Seismicity in volcanic areas is commonly analyzed to understand the dynamic processes occurring at different scales and to monitor the seismic hazard. In subduction zones seismic signals include purely tectonic events, tremors due to hydrothermalism, magma migration or plate interaction, volcano-tectonic events, swarms and transients (Eiler, 2003). Therefore it is important to identify the source of the different types of signals also from a volcanic and seismic hazard perspective, particularly where monitoring systems are poorly developed or lacking. The southern Tyrrhenian Sea-Calabrian Arc-Ionian Sea subduction setting is characterized by the volcanic arc including Lipari, Vulcano and Salina islands, which form a NNW-SSE elongated volcanic ridge crossing the central portion of the Aeolian Archipelago (Figures 1A,B) (De Astis et al., 2003). The volcanic ridge formed in Quaternary times along the Aeolian Tindari-Letojanni (ATL) fault system, a Subduction-Transform-Edge Propagator (STEP) fault that bounds the western edge of the subduction of the Ionian Sea below the Calabrian Arc and transfers stress across northeastern Sicily (Govers and Wortel, 2005; Billi et al., 2006; Scarfì et al., 2018).

The ATL is a complex and heterogeneous crustal discontinuity consisting of a broad NNW-SSE- to NW-SE-trending fault system that cuts the south-western flank of Lipari and Salina, borders both the western and eastern flanks of Vulcano and extends southward to the Ionian coast of Sicily (Figure 1B) (Palano et al., 2015). The Sisifo-Alicudi (SA) fault forms the eastern boundary of an E-W-trending transpressional belt which extends from far west in the southern Tyrrhenian Sea (Figure 1A) to Lipari-Vulcano (Figure 1B) absorbing a portion of the Africa-Eurasia convergence rate (Palano et al., 2012). A large number of geological and geophysical observations have suggested that magma channeling and uprising in western and central Aeolian Islands have been controlled by the SA (∼1.3 Myr) and the ATL fault systems (∼0.4 Myr; De Astis et al., 2003; Ventura et al., 2013). Spatial distribution of crustal earthquake clusters along both tectonic structures and the largest events occurred in 1978 (M5.6) and 1980 (M5.7) on ATL and SA, respectively (Neri et al., 1996). Focal mechanisms of earthquakes are consistent with right-lateral slip in NNW-SSE direction in response to a strike-slip stress regime characterized by a N-S compression (Cintorrino et al., 2019). Deep events, down to 200 km, also occur to the east of ATL and are associated with the Ionian subduction zone (Milano et al., 1994).

Ground deformation measurements (Bonaccorso, 2002; Mattia et al., 2008; Esposito et al., 2015) indicate the N-S contraction of Lipari and Vulcano and an overall subsidence with rates decreasing from the northernmost GPS stations to the southernmost ones. Recent studies (Alparone et al., 2019; Cintorrino et al., 2019) show that the surface ground deformation is explained by the joint contribution of regional (tectonic, ATL) and local (magmatic, a deflating source located under Vulcano at a depth of ∼4 km) sources. Furthermore, archaeological data indicate that the southern sector of Lipari has been undergoing subsidence for the last 2,100 years at rates of up to ∼11 mm/yr (Anzidei et al., 2016; Anzidei et al., 2017).

