Somma-Vesuvio is a stratovolcano consisting of an older cone (Somma) with a polygenetic caldera where the Vesuvio (or Vesuvius) cone has been built up during the last 2000 years. The Somma activity started about 33 ka BP with prevailing emission of lava flows and scoriae. In the last 22 ka, the volcanic activity become highly explosive producing tens of 4/5 Volcanic Explosivity Index (VEI) eruptions, including the last plinian eruption of Pompeii in 79 AD (Sigurdsson et al., 1985), and, most recently, the sub-plinian 472 (Sulpizio et al., 2005) and 1,631 (Braccini, 1632) eruptions. After the 1,631 eruption, Vesuvius remained in open conduit condition and was characterized by almost continuous intra-crater activity, until the last eruption, which occurred in 1944. On March 18, 1944, the eruption began with a lava flow that partially destroyed the towns of Massa di Somma and San Sebastiano al Vesuvio and was followed by an explosive phase, a few days later (March 22). An eruptive column of approximately 6 km height developed causing abundant volcanic ash-fall that reached large distance from the vent (Pappalardo et al., 2014; Cubellis et al., 2013, Cubellis et al., 2016). After this eruption, Vesuvius changed from open to closed conduit conditions and, nowadays, it persists in a quiescent state. The activity of Vesuvius from 1,631 onwards is well documented by contemporary chronicles (Baratta, 1897; Alfano and Friedlaender, 1929; Santacroce, 1987 and reference therein; Marturano and Scaramella, 1998; Guidoboni, 2008), allowing us to establish the precursory phenomena, which generally preceded Vesuvian eruptions in the last four centuries. Among the precursors, earthquakes were the most common ones (Cubellis et al., 2007; Cubellis and Marturano, 2013 and reference therein; Marturano, 2006; Scandone and Giacomelli, 2008). Particularly, the seismicity of Vesuvius before and during the 1944 eruption was recorded by a seismic station (Giudicepietro et al., 2010) installed in the historic site of the Osservatorio Vesuviano (OVO site in Figure 2) and described by Imbò. (1954) and more recently re-evaluated by Pappalardo et al. (2014). Seismic data from that period include Long Period (LP) events, explosion and lava fountain signals. After 1944 the seismicity disappeared for some years. In 1964 a series of landslides occurred in the crater of Vesuvius, accompanied by a partial collapse of the bottom of the crater (Imbò, 1964; Imbò et al., 1964). After this event, the seismicity resumed in 1966.

Currently, Vesuvius shows moderate seismicity with some hundreds of earthquakes per year, with magnitudes typically between −1.0 and 2.0, mainly located in the axial zone of the crater. This zone extends to a depth of about 4 km (Cubellis and Marturano, 2002; Giudicepietro et al., 2010; Cubellis and Marturano, 2013; D’Auria et al., 2013; D’Auria et al., 2014; D’Auria and Massa, 2015; Ricco et al., 2021). The geochemical interpretation of the fumarolic compositions reveals the presence of a hydrothermal system, with temperatures as high as 400–450°C, possibly hosted between 1.5 and 4 km depth within the still hot volcanic conduits of the recent Vesuvius eruptions (Chiodini et al., 2001b).

