Between 19 and 26 August 2017, the south-western slope of Etna was affected by seismic activity with three different swarms.
The first, which occurred on August 19, with 70 earthquakes (of which 36 located with a magnitude ML between 1.0 and 3.3) in the space of about two and a half hours (from 13:12 to 15:38 pm Italian time) and with hypocentral depths included between 15 and 20 km, it involved the western slope of Etna, near the town of Adrano. The strongest earthquake, of magnitude ML 3.3, occurred at 13:38:10 Italian time on August 19, 2017 at a depth of 17 km.
The second swarm, between 25 and 26 August, with 23 recorded earthquakes (of which 10 located with a magnitude ML between 1.0 and 3.3) affected an area south of Adrano, between the villages of Ragalna and S. Maria di Licodia (CT) with hypocentres within the first 5 km of depth. The main event of this seismic swarm took place on 25 August at 23:57:47 Italian time with a magnitude of ML 3.3 and a depth of 2.6 km.
The third occurred on August 26, in the same area affected by the swarm of August 19, with over 60 events (of which 17 localized with magnitude ML between 1.0 and 3.1) and depths between 15 and 20 km. The strongest earthquake, of magnitude ML 3.1, was located at 22:46:21 Italian time near Adrano (CT).
The locations of the events of the three seismic swarms and the focal solutions of the three main events (the epicenters are represented with different colors according to their time of occurrence).
The figure shows the locations of the events of the three seismic swarms and the focal solutions of the three main events. The focal mechanisms show a strike-slip kinematics, with sub-horizontal P axes with a NS to NE-SW direction, consistent with the structural arrangement of the area reported in the literature.
The described seismicity is one of the volcano-tectonic events typically recorded at Etna, generated by regional tectonic stresses and/or local stresses deriving from migrations of magma within the crust. In general, the spatial distribution of Etna seismicity allows to identify different seismogenic volumes characterized by different seismicity rates and focal depths. In particular, on the eastern side the earthquakes are essentially localized within the first 10 km of depth; otherwise, earthquakes with hypocentres located up to 30 km deep are recorded on the western side.
Simultaneously with the occurrence of the seismicity described up to now, no significant variation of the volcanic activity at the summit craters has been recorded.
However, in the past, literature studies have correlated the deep seismicity of the mid-southwestern flank of the volcano to phases of deep magma recharge. In particular, the reactivation of NNW-SSE and NE-SW oriented structures, located in this sector of the volcano, seems to be associated with changes in the state of activity of the same (Alparone et al., 2012 and references within).
From an instrumental point of view, it is important to highlight that the energy release, associated with the seismic activity recorded between 19 and 26 August, is the greatest so far observed since the year 2000 in this sector of the volcano (Alparone et al., 2015 ; Catalog_2000-2010, Catalog of earthquakes in Eastern Sicily - Southern Calabria (1999-2017), INGV-Catania).
Among the phenomena contextual to the occurrence of the seismic swarms described above, it should be mentioned the beginning of a new phenomenon of "secondary volcanism" at the Salinelle di Contrada Cappuccini Vecchi, near the Falcone-Borsellino stadium in Paternò (CT). In general, these are emissions of brackish water and sludge, conveyed by essentially magmatic gases. Historically the Salinelle have presented paroxysmal phases of degassing activity which have been monitored by INGV in recent years, and correlated with the deep dynamics of Etna. In fact, the main gas supply tanks of the Salinelle would be located at depths greater than 10 km. A new eruptive phase began on 24 August with an intense emission of mud which partially invaded the perimeter road of the Paternò stadium. From an inspection carried out on August 28, by staff of the INGV-Osservatorio Etneo, it was found that the phenomenon had decreased in intensity compared to its initial stages. This episode, although in the context of a period of relative quiescence in recent months, is normal for the paroxysmal phenomena of the Salinelle, both in terms of intensity and in the location of the eruptive vents.
It cannot be excluded that the recent superficial seismic activity, near the villages of Ragalna – S. Maria di Licodia, may have influenced the state of stress acting on the ascent ducts of the fluids that feed the Salinelle di Paternò, modifying the level of activity.
The detailed analyzes relating to the three swarms are still in progress by the Seismic Data Analysis and Catalogs group of the INGV-Osservatorio Etneo.
Link to the INGVterremoti Blog:
https://ingvterremoti.wordpress.com/2017/09/01/aggiornamento-dellattivita-sismica-del-versante-sud-occidentale-delletna-tra-il-19-e-il-26-agosto-2017/