Burst-like swarms and periodical VT events in the accelerating unrest phase of Campi Flegrei caldera (2021-2024)

Burst-like swarms and periodical VT events in the accelerating unrest phase of Campi Flegrei caldera (2021-2024) | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Burst-like swarms and periodical VT events in the accelerating unrest phase of Campi Flegrei caldera (2021-2024) Giovanni Macedonio, Flora Giudicepietro, Rosario Avino, Eliana Bellucci Sessa, and 23 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4708123/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 11 Feb, 2025 Read the published version in Nature Communications → Version 1 posted You are reading this latest preprint version Abstract Since 2021, peculiar seismic sequences became evident and frequent in Campi Flegrei caldera (Italy), while deformation, seismicity and gas emission showed an acceleration. We distinguished burst-like swarms and periodical VT sequences. The earthquakes of both types of sequences resulted located in an area that includes the main hydrothermal field, and a zone affected by a geodetic anomaly, which clearly appeared in 2021. Burst-like swarms (max Md = 4.4) are accompanied by a pseudo-tremor, suggesting a mechanism involving near-continuous brittle failure. The periodical VT sequences are shallow and appear linked to the dynamics of the Mt Olibano lava dome, which deforms non-uniformly compared to the rest of the caldera and coincides with the geodetic anomaly. This peculiar seismicity, described in the Campi Flegrei for the first time in this study, has been associated with phreatic explosions and critical phases of unrest in other volcanoes, and currently characterizes the rapidly evolving state of activity of this high-risk volcano. Earth and environmental sciences/Solid Earth sciences/Volcanology Earth and environmental sciences/Natural hazards Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Introduction The volcanic area of Campi Flegrei (Italy) is a subcircular caldera with a diameter of approximately 12 km ( 1 , 2 ). The last eruption occurred in 1538 ( 3 ) and formed the volcanic edifice of Monte Nuovo. Today, the Campi Flegrei area is densely inhabited, with about 500,000 people living in this area ( 4 ). The main city is Pozzuoli, which is located in the central part of the caldera (Fig. 1 ). Ground deformations are typical of this caldera and, in the past, they have resulted in phases of subsidence alternating with phases of uplift. This phenomenon is called bradyseism and has been studied since the 19th century by the major scientists of that time ( 5 , 6 ). In the first half of the 19th century, relative measurements of sea level compared to ground level ( 7 ) demonstrated that the Campi Flegrei caldera was in subsidence. Since then, the first uplift phase began in the 1950s ( 8 ). Then, two other bradyseism crises occurred in the 1969–1970 period ( 9 ) and during the 1982–1984 time interval ( 10 – 14 ), accompanied by significant uplift (1.7 and 1.8 m respectively) and seismicity. These bradyseism crises were followed by temporary subsidence ( 15 , 16 ). Finally, the last phase of bradyseism began around 2005 and is still ongoing. The current phase is characterized by increasing seismicity ( 17 – 21 ) and fumarolic tremor amplitude, which is indicative of hydrothermal activity ( 22 – 26 ). Moreover, increasing fluid emission ( 27 – 36 ) and significant ground deformations ( 14 , 37 – 45 ), which produced about 1.2 m uplift in the central sector of the caldera, are typical of the current unrest (Fig. 1 ). During the bradyseism crises of the second half of the 20th century and in the current one the deformation pattern has always shown a bell-shaped radial pattern with the maximum uplift in the central area of the caldera, where the RITE GNSS station is presently located (Fig. 1 ). In the 1982-84 crisis, deformation measurements were mainly based on topographic leveling, which provided vertical deformation measurements. For that period the deformation pattern was modeled as a Mogi source ( 10 , 11 ) or as a horizontal crack ( 13 , 13 ). The deformation pattern of the current unrest, which is measured through the GNSS network, managed by the Osservatorio Vesuviano of the Istituto Nazionale di Geofisica e Vulcanologia (INGV-OV) and through DInSAR measurements, has often been modeled as a horizontal crack (e.g. 40,41,45,46), or a horizontal thermo-poro-elastic zone ( 47 ). As a consequence of the current unrest, in December 2012 the Italian Civil Protection decided to move to the yellow alert phase, which is the second of four alert levels (green = basic; yellow = attention; orange = pre-alarm; red = alarm). From 2012 onwards, the unrest continued and accelerated (Fig. 1 c,d,e) in terms of increase in the earthquake occurrence rate, earthquake magnitude ( 17 , 18 , 25 ), ground deformation rate ( 18 , 19 ) and variations in geochemical parameters ( 36 ). In particular, Bevilacqua et al. ( 19 ) have shown that the increase in the cumulative number of earthquakes (in total or above a given Md) versus the vertical uplift of the caldera is well fitted with two exponential functions and that the “connecting time” between these functions falls in the period between 4/2020 and 9/2022. This transition between the two exponential functions, the second having a larger exponent, marks a clear acceleration of the seismicity in the caldera. Furthermore, analyzing in detail the deformation pattern, Giudicepietro et al. ( 46 ) discovered a geodetic anomaly that clearly manifested in 2021, with an uplift deficit of approximately 9 cm in the Mt. Olibano area (East of Pozzuoli) compared to the surrounding areas. The aforementioned recent studies indicate that during 2021 the unrest of the Campi Flegrei caldera has intensified. For this reason, in this work we analyzed the seismicity that occurred in 2021–2024 with the aim of investigating the relationships between the characteristics of the earthquakes, the seismogenic processes, the evolution of the deformation field and the geochemical variations of the Campi Flegrei caldera. In particular, we focused our attention on several seismic sequences characterized by very short inter-event times, of even a few seconds. Within this category, we distinguished two types of sequences which we call burst-like swarms and periodical Volcano-Tectonic (VT) events, following the classic literature of volcanic seismology (i.e. 48,49). These types of sequences can provide clues on the seismogenic processes in relation to the evolution of the current accelerating unrest, which is controlled by intense ground deformation and escalating hydrothermal activity. Results In Campi Flegrei the INGV-OV seismic network has recorded approximately 18,500 earthquakes (max Md = 4.4; average Md = 0.26, minimum Md= -1.6) from March 2005 to May 2024. Not all of these earthquakes are located because part of them are too small to obtain a reliable location (those located are around 10,200). The locations show a distribution that approximately describes an ellipse in the central area of the caldera, however the majority of earthquakes (about 90%) are located at Pozzuoli and east of Pozzuoli, in an area that includes the Solfatara-Pisciarelli hydrothermal system and the lava domes of Mt. Olibano (see the zone marked as A in Fig. 1 a). A minor group of earthquakes (about 2% of the total) is located in the Gulf of Pozzuoli (zone B in Fig. 1 a). The cumulative earthquake count highlights the progressive increase in seismicity rate over time (Fig. 1 c). We focused on the seismicity in the period from 2021 to 2024, when the current unrest showed an intensification in terms of seismicity rate and uplift velocity. Specifically, we looked at the types of seismic events that show peculiar characteristics. The seismic events recorded in the Campi Flegrei caldera from 2021 to 2024 are generally of VT type. In the context of VT seismicity, we recognized sequences characterized by a very short time interval between two consecutive events, which we call here intertime (even less than a few seconds). In particular, we distinguished sequences of periodical VT events, which can be considered similar to drumbeat type events ( 49 , 50 ) as regards the regularity of occurrence, although in the case of the Campi Flegrei these sequences contain a limited number of events. Moreover, we recognized earthquake sequences that we call burst-like swarms, following the definition of Hill et al. ( 48 ). Periodical VT type sequences (Fig. 2 a-c) are generally characterized by events of short duration (from a few seconds to a few tens of seconds), with impulsive onset, which occur with earthquake intertimes comparable to each other (e.g. a few seconds or tens of seconds). Generally, no major events are associated with pure periodical VT sequences. Burst-like swarm sequences (Fig. 2 e-h) are characterized by widely overlapping earthquakes, with intertimes that are sometimes not easily recognizable, and are generally accompanied by a continuous tremor-like background signal. However, the background signal cannot be defined as volcanic tremor because it does not show distinct frequency peaks as volcanic tremor typically exhibits. Therefore, we define this background signal as pseudo-tremor. The spectrogram analysis, which highlights the wide frequency band that characterizes the pseudo-tremor, suggests that it can be due to signals generated by a mechanism involving near-continuous brittle failure. Examples of this type of background signal are depicted in Fig. 2 f, which shows a burst-like swarm recorded on 19 April 2023, starting with a pseudo-tremor signal, with increasing amplitude, and Fig. 2 e, which shows the pseudo-tremor type signal recorded on 22 July 2022, with remarkable amplitude, in the second part of the recording. Burst-like swarm sequences can be accompanied by larger events. In particular, the earthquake with the greatest duration magnitude (Md = 4.4), recorded in the Campi Flegrei on 20 May 2024 at 18:10 (UTC), belongs to this type of sequence (Fig. 2 h). The spectrogram helps to distinguish the different events in the burst. For comparison, the seismogram and spectrogram of an earthquake not associated with a burst-like swarm is shown in Fig. 2 d. The selected event is the largest one (Md = 3.9) of the group of events located offshore in the Gulf of Pozzuoli (zone B in Fig. 1 a). Looking at Fig. 2 it can be noted that the two categories of sequences in certain cases show common characteristics. For example, the burst-like swarm in Fig. 2 f shows a regular sequence of VTs in the second half of the recording. To better investigate these seismic sequences, we selected two well-defined examples of the two types. Figure 3 shows the comparison between the periodical VT sequence recorded on 12 October 2023 and the burst-like swarm recorded on 14 April 2024. The burst-like swarm includes two earthquakes with Md > 3.0 and shows the background signal here defined as pseudo-tremor, well recognizable in the spectrogram (Fig. 3 d). In the 5-minute signal windows of the two sequences reported in Fig. 3 c-d, we counted 38 events in the regular VT type sequence (Fig. 3 c), and 36 events in the burst-like swarm sequence, with an average earthquake intertime of approximately 8 seconds for both sequences. We compared the locations of the events belonging to