Campi Flegrei produced at least six large-scale explosive eruptions in the last 250 ka (e.g., Albert et al., 2019; De Vivo et al., 2001). The largest of these events are the two ignimbrite eruptions of the Campanian Ignimbrite (CI, 300 km³ DRE, 40 ka, Gebauer et al., 2014) and the Neapolitan Yellow Tuff (40 km³, 14.9 ka, following Deino et al., 2004) that caused the collapse of the 12 km-wide Campi Flegrei caldera. In the last 15 ka this nested collapse structure was the site of a monogenetic volcanic activity producing about 70 eruptions with variable VEI, spanning from 0 to 5. The last Monte Nuovo (Figure 3) eruption (VEI 2) occurred in 1538 AD after a period of ca 3,000 years of quiescence and was preceded by intense and long-term precursory phenomena (Di Vito et al., 2016) such as ground deformation and seismic activity. Seismicity continued even after the eruptive event at least until the end of the 16th century (Guidoboni and Ciuccarelli, 2011). After the 16th century the area was generally under subsidence (Niccolini, 1829, Niccolini, 1839, Niccolini, 1846). The subsidence was interrupted in 1945–1953 when a first bradyseism crisis occurred, with an uplift of about 0.5 m, followed by the two main episodes of 1969–1972 (about 1.7 m uplift) and 1982–1984 (about 1.8 m uplift), which led to a total ground uplift of about 3.5 m in the central sector of the caldera (Del Gaudio et al., 2010). After 1985 subsidence reappeared and continued until 2000 leading to a ground lowering of about 85 cm. During the bradyseism crisis of 1982–84, the ground uplift was accompanied by a remarkable seismicity (more than 16,000 earthquakes, most of them with magnitude < 3, max Md = 4.0). After 1985, the seismicity in Campi Flegrei almost disappeared and resumed in July 2000 with a swarm of LP events (Saccorotti et al., 2001; D’Auria et al., 2011). From 2005 a gradual resumption of uplift occurred in Campi Flegrei with a significant increase of its rate at the end of 2012 when the Italian Department of Civil Protection raised the alert level from the base (green) to yellow (attention).

Campi Flegrei caldera is also characterized by an intense hydrothermal activity highlighted by the emission of large amounts of deeply derived gases from soil diffuse degassing and from strong fumarolic vents at Solfatara-Pisciarelli (Chiodini et al., 2001a; Cubellis et al., 2002; D’Auria et al., 2011; Chiodini et al., 2012; Amoruso et al., 2014; Cardellini et al., 2017; Tamburello et al., 2019). Seismic activity is currently increasing and it is mainly concentrated in the same hydrothermal sites of Solfatara-Pisciarelli, with hypocentral depths rarely exceeding 2 km and M < = 3.3 (Chiodini et al., 2021; Giudicepietro et al., 2021; Tramelli et al., 2021).

Ischia is the emerged part of an active volcanic field, which rises more than 1,000 m above the seafloor (Orsi et al., 1999; Bruno et al., 2002), along the margin of an E-W trending scarp that borders to the south the Phlegraean volcanic district. At Ischia, volcanism began before 150 ka BP and has continued intermittently, with quiescent periods lasting centuries to millennia, until the last eruption in AD 1302 (Vezzoli, 1988; Orsi et al., 1996; de Vita et al., 2010). The volcanic and deformation history is dominated by the caldera-forming Monte Epomeo Green Tuff (MEGT) eruption at about 55 ka (Vezzoli, 1988), during a period of activity that generated the most voluminous and devastating eruptions of the island (Brown et al., 2008, and references therein). The MEGT caldera floor was later affected by resurgence that generated uplift of about 900 m, probably over the past 33 ka (the age of the oldest deformed rocks). Orsi et al. (1991) explain this remarkable resurgence through a simple-shearing mechanism, and tilting of differentially displaced blocks; the most uplifted one is Monte Epomeo, whereas other authors (e.g. Acocella and Funiciello, 1999), on the basis of more recent data, interpreted it as due to a trap-door like mechanism. Mount Epomeo plays an important role in the seismic activity of Ischia. This structure was subject to a remarkable uplift in the period between 33 ka and about 5 ka and was interpreted as a resurgent block (Vezzoli, 1988). It is not easy to date the end of the resurgence of the Mt. Epomeo block (e.g. Carlino et al., 2006; Carlino, 2012; Trasatti et al., 2019; Cubellis et al., 2020). Currently a subsidence of about 1 cm/year occurs in the northern sector of the Island, as recorded from GPS, optical leveling and DInSAR measurements (see eg: De Martino et al., 2011; Manzo et al., 2006, De Novellis et al., 2018; Trasatti et al., 2019). The post-MEGT volcanic history has been subdivided into three periods of activity based on structural and volcanological evidence, as well as geochemical and isotopic variations of the magmas erupted through time (Civetta et al., 1991). Even if, since the last eruption occurred in AD 1302, no evidence of renewal of uplift has been recorded in concurrence with more recent minor mass movements, the magmatic system of Ischia has to be considered still active. The presence of a hydrothermal system characterized by energetic circulation underneath Ischia is demonstrated by the numerous surface manifestations, including fumaroles, thermal waters, steaming grounds and mud pools. Since accurate geodetic measurements have been made (early 20th century), central-southern portion of the island has subsided. Generally, Ischia has a low level of seismicity, characterized by few events per year with a magnitude typically between -1 and 2.5 (see the catalog included in the supplementary Datasheet S1), which is concentrated in the Casamicciola area (D’Auria et al., 2018). However, as known, in this area, events with a relatively greater magnitude can occur (Carlino et al., 2006; Cubellis et al., 2004, 2020; De Novellis et al., 2018; D’Auria et al., 2018; Selva et al., 2021), which are potentially capable of causing severe damage to the town and population. This happened in 1883, when an earthquake with intensity XI MCS degree and magnitude of about 5 destroyed the town of Casamicciola, causing approximately 2,300 victims, and in August 2017, when an M4.0 earthquake caused severe damage to the town and two victims. The earthquake of August 2017 is the last seismic event with a significant impact on the island.

