RING stations (Table 3) from the time series of the NeVoCGPS network. The tectonic contribution to the vertical component was assumed to be negligible and no correction was applied.
To take into account the motion of the buoys, the MEDUSA cGPS data were processed in kinematic mode with the RTKLIB ver. 2.4.2 software [62] to obtain positions of the buoys every 30 s. The cGPS station LICO (see Figure 2a) was the reference in data processing [45][46][47].
To reduce the noise due to weather and sea conditions on the horizontal components [45][46][47], we applied cleaning algorithms to outlier detection and removal, and weekly average to produce less frequent but more accurate positions (a full description of kinematic GPS time series analysis is reported in [47]).

Results
The outwards ( Figure 5). The horizontal displacements show a radial symmetry centered on Pozzuoli (Figures 3 and 4).      Figure 6 shows the weekly filtered MEDUSA GPS time series that highlight the vertical and horizontal displacements of the CF marine sector since 2016 [47].   Figure 6 shows the weekly filtered MEDUSA GPS time series that highlight the vertical and horizontal displacements of the CF marine sector since 2016 [47]. The seafloor deformation pattern is consistent with that observed by the on-land NeVoCGPS stations. The uplift ranges between 8 and 20 cm and the horizontal displacements show a radial trend, despite the data being affected by noise associated with meteorological conditions [47]. It is known that software-derived formal positioning errors are overly optimistic, and they should be rescaled by a scaling factor [63]. The mean values of the formal errors for the horizontal and vertical positions are 0.3 and 1 mm, respectively. We used a scaling factor of 10 to obtain a mean, and more realistic accuracies of 3 and 10 mm for the daily horizontal and vertical components, respectively, of the NeVoCGPS stations. For clarity, the rescaled errors are not shown in Figures 3-5, Figures 7 and 8, but are reported in data files in the Supplementary Materials.

Campi Flegrei Caldera
The final daily position time series of the 21 stations of the NeVoCGPS network operating in the CF area are shown in Figures 3-5, displaying the ground deformation history of the recent volcanic caldera unrest.
The first cGPS stations installed in the CF area detected the 2000 mini-uplift, recording a maximum vertical displacement of about 4 cm [64].
After this episode, the subsidence following the 1982-1984 unrest continued until 2005. Then, the uplift restarted and is ongoing, with different rates in time [16,65,66].
Starting from November 2005, the RITE cGPS station (Figure 2a) located at Rione Terra-Pozzuoli recorded the largest uplift, measuring about 65 cm (Figures 1 and 5). The other cGPS stations show a decrease in the uplift displacement from the caldera center outwards ( Figure 5). The horizontal displacements show a radial symmetry centered on Pozzuoli (Figures 3 and 4). Figure 6 shows the weekly filtered MEDUSA GPS time series that highlight the vertical and horizontal displacements of the CF marine sector since 2016 [47].
The seafloor deformation pattern is consistent with that observed by the on-land Ne-VoCGPS stations. The uplift ranges between 8 and 20 cm and the horizontal displacements show a radial trend, despite the data being affected by noise associated with meteorological conditions [47].

Ischia and Procida Islands
The final daily position time series of the seven stations of the NeVoCGPS network operating at Ischia and Procida islands are presented in Figure 7. The vertical and horizontal components of the time series are characterized by constant trends. Following the Ischia earthquake, which occurred on 21 August 2017, some cGPS stations (MEPO and OSCM in Figure 2b) recorded detectable coseismic deformations [44,67]. The offsets in time series in Figure 7 have been corrected.  All stations display a general subsidence, with higher values on the summit volcano edifice [43,68]. Only the BKNO, BKE1, and OSVE stations have significant horizontal displacements; the other stations show negligible horizontal trends.

Discussion and Conclusions
The ground and seafloor deformation pattern highlighted by the GPS time series at CFc in the last 20 years is characterized by the invariance of the uplifted area with the persistence of a bell-shaped geometry (Figure 9). The horizontal displacements show a

Ischia and Procida Islands
The final daily position time series of the seven stations of the NeVoCGPS network operating at Ischia and Procida islands are presented in Figure 7.
The vertical and horizontal components of the time series are characterized by constant trends. Following the Ischia earthquake, which occurred on 21 August 2017, some cGPS sta-tions (MEPO and OSCM in Figure 2b) recorded detectable coseismic deformations [44,67]. The offsets in time series in Figure 7 have been corrected. Table 4 shows the results in terms of horizontal and vertical velocities and uncertainties derived from the Ischia-Procida time series analysis with the Hector software package [59]. A power law plus white noise model was used to take into account the temporal correlated noise in the time series. An annual and semiannual seasonal signal were estimated and removed.
Regarding the north component (Figure 7a), the displacements of the SERR and MEPO stations are northward. The FORI, IPRO, OSCM, and AQMO stations have displacements southward, and SANT shows a negligible N-S trend.
Moreover, it is evident that for the east component (Figure 7b), the AQMO, SANT, and IPRO stations move westward. The displacements of OSCM, MEPO, and SERR are also westward, but at smaller rates. Only the FORI station seems to show a negligible E-W trend.
Regarding the vertical component (Figure 7c), all stations, except IPRO, show subsidence. In particular, the MEPO and SERR stations display a significant subsidence. The other sites also experience subsidence, but at a smaller rate.