In the last decades, a number of studies based on seismic reflection, marine morpho-structural and bathymetric data (Favalli et al., 2005; Argnani et al., 2007; Bortoluzzi et al., 2010; Cultrera et al., 2017) have mapped main fault segments, therefore providing constraints on the structural trends and tectonic arrangement of the southern Tyrrhenian region. These data, complemented with the structural data on land, have shown that Lipari and Vulcano are primary controlled by NNW to NW-SE strike-slip faults, coupled with secondary NE-SW and N-S striking faults (Mazzuoli et al., 1995; Ventura et al., 1999; De Astis et al., 2003; Ruch et al., 2016). Moreover, the N-S faults control the location of the shallow magmatic reservoirs responsible for the more recent (<55 kyr) volcanism at Lipari and Vulcano (Ventura et al., 1999; Ruch et al., 2016). In the case of Lipari, volcanism occurred between 267 ka and 1,220 AD (Forni et al., 2013) on a 20 km thick continental crust (Ventura et al., 1999; Calò et al., 2013). Volcanic activity developed in three main phases: 1) the early activity (267 ka to about 150 ka), mainly focused in the western sector, with the eruption of lava flows and scorias; 2) the 119–81 ka volcanism concentrated in the central sector with the emplacements of lavas and pyroclastics (Monte S. Angelo and Monte Chirica volcanoes, close to the 121 and 145 nodes, respectively, Figure 1C); 3) the more recent volcanism, 42 ka to 1,220 AD, affected the southern and eastern sectors and comprises pyroclastic fall and flow deposits, lava flows and domes, the latter concentrated in the southern sector. The last eruption occurred in the NE corner of the island with the formation of a rhyolitic pumice cone and the emplacement of obsidian lava. A hydrothermal field is located in Lipari’s western sector, where hydrothermal activity is concentrated on faults and fractures that align along a N-S direction (Ruch et al., 2016; Cucci et al., 2017). Hydrothermal output mainly consists of CO₂ of magmatic origin (Cioni et al., 1998). Although the last volcanic eruption in Lipari occurred in 1,220 AD, volcanic features such as hot springs and fumaroles observed at the surface prove that volcanism is still active (Cioni et al., 1998). Very few studies have focused on the Lipari hydrothermal system, among them Bruno et al. (2000) observed that the fluid circulation concentrates along the ENE-trending faults located near the Terme di San Calogero resort (south of the fumaroles area shown in Figure 1C). Cucci et al. (2017), analyzing the gypsum-filled vein networks in the kaolin area (outlined black square in Figure 1C), have concluded that the decrease of the fluid discharge in hydrothermal fields may reflect pressurization at depth potentially preceding hydrothermal explosions. Fumaroles and hot water, along with a kaolin-dominated alteration zone, describe the N-S hydrothermal belt that is characterized by magmatic CO₂ with equilibrium temperatures of 170–180°C and SO₄ ²⁻ up to 780 ppm (Cioni et al., 1988). The more recent, still active surface hydrothermal alteration episode started at about 27 ka with temperatures up to 90°C. The presence of active structures has also been related to anomalies in the spatial distribution of soil gas emissions (Camarda et al., 2016). The hydrothermal activity accounts for the shallow depth of the magnetic bottom (Curie isotherm) at Lipari, which is at about 1 km depth (De Ritis et al., 2013). The spatial distribution of the seismicity and the shallow Curie isotherm led De Ritis et al. (2013) to put forward the hypothesis of the existence of a “soft” layer due to melts and/or high temperature fluids beneath Lipari.

To investigate the seismicity in Lipari, we carried out a passive seismic experiment from October 16 to November 14, 2018. We deployed a nodal seismic array of 48 FairfieldNodal ZLand three-component nodes (stations) with a 5 Hz low-corner frequency, nominal sensitivity of 78.7 Vm⁻¹ s⁻¹ and battery life of about one month (Di Luccio et al., 2019).

All the nodes, placed at distances between 0.1 and 1.5 km on the island (Figure 1C), recorded continuously at a 4 ms sampling rate. Nodes were buried beneath a few centimeters of soil allowing the GPS signal to arrive at each node for time synchronization; most of them were installed on the street side, others were placed with homeowners, hotel owners, at the Lipari Observatory and Lipari Museum (Figure 1D). Site selection was done before the deployment that took two days and was completed by two groups, each composed of two persons. The deployment and retrieval of the cable-free node system was faster than it would have been with a cabled system, and also cheaper in terms of costs and personnel required. Each node is a complete recording system, including the data logger, a flash memory card and the GPS, without any required connection to any external device, once it has been programmed for the specific acquisition in the laboratory.

The Lipari array recorded over 300 Gb of data, including local and distant earthquakes, as for instance the October 25, 2018 M_w6.8 Peloponnese event and its aftershocks (Di Luccio et al., 2019). This is the first time that such a dense seismic array has been deployed on Lipari Island. In Figure 2 one-day station plots show the quality of the recorded data as well as the day and night noise level.