The Osservatorio Vesuviano seismic network (INGV-OV) currently consists of 34 broadband digital stations and 18 short-period seismic stations (5 with only vertical component and 13 triaxial), which continuously transmit signals to the Monitoring Center. Some of them are co-located in the same site for redundancy and continuity in the catalogs. The network is designed for monitoring the active Neapolitan volcanic areas, Vesuvius (Figure 2), Campi Flegrei (Figure 3), Ischia (Figure 4) and provides information on seismicity at a regional scale in collaboration with the national seismic network managed by Osservatorio Nazionale Terremoti (INGV-ONT). In the period 2010–2012 the seismic network was densified on the three volcanoes. Moreover, three OBS have been installed in Campi Flegrei in recent years (Iannaccone et al., 2018). The current geometry of the network of the Neapolitan volcanoes, in low seismic noise condition, allows to reliably locate seismic events with M < 0, in Vesuvius, Campi Flegrei and Ischia.

The data are collected at Osservatorio Vesuviano in Naples through intermediate data centers (e.g. in Ercolano). The analogue station data are acquired at Posillipo data center and then transmitted to the Osservatorio Vesuviano headquarter via TCP/IP protocol using a dedicated WiFi link. Here, an Earthworm-based system (Johnson et al., 1995; Johnson 2020) is devoted to the automatic detection and location of seismic events. This system provides a good quality location in a few tens of seconds after the detection of an earthquake.

In this article we consider the seismic catalogs of Vesuvius, Campi Flegrei and Ischia, that are routinely updated by the Osservatorio Vesuviano seismic laboratory and are available on the website www.ov.ingv.it (last accessed on March 27, 2021). The catalogs are based on single-station detections, and exploit the long historical record provided by the most ancient stations such as OVO (Vesuvius), STH (Campi Flegrei) and OC9 (Ischia). These stations are located in sites close to the main seismogenic areas of each volcano, which are the summit cone for Vesuvius, Solfatara-Pisciarelli for Campi Flegrei and the area of Casamicciola Terme for Ischia. Numerous articles analyzed in detail the catalogs of Campi Flegrei (D'Auria et al., 2011; Giudicepietro et al., 2019; Tramelli et al., 2021), Vesuvius (Giudicepietro et al., 2010; D'Auria et al., 2013) and Ischia (D'Auria et al., 2018; Selva et al., 2021). We refer to these works for an in-depth analysis of the seismological statistics of these three volcanoes. However, here we present the estimate of the completeness thresholds and the b-values of the entire catalogs and other basic elaborations, such as the monthly and annual distribution of all the detected earthquakes of the three areas, in order to have elements to compare the seismicity in the three volcanoes. The catalogs used in this work are also included in the supplementary Datasheet S1.

