On Mt. Etna, the 1991-93, 2001, 2002, 2008-09 and 2018 eruptions represent the main lateral eruptions occurring over the last 30 years. For each episode, a dike intrusion accompanied by an intense seismic swarm, preceded the opening of eruptive fractures. Indeed, the dike propagation changes the stress field around the intrusion, causing a relevant number of earthquakes that occur close to the dike and activating structures and discontinuities in the nearby areas. This propagation is evidenced by changes in ground deformation measurements, whose modelling allows locating and characterizing the intrusive source (Fig. 5). On Etna, the ability to obtain reliable models began in 1990 with the expansion and improvement of the continuous tilt and GPS permanent networks (Ferro et al. 2011; Gambino et al. 2014; Palano et al. 2010). The 1991-93, 2001, 2002, 2008 and 2018 eruptions have been extensively studied in dozens of papers.
The 1991-93 eruption was characterized by low-explosive activity and produced a very large lava field (total volume of magma of about 235*106 m3). A shallow intrusion on the upper eastern flank, inferred by Bonaccorso et al, (1996), was accompanied by a seismic swarm of 197 recorded events (Patané et al. 1994) occurring in the same area 5 hours before the fractures opened on December 14, 1991 (Gambino et al. 2018).
The eccentric eruption of 2001 was caused by the forceful uprising of a vertical dike (Bonaccorso et al. 2002; Bonforte et al. 2009) below the south flank (Fig. 5), with magma ascending from a reservoir within the sedimentary substratum (Benchke and Neri 2003). It was preceded by a seismic swarm occurring from July 12 with 2645 events (with M > 1.0) that affected the southern sector southern sector of the volcano in a main cluster, related to the dike intrusion, with a minor cluster on the upper S-SE sector of the volcano (Patanè et al. 2003; Gambino 2004; Bonforte et al. 2009).
In particular, earthquakes of the main cluster from 13 July to 14 July occurred at a depth between 2.5 and 1.2 km b.s.l. while from 15 July (at 10:00 GMT) seismicity is very shallow (confined at ~ 0.5 km a.s.l.), shifting just below the eruptive fractures (Bonforte et al. 2009).
The 2002–2003 eruption was characterized by a first dike that ascended vertically in the southern flank close to the 2001 eruption site (2002s in Fig. 5) and a second horizontal dike propagating radially in the NE flank, along the NE rift (2002 in Fig. 5). Seismicity started at 20:12 UTC on October 26, 2002. During the first 3–4 h, it took place in the southern-upper part of the volcano when a dike-forming intrusion ascended vertically through the edifice close to the 2001 eruption site (Aloisi et al. 2003; 2006). Successively, the epicentral seismic pattern showed a northeastward migration of earthquakes along the NE Rift with a second dike-forming intrusion propagated laterally along the NE Rift (Aloisi et al. 2006). At 01.28 GMT on 27 October a Md = 3.5 involved the western tip of the Pernicana Fault destroying the Piano Provenzana skiing station (Alparone et al. 2015) and successively an exceptional increase of the seaward movement characterized the eastern flank (Bonaccorso et al. 2006).
The 2008 and 2018 eruptions were characterized by dike intrusions and the opening of lateral eruptive fissures located on the upper eastern flank (Fig. 5).
On May 13, 2008 at 08:40 (GMT), a seismic swarm preceded and accompanied the beginning of the 2008–2009 eruption, when more than 240 events were recorded in about 6–7 h, most of them occurring in the first two hours (Alparone et al., 2012). The eruption started with a fracture system between 3055 m and 2620 m a.s.l. (Aloisi et al. 2009), propagating from the summit craters toward the western wall of the Valle del Bove (e.g. Bonaccorso et al. 2011).