We investigated the seismicity of Lipari Island by using different techniques on a one month dataset recorded by a 48 node array (Di Luccio et al., 2019). We located a seismic swarm that occurred on November 4, 2018 off the western coast of Lipari (Figure 1C) where the volcano-tectonic events define a NE-SW striking rupture zone. According to the structural model of Lipari proposed by Mazzuoli et al. (1995), faults with NE-SW strike represent second-order shears moving in response to the strike-slip movements of the main, NNW-SSE striking ATL shear zone. Swarms with similar characteristics in the frequency and time domains to those detected at Lipari have been observed in other volcanic and hydrothermal areas, such as Nisyros, Greece (Caliro et al., 2005), where both VT and long period events are related to fluid migration caused by the reactivation of pre-existing fractures (Caliro et al., 2005). The occurrence of swarms in other active volcanic and tectonic areas has been related to hydrothermal alteration of rocks weakened by water-rich CO₂ solutions. In these weak microfractured zones, swarms can occur by stress accumulation or triggered by pore pressure changes (Heinicke et al., 2009).

The hybrid event that occurred in Lipari on October 26, 2018 has been located near node 114, close to the hydrothermal field (Figure 1C), at very shallow depth (<1 km). This depth corresponds to that of the magnetic bottom below Lipari, where the 500°C isotherm is supposed to be located (De Ritis et al., 2013). Therefore the source mechanism of the October 26, 2018 event could be explained as due to a fluid-filled crack process in agreement with observations in laboratory experiments (Julian, 1994; Fazio et al., 2017), which may account for the distinctive surface waves packet following the onset of the initial waveform. Seismic signals with similar characteristics in time and frequency domains have been recorded in the hydrothermal areas of Vulcano Island, south of Lipari and interpreted as due to fluid migration processes (Alparone et al., 2010). Hybrid earthquakes have been also detected close to the hydrothermal system of Deception Island, related to the local response of the volcanic complex to regional tectonics (Carmona et al., 2012). According to Harrington and Brodsky (2007) shallow events like that of October 26, 2018 at Lipari could be also due to shallow tectonic sources combined with low rupture velocities, without excluding the possible involvement of fluids. We conclude that the shallow Lipari earthquake is associated with the dynamics of the local hydrothermal system and, in particular, with fluid pressurization within pre-existing cracks. Field evidence of such processes in the Lipari hydrothermal area is provided by the occurrence of gypsum-filled cracks formed by the pressurized injection of brine-type fluids along a N-S striking fault zone (Cucci et al., 2017). The shallow depth of the magnetic bottom (De Ritis et al., 2013), the equilibrium temperature of hydrothermal fluids (170–180°C; Cioni et al., 1988) and the active gas emissions from fumaroles suggest the involvement of high temperature fluids.

The increase in seismic noise detected on October 22, 26 and 30 and November 4 and 5 during periods of sea wave heights indicates that Lipari is subjected to shaking during storms. Generally, sea cliffs are zones mainly exposed to the ground motion generated by the direct sea wave impact and nearshore wave period (Adams et al., 2005). During storm events, microcracking episodes can generate microseismic ground displacements that reduce the rock cohesion. This does not occur during periods of normal sea level (Adams et al., 2005; Brain et al., 2014). Such weakening may act as an additional driver to coastal erosion processes, favoring cliff collapse as observed in Southern California (Young et al., 2016). These phenomena are also common on Lipari and mostly affect the eastern coast of the island (https://www.geologidisicilia.it/public/bollettino/pdf/giugno-2016.pdf).

The main conclusions of this study are:

• Lipari is a dynamically active volcanic area as derived from seismological analysis, implying the importance of the systematic survey of the seismic activity and the geochemical monitoring, which can provide useful information for the overall comprehension of the seismic signals. The day plots, cluster analysis and the time evolution of the seismicity in one month of data show that sea wave dynamics, in terms of wave height and frequency of occurrence, causes an increase of the background seismic noise due to the shaking of the island.

Seismological and cluster analyses suggest the coeval action of different processes such as hydrothermal fluid migration, regional tectonics, sea erosion and subsequent cliff instabilities in volcanic islands. In the Southern Tyrrhenian Sea the detailed investigation of seismic signals at the local scale allows us to infer interesting features of the distribution and evolution of the seismicity in one month of recordings. We propose that a monitoring system combining geochemical campaigns, tide gauge and seismic arrays like the one adopted here could contribute to better evaluate the progressive weakening of the rocks and study the steps needed to reduce the hazard related to rock instabilities in coastal areas (Ramalho et al., 2013). In conclusion, integrating measurements from both seismic and geochemical monitoring systems at Lipari will allow for a full investigation of the dynamics affecting the area with direct benefits for hazard evaluation.