The traditional seismic catalog of Vesuvius is based on the earthquakes recorded at OVO station (in the historical building of Osservatorio Vesuviano in Ercolano). It begins in 1972 and includes 11,610 earthquakes, until December 2020. The catalog is based on a single station and is generated using the duration magnitude (Md), suitable for local earthquakes. The duration of the earthquakes is determined by the analysts of the seismic laboratory through visual analysis of seismograms. The same methodology is also used for the catalogs of the other volcanoes in the Neapolitan area, which are included in the supplementary Datasheet S1. Thanks to the BKE station (Bunker Est), which was installed on the top of Mount Vesuvius in 1999, an additional catalog of the events recorded near the crater is available. The BKE catalog (1999–2020) currently includes 18,663 events, many of which have magnitude < 1 and can only be recorded at the summit of Mount Vesuvius.

The revised catalog of Campi Flegrei covers the period from January 2007 to December 2020. It contains 4,473 earthquakes recorded at the STH reference station, installed near the Solfatara-Pisciarelli area. When the background seismic noise is particularly low, this station can detect earthquakes with magnitude < = −0.2. However, smaller earthquakes are often not locatable. Actually, only 1,437 of the 4,473 earthquakes in the catalog of the Campi Flegrei are located.

Finally, the seismic catalog of Ischia includes 147 earthquakes recorded in the 1999–2020 interval at OC9 station installed in the historical Osservatorio Geofisico della Gran Sentinella in Casamicciola Terme (Ischia), founded in 1885 by the geophysicist Giulio Grablovitz. Most of these earthquakes are located in the Casamicciola area.

We consider a period of 14 years, from January 2007 to November 2020, to compare the time evolution of the seismicity of the three volcanoes. During the selected period, the permanent seismic network of Osservatorio Vesuviano recorded a total of 16,804 events, of which 12,208 in Vesuvius, 4,473 in Campi Flegrei and 123 in Ischia.

In addition to the catalog data, we use a selection of the locations of the Campi Flegrei earthquakes, the best quality ones, e g., A and B quality with location errors typically in the range of 200–300 m (locations and related errors are reported in the supplementary Datasheet S1) from the INGV Osservatorio Vesuviano database. We use the hypocentral parameters of the INGV database also as initial locations to relocate the earthquakes of the April 26, 2020 swarm, the largest one that occurred in Campi Flegrei since 1984 in terms of energy released. Moreover, we calculated ex novo the locations of the Vesuvius earthquakes from 2015 to 2020 and in 1999, to compare the most relevant swarm, which occurred in October 1999, with that occurred in November-December 2018 (see. “Methods”). For Ischia we used the locations recently published in D’Auria et al. (2018) and Selva et al. (2021).

We use the data of few selected stations of the permanent GPS network dedicated to monitoring the Neapolitan volcanoes (De Martino et al., 2011; Tammaro et al., 2013; De Martino et al., 2014; Devoti et al., 2018; De Martino et al., 2020). In particular, to highlight the vertical ground displacements in the three Neapolitan volcanoes, we used the daily average of the vertical component from the RITE station (Figure 3) in Campi Flegrei (between January 2010 and November 2020); BKNO (Figure 2) at Vesuvius (between June 2010 and November 2020) and SERR (January 2004–November 2020) and MEPO (February 2017 - November 2020) in Ischia (Figure 4).

The fumaroles located in the bottom of the Vesuvius crater are sampled systematically in the frame of the surveillance of the volcano. Here we consider 68 samples (collected from 2010 to November 2020) for which we estimate the equilibrium temperatures within the gas system H₂O-H₂-CO₂-CH₄-CO using the method described in Chiodini et al. (2001a) and Caliro et al. (2011). The estimated temperatures, in the range from 379 to 441°C, are regularly reported in the Osservatorio Vesuviano surveillance reports (see, e.g., Figure. 4.3 in www.ov.ingv.it/ov/bollettini-mensili-campania/Bollettino_Mensile_Vesuvio_2020_12.pdf).