these two sequences (Fig. 3 b). For each sequence, we selected the locations of the events contained in the half hour of signal shown in Fig. 3 a. For the periodical VT sequence of 12 October 2023, we obtained 10 locations with depths between 700 and 900 m and Md ranging between − 0.5 and 1.7. For the burst-like swarm sequence of 14 April 2024, we found 37 locations with depths between 730 and 2870 m and Md between − 0.3 and 3.7. Both sequences fall within the Solfatara-Olibano area (zone A in Fig. 1 a). Finally, we selected all the sequences with very short earthquake intertimes that we recognized in the period between January 2021 and May 2024. In this time interval this type of seismic sequences has become more frequent and more evident than in previous years. In our selection we did not always distinguish between the two types as the distinction is not clear for all the cases. It is appreciable only in the clearest sequences, such as those reported in the examples of Figs. 2 and 3 . We show the distribution of event epicenters belonging to the uncategorized very short earthquake intertimes sequences (light gray circles) in Fig. 4 a, together with those classified as burst-like swarms (red circles) and those classified as periodical VTs (light blue circles). The burst-like swarms are those shown in Fig. 2 e-h, the April 14, 2024 sequence reported in Fig. 3 a, and a sequence recorded on June 2, 2023 (total number of events = 70; depth ranging between 450 and 2870 m and Md between − 0.8 and 4.4). The periodical VTs are the events in Fig. 2 a-c and the sequence of October 12, 2023, shown in Fig. 3 a (total number of events = 47; depth ranging between 210 and 1410 m and Md between − 0.1 and 2.7). Moreover, in Fig. 4 a, we delimited the area of the geodetic anomaly discovered in Giudicepietro et al. ( 46 ) (solid line) and the Solfatara-Pisciarelli hydrothermal area (dashed line), based on the Solfatara Diffuse Degassing Structure (Solfatara DDS, 51). It is worth noting that the two types of events are closely spatially co-related as already shown in the examples in Fig. 3 . They partly share the same seismogenic volume, especially in the border sector between the Solfatara-Pisciarelli hydrothermal system and the area of the Mt. Olibano geodetic anomaly. Actually, as a result of our analysis, it emerges that burst-like swarms often also contain periodical VT events which are generally localized at smaller depths (Fig. 3 ). However, the locations of burst-like events are generally slightly north of the locations of pure periodical VT sequences. This distribution indicates a greater concentration of burst-like swarms in the Solfatara-Pisciarelli hydrothermal area and the preferential locations of periodical VT events around the area of the geodetic anomaly (Fig. 4 a, b). In this zone of the caldera, where the majority of the Campi Flegrei earthquakes are located, the diffuse degassing from the Solfatara crater increased significantly in the period 2021–2024 (Fig. 4 b, d), when the burst-like and periodical VT sequences became evident. In the same period, the geodetic anomaly at the lava dome of Mt. Olibano became considerably more pronounced, as evidenced by the residual between the actual uplift of the ACAE GNSS station and the estimated one (Fig. 4 c; see Materials and Methods). To frame this type of sequences in the general context of the ongoing unrest in the Campi Flegrei caldera, we considered the geodetic, GNSS and DInSAR measurements and the geochemical measurements and compared them with the ongoing seismicity. To show the distribution of earthquakes in the caldera, in relation to ground deformation, we selected earthquakes with Md > 1.0 and plotted them on the uplift map obtained with Sentinel 1 data (Fig. 5 ). We updated the processing performed in Giudicepietro et al.( 46 ) to obtain the map of the vertical displacement of all the correlated pixels of the Sentinel 1 data from 2015 to May 2024 (maximum uplift approximately equal to 103 cm). We reported on the map the indication of the deformation source obtained in Giudicepietro et al. ( 46 ) whose parameters are: latitude = 40.817814 +/- 13 m; longitude = 14.126922 +/- 17 m; depth = 3826 +/- 45 m. The spatial distribution of seismicity in the Campi Flegrei caldera highlights the occurrence of earthquakes in an elliptical ring around the location of the ground deformation source (blue star in Fig. 5 ). We also added the focal mechanisms of two earthquakes, calculated with the FPFIT program ( 53 ), one falling in zone A (2024-04-14 08:01:44; Md = 3.0) and the other in zone B (2023-02-05 00:45:36, Md = 3.0) of Fig. 1 a. The focal mechanism of the earthquake in zone A is extensive whereas the focal mechanism of the earthquake in zone B is compressive. This characteristic is also common to other earthquakes recorded in the two zones, which for simplicity of representation we do not show in the figure but which are reported in the INGV-OV surveillance bulletins ( https://www.ov.ingv.it/index.php/monitoraggio-e-infrastrutture/bollettini-tutti/bollett-mensili-cf , last accessed 8 May 2024). However, the concentration of locations in zone A of Fig. 1 a suggests the overlap of multiple seismogenic processes. In particular, in zone A there is the Solfatara-Pisciarelli hydrothermal area, with CO 2 diffuse degassing which shows a progressive increase over time and is currently comparable with the average flux of CO 2 in the plume of active volcanoes with continuous degassing ( 36 ). In recent years, the diffuse emission of CO 2 from the target area, which is systematically monitored, increased from 200–300 t/d in 2010 to values higher than 1200 t/d in the 2024 measurement campaigns (Fig. 1 e). Furthermore, at the southern edge of the Solfatara crater, there is the geodetic anomaly of Mt. Olibano. Taking advantage of the high temporal sampling of the GNSS network, we calculated the vertical displacement deficit (uplift deficit) of the ACAE station, which is located in the geodetic anomaly, compared to the expected one. To do this, we applied a method similar to the one used with the DInSAR Sentinel 1 data to spatially map the geodetic anomaly in Giudicepietro et al. ( 46 ). In this case, we used RITE GNSS station, which well represents the uplift of the central area of the caldera, as a reference to derive the correlation coefficient to estimate the expected uplift of the ACAE station, from the time series relevant to the 2004–2024 period, therefore well before the start of the 2021 acceleration. We obtained the residuals by subtracting the expected uplift of the ACAE station from the observed uplift data (Fig. 4 c). Finally, we defined the opposite of the residuals as the “uplift deficit" (see Material and Methods for details). This allowed us to retrieve the temporal evolution of the uplift deficit of the ACAE station located in the geodetic anomaly. Thus, we were able to correlate the uplift deficit in the anomaly area with the cumulative earthquake count, which is dominated by events occurring in area A of Fig. 1 a. We found a very high correlation between the uplift deficit and the number of earthquakes (r = 0.998) (Fig. 6 a, b). We also compared the vertical component of the RITE station with the cumulative earthquake count and obtained a slightly lower correlation between the two parameters (r = 0.988) and a non-linear relationship (Fig. 6 c, d); this latter is consistent with the exponential-type trend analyzed in Bevilacqua et al.( 55 ). Discussion The picture that emerges from this study indicates that the current phase of bradyseism taking place in the Campi Flegrei caldera is accelerating. The process is controlled by the inflation of a source at a depth of about 3800 meters in the central area of the caldera (Fig. 5 ), whose evolution seems to justify the seismicity of the elliptical ring at the center of the caldera. This can be interpreted as the effect of a system of faults that border the uplifted block in the center of the caldera and create a ring fault-type structure. The focal mechanisms of the two earthquakes reported in Fig. 5 , extensive in zone A and compressive in zone B, represent a common characteristic of the earthquakes that occur in these two areas. In particular, extensive mechanisms in zone A are reported in numerous articles (e.g.24,54) as well as in the INGV-OV surveillance bulletins. The compressive mechanisms in zone B are also reported in the surveillance bulletins, however some of these are less constrained than those in zone A, that is better covered by the seismic network. In any case, the mutual positions of these focal mechanisms suggest a possible dislocation of the block in the center of the caldera, which in depth could behave like a trapdoor-type structure. However, we remark that the interpretation of the deformation source located in the center of the caldera at a depth of approximately 3800 m is still speculative. It could be due to an accumulation zone of magmatic fluids, the expansion of a porous medium subject to an increase in pressure or the intrusion of magma. The seismic sequences with short earthquake intertimes, which are the main object of this study, are mainly concentrated in the seismogenic area A. Burst-like swarms has been observed in other volcanoes such as Mammoth Mountain (United States), a hydrothermally active lava dome complex in the Long Valley caldera ( 55 ). For them, Hill et al. ( 48 ) hypothesized a process driven by a transient increase in local fluid pressure, based on the similarity of these sequences with signals associated with pressure transients generated by shutdown operations on production wells in Japanese geothermal fields ( 56 ). Nishi et al. ( 57 ) recorded sequences of this type on White Island and observed that they were located in the same seismogenic volume as volcano-tectonic earthquakes. The mechanism that caused them was attributed to brittle failure due to rapid fluid pressure fluctuations. Also, Lin et al. ( 58 ) recorded burst-like swarms on the Tatun Volcano Group (Taiwan) and interpreted them as the response of an extended network of cracks to the pressure variations due to fluid injection. Finally, a close association between these signals and hydrothermal activity is also suggested by McCausland et al.