GNGTS 2019 - Atti del 38° Convegno Nazionale

GNGTS 2019 S essione 1.3 201 The definition of the basal sliding decollement of Mt. Etna is widely debated and a number of different models has been proposed in the last 3 decades. For instance, Lo Giudice and Rasà (1992) postulate a shallow (~1.5 km) surface with a listric geometry located beneath the volcanic pile of Mt. Etna. Borgia et al. (1992) suggest an approximately 5-km-deep sub- horizontal décollement. Tibaldi and Groppelli (2002) suggest the possibility of alternating or contemporaneous movements on both shallow and deep slip surfaces under the effects of different source mechanisms. GPS-based models point to a décollement surface located at a depth ranging from 0 to 4 km b.s.l. (see Palano 2016 and references therein). Geodetic data. All available continuous GPS data on the eastern flank of Mt. Etna edifice were collected and analyzed in order to take into account the best spatially and temporally station coverage. To this aim we collected data from several national (Italpos, NetGEO) and regional (Etn@net, Siornet) networks. All collected data were processed using the GAMIT/ GLOBK software with IGS precise ephemerides. To improve the overall configuration of the network and tie the regional measurements to an external global reference frame, data coming from 15 continuously operating IGS stations were introduced in the processing. We used the latest absolute receiver antenna models by the IGS and we adopted the Saastamoinen (1972) atmospheric zenith delay models, coupled with the Global Mapping Functions (Böhm et al. , 2006) for the neutral atmosphere. Estimated GPS daily time series and displacements for specified time intervals were referred to the “Etn@ref” reference frame (Palano et al. , 2010; a local reference frame computed to isolate the Mt. Etna volcanic deformation from the background tectonic pattern). From the visual inspection of daily-based time series of stations installed on the lower part of the unstable flank of the volcano, we detected 11 SSEs with duration ranging from 2 to 67 days (Fig. 2). On the analyzed time interval, the first SSE occurred in April 2006, while the last one in April 2016; no other significant SSEs have been detected in the last 2 years. For each recognized SSE, we determine the amount of displacements by averaging site position in the 3 days preceding the event and in the 5 days following its end. Modelling. The deformation fields were used to constrain isotropic half-space elastic inversion models in order to infer the spatial distribution of modelled slip on a décollement surface. In a first step, to determine how well the spatial distribution of slip is estimated from our data set as well as from the GPS stations distribution on the eastern flank of the volcano, we performed some resolution tests. To this aim, after extending the décollement surface (dipping seaward) inferred in Palano (2016) in the length direction by 4 km, we subdivided it into 12 (along-strike) by 10 (downdip) squared patches (dimension of 2 x 2 km). We therefore Fig. 2 - Time-series of the East component of ELAC, SCAC, SERB and MASC stations (see Fig. 1 for location). Detected slow-slip events are also reported.