Over the past 2 decades a gradual increase in seismicity has occurred in Campi Flegrei (Tramelli et al., 2021). Until 2014, earthquakes were rare and occurred in swarms of numerous events with low magnitudes, such as on January 23, 2009 (173 events, M_max = 0.0), March 30, 2010 (141 events, M_max = 1.2) and September 7, 2012 (188 events, M_max = 1.7) (Figure 5A). After 2014, the seismicity occurrence changed: the events became more frequent over time (Figures 5A,C), the seismic energy release and the “background seismicity” (Chiodini et al., 2017b) increased (Figure 5C), the number of events in a single swarm decreased even if earthquakes with greater magnitude characterized the swarms. The histogram in Figure 5B shows the magnitude distribution (M_mean = −0.2). The magnitude-frequency distribution can be consistent with a Gutenberg-Richter distribution with the b-value = 0.95 (see Method section). The red curve in Figure 5A shows the monthly maximum magnitude. The fumarolic tremor that is a robust indicator of the activity of the Pisciarelli-Solfatara hydrothermal system (Chiodini et al., 2017a; Giudicepietro et al., 2019; Giudicepietro et al., 2021) showed a continuous increase and reached its maximum amplitude in September 2020 (Figure 5D).

A relationship between seismicity and injection of fluids into the hydrothermal system was evidenced for the two swarms of October 7, 2015 and December 6, 2019 in Giudicepietro et al. (2021). This relationship was highlighted by a sudden increase in the amplitude of the fumarolic tremor at Pisciarelli (Giudicepietro et al., 2021), recognized as a proxy of the activity of the hydrothermal system at Campi Flegrei (Chiodini et al., 2016).

In recent decades, after the 1982–1984 crisis, the most energetic earthquake swarm occurred on April 26, 2020. Here, we analyzed that swarm comparing it with the two swarms occurred on October 7, 2015 and December 6, 2019, studied in Giudicepietro et al. (2021), which are located in the same seismogenic volume. We compared the waveform of the April 26, 2020 major earthquake (Md 3.3, the largest one since 1984) with two of the largest events belonging to the swarms of October 2015 and December 2019, which form a distinct family of earthquakes (Giudicepietro et al., 2021). We calculated the cross-correlation of a 1s-sliding window of the April 26, 2020 (Md 3.3) earthquake onset (CAAM station vertical component) with the two selected events of the swarms of October 7, 2015 and December 6, 2019. We found a correlation of 0.9 with the onset of both events. This suggests that the April 26, 2020 (Md 3.3) earthquake belongs to the same family as the largest events of the swarms that occur in Pisciarelli area. Figure 6 shows the spectrogram and waveform of this earthquake and the comparison of its onset with the two events of the October 7, 2015 and December 6, 2019 swarms. The spectrogram (Figure 6A) is typical of VT earthquakes with an impulsive onset characterized by a wide frequency range up to 30 Hz. The comparison of the onsets (Figure 6B) confirms that the main event of the April 26, 2020 swarm shares with the major earthquakes occurred in recent years similar waveform characteristics. Furthermore, we calculated the focal mechanism of the April 26, 2020 Md 3.3 earthquake using FPFIT software (Reasenberg and Oppenheimer, 1985). It shows a solution that can be associated with normal-type mechanisms with an oblique component (Figure 7D) and it is similar to those found in Giudicepietro et al. (2021) for the major earthquakes of the October 2015 and December 2019 swarms. This further similarity with the major earthquakes of the swarms located in the Solfatara–Pisciarelli area shows that the same seismogenic structure has been reactivated several times in recent years.