( 59 ) who consider them possible precursors of phreatic explosions when recorded in association with low-frequency and hybrid events. This leads us to interpret the burst-like swarms of the Campi Flegrei as the brittle response to the increase in the hydrothermal system fluid pressure. This interpretation is compatible with the model proposed by Fournier ( 60 ) for the movement of fluid from a shallow magmatic zone to the hydrothermal environment. Furthermore, we have evidence that this variation in hydrothermal fluid pressure occurs in extensive stress regime. Indeed, the burst-like swarms share the same source region as other earthquakes occurring in zone A, which have extensive focal mechanisms indicating an extensive stress regime. Furthermore, the burst-like swarm locations are close to the Mt Olibano area, where a geodetic anomaly is located, which is a further evidence of the local extensive stress regime. Periodical VT sequences are also concentrated in the zone A. In particular they are distributed around the lava block of Mt Olibano at depths generally less than 1000 m. The regular occurrence of seismic events in volcanic environments has been associated with a stick-slip mechanism between an extruding lava dome and conduit walls ( 61 – 63 ). This hypothesis could also be suitable to explain the regular VT sequences recorded at Campi Flegrei. In this case there is not an extruding lava dome, but a dome in relative subsidence compared to the general uplift of the surrounding areas, as evidenced by the uplift deficit of Mt Olibano, which correlates closely with seismicity (Fig. 6 ) and degassing (Fig. 4 c). In general, in the Campi Flegrei current unrest, the spatial distribution of earthquakes and the characteristics of the seismic sequences suggest the strong interconnection of the geodetic, hydrothermal and seismic phenomena, that are controlled by the deformation source located at the center of the caldera. When the uplift of the center of the caldera (RITE GNSS station) accelerates, the uplift deficit of the geodetic anomaly (ACAE GNSS station) and the cumulative earthquake count also accelerate. However, previous studies ( 18 , 19 ) have shown that seismicity, as represented by the cumulative earthquake count (total or above a given magnitude), increases more rapidly, in particular with two exponential regressions with increasing exponent in time, than GNSS data, such as the vertical component of the RITE station, which shows the greatest uplift among the GNSS stations. This difference in the temporal evolution of the two parameters is also observed in the period 2021–2024 (Fig. 6 c-d). On the other hand, in this study we calculated the daily time series of the uplift deficit in the Mt Olibano area, identified as a geodetic anomaly ( 46 ), exploiting the high acquisition rate of data from the INGV-OV GNSS network. It is noteworthy that this uplift deficit, due to the different uplift rate of the lava block of Mt Olibano compared to the surrounding areas, evolves similarly to seismicity, and shows a linear relationship with the cumulative earthquake count (Fig. 6 a, b). This linear relationship suggests that the uplift deficit is a good indicator of the inelastic component of the behavior of this localized sector of the caldera in response to the overall deformation process. Therefore, we interpret this finding as the effect of the lithological and mechanical discontinuity of the crustal rock, which in this area begins to deviate from elastic behavior when subjected to deformation. This interpretation aligns with the recent study on the spatial distribution of the b value ( 21 ), which highlights that the b value of the Mt. Olibano earthquakes is different from that found for the Solfatara-Piscairelli earthquakes, showing different physical characteristics of the medium. Methods The seismic network of the Campi Flegrei is developed and managed by the INGV-OV. It includes 27 stations distributed within the rim of the caldera. Moreover, four stations were installed in the Gulf of Pozzuoli ( 64 ). Most of the stations are equipped with three-component broadband seismometers, with a sampling rate of 100 samples per second and with accelerometers, whose signals are acquired at 200 samples per second. The data is transmitted in real time to the INGV-OV acquisition center. In 2020, the location sensibility in space of the Campi Flegrei seismic network was characterized by a magnitude threshold ranging between 0 and 0.5 ( 17 ). In the Solfatara-Pisciarelli area, where a very local seismicity has become increasingly evident since 2010, the seismic network is denser and allows us to locate earthquakes with magnitude (Md) < 0, when the seismic noise is particularly low. To describe the general trend of the current long-term unrest, we used the locations of the earthquakes from 2005 to 2024, and the seismic catalogue of the Campi Flegrei from 2004 to 2024 (Fig. 1 a,b,c). The locations and the seismic catalogue are extracted from the INGV-OV seismological database ( https://terremoti.ov.ingv.it/gossip/flegrei/index.html , last accessed 30 May 2024). The velocity model used by INGV-OV is reported in Calò and Tramelli ( 65 ). Since the burst-like and regular VT seismic sequences share the same source region as the other A-zone earthquakes, shown in Fig. 1 , we also created a specific A-zone earthquake catalog. This catalog includes only earthquakes with first arrival recorded by a seismic station located in the A-zone (see the seismic station map in Fig. 1 of the supplementary material). In selecting the A-zone earthquakes, we considered both localized and non-localized events, including those recorded by only one or two stations, which are typically very small. Thus, we generated plots similar to those in Fig. 6 , both with the cumulative count of all earthquakes recorded in the entire caldera and with the cumulative count of A-zone earthquake catalog. The two counts show negligible differences in their temporal evolution (see Fig. 2 of the supplementary materials), therefore, we no longer took into account the A-zone earthquake catalog, and in Fig. 6 , we only showed the count of all earthquakes in the caldera (which is evidently dominated by the seismicity of zone A). The selection of the burst-like swarms and VT regular sequences was conducted through visual analysis of the seismograms. The spectrogram analysis and time series correlations were performed using ObsPy ( 66 ), a system for seismic data analysis, along with the Python packages numpy, scipy, and matplotlib for figure creation. Additionally, since the ground deformations and geochemical changes taking place at Campi Flegrei are significant, and are closely correlated with each other and with seismicity, we also used data from the INGV-OV GNSS network and DInSAR Sentinel-1 data. Data from the continuous GNSS network of Campi Flegrei were processed using Bernese GNSS software on a daily basis with the IGS final and reprocessing products to obtain homogeneous results. To remove the regional tectonic background from the volcanic deformation pattern, the time series and velocity fields were transformed into a local reference frame including six stations from the INGV RING network, located outside the Neapolitan volcanic area. No correction was applied to the vertical component, as the tectonic contribution was considered negligible. Furthermore, seasonal signals were removed (for more details on the data processing of the Campi Flegrei GNSS network see ( 67 ). To obtain the time series of the vertical residuals of the ACAE GNSS station (Fig. 4 ), we applied the geodetic anomaly identification method from Giudicepietro et al. ( 46 ), based on DInSAR data, to the GNSS data. Considering the RITE station is located in the area of maximum uplift of the radially symmetrical bell-shaped deformation pattern of Campi Flegrei, and the ACAE station is located in Mt Olibano, we used these two stations to derive the uplift deficit in the area characterized by the geodetic anomaly. Specifically, we: 1) assumed RITE as a reference for the uplift trend in the central area of the caldera; 2) calculated a proportionality coefficient (α) between the RITE and ACAE uplift time series using data from 2015–2018, when the ACAE area was not yet affected by the geodetic anomaly; 3) estimated the expected ACAE uplift time series by multiplying the RITE uplift time series by α; 4) subtracted the expected ACAE uplift time series from the actual ACAE uplift time series, obtaining the residual series from 2004 to 2024 as shown in Fig. 4 . Then, we calculated the opposite of the residuals to obtain the time series of the “uplift deficit” shown in Fig. 6 a. The DInSAR data were generated by applying the Parallel Small BAseline Subset (P-SBAS) approach ( 68 , 69 ). In particular, we separately processed 430 ascending (Track 44) and 429 descending (Track 22) orbits data acquired by Sentinel-1 constellation from March 24, 2015 to May 24, 2024 to obtain the Line of Sight (LOS) displacement time series for each coherent pixel. Then, the vertical displacement component was calculated by combining the LOS P-SBAS time series using the method described in Casu and Manconi ( 70 ). The mean velocity of the vertical component computed for the 2021–2024 period represented as DInSAR fringes (every color cycle corresponds to a velocity variation of 4 cm/year) is shown in Fig. 7 a. It clearly highlights that the uplift deficit in the Mt. Olibano area, starting form 2021, makes the fringes to deviate from a radial pattern, which instead is preserved in the whole caldera. The vertical component of the Sentinel-1 data, updated to May 24, 2024, is also shown in blue scale in Fig. 5 . Finally, we used geochemical data from the INGV-OV geochemical monitoring network. The data consists in the CO 2 emission (in t/d) from an area of ~ 90000 m 2 (‘target area’ in Fig. 8 ) located inside the Solfatara crater. The CO 2 emission is monthly computed through the measurement of 63 fixed points with the accumulation chamber method ( 71 ). This monitoring activity started in 2004 and is still ongoing. The data show a marked increase of the CO 2 emission in the last years (Fig. 4 d). Further details of the technique measurement and data treatment are reported in Chiodini et al.( 36 ) and Cardellini et al. ( 52 ). Declarations Acknowledgments This study has been partly funded by the Italian DPC, in the frame of INGV-DPC (2022–2025) and IREA-DPC (2022–2024) agreements, although it does not necessarily represent DPC official opinion and policies. We also acknowledge the support of EPOS-RI, including the one obtained through the EPOS-Italia JRU. This work benefited from the Progetto ORME, INGV Project “Pianeta Dinamico” - Working Earth (CUP 1466 D53J19000170001 - legge 145/2018) (Scientific Responsibility: F.G.). This research was also partially funded by the European Union - NextGeneratonEU through the following projects: NRRP - MEET (National Recovery and Resilience Plan - Monitoring Earth's Evolution and Tectonics), ICSC - CN-HPC - PNRR M4C2 Investimento 1.4 - CN00000013, GeoSciences IR - PNRR M4C2 Investimento 3.1 - IR0000037, CN-MOST - PNRR M4C2 Investimento 1.4 - CN00000023. This work benefited also from the project Progetto Strategico Dipartimentale INGV 2019. “LOVE-CF”. This research has been supported by the GRINT (PIR01_00013) and IBiSCo (PIR01_00011) projects (PIR01_00013), funded by the National Operational Programme Infrastructures and Networks 2014/2020 of the Italian Ministry of Infrastructure and Transports. A pre-print version of this work is available at: https://www.techrxiv.org/doi/full/10.36227/techrxiv.171043391.14447454/v1. The authors would like to thank all the colleagues who contribute to the Campi Flegrei monitoring system. In particular, we thank the INGV technical staff who ensure the 24/7 operation of the seismic and GNSS monitoring networks. Author contributions: Conceptualization: FG, GC, RL, GM, FC, AN, MDV Methodology: FG, GC, RL, SC, RA, AB, CDL, PDM, CM, PR, FC, GS, MD Investigation: FM, AB, FR, FG, AS, PS, FDT, AME, AT, Visualization: EBS, WDC, FG, FC, GC, GS, MD Supervision: FG, GC, RL, GM, FC, AN, MDV Writing—original draft: FG, GC, RL, GM, FC Writing—review & editing: FG, GC, RL, GM, FC Competing interests: Authors declare that they have no competing interests. Data and materials availability: All data are available in the main text or the supplementary materials. 