Somma-Vesuvius Volcano
The final daily position time series of the nine stations of the NeVoCGPS network operating at the Somma-Vesuvius volcano are shown in Figure 8. Additionally, these cGPS stations are characterized by constant and linear trends. Table 5 shows station velocities and uncertainties derived from the Somma-Vesuvius time series analysis with the Hector software package [59]. A power law plus white noise model was used, and an annual and semiannual seasonal signal were estimated and removed. All stations display a general subsidence, with higher values on the summit volcano edifice [43,68]. Only the BKNO, BKE1, and OSVE stations have significant horizontal displacements; the other stations show negligible horizontal trends.

Discussion and Conclusions
The ground and seafloor deformation pattern highlighted by the GPS time series at CFc in the last 20 years is characterized by the invariance of the uplifted area with the persistence of a bell-shaped geometry (Figure 9). The horizontal displacements show a radial pattern (i.e., axial symmetry) from the zone of maximum vertical deformation (Figure 9a) located at Pozzuoli town in the central part of the caldera (Figure 2a).
Remote Sens. 2021, 13, x FOR PEER REVIEW 12 of 18 radial pattern (i.e., axial symmetry) from the zone of maximum vertical deformation (Figure 9a) located at Pozzuoli town in the central part of the caldera (Figure 2a). The shape of the vertical and horizontal ground deformation pattern is remarkably constant (Figure 9), and independent from the total amount of displacement, as noted in several papers referring to both uplift and subsidence episodes during the past 50 years ( [69,70], and references therein).
This characteristic pattern seems to suggest a stable source location during the time interval from 2000 to 2019. Acocella et al., 2015 [71], studying unrest episodes at calderas over an interval of 26 years (1988-2014), concluded that many unrest episodes do not lead to eruptions, but an eruption is always preceded by an unrest episode. Therefore, it is critically important to understand the source from which the uplift episodes at CFc are produced.
By mainly using leveling, GPS, and InSAR data, several authors have proposed different models to describe the source of the ground deformations at CFc ( [72] and references therein). The shape of the vertical and horizontal ground deformation pattern is remarkably constant (Figure 9), and independent from the total amount of displacement, as noted in several papers referring to both uplift and subsidence episodes during the past 50 years ( [69,70], and references therein). This characteristic pattern seems to suggest a stable source location during the time interval from 2000 to 2019. Acocella et al., 2015 [71], studying unrest episodes at calderas over an interval of 26 years (1988-2014), concluded that many unrest episodes do not lead to eruptions, but an eruption is always preceded by an unrest episode. Therefore, it is critically important to understand the source from which the uplift episodes at CFc are produced.
By mainly using leveling, GPS, and InSAR data, several authors have proposed different models to describe the source of the ground deformations at CFc ( [72] and references therein).
Despite the large amount of deformation, geochemical, and geophysical data acquired in the last years, the magmatic versus hydrothermal origin of the unrest episodes at Campi CFc is still a matter of debate ( [69,70,72], and references therein).
Several authors have interpreted uplift episodes as induced by a magmatic source [73] or as driven by hydrothermal system processes as deeper fluids released by the intruding magma affect the hydrothermal system [74][75][76][77]. Figure 10 shows the horizontal and vertical velocity field retrieved from the GPS time series at Ischia and Procida islands (Table 4). In the eastern part of Ischia island, the horizontal displacements are mainly westward with rates of 2.9 ± 0.1 mm/y in a SW direction (AQMO station) and 2.8 ± 0.1 mm/y in a W direction (SANT station). Procida island shows a horizontal displacement with rate of 3.7 ± 0.1 mm/y in a SW direction (IPRO station). The FORI station, located in western part of the island, displays a rate of 1.1 ± 0.1 mm/y in a S direction, indicating a condition of greater stability in this part of the island. The velocity vectors of MEPO station (2.6 ± 0.3 mm/y), located on summit of Mount Epomeo, and SERR station (3.6 ± 0.1 mm/y) on the central-western part of the island show velocity pointing towards N-NW, whereas OSCM station, in the north part of the island, has a rate of 3.1 ± 0.1 mm/y in a SW direction. It seems that the area between Monte Epomeo and Casamicciola is an area of accumulation of compressive strain [29,30]. This area is precisely the one affected by the earthquake of 21 August 2017 [29,30,44].
Remote Sens. 2021, 13, x FOR PEER REVIEW 13 of 18 Despite the large amount of deformation, geochemical, and geophysical data acquired in the last years, the magmatic versus hydrothermal origin of the unrest episodes at Campi CFc is still a matter of debate ( [69,70,72], and references therein).
