Detailed investigation of crustal structure in the Sicilian region (southern Italy) from local earthquake tomography

We present a new tomographic velocity model for the crust and uppermost mantle beneath Sicily (Southern Italy) and surrounding areas. The Sicilian region represents a portion of the Apennine–Maghrebide fold-and-thrust belt developed in an area dominated by both the convergence between the European and Nubia plates and the extensional processes linked to the opening of the Tyrrhenian basin (Malinverno and Ryan, 1986; Faccenna et al., 1996). The major seismogenic domains identified on the basis of seismological evidences (Presti et al., 2013) are: (I) the Southern Tyrrhenian E–W striking domain, located in correspondence with the northern Sicily off-shore, undergoing N–S compression; (II) the northern Sicily domain presenting extensional regimes that vary from N–S, to the west, to WNW–ESE to the east; (III) the mainland Sicily domain comprising the Etna area and showing a mainly transpressional regime; (IV) the Hyblean Foreland domain in southeastern Sicily with primarily strike-slip deformation. To perform the tomographic inversion, we selected ca. 7100 earthquakes that have occurred in the last twenty years in the depth range 0-100km and with a minimum of 10 observations. Data and recordings have been derived by the Italian recording networks managed by INGV (www.ingv.it). By using the LOTOS code for passive tomography inversion (Koulakov, 2009), we computed the distribution of Vp, Vs, and the Vp/Vs ratio in the study area. In the inversion grid construction, node distribution has a regular horizontal step (5 km in our case). In vertical direction, the grid spacing depends on the data density, but it cannot be smaller than