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Supplementary Files dataset1locallsequences.txt Dataset 1 dataset2locburstlikeswarms.txt Dataset 2 dataset3locperiodicalVT.txt Dataset 3 dataset4databaseCO2flux.xls Dataset 4 supplementaryinfo.docx Cite Share Download PDF Status: Published Journal Publication published 11 Feb, 2025 Read the published version in Nature Communications → Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. 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08:57:18","extension":"jpeg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":335492,"visible":true,"origin":"","legend":"\u003cp\u003ea) Shaded map of Campi Flegrei with the indication of the horizontal displacement of the GNSS stations in the period 2016-2024 (black arrows) and locations of the earthquakes that occurred between 2005 and 2024 (red dots). The RITE GNSS station, which is located in the center of the city of Pozzuoli, is indicated with a black square. Ellipses A and B mark the zones with the greatest concentration of epicentres, corresponding to the main seismogenic volumes, at crustal level. b) East-West section of the Campi Flegrei caldera with the hypocenters of the earthquakes recorded between 2005 and 2024. c) Temporal evolution of the cumulative earthquake count. d) Uplift measured from the vertical component of the GNSS RITE station. e) Soil CO\u003csub\u003e2\u003c/sub\u003e flux in the Solfatara area (‘target area’, (36))\u003c/p\u003e","description":"","filename":"image1.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-4708123/v1/c1aad0e2fa78ece258cb2634.jpeg"},{"id":60979382,"identity":"f85697bc-1372-438f-95ec-86a46c7ba424","added_by":"auto","created_at":"2024-07-24 08:49:19","extension":"jpeg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":526177,"visible":true,"origin":"","legend":"\u003cp\u003eSeismograms and spectrograms of examples of periodical VT sequences (a-c). d) Seismogram and spectrogram of the earthquake of 27 April 2024, 03:44:56 UTC (Md = 3.9) located in the Gulf of Pozzuoli (group B in Figure 1a) reported as an example of a Campi Flegrei earthquake not included in a burst-like swarm. (e–h) Seismograms and spectrograms of examples of burst-like swarms. These sequences also include earthquakes with relatively high magnitudes such as the one with Md 4.2 occurred on 27 September 2023 at 01:35 (UTC) (panel g) and the largest Campi Flegrei earthquake, with Md=4.4, recorded on May 20, 2024 at 18:10 (UTC) (panel h).\u003c/p\u003e","description":"","filename":"image2.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-4708123/v1/5d4e0e593f9b94169447c3c6.jpeg"},{"id":60979374,"identity":"52bc2961-ac09-4205-acd7-60255598a285","added_by":"auto","created_at":"2024-07-24 08:49:18","extension":"jpeg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":407376,"visible":true,"origin":"","legend":"\u003cp\u003eComparison between the regular VT sequence of 12 October 2023 and the burst-like swarm of 14 April 2024. a) 30-minute seismogram of the two sequences. b) Location of the events of the regular VT sequence (blue circles) and the burst-like swarm (semi-transparent red circles). c) Seismogram and spectrogram of the 5-minute window of the regular VT sequence highlighted with a yellow rectangle in panel a); seismogram and spectrogram of the 5-minute window of the burst-like swarm highlighted with a yellow rectangle in panel a).\u003c/p\u003e","description":"","filename":"image3.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-4708123/v1/ba88037a140a4e8865f0c3dd.jpeg"},{"id":60979375,"identity":"73485ad4-f708-4949-8e52-1287a2e0f6e3","added_by":"auto","created_at":"2024-07-24 08:49:18","extension":"jpeg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":322641,"visible":true,"origin":"","legend":"\u003cp\u003ea) Map of the locations of the events of the sequences with short earthquake intertime (see the square in the inset). Red circles: burst-like swarm events; blue circles: periodical VTs; light gray circles: uncategorized. The dashed line includes the Solfatara-Pisciarelli hydrothermal area. The solid line contains the area of the Mt. Olibano geodetic anomaly. The earthquake locations are included in the supplementary material b) Red scale: residuals from the inversion of DInSAR Sentinel 1 data estimated in (46), highlighting the geodetic anomaly. The residuals are negative and are concentrated in the Monte Olibano area with a minimum of approximately -10 cm. The image of the Solfatara diffuse degassing structure, modified after Cardellini et al. (52), is shown on the map. c) Residuals between observed and expected vertical displacements of the ACAE GNSS station, in the area of the geodetic anomaly. d) CO\u003csub\u003e2\u003c/sub\u003e flux in the target area of the Solfatara diffuse degassing structure, measured monthly by the INGV-OV staff.\u003c/p\u003e","description":"","filename":"image4.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-4708123/v1/42030357fdf4d6fb0110bf87.jpeg"},{"id":60979377,"identity":"5f9d82db-f539-49dc-ab85-1f0b37e3b001","added_by":"auto","created_at":"2024-07-24 08:49:18","extension":"jpeg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":255260,"visible":true,"origin":"","legend":"\u003cp\u003eMap with the locations of earthquakes with Md \u0026gt;1.0, which are distributed on an elliptical ring. The vertical displacement of the ground from 2015 to May 2024 obtained from DInSAR data is shown in blue scale on the map. The blue star indicates the location of the source of the deformation (depth 3800 +/-50 m). 1 focal mechanism of the earthquake of April 14, 2024 at 08:01:44, Md=3.0. 2 focal mechanism of the earthquake of February 5, 2023 at 00:45:36, Md=3.0.\u003c/p\u003e","description":"","filename":"image5.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-4708123/v1/367be18b77b7463fe81c7a61.jpeg"},{"id":60979378,"identity":"0e3536f1-285b-4d3d-ad34-83c12a8e49bc","added_by":"auto","created_at":"2024-07-24 08:49:18","extension":"jpeg","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":176543,"visible":true,"origin":"","legend":"\u003cp\u003ea) Comparison between the uplift deficit of the GNSS ACAE station (cyan line), representative of the geodetic anomaly of Mt. Olibano, and the cumulative earthquake count in the period January 2021 - April 2024. The black line indicates the cumulative count of earthquakes recorded in the caldera. The correlation coefficients between earthquakes and ACAE uplift deficit is r = 0.998. b) cumulative earthquake count in the period January 2021 - April 2024 versus uplift deficit of the GNSS ACAE station. c) Comparison between the uplift of the GNSS RITE station (orange line), representative of the general deformation process (see deformation source Figure 5), and the cumulative earthquake count in the period January 2021 - April 2024. The correlation coefficients between all earthquakes and RITE up is r = 0.989. d) cumulative earthquake count in the period January 2021 - April 2024 versus RITE station uplift.\u003c/p\u003e","description":"","filename":"image6.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-4708123/v1/332ad88c67600d6ad983dc1e.jpeg"},{"id":60979979,"identity":"ef257fd5-27c7-447a-92ae-ce4e302c835c","added_by":"auto","created_at":"2024-07-24 08:57:19","extension":"jpeg","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":311424,"visible":true,"origin":"","legend":"\u003cp\u003ea) Mean vertical velocity computed for the 2021-2024 period (every color cycle represents a velocity variation of 4 cm/year). Inset shows a closed look to the velocity patterns in the Solfatara - Mt. Olibano area. b) Difference between the actual and the expected displacement time series for the vertical component for a pixel representative of the deformation behavior in the Mt. Olibano area. Dashed red lines correspond to 2s (standard deviation) error bar computed before 2021.\u003c/p\u003e","description":"","filename":"image7.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-4708123/v1/a1e5911b2b9cb97c3c03054f.jpeg"},{"id":60979978,"identity":"2d77112f-249b-48ef-9604-d7d7939629dc","added_by":"auto","created_at":"2024-07-24 08:57:18","extension":"jpeg","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":368555,"visible":true,"origin":"","legend":"\u003cp\u003eSolfatara diffuse degassing structure (DDS) and locations of the target area and of the 63 points that are monthly measured. The map is modified from Cardellini et al. (52) and Chiodini et al. (36) and shows the probability that the CO\u003csub\u003e2\u003c/sub\u003e flux is \u0026gt; 50 g m\u003csup\u003e-2\u003c/sup\u003e d\u003csup\u003e-1\u003c/sup\u003e, a suitable threshold for a pure biogenic CO\u003csub\u003e2\u003c/sub\u003e flux. Above this threshold the CO\u003csub\u003e2\u003c/sub\u003e emission is at least partially fed by the hydrothermal-magmatic source.\u0026nbsp; Coordinates are expressed in UTM-WGS84\u003c/p\u003e","description":"","filename":"image8.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-4708123/v1/ca840f5ebcc45c5eaebdd40e.jpeg"},{"id":76088328,"identity":"00627d47-b83e-4930-9a6e-53622dec448d","added_by":"auto","created_at":"2025-02-12 08:05:40","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":3394907,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4708123/v1/132eced1-5927-4a86-ad17-eb2446adddbc.pdf"},{"id":60979384,"identity":"d0c515b8-7a15-459d-a88c-f716f28363ce","added_by":"auto","created_at":"2024-07-24 08:49:19","extension":"txt","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":124035,"visible":true,"origin":"","legend":"Dataset 1","description":"","filename":"dataset1locallsequences.txt","url":"https://assets-eu.researchsquare.com/files/rs-4708123/v1/e360615bafbdd4f6b0762fb6.txt"},{"id":60979371,"identity":"c2682a62-927e-46bd-8875-fbda38ddb812","added_by":"auto","created_at":"2024-07-24 08:49:18","extension":"txt","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":8815,"visible":true,"origin":"","legend":"Dataset 2","description":"","filename":"dataset2locburstlikeswarms.txt","url":"https://assets-eu.researchsquare.com/files/rs-4708123/v1/50fd5d747bd1239c7a632f7e.txt"},{"id":60979980,"identity":"d1cca319-e332-4010-8d0a-e7a0d4f3a17c","added_by":"auto","created_at":"2024-07-24 08:57:19","extension":"txt","order_by":3,"title":"","display":"","copyAsset":false,"role":"supplement","size":8897,"visible":true,"origin":"","legend":"Dataset 3","description":"","filename":"dataset3locperiodicalVT.txt","url":"https://assets-eu.researchsquare.com/files/rs-4708123/v1/45e14458072714c09854d731.txt"},{"id":60979376,"identity":"4ee45298-3f10-4f37-a525-ef2c19a6d747","added_by":"auto","created_at":"2024-07-24 08:49:18","extension":"xls","order_by":4,"title":"","display":"","copyAsset":false,"role":"supplement","size":70144,"visible":true,"origin":"","legend":"Dataset 4","description":"","filename":"dataset4databaseCO2flux.xls","url":"https://assets-eu.researchsquare.com/files/rs-4708123/v1/8628da599b027a60b29483a0.xls"},{"id":60979380,"identity":"f7578dab-1ee4-4ff7-8699-7a5b28629132","added_by":"auto","created_at":"2024-07-24 08:49:19","extension":"docx","order_by":5,"title":"","display":"","copyAsset":false,"role":"supplement","size":571437,"visible":true,"origin":"","legend":"","description":"","filename":"supplementaryinfo.docx","url":"https://assets-eu.researchsquare.com/files/rs-4708123/v1/d02f8ce6b6d7f20d9e4c479d.