Several authors have interpreted uplift episodes as induced by a magmatic source [73] or as driven by hydrothermal system processes as deeper fluids released by the intruding magma affect the hydrothermal system [74][75][76][77]. Figure 10 shows the horizontal and vertical velocity field retrieved from the GPS time series at Ischia and Procida islands (Table 4). In the eastern part of Ischia island, the horizontal displacements are mainly westward with rates of 2.9  0.1 mm/y in a SW direction (AQMO station) and 2.8  0.1 mm/y in a W direction (SANT station). Procida island shows a horizontal displacement with rate of 3.7  0.1 mm/y in a SW direction (IPRO station). The FORI station, located in western part of the island, displays a rate of 1.1  0.1 mm/y in a S direction, indicating a condition of greater stability in this part of the island. The velocity vectors of MEPO station (2.6  0.3 mm/y), located on summit of Mount Epomeo, and SERR station (3.6  0.1 mm/y) on the central-western part of the island show velocity pointing towards N-NW, whereas OSCM station, in the north part of the island, has a rate of 3.1  0.1 mm/y in a SW direction. It seems that the area between Monte Epomeo and Casamicciola is an area of accumulation of compressive strain [29,30]. This area is precisely the one affected by the earthquake of 21 August 2017 [29,30,44]. The vertical GPS velocity field ( Figure 10) displays a general subsidence, with maximum rates recorded at the MEPO (−11.2  1 mm/y) and SERR (−6.1  0.1 mm/y) stations. The AQMO, FORI, and SANT stations also show subsidence, but at smaller rates. The minimum value is recorded at FORI with −1.3  0.2 mm/y, whereas the IPRO station shows a negligible vertical trend.
Therefore, in the period 2001-2019, the vertical velocity field indicates the general subsidence of the island, whereas the horizontal field shows that the stations in the western part of island have displacement vectors pointing towards the central part, where the highest values of subsidence are recorded. The vertical GPS velocity field ( Figure 10) displays a general subsidence, with maximum rates recorded at the MEPO (−11.2 ± 1 mm/y) and SERR (−6.1 ± 0.1 mm/y) stations. The AQMO, FORI, and SANT stations also show subsidence, but at smaller rates. The minimum value is recorded at FORI with −1.3 ± 0.2 mm/y, whereas the IPRO station shows a negligible vertical trend. Therefore, in the period 2001-2019, the vertical velocity field indicates the general subsidence of the island, whereas the horizontal field shows that the stations in the western part of island have displacement vectors pointing towards the central part, where the highest values of subsidence are recorded. Figure 11 shows the horizontal and vertical GPS velocity field at the Somma-Vesuvius volcano in the time span of 2001-2019 (Table 5).
Remote Sens. 2021, 13, x FOR PEER REVIEW 14 of 18 part of the volcano, also display subsidence, but at smaller rates ( Table 5). The GPS stations with the highest rates are those installed at higher altitudes, i.e., those with highest potential energy [34]. From Vesuvius to Somma, the subsidence rate decreases until reaching the minimum values on the coast. These values are in agreement with data from the tide gauge network [78] and from the levelling measurements [68]. These last two considerations show how the vertical displacements are influenced by the topography of the volcano [79,80]. The presence of the volcano does not appear to affect the horizontal velocities of the GPS stations ( Figure 11). In fact, as already mentioned, only the BKNO (3.4  0.1 SW), BKE1 (1.6  0.1 W), and OSVE (1.2  0.1 SE) stations have horizontal velocities greater than 1 mm/y, whereas the other stations show a negligible horizontal trend.
The complete dataset of the GPS time series from January 2000 to December 2019 presented in this study is available in the Supplementary Materials. These data can be useful for the scientific community to improve the knowledge on volcano dynamics and volcanic and seismic source models, in addition to possibly discriminating a magmatic versus a hydrothermal origin of the uplift and subsidence episodes at Campi Flegrei caldera; the data can also provide useful information for volcanic hazard assessment of the Neapolitan area.  The dynamics of the volcano are characterized by a general subsidence with maximum rates recorded at the BKNO (7.2 ± 0.2 mm/y) and BKE1 (−6.2 ± 0.3) stations, located on Vesuvius, the younger part of the volcano. The stations installed on Somma, the oldest part of the volcano, also display subsidence, but at smaller rates ( Table 5). The GPS stations with the highest rates are those installed at higher altitudes, i.e., those with highest potential energy [34]. From Vesuvius to Somma, the subsidence rate decreases until reaching the minimum values on the coast. These values are in agreement with data from the tide gauge network [78] and from the levelling measurements [68]. These last two considerations show how the vertical displacements are influenced by the topography of the volcano [79,80].
The presence of the volcano does not appear to affect the horizontal velocities of the GPS stations ( Figure 11). In fact, as already mentioned, only the BKNO (3.4 ± 0.1 SW), BKE1 (1.6 ± 0.1 W), and OSVE (1.2 ± 0.1 SE) stations have horizontal velocities greater than 1 mm/y, whereas the other stations show a negligible horizontal trend.
The complete dataset of the GPS time series from January 2000 to December 2019 presented in this study is available in the Supplementary Materials. These data can be useful for the scientific community to improve the knowledge on volcano dynamics and volcanic and seismic source models, in addition to possibly discriminating a magmatic versus a hydrothermal origin of the uplift and subsidence episodes at Campi Flegrei caldera; the data can also provide useful information for volcanic hazard assessment of the Neapolitan area.