The earthquakes (the largest being ML = 3.9, on May 13, 2008 at 10:07 GMT) were located in the northeastern summit area at depths ranging between 1.5 km b.s.l. and 1.5 km a.s.l. (Alparone et al., 2012). In less than two hours, there was a clear migration of the seismic events toward the north (Patanè 2008), suggesting a propagation of a shallow intrusion toward this sector of the volcano (Aloisi et al. 2009). The intrusion was very fast and was marked by ground deformation recorded at permanent tilt and GPS stations (Aloisi et al. 2009).
In 2018, the eruptive fissures opened at the base of summit craters, with a small lateral eruption though the seismic swarm and ground deformation were very strong. The initial seismicity (December 24 at 8:30 GMT) was located beneath the summit craters and along the eruptive fracture with epicentres aligned in a N-S direction, between − 1 km a. s. l. and 1 km b.s.l. (Alparone et al. 2020) involving the PFS with an ML 3.5 at 10:27 GMT. From 8:30 to 11:10, Aloisi et al. (2020) inferred one propagating magmatic intrusion (2018a in Fig. 5) that ascended near vertically from sea level toward the ground surface where eruptive fissures opened.
However, continuous ground deformation few hours after revealed a new elongated intrusion in the southern flank, matching the southwards migration of seismicity with a deepening of the hypocentres up to 3 km b.s.l. (Alparone et al. 2020) that did not reach the ground surface (2018b in Fig. 5).
After the 24 December 2018 the eastern flank was characterized by strong increased seaward velocity, clearly shown in the slope of the GNSS time series (Mattia et al. 2020).
Therefore, seismic swarms accompanying these five episodes consisted of hundreds of events that took place during the dike propagation and were testified by contemporary ground deformation changes, Table 2 reports some parameters of these seismic episodes (duration, number of events recorded, earthquake hourly frequency and seismic moment).
Table 2
Intrusive seismic swarm parameters. The duration represents the time-interval in which there is contemporaneity between seismicity and ground deformation (Gambino et al, 2018 and references therein; Aloisi et al. 2020; Gruppo Analisi Dati Sismici 2020). The seismic moment Mo was obtained by using (1) and (2) relationships. The 2018 values comprise an estimate of 2.0*1015 Nm for the 1425 events with M < 1.4 not included in the Bonaccorso and Giampiccolo (2020) calculus. The last row reports data of the first phase of the October 1984 seismic swarm (16th at 14:00-18th at 12:00).
Eruptive episode
|
Type
|
Duration
(hours)
|
Seismic events
(n°)
|
Seismic events
(events/hour)
|
Mo
(N/m)
|
1991-93
|
Lateral
|
5.0
|
197
|
39.40
|
4.09E + 15
|
2001
|
Eccentric
|
113.0
|
2694
|
23.84
|
4.35E + 16
|
2002
|
Lateral
|
19.5
|
294
|
15.08
|
1.89E + 16
|
2008
|
Lateral
|
6.4
|
230
|
35.94
|
6.80E + 15
|
2018
|
Lateral
|
31.5
|
1560
|
49.52
|
1.36E + 16
|
1984
|
No eruption
|
43.0
|
813
|
18.91
|
1.55E + 16
|
On Mt. Etna, the duration of an intrusive process is highly variable, from 5 to 113 hours, while the number of earthquakes ranges between a couple of hundred events to a couple of thousand events with an occurrence between 15 and 50 events/hour. The seismic moment varies within an order of magnitude between ca. 4.0*1015 Nm and 4.0*1016 Nm (Gambino et al. 2018; Bonaccorso and Giampiccolo 2020).
In the last row of Table 2 are reported the same parameters obtained for the first phase of the October 1984 seismic swarm (813 events in 43 hours, an hour-frequency of ca. 19 events per hour and an estimated cumulative seismic moment of 1.55*1016 Nm).
The seismic moment has been obtained with the Giampiccolo et al. (2007) equation:
Log (Mo) = (17.60 ± 0.37) + (1.12 ± 0.10)*ML (1)
where ML is the local magnitude of each earthquake that we obtained by converting Md in ML using the Tuvè et al. (2015) relationship:
ML = 1.164 (± 0.011) * Md − 0.337 ((± 0.020) (2)