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Burst-like swarms and periodical VT events in the accelerating unrest phase of Campi Flegrei caldera (2021-2024)","fulltext":[{"header":"Introduction","content":"\u003cp\u003eThe volcanic area of Campi Flegrei (Italy) is a subcircular caldera with a diameter of approximately 12 km (\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e). The last eruption occurred in 1538 (\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e) and formed the volcanic edifice of Monte Nuovo. Today, the Campi Flegrei area is densely inhabited, with about 500,000 people living in this area (\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e). The main city is Pozzuoli, which is located in the central part of the caldera (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). Ground deformations are typical of this caldera and, in the past, they have resulted in phases of subsidence alternating with phases of uplift. This phenomenon is called bradyseism and has been studied since the 19th century by the major scientists of that time (\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e, \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e). In the first half of the 19th century, relative measurements of sea level compared to ground level (\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e) demonstrated that the Campi Flegrei caldera was in subsidence. Since then, the first uplift phase began in the 1950s (\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e). Then, two other bradyseism crises occurred in the 1969\u0026ndash;1970 period (\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e) and during the 1982\u0026ndash;1984 time interval (\u003cspan additionalcitationids=\"CR11 CR12 CR13\" citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e), accompanied by significant uplift (1.7 and 1.8 m respectively) and seismicity. These bradyseism crises were followed by temporary subsidence (\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e, \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e). Finally, the last phase of bradyseism began around 2005 and is still ongoing. The current phase is characterized by increasing seismicity (\u003cspan additionalcitationids=\"CR18 CR19 CR20\" citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e) and fumarolic tremor amplitude, which is indicative of hydrothermal activity (\u003cspan additionalcitationids=\"CR23 CR24 CR25\" citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e). Moreover, increasing fluid emission (\u003cspan additionalcitationids=\"CR28 CR29 CR30 CR31 CR32 CR33 CR34 CR35\" citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e) and significant ground deformations (\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e, \u003cspan additionalcitationids=\"CR38 CR39 CR40 CR41 CR42 CR43 CR44\" citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e), which produced about 1.2 m uplift in the central sector of the caldera, are typical of the current unrest (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eDuring the bradyseism crises of the second half of the 20th century and in the current one the deformation pattern has always shown a bell-shaped radial pattern with the maximum uplift in the central area of the caldera, where the RITE GNSS station is presently located (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). In the 1982-84 crisis, deformation measurements were mainly based on topographic leveling, which provided vertical deformation measurements. For that period the deformation pattern was modeled as a Mogi source (\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e, \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e) or as a horizontal crack (\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e, \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e). The deformation pattern of the current unrest, which is measured through the GNSS network, managed by the Osservatorio Vesuviano of the Istituto Nazionale di Geofisica e Vulcanologia (INGV-OV) and through DInSAR measurements, has often been modeled as a horizontal crack (e.g. 40,41,45,46), or a horizontal thermo-poro-elastic zone (\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eAs a consequence of the current unrest, in December 2012 the Italian Civil Protection decided to move to the yellow alert phase, which is the second of four alert levels (green\u0026thinsp;=\u0026thinsp;basic; yellow\u0026thinsp;=\u0026thinsp;attention; orange\u0026thinsp;=\u0026thinsp;pre-alarm; red\u0026thinsp;=\u0026thinsp;alarm). From 2012 onwards, the unrest continued and accelerated (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec,d,e) in terms of increase in the earthquake occurrence rate, earthquake magnitude (\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e, \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e, \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e), ground deformation rate (\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e, \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e) and variations in geochemical parameters (\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e). In particular, Bevilacqua et al. (\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e) have shown that the increase in the cumulative number of earthquakes (in total or above a given Md) versus the vertical uplift of the caldera is well fitted with two exponential functions and that the \u0026ldquo;connecting time\u0026rdquo; between these functions falls in the period between 4/2020 and 9/2022. This transition between the two exponential functions, the second having a larger exponent, marks a clear acceleration of the seismicity in the caldera. Furthermore, analyzing in detail the deformation pattern, Giudicepietro et al. (\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e) discovered a geodetic anomaly that clearly manifested in 2021, with an uplift deficit of approximately 9 cm in the Mt. Olibano area (East of Pozzuoli) compared to the surrounding areas.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe aforementioned recent studies indicate that during 2021 the unrest of the Campi Flegrei caldera has intensified. For this reason, in this work we analyzed the seismicity that occurred in 2021\u0026ndash;2024 with the aim of investigating the relationships between the characteristics of the earthquakes, the seismogenic processes, the evolution of the deformation field and the geochemical variations of the Campi Flegrei caldera. In particular, we focused our attention on several seismic sequences characterized by very short inter-event times, of even a few seconds. Within this category, we distinguished two types of sequences which we call burst-like swarms and periodical Volcano-Tectonic (VT) events, following the classic literature of volcanic seismology (i.e. 48,49). These types of sequences can provide clues on the seismogenic processes in relation to the evolution of the current accelerating unrest, which is controlled by intense ground deformation and escalating hydrothermal activity.\u003c/p\u003e"},{"header":"Results","content":"\u003cp\u003eIn Campi Flegrei the INGV-OV seismic network has recorded approximately 18,500 earthquakes (max Md\u0026thinsp;=\u0026thinsp;4.4; average Md\u0026thinsp;=\u0026thinsp;0.26, minimum Md= -1.6) from March 2005 to May 2024. Not all of these earthquakes are located because part of them are too small to obtain a reliable location (those located are around 10,200). The locations show a distribution that approximately describes an ellipse in the central area of the caldera, however the majority of earthquakes (about 90%) are located at Pozzuoli and east of Pozzuoli, in an area that includes the Solfatara-Pisciarelli hydrothermal system and the lava domes of Mt. Olibano (see the zone marked as A in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea). A minor group of earthquakes (about 2% of the total) is located in the Gulf of Pozzuoli (zone B in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea). The cumulative earthquake count highlights the progressive increase in seismicity rate over time (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec). We focused on the seismicity in the period from 2021 to 2024, when the current unrest showed an intensification in terms of seismicity rate and uplift velocity. Specifically, we looked at the types of seismic events that show peculiar characteristics.\u003c/p\u003e \u003cp\u003eThe seismic events recorded in the Campi Flegrei caldera from 2021 to 2024 are generally of VT type. In the context of VT seismicity, we recognized sequences characterized by a very short time interval between two consecutive events, which we call here intertime (even less than a few seconds). In particular, we distinguished sequences of periodical VT events, which can be considered similar to drumbeat type events (\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e, \u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e50\u003c/span\u003e) as regards the regularity of occurrence, although in the case of the Campi Flegrei these sequences contain a limited number of events. Moreover, we recognized earthquake sequences that we call burst-like swarms, following the definition of Hill et al. (\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e).\u003c/p\u003e \u003cp\u003ePeriodical VT type sequences (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea-c) are generally characterized by events of short duration (from a few seconds to a few tens of seconds), with impulsive onset, which occur with earthquake intertimes comparable to each other (e.g. a few seconds or tens of seconds). Generally, no major events are associated with pure periodical VT sequences. Burst-like swarm sequences (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ee-h) are characterized by widely overlapping earthquakes, with intertimes that are sometimes not easily recognizable, and are generally accompanied by a continuous tremor-like background signal. However, the background signal cannot be defined as volcanic tremor because it does not show distinct frequency peaks as volcanic tremor typically exhibits. Therefore, we define this background signal as pseudo-tremor. The spectrogram analysis, which highlights the wide frequency band that characterizes the pseudo-tremor, suggests that it can be due to signals generated by a mechanism involving near-continuous brittle failure. Examples of this type of background signal are depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ef, which shows a burst-like swarm recorded on 19 April 2023, starting with a pseudo-tremor signal, with increasing amplitude, and Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ee, which shows the pseudo-tremor type signal recorded on 22 July 2022, with remarkable amplitude, in the second part of the recording. Burst-like swarm sequences can be accompanied by larger events. In particular, the earthquake with the greatest duration magnitude (Md\u0026thinsp;=\u0026thinsp;4.4), recorded in the Campi Flegrei on 20 May 2024 at 18:10 (UTC), belongs to this type of sequence (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eh). The spectrogram helps to distinguish the different events in the burst. For comparison, the seismogram and spectrogram of an earthquake not associated with a burst-like swarm is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ed. The selected event is the largest one (Md\u0026thinsp;=\u0026thinsp;3.9) of the group of events located offshore in the Gulf of Pozzuoli (zone B in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea).\u003c/p\u003e \u003cp\u003eLooking at Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e it can be noted that the two categories of sequences in certain cases show common characteristics. For example, the burst-like swarm in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ef shows a regular sequence of VTs in the second half of the recording.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTo better investigate these seismic sequences, we selected two well-defined examples of the two types. Figure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e shows the comparison between the periodical VT sequence recorded on 12 October 2023 and the burst-like swarm recorded on 14 April 2024. The burst-like swarm includes two earthquakes with Md\u0026thinsp;\u0026gt;\u0026thinsp;3.0 and shows the background signal here defined as pseudo-tremor, well recognizable in the spectrogram (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ed). In the 5-minute signal windows of the two sequences reported in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec-d, we counted 38 events in the regular VT type sequence (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec), and 36 events in the burst-like swarm sequence, with an average earthquake intertime of approximately 8 seconds for both sequences.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eWe compared the locations of the events belonging to these two sequences (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb). For each sequence, we selected the locations of the events contained in the half hour of signal shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea. For the periodical VT sequence of 12 October 2023, we obtained 10 locations with depths between 700 and 900 m and Md ranging between \u0026minus;\u0026thinsp;0.5 and 1.7. For the burst-like swarm sequence of 14 April 2024, we found 37 locations with depths between 730 and 2870 m and Md between \u0026minus;\u0026thinsp;0.3 and 3.7. Both sequences fall within the Solfatara-Olibano area (zone A in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea).\u003c/p\u003e \u003cp\u003eFinally, we selected all the sequences with very short earthquake intertimes that we recognized in the period between January 2021 and May 2024. In this time interval this type of seismic sequences has become more frequent and more evident than in previous years. In our selection we did not always distinguish between the two types as the distinction is not clear for all the cases. It is appreciable only in the clearest sequences, such as those reported in the examples of Figs.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e and \u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. We show the distribution of event epicenters belonging to the uncategorized very short earthquake intertimes sequences (light gray circles) in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea, together with those classified as burst-like swarms (red circles) and those classified as periodical VTs (light blue circles). The burst-like swarms are those shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ee-h, the April 14, 2024 sequence reported in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea, and a sequence recorded on June 2, 2023 (total number of events\u0026thinsp;=\u0026thinsp;70; depth ranging between 450 and 2870 m and Md between \u0026minus;\u0026thinsp;0.8 and 4.4). The periodical VTs are the events in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea-c and the sequence of October 12, 2023, shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea (total number of events\u0026thinsp;=\u0026thinsp;47; depth ranging between 210 and 1410 m and Md between \u0026minus;\u0026thinsp;0.1 and 2.7). Moreover, in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea, we delimited the area of the geodetic anomaly discovered in Giudicepietro et al. (\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e) (solid line) and the Solfatara-Pisciarelli hydrothermal area (dashed line), based on the Solfatara Diffuse Degassing Structure (Solfatara DDS, 51). It is worth noting that the two types of events are closely spatially co-related as already shown in the examples in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. They partly share the same seismogenic volume, especially in the border sector between the Solfatara-Pisciarelli hydrothermal system and the area of the Mt. Olibano geodetic anomaly. Actually, as a result of our analysis, it emerges that burst-like swarms often also contain periodical VT events which are generally localized at smaller depths (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). However, the locations of burst-like events are generally slightly north of the locations of pure periodical VT sequences. This distribution indicates a greater concentration of burst-like swarms in the Solfatara-Pisciarelli hydrothermal area and the preferential locations of periodical VT events around the area of the geodetic anomaly (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea, b). In this zone of the caldera, where the majority of the Campi Flegrei earthquakes are located, the diffuse degassing from the Solfatara crater increased significantly in the period 2021\u0026ndash;2024 (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb, d), when the burst-like and periodical VT sequences became evident. In the same period, the geodetic anomaly at the lava dome of Mt. Olibano became considerably more pronounced, as evidenced by the residual between the actual uplift of the ACAE GNSS station and the estimated one (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec; see Materials and Methods).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTo frame this type of sequences in the general context of the ongoing unrest in the Campi Flegrei caldera, we considered the geodetic, GNSS and DInSAR measurements and the geochemical measurements and compared them with the ongoing seismicity.\u003c/p\u003e \u003cp\u003eTo show the distribution of earthquakes in the caldera, in relation to ground deformation, we selected earthquakes with Md\u0026thinsp;\u0026gt;\u0026thinsp;1.0 and plotted them on the uplift map obtained with Sentinel 1 data (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e). We updated the processing performed in Giudicepietro et al.(\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e) to obtain the map of the vertical displacement of all the correlated pixels of the Sentinel 1 data from 2015 to May 2024 (maximum uplift approximately equal to 103 cm). We reported on the map the indication of the deformation source obtained in Giudicepietro et al. (\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e) whose parameters are: latitude\u0026thinsp;=\u0026thinsp;40.817814 +/- 13 m; longitude\u0026thinsp;=\u0026thinsp;14.126922 +/- 17 m; depth\u0026thinsp;=\u0026thinsp;3826 +/- 45 m. The spatial distribution of seismicity in the Campi Flegrei caldera highlights the occurrence of earthquakes in an elliptical ring around the location of the ground deformation source (blue star in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e). We also added the focal mechanisms of two earthquakes, calculated with the FPFIT program (\u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e53\u003c/span\u003e), one falling in zone A (2024-04-14 08:01:44; Md\u0026thinsp;=\u0026thinsp;3.0) and the other in zone B (2023-02-05 00:45:36, Md\u0026thinsp;=\u0026thinsp;3.0) of Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea. The focal mechanism of the earthquake in zone A is extensive whereas the focal mechanism of the earthquake in zone B is compressive. This characteristic is also common to other earthquakes recorded in the two zones, which for simplicity of representation we do not show in the figure but which are reported in the INGV-OV surveillance bulletins (\u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://www.ov.ingv.it/index.php/monitoraggio-e-infrastrutture/bollettini-tutti/bollett-mensili-cf\u003c/span\u003e\u003cspan address=\"https://www.ov.ingv.it/index.php/monitoraggio-e-infrastrutture/bollettini-tutti/bollett-mensili-cf\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e, last accessed 8 May 2024).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eHowever, the concentration of locations in zone A of Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea suggests the overlap of multiple seismogenic processes. In particular, in zone A there is the Solfatara-Pisciarelli hydrothermal area, with CO\u003csub\u003e2\u003c/sub\u003e diffuse degassing which shows a progressive increase over time and is currently comparable with the average flux of CO\u003csub\u003e2\u003c/sub\u003e in the plume of active volcanoes with continuous degassing (\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e). In recent years, the diffuse emission of CO\u003csub\u003e2\u003c/sub\u003e from the target area, which is systematically monitored, increased from 200\u0026ndash;300 t/d in 2010 to values higher than 1200 t/d in the 2024 measurement campaigns (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ee). Furthermore, at the southern edge of the Solfatara crater, there is the geodetic anomaly of Mt. Olibano. Taking advantage of the high temporal sampling of the GNSS network, we calculated the vertical displacement deficit (uplift deficit) of the ACAE station, which is located in the geodetic anomaly, compared to the expected one. To do this, we applied a method similar to the one used with the DInSAR Sentinel 1 data to spatially map the geodetic anomaly in Giudicepietro et al. (\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e). In this case, we used RITE GNSS station, which well represents the uplift of the central area of the caldera, as a reference to derive the correlation coefficient to estimate the expected uplift of the ACAE station, from the time series relevant to the 2004\u0026ndash;2024 period, therefore well before the start of the 2021 acceleration. We obtained the residuals by subtracting the expected uplift of the ACAE station from the observed uplift data (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec). Finally, we defined the opposite of the residuals as the \u0026ldquo;uplift deficit\" (see Material and Methods for details). This allowed us to retrieve the temporal evolution of the uplift deficit of the ACAE station located in the geodetic anomaly. Thus, we were able to correlate the uplift deficit in the anomaly area with the cumulative earthquake count, which is dominated by events occurring in area A of Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea. We found a very high correlation between the uplift deficit and the number of earthquakes (r\u0026thinsp;=\u0026thinsp;0.998) (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ea, b). We also compared the vertical component of the RITE station with the cumulative earthquake count and obtained a slightly lower correlation between the two parameters (r\u0026thinsp;=\u0026thinsp;0.988) and a non-linear relationship (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ec, d); this latter is consistent with the exponential-type trend analyzed in Bevilacqua et al.