GNGTS 2018 - 37° Convegno Nazionale

126 GNGTS 2018 S essione 1.1 REGIONALIZATION AND DEPENDENCE OF CODA Q ON FREQUENCY IN THE SEISMICALLY ACTIVE HYBLEAN PLATEAU REGION (SOUTHEASTERN SICILY, ITALY) E. Giampiccolo 1 , D. Ferrantelli 2 , T. Tuvè 1 , E. Del Pezzo 3,4 , S. Gresta 2 1 INGV Osservatorio Etneo, Catania, Italy 2 Dipartimento di Scienze Biologiche, Geologiche e Ambientali, Università di Catania, Italy 3 INGV Osservatorio Vesuviano, Napoli, Italy 4 Istituto Andaluz de Geofisica, Universidad de Granada, Spain Introduction. The Hyblean Plateau is one of the most seismically active areas in Italy. It is part of the orogenic foreland deformed by extensional and strike-slip faults. The tectonic setting (Fig.1) is mainly characterized by a series of NW–SE and NNW–SSE oriented horst and graben, linked to the Malta Escarpment Fault System (MEFS). Secondary faults [i.e., Avola fault (AF), Scordia-Lentini Graben (SLG), Rosolini-Pozzallo System (RPS), and the Scicli- Ragusa System (SRS)] strike along an overall NE–SW and NNE–SSW direction and represent the southern branch of the Siculo-Calabrian Rift (i.e. Monaco et al. 1997), an active rift zone extending from southern Italy to the Ionian shore of Sicily for a total length of about 370 km. The seismicity is sparse in space and time and it is mainly related to the offshore MEFS, whereas weaker shocks are due to the secondary inland faults. In the last millennium, two destructive earthquakes reached macroseismic magnitude 7 (I = X-XI MCS): the 1169 February 4, and 1693 January 9 and 11 events (Boschi et al., 1995). Three other earthquakes (in 1125, 1542 and 1818) exceeded macroseismic magnitude 5.8 (Azzaro and Barbano 2000). The strongest instrumentally recorded seismic event took place on 1990 December 13 (ML = 5.4), few kilometres offshore of Siracusa (Amato et al. , 1995) (Fig. 1). As a consequence, the quantification of the seismic wave attenuation mechanisms in the medium in this seismically active area plays an important role for seismic risk evaluation as well as being useful for a correct estimation of the seismic source spectra, the observational basis of the seismic source studies. Data and Method. We performed an in depth analysis of attenuation by using the “coda” of local earthquakes which is traditionally defined as the wave train following the direct S-waves in a local earthquake seismogram. That in order to update the knowledge of seismic attenuation in the Hyblean Plateau, through: i) the estimate of the frequency-dependent Q C relationship; ii) the regionalization of Q C to better understand the relation between spatial variation of seismic attenuation and geological and tectonic features in the whole region. Our results are jointly interpreted with those obtained from recently published 3D velocity tomographies for further insights. We applied the single back scattering model of Sato (1977) to about 400 local earthquakes recorded at the seismic network run by Istituto Nazionale di Geofisica e Vulcanologia – Osservatorio Etneo (INGV-OE). The dataset was divided into two subsets, according to the focal depth (h ≤ 20 km; h > 20 km). The waveforms of each subset were filtered in frequency bands centred at f equal to 1.5, 3, 6 and 12 Hz, with a bandwidth of an octave. Fig. 1 - Structural map of the Hyblean Plateau and distribution of the earthquakes selected for the present study (empty circles).