GNGTS 2018 - 37° Convegno Nazionale

GNGTS 2018 S essione 1.1 153 ANALYSIS OF SEISMOLOGICAL AND GEOLOGICAL OBSERVATIONS FOR THE 2016 SOUTHEASTERN SICILY EARTHQUAKE SEQUENCE C. Musumeci 1 , L. Scarfì 1 , G. Tusa 1 , G. Barberi 1 , G. Barreca 2 , F. Cannavò 1 , S. Gresta 2 1 Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Catania, Osservatorio Etneo, Italy 2 Dipartimento di Scienze Biologiche, Geologiche e Ambientali, Università di Catania, Italy Introduction. Between February 6 and 12, 2016 the Hyblean foreland domain in SE Sicily (HP in Fig. 1A) experienced an earthquake sequence that affected mainly the provinces of Siracusa and Ragusa. The seismogenic region of the swarm lies a few kilometers east of the roughly N-S oriented shear zone, known as Scicli-Ragusa Fault System, characterized by a bulk left-transtensive regime since the late Quaternary time. We are talking about a complex tectonic area (Fig. 1B), where events of macroseismic intensity up to 11 have been recorded in zones immediately adjacent to the seismic sequence. The seismological recordings, collected by the seismic network operated by the Istituto Nazionale di Geofisica e Vulcanologia, allowed to record approximately 120 shocks in just a few days, whereas the geological field observations provided a picture of the structural features. In addition, earthquake ground motion simulations have been provided in order to generate earthquake scenarios in terms of ground motion parameters for the strongest event of the seismic sequence. Despite the low magnitude of most events, the sequence represents one of the largest episode of strain release in the Hyblean area over the last 20 years (Gruppo Analisi Dati Sismici, 2018). In particular, the mainshock (M L =4.3 on February 8), located 10 km NNE from the town of Ragusa, was felt throughout the southeastern Sicily, in a range of about 70 km from the epicenter, by causing very slight damage in the vicinity. On the basis of earthquake locations, fault plane solutions, and geological mapping we explored the possible correlation between the earthquake causative fault planes and the faults exposed in the area. Data analysis and results. Since waveform analysis revealed that the events are multiplets (i.e. events with very similar waveform signatures that are commonly interpreted as due to repeated slip on the same fault plane), a cross-correlation method was applied to obtain precise readings of the wave onsets. This enabled substantial improvement in precision of earthquake locations that indicates the activation of a NS structure at about 8 km of depth (Fig. 1C-E). Results well match with the focal solutions characterized by one roughly N-S striking nodal plane, underling a left-lateral kinematics (Fig. 1F). Stochastic ground-motion simulations for the M L 4.3 event have been performed through the code EXIM (Motazedian and Atkinson, 2005; Boore, 2009). Firstly, for validation purposes, we have computed synthetic seismograms at 10 recording sites, within 50 km from the epicenter, where are installed the accelerometric stations belonging to the Italian Accelerometric Network (RAN) and considering hard rock conditions (soil A). The rupture plane is well constrained from the results of this study, while both geometrical spreading and anelastic attenuation are known from seismological literature. However, to fully account for the path effects, the distance-dependent duration model is needed. Therefore, we have considered the effective duration (T90, Trifunac and Brady, 1975) and tested several different path duration models (see Fig. 2A). Fig. 2B and Fig. 2C show the comparison between recorded and synthetic PGAs and PGVs obtained with the inversion that leads to less root mean squared error. Finally, in order to generate the PGA shakemap (Fig. 2D), we computed the synthetic ground motion at virtual stations placed on a regular grid (2 km sized) within 25 km from the source through interpolation technique (Inverse Distance Weighting). The PGA and PGV values estimated at the epicenter are 53 gal and 1.94 cm/s, respectively. Conclusions. The characterization of the active geological structure responsible for the 2016 seismic sequence is of great interest to improve the understanding of seismic hazard