We also performed relative locations of the April 26, 2020 swarm (blue circles in Figure 7) using the hypoDD software (Waldhauser and Ellsworth, 2000). The relocation confirmed that the April 26, 2020 swarm occurred along the same conduit-like path already identified in Giudicepietro et al. (2021) for the October 7, 2015 and December 6, 2019 swarms (Figure 7).

Typically the relatively high magnitude events (Md max = 3.3) of the swarms occurred in Solfatara–Pisciarelli area, and in particular for the three main swarms considered in this work (Oct 2015, Dec 2019 and Apr 2020), are deeper than those of the ordinary Campi Flegrei seismicity (Chiodini et al., 2021). In the December 6, 2019 swarm a clear migration of the hypocenters toward the surface was also observed (Giudicepietro et al., 2021).

The double-difference locations of the swarms of October 7, 2015 and December 6, 2019 are published as supplementary Datasheet S1 in Giudicepietro et al. (2021). The double-difference locations of the events of the swarm that occurred on April 26, 2020 are included in the supplementary Datasheet S1 of this article. The locations of the earthquakes from 2000 to 2020 are available online on the Osservatorio Vesuviano (INGV) website (www.ov.ingv.it), as well as the seismic catalogs.

More recently, four earthquakes with magnitude Md > 2 were recorded in December 2020 (see supplementary Datasheet S1, Table 1). Two of these earthquakes belong to a swarm of 51 events, which occurred on December 19, 2020. In this case a sudden increase in the amplitude of the fumarolic tremor was observed during and after the swarm, confirming a mechanism linked to fluid injection in the hydrothermal system, similar to that observed for the swarms of the October 7, 2015 and December 6, 2019 (Giudicepietro et al., 2021). This shows that at least since 2015 a seismicity directly linked to injections of fluids in conduit-type paths has occurred in the Campi Flegrei, even if this is not the only source of earthquakes acting in the Phlegraean caldera.

Vesuvius is a very active volcano, with a predominantly explosive eruptive style (Arnò et al., 1987; Scandone and Giacomelli, 2008), that is located in a densely populated area (Figure 2). These factors make Vesuvius one of the highest risk volcanoes in the world.

After the 1944 eruption the seismicity of Vesuvius was very low, with few earthquakes per year, until 1966, when a local seismicity appeared, consisting of Volcano Tectonic events (VT), located in axis with the crater. The complete catalog from 1944 was reconstructed in Giudicepietro et al. (2010). Since 1972 the estimation of Vesuvian earthquake magnitude became systematic and allowed to create the seismic catalog (Figure 8A) based on the OVO station, installed at the Osservatorio Vesuviano historical site (Figure 2).

The study of Vesuvius seismicity that occurred in the last 50 years and the densification of the seismic network in the summit area (Figure 2) made it possible to recognize two seismogenic volumes (Giudicepietro et al., 2010; D’Auria et al., 2013), one at a depth of about 4 km below the top of the edifice and the other one much shallower, where very small magnitude earthquakes are concentrated. The OVO station is mainly sensitive to earthquakes originated in the deep seismogenic zone whereas the BKE station can record even the shallow small earthquakes. Therefore, the comparison of OVO and BKE catalogs gives a picture of the two seismogenic volume temporal evolutions. Since 2008, earthquakes located in the deepest seismogenic zone decreased whereas shallow earthquakes were increasingly recorded (Figure 8B).

The implementation of a dense network in the summit area of Vesuvius between 2009 and 2011 made it possible to locate the very small magnitude (typically <0.5) earthquakes recorded at the top of Vesuvius, and so to discover the shallow seismogenic volume within the volcanic edifice (Figure 9A).