(\u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e55\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"Discussion","content":"\u003cp\u003eThe picture that emerges from this study indicates that the current phase of bradyseism taking place in the Campi Flegrei caldera is accelerating. The process is controlled by the inflation of a source at a depth of about 3800 meters in the central area of the caldera (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e), whose evolution seems to justify the seismicity of the elliptical ring at the center of the caldera. This can be interpreted as the effect of a system of faults that border the uplifted block in the center of the caldera and create a ring fault-type structure. The focal mechanisms of the two earthquakes reported in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e, extensive in zone A and compressive in zone B, represent a common characteristic of the earthquakes that occur in these two areas. In particular, extensive mechanisms in zone A are reported in numerous articles (e.g.24,54) as well as in the INGV-OV surveillance bulletins. The compressive mechanisms in zone B are also reported in the surveillance bulletins, however some of these are less constrained than those in zone A, that is better covered by the seismic network. In any case, the mutual positions of these focal mechanisms suggest a possible dislocation of the block in the center of the caldera, which in depth could behave like a trapdoor-type structure. However, we remark that the interpretation of the deformation source located in the center of the caldera at a depth of approximately 3800 m is still speculative. It could be due to an accumulation zone of magmatic fluids, the expansion of a porous medium subject to an increase in pressure or the intrusion of magma.\u003c/p\u003e \u003cp\u003eThe seismic sequences with short earthquake intertimes, which are the main object of this study, are mainly concentrated in the seismogenic area A. Burst-like swarms has been observed in other volcanoes such as Mammoth Mountain (United States), a hydrothermally active lava dome complex in the Long Valley caldera (\u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e55\u003c/span\u003e). For them, Hill et al. (\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e) hypothesized a process driven by a transient increase in local fluid pressure, based on the similarity of these sequences with signals associated with pressure transients generated by shutdown operations on production wells in Japanese geothermal fields (\u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e56\u003c/span\u003e). Nishi et al. (\u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e57\u003c/span\u003e) recorded sequences of this type on White Island and observed that they were located in the same seismogenic volume as volcano-tectonic earthquakes. The mechanism that caused them was attributed to brittle failure due to rapid fluid pressure fluctuations. Also, Lin et al. (\u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e58\u003c/span\u003e) recorded burst-like swarms on the Tatun Volcano Group (Taiwan) and interpreted them as the response of an extended network of cracks to the pressure variations due to fluid injection. Finally, a close association between these signals and hydrothermal activity is also suggested by McCausland et al.(\u003cspan citationid=\"CR59\" class=\"CitationRef\"\u003e59\u003c/span\u003e) who consider them possible precursors of phreatic explosions when recorded in association with low-frequency and hybrid events. This leads us to interpret the burst-like swarms of the Campi Flegrei as the brittle response to the increase in the hydrothermal system fluid pressure. This interpretation is compatible with the model proposed by Fournier (\u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e60\u003c/span\u003e) for the movement of fluid from a shallow magmatic zone to the hydrothermal environment. Furthermore, we have evidence that this variation in hydrothermal fluid pressure occurs in extensive stress regime. Indeed, the burst-like swarms share the same source region as other earthquakes occurring in zone A, which have extensive focal mechanisms indicating an extensive stress regime. Furthermore, the burst-like swarm locations are close to the Mt Olibano area, where a geodetic anomaly is located, which is a further evidence of the local extensive stress regime.\u003c/p\u003e \u003cp\u003ePeriodical VT sequences are also concentrated in the zone A. In particular they are distributed around the lava block of Mt Olibano at depths generally less than 1000 m. The regular occurrence of seismic events in volcanic environments has been associated with a stick-slip mechanism between an extruding lava dome and conduit walls (\u003cspan additionalcitationids=\"CR62\" citationid=\"CR61\" class=\"CitationRef\"\u003e61\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR63\" class=\"CitationRef\"\u003e63\u003c/span\u003e). This hypothesis could also be suitable to explain the regular VT sequences recorded at Campi Flegrei. In this case there is not an extruding lava dome, but a dome in relative subsidence compared to the general uplift of the surrounding areas, as evidenced by the uplift deficit of Mt Olibano, which correlates closely with seismicity (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e) and degassing (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec).\u003c/p\u003e \u003cp\u003eIn general, in the Campi Flegrei current unrest, the spatial distribution of earthquakes and the characteristics of the seismic sequences suggest the strong interconnection of the geodetic, hydrothermal and seismic phenomena, that are controlled by the deformation source located at the center of the caldera. When the uplift of the center of the caldera (RITE GNSS station) accelerates, the uplift deficit of the geodetic anomaly (ACAE GNSS station) and the cumulative earthquake count also accelerate. However, previous studies (\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e, \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e) have shown that seismicity, as represented by the cumulative earthquake count (total or above a given magnitude), increases more rapidly, in particular with two exponential regressions with increasing exponent in time, than GNSS data, such as the vertical component of the RITE station, which shows the greatest uplift among the GNSS stations. This difference in the temporal evolution of the two parameters is also observed in the period 2021\u0026ndash;2024 (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ec-d). On the other hand, in this study we calculated the daily time series of the uplift deficit in the Mt Olibano area, identified as a geodetic anomaly (\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e), exploiting the high acquisition rate of data from the INGV-OV GNSS network. It is noteworthy that this uplift deficit, due to the different uplift rate of the lava block of Mt Olibano compared to the surrounding areas, evolves similarly to seismicity, and shows a linear relationship with the cumulative earthquake count (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ea, b). This linear relationship suggests that the uplift deficit is a good indicator of the inelastic component of the behavior of this localized sector of the caldera in response to the overall deformation process. Therefore, we interpret this finding as the effect of the lithological and mechanical discontinuity of the crustal rock, which in this area begins to deviate from elastic behavior when subjected to deformation. This interpretation aligns with the recent study on the spatial distribution of the b value (\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e), which highlights that the b value of the Mt. Olibano earthquakes is different from that found for the Solfatara-Piscairelli earthquakes, showing different physical characteristics of the medium.\u003c/p\u003e"},{"header":"Methods","content":"\u003cp\u003eThe seismic network of the Campi Flegrei is developed and managed by the INGV-OV. It includes 27 stations distributed within the rim of the caldera. Moreover, four stations were installed in the Gulf of Pozzuoli (\u003cspan citationid=\"CR64\" class=\"CitationRef\"\u003e64\u003c/span\u003e). Most of the stations are equipped with three-component broadband seismometers, with a sampling rate of 100 samples per second and with accelerometers, whose signals are acquired at 200 samples per second. The data is transmitted in real time to the INGV-OV acquisition center. In 2020, the location sensibility in space of the Campi Flegrei seismic network was characterized by a magnitude threshold ranging between 0 and 0.5 (\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e). In the Solfatara-Pisciarelli area, where a very local seismicity has become increasingly evident since 2010, the seismic network is denser and allows us to locate earthquakes with magnitude (Md)\u0026thinsp;\u0026lt;\u0026thinsp;0, when the seismic noise is particularly low. To describe the general trend of the current long-term unrest, we used the locations of the earthquakes from 2005 to 2024, and the seismic catalogue of the Campi Flegrei from 2004 to 2024 (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea,b,c). The locations and the seismic catalogue are extracted from the INGV-OV seismological database (\u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://terremoti.ov.ingv.it/gossip/flegrei/index.html\u003c/span\u003e\u003cspan address=\"https://terremoti.ov.ingv.it/gossip/flegrei/index.html\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e, last accessed 30 May 2024). The velocity model used by INGV-OV is reported in Cal\u0026ograve; and Tramelli (\u003cspan citationid=\"CR65\" class=\"CitationRef\"\u003e65\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eSince the burst-like and regular VT seismic sequences share the same source region as the other A-zone earthquakes, shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, we also created a specific A-zone earthquake catalog. This catalog includes only earthquakes with first arrival recorded by a seismic station located in the A-zone (see the seismic station map in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e of the supplementary material). In selecting the A-zone earthquakes, we considered both localized and non-localized events, including those recorded by only one or two stations, which are typically very small. Thus, we generated plots similar to those in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e, both with the cumulative count of all earthquakes recorded in the entire caldera and with the cumulative count of A-zone earthquake catalog. The two counts show negligible differences in their temporal evolution (see Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e of the supplementary materials), therefore, we no longer took into account the A-zone earthquake catalog, and in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e, we only showed the count of all earthquakes in the caldera (which is evidently dominated by the seismicity of zone A).