Intrusive Seismic Swarms as Possible Precursors of Destructive Earthquakes on Mt. Etna’s Eastern Flank

The Timpe Fault System (TFS) represents the source of shallow earthquakes that strike numerous towns and villages on Mt. Etna eastern ank. In the last 40 years, three destructive seismic events reached I 0 = VIII EMS (heavily damaging) - in 1984 (October 25), 2002 (October 29) and 2018 (December 26). These events followed a few days after the occurrence of strong seismic swarms and the sudden acceleration of the eastern ank seaward. However, if the 2002 and 2018 events were caused by stress induced by eruptive dike propagation, in October 1984 no eruption occurred. In this work, parameters such as localization, cumulative seismic moment and hourly occurrence frequency of the 1984 seismic swarm, have been analyzed and shown to have typical values of Mt. Etna intrusive seismic swarms. This suggests that the 1984 episode may have been an aborted intrusive magma episode that triggered similar processes (long and powerful intrusions with acceleration of the eastern ank movement and destructive earthquakes), as in 2002 and 2018. These three episodes suggest that an evaluation of some seismic parameters during future intrusive swarms may furnish indications of a possible re-activation of the TFS.


Introduction
Mt. Etna is a stratovolcano of about 3300 meters, built up over the past 500 ka on the eastern coast of Sicily, and is one of the most active volcanoes on Earth (Fig. 1). At the intersection of two regional structural trends, NE-SW and NNW-SSE oriented (inset in Fig. 1), Mt. Etna is the result of a complex interaction between regional tectonics, ank instability processes and basement geometry (e.g. Bousquet and Lanzafame 2004; Neri et al. 2005).
Volcanic activity on Mt. Etna comprises a persistent activity with episodic paroxysmal events, which occur close to the summit area, and hazardous ank eruptions that are generally preceded by intense intrusive seismic swarms and ground deformation (e.g. Allard et al. 2006).
Its southeastern ank, with an overall area of about 700 km 2 , moves ESE seaward continuously, at a rate of a few centimeters per year (e.g. Borgia et al. 1992; Bonforte et al. 2011). This instability is mainly linked to eruptive activity (intrusion and magma pressurization), regional stress and gravitational forces  (Fig. 1, Fig. 2) characterized by normal and right-lateral dynamics (Azzaro, 1999;Azzaro et al. 2000; Monaco et al. 1997). These structures normally show right-lateral kinematics and normal dip-slip with slip-rates (3.0 to 5.0 mm/y).
However, stress changes due to dike intrusion and accelerations of the southern-eastern ank velocity may cause fast ruptures with displacements up to some tens of centimeters with strong seismic releases (Azzaro, 2004  The swarms started on October the 16th at about 14:00 GMT and ca. 2143 events were recorded until the 31st. The October 1984 swarm is one of the most powerful swarms known on Mt Etna, recording more than 50 events with M ≥ 3.0 in 15 days. Figure 3 reports the hourly occurrence of earthquakes at the CTS station that evidences two distinct phases of this seismicity: the rst, from the 16th at 14:00 to the 18th at 12:00, with a high events occurrence (18.3 events/hours), and a successive phase with an average of about 5 events/hour with a slight increase only close to the 25th. This change occurred after the M = 4.2 earthquakes located on the Pernicana Fault on the 18th at 11:58 GMT.
Of these events about 600 earthquakes with M > 1.8 were located by using Hypoellipse (Lahr 1989) and the velocity model of Hirn et al. (1991). These locations are very scattered because they were obtained through manual P and S waves picking on paper seismograms. Therefore, in order to have a more reliable picture of the seismic source locations, data has been considerably reduced by only taking account of the events with more than 7 pickings, RMS > 0.5, GAP > 180°, ERH < 2.5 Km and ERZ < 3.0 Km, obtaining 30 located events for the rst phase and 55 for the second one (Fig. 3). The events belonging to the rst group are mainly located a few kilometres east with respect to the summit craters, while the events after the 18th at 12:00 are more distributed on the eastern ank of the volcano (Fig. 4). Therefore, seismic swarms accompanying these ve episodes consisted of hundreds of events that took place during the dike propagation and were testi ed 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).  (1) and (2)  In the last row of Table 2  In October 1984 however, no eruption occurred but the seismicity beginning on the 16th showed some common features with intrusive seismic swarms: 1) Location of the 1984 rst phase (from the 16th at 14:00 to the 18th at 12:00) is on the high eastern ank of Mt. Etna, similarly to what was observed for other intrusive episodes such as the 1991, 2008 and 2018a events (Fig. 6) involving similar volumes at depths between 1.5 km a.s.l and 2.0 km b.s.l.
2) Seismic parameters, such as hourly seismic events (19 events/hour) and cumulative seismic moment (1.55*10 16 Nm) are typical of the range of intrusive swarms at Mt Etna (Table 2).
These considerations suggest that the 1984 seismic swarm may be the result of an aborted intrusive magma episode that caused the acceleration of the eastern ank and the destructive earthquakes on TFS (Fig. 7). No eruptive intrusion at Mt. Etna has been recognized during the 2018 eruptions (2018b in Fig. 4), but also in 1989 (Bonaccorso and Davis 1993) and in January 1998 (Bonaccorso and Patanè 2001).
If so, it is interesting to note that the 1984, 2002 and 2018 swarms show common elements, such as: 1) a duration greater than 15-20 hours (Table 2); 2) a cumulative seismic moment (Fig. 8)  employed and the presentation of the material on this map do not imply the expression of any opinion whatsoever on the part of Research Square concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors.

Figure 2
Macroseismic location of earthquakes with epicentral intensities Io ≥ VII EMS occurring from 1980 to 2019 in the southeastern ank of Mt. Etna (Azzaro and D'Amico 2019). Numbers are referred to Tab. 1.
Note: The designations employed and the presentation of the material on this map do not imply the expression of any opinion whatsoever on the part of Research Square concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors.

Figure 3
Earthquakes hourly frequency at CTS station from October 16 to October 31.

Figure 4
Epicentral location of selected earthquakes for the rst (blue) and second (orange) phase. Note: The designations employed and the presentation of the material on this map do not imply the expression of any opinion whatsoever on the part of Research Square concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors.   Sequence of the events occurred in October 1984: 1) the seismic swarm is the result of an aborted magmatic intrusion that caused 2) the acceleration of the eastern ank (with PFS activation) and 3) the destructive earthquakes on TFS. Note: The designations employed and the presentation of the material on this map do not imply the expression of any opinion whatsoever on the part of Research Square concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors.

Figure 8
Cumulative seismic moment obtained for the intrusive seismic swarms.

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