The most significant seismic crisis recorded at Vesuvius after the last eruption was on October 9, 1999 when a swarm of earthquakes with a relatively large magnitude occurred (M_max = 3.6, the highest magnitude recorded so far at Vesuvius) (Figure 9C). In more recent times, a significant swarm occurred between November 29 and December 1, 2018 (Figure 9B). We compared the earthquake locations of the two swarms and we found that the events of both swarms are predominantly located in the deep seismogenic volume (Figure 9D). In addition, some events of the 2018 swarm were located in the shallower seismogenic zone. The lack of earthquakes in the shallow zone during the 1999 swarm (Figure 9C) is due to the seismic network configuration, which did not allow their location at that time. Actually, they were recorded by the BKE station, but it was not possible to locate them because the other stations did not register them.

Vesuvius seismic events are generally VT earthquakes (Figure 10A) with a magnitude that rarely exceeds 2 (Figure 10B). The magnitude-frequency distribution in Figure 10B is reasonably consistent with a Gutenberg-Richter distribution with the b-value = 1.5 (see Method section). Recently, the Vesuvius network recorded three seismic transients characterized by frequencies between 1 and 5 Hz that occurred between 5:58 and 6:00 (UTC) on November 16, 2020. These events, which can be classified as LP events based on their frequency content (Figure 10C), have emergent onset and duration of about 30 s each. However, the second has a clearer onset than the others so we located it by calculating the time delay of its onset at different stations using cross-correlation (Figure 10D). The location falls at the base of the deep seismogenic zone (cyan circle in Figure 9).

The seismicity of Ischia island has been instrumentally monitored since 1993. Before then we can still count on a historical catalog that goes up to the 8th century BC (Figure 11). This richness of historical information is to be attributed to the tourist attraction that the Ischia island has always exercised on people who have left written traces of what they have seen and heard during their holidays. Besides, since 1885 a seismic tank was working in the Geophysical Observatory of Casamicciola. This innovative instrument capable of measuring and recording the oscillations of the water contained in a tank was installed by the scientist Giulio Grablovitz (1846–1928) and maintained until 1923 (Luongo et al., 2012; Cubellis and Luongo, 2018; D’Auria et al., 2018). After a gap of 70 years, the first seismic station was installed in the same place of the seismic tank, Casamicciola Observatory (OC9 in Figure 4).

The network was gradually improved until the four seismic sites equipped in 2011. After the 2017 M4.0 earthquake the network was improved reaching a total of 9 permanent and 5 mobile seismic stations. The complete (hystorical and instrumental) catalog has been recently revised by Selva et al. (2021). Their analysis evidenced that the Ischia seismicity is not stationary: the annual rate for earthquakes with a magnitude higher than 3.6 in the past (1750–1884) was six times larger than the current one (1885–2019) (Selva et al., 2021). These authors estimated a b-value higher than 1, as usual for volcanic areas, but the non-stationarity of the distribution of the seismic events makes the Gutenberg-Richter not the most appropriate relationship to describe the process. For the instrumental catalog the magnitude is estimated from the duration of the seismogram recorded at a seismic station located in the Casamicciola Observatory (D’Auria et al., 2018). The magnitude of the historical earthquakes has been converted from the intensity (Selva et al., 2021 and reference therein). As expected, the completeness magnitude highly decreases with time thanks to the improvement of the seismic network in the last decades.

The seismicity of Ischia is mainly associated with the faults of the northern boundary of the Mt. Epomeo block. Ischia indeed experienced one of the highest uplift measured at any volcano (more than 900 m in 35–55 ka) (Trasatti et al., 2019 and references therein), which produced a fault-bounded block (the Mt. Epomeo block) with topmost uplift to the NW and an overall tilt downward SE (Acocella and Funiciello, 1999). Currently the Epomeo block is subject to subsidence.

Several destructive earthquakes have been located in the northern part of the island with shallow hypocentre and estimated magnitude between 3.6 and 5.2: in 1,228 (700 casualties), 1796 (7 casualties), 1828 (30 casualties), 1881 (126 casualties), 1883 (2,333 casualties), and 2017 (2 casualties) (Cubellis and Luongo, 1998, 2018; D’Auria et al., 2018; De Novellis et al., 2018; Cubellis et al., 2020; Selva et al., 2021; Carlino et al., 2021).