\u003c/p\u003e \u003cp\u003eThe selection of the burst-like swarms and VT regular sequences was conducted through visual analysis of the seismograms. The spectrogram analysis and time series correlations were performed using ObsPy (\u003cspan citationid=\"CR66\" class=\"CitationRef\"\u003e66\u003c/span\u003e), a system for seismic data analysis, along with the Python packages numpy, scipy, and matplotlib for figure creation. Additionally, since the ground deformations and geochemical changes taking place at Campi Flegrei are significant, and are closely correlated with each other and with seismicity, we also used data from the INGV-OV GNSS network and DInSAR Sentinel-1 data.\u003c/p\u003e \u003cp\u003eData from the continuous GNSS network of Campi Flegrei were processed using Bernese GNSS software on a daily basis with the IGS final and reprocessing products to obtain homogeneous results. To remove the regional tectonic background from the volcanic deformation pattern, the time series and velocity fields were transformed into a local reference frame including six stations from the INGV RING network, located outside the Neapolitan volcanic area. No correction was applied to the vertical component, as the tectonic contribution was considered negligible. Furthermore, seasonal signals were removed (for more details on the data processing of the Campi Flegrei GNSS network see (\u003cspan citationid=\"CR67\" class=\"CitationRef\"\u003e67\u003c/span\u003e). To obtain the time series of the vertical residuals of the ACAE GNSS station (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e), we applied the geodetic anomaly identification method from Giudicepietro et al. (\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e), based on DInSAR data, to the GNSS data. Considering the RITE station is located in the area of maximum uplift of the radially symmetrical bell-shaped deformation pattern of Campi Flegrei, and the ACAE station is located in Mt Olibano, we used these two stations to derive the uplift deficit in the area characterized by the geodetic anomaly. Specifically, we: 1) assumed RITE as a reference for the uplift trend in the central area of the caldera; 2) calculated a proportionality coefficient (α) between the RITE and ACAE uplift time series using data from 2015\u0026ndash;2018, when the ACAE area was not yet affected by the geodetic anomaly; 3) estimated the expected ACAE uplift time series by multiplying the RITE uplift time series by α; 4) subtracted the expected ACAE uplift time series from the actual ACAE uplift time series, obtaining the residual series from 2004 to 2024 as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e. Then, we calculated the opposite of the residuals to obtain the time series of the \u0026ldquo;uplift deficit\u0026rdquo; shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ea.\u003c/p\u003e \u003cp\u003eThe DInSAR data were generated by applying the Parallel Small BAseline Subset (P-SBAS) approach (\u003cspan citationid=\"CR68\" class=\"CitationRef\"\u003e68\u003c/span\u003e, \u003cspan citationid=\"CR69\" class=\"CitationRef\"\u003e69\u003c/span\u003e). In particular, we separately processed 430 ascending (Track 44) and 429 descending (Track 22) orbits data acquired by Sentinel-1 constellation from March 24, 2015 to May 24, 2024 to obtain the Line of Sight (LOS) displacement time series for each coherent pixel. Then, the vertical displacement component was calculated by combining the LOS P-SBAS time series using the method described in Casu and Manconi (\u003cspan citationid=\"CR70\" class=\"CitationRef\"\u003e70\u003c/span\u003e). The mean velocity of the vertical component computed for the 2021\u0026ndash;2024 period represented as DInSAR fringes (every color cycle corresponds to a velocity variation of 4 cm/year) is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ea. It clearly highlights that the uplift deficit in the Mt. Olibano area, starting form 2021, makes the fringes to deviate from a radial pattern, which instead is preserved in the whole caldera. The vertical component of the Sentinel-1 data, updated to May 24, 2024, is also shown in blue scale in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFinally, we used geochemical data from the INGV-OV geochemical monitoring network. The data consists in the CO\u003csub\u003e2\u003c/sub\u003e emission (in t/d) from an area of ~\u0026thinsp;90000 m\u003csup\u003e2\u003c/sup\u003e (\u0026lsquo;target area\u0026rsquo; in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e) located inside the Solfatara crater. The CO\u003csub\u003e2\u003c/sub\u003e emission is monthly computed through the measurement of 63 fixed points with the accumulation chamber method (\u003cspan citationid=\"CR71\" class=\"CitationRef\"\u003e71\u003c/span\u003e). This monitoring activity started in 2004 and is still ongoing. The data show a marked increase of the CO\u003csub\u003e2\u003c/sub\u003e emission in the last years (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ed). Further details of the technique measurement and data treatment are reported in Chiodini et al.(\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e) and Cardellini et al. (\u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e52\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgments\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis study has been partly funded by the Italian DPC, in the frame of INGV-DPC (2022\u0026ndash;2025) and IREA-DPC (2022\u0026ndash;2024) agreements, although it does not necessarily represent DPC official opinion and policies. We also acknowledge the support of EPOS-RI, including the one obtained through the EPOS-Italia JRU. This work benefited from the Progetto ORME, INGV Project \u0026ldquo;Pianeta Dinamico\u0026rdquo; - Working Earth (CUP 1466 D53J19000170001 - legge 145/2018) (Scientific Responsibility: F.G.). This research was also partially funded by the European Union - NextGeneratonEU through the following projects: NRRP - MEET (National Recovery and Resilience Plan - Monitoring Earth\u0026apos;s Evolution and Tectonics), ICSC - CN-HPC - PNRR M4C2 Investimento 1.4 - CN00000013, GeoSciences IR - PNRR M4C2 Investimento 3.1 - IR0000037, CN-MOST - PNRR M4C2 Investimento 1.4 - CN00000023. This work benefited also from the project Progetto Strategico Dipartimentale INGV 2019. \u0026ldquo;LOVE-CF\u0026rdquo;. This research has been supported by the GRINT (PIR01_00013) and IBiSCo (PIR01_00011) projects (PIR01_00013), funded by the National Operational Programme Infrastructures and Networks 2014/2020 of the Italian Ministry of Infrastructure and Transports. A pre-print version of this work is available at: https://www.techrxiv.org/doi/full/10.36227/techrxiv.171043391.14447454/v1. The authors would like to thank all the colleagues who contribute to the Campi Flegrei monitoring system. In particular, we thank the INGV technical staff who ensure the 24/7 operation of the seismic and GNSS monitoring networks.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor contributions:\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eConceptualization: FG, GC, RL, GM, FC, AN, MDV\u003c/p\u003e\n\u003cp\u003eMethodology: FG, GC, RL, SC, RA, AB, CDL, PDM, CM, PR, FC, GS, MD\u003c/p\u003e\n\u003cp\u003eInvestigation: FM, AB, FR, FG, AS, PS, FDT, AME, AT,\u003c/p\u003e\n\u003cp\u003eVisualization: EBS, WDC, FG, FC, GC, GS, MD\u003c/p\u003e\n\u003cp\u003eSupervision: FG, GC, RL, GM, FC, AN, MDV\u003c/p\u003e\n\u003cp\u003eWriting\u0026mdash;original draft: FG, GC, RL, GM, FC\u003c/p\u003e\n\u003cp\u003eWriting\u0026mdash;review \u0026amp; editing: FG, GC, RL, GM, FC\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests:\u003c/strong\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eAuthors declare that they have no competing interests.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData and materials availability:\u003c/strong\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eAll data are available in the main text or the supplementary materials.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eRosi M, Sbrana A, Principe C (1983) The Phlegraean Fields: Structural evolution, volcanic history and eruptive mechanisms. 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Appl Geochem 13:543\u0026ndash;552\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"nature-portfolio","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"","title":"Nature Portfolio","twitterHandle":"","acdcEnabled":false,"dfaEnabled":false,"editorialSystem":"ejp","reportingPortfolio":"","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-4708123/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4708123/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eSince 2021, peculiar seismic sequences became evident and frequent in Campi Flegrei caldera (Italy), while deformation, seismicity and gas emission showed an acceleration. We distinguished burst-like swarms and periodical VT sequences. The earthquakes of both types of sequences resulted located in an area that includes the main hydrothermal field, and a zone affected by a geodetic anomaly, which clearly appeared in 2021. Burst-like swarms (max Md\u0026thinsp;=\u0026thinsp;4.4) are accompanied by a pseudo-tremor, suggesting a mechanism involving near-continuous brittle failure. The periodical VT sequences are shallow and appear linked to the dynamics of the Mt Olibano lava dome, which deforms non-uniformly compared to the rest of the caldera and coincides with the geodetic anomaly. This peculiar seismicity, described in the Campi Flegrei for the first time in this study, has been associated with phreatic explosions and critical phases of unrest in other volcanoes, and currently characterizes the rapidly evolving state of activity of this high-risk volcano.\u003c/p\u003e","manuscriptTitle":"Burst-like swarms and periodical VT events in the accelerating unrest phase of Campi Flegrei caldera (2021-2024)","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-07-24 08:49:13","doi":"10.21203/rs.3.rs-4708123/v1","editorialEvents":[],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"nature-communications","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"NCOMMS","sideBox":"Learn more about [Nature Communications](http://www.nature.com/ncomms/)","snPcode":"","submissionUrl":"https://mts-ncomms.nature.com/","title":"Nature Communications","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"ejp","reportingPortfolio":"Nature Communications","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"38829846-0842-438f-bae5-e5e33aeeae02","owner":[],"postedDate":"July 24th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[{"id":35015801,"name":"Earth and environmental sciences/Solid Earth sciences/Volcanology"},{"id":35015802,"name":"Earth and environmental sciences/Natural hazards"}],"tags":[],"updatedAt":"2025-02-12T08:05:35+00:00","versionOfRecord":{"articleIdentity":"rs-4708123","link":"https://doi.org/10.1038/s41467-025-56723-y","journal":{"identity":"nature-communications","isVorOnly":false,"title":"Nature Communications"},"publishedOn":"2025-02-11 05:00:00","publishedOnDateReadable":"February 11th, 2025"},"versionCreatedAt":"2024-07-24 08:49:13","video":"","vorDoi":"10.1038/s41467-025-56723-y","vorDoiUrl":"https://doi.org/10.1038/s41467-025-56723-y","workflowStages":[]},"version":"v1","identity":"rs-4708123","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4708123","identity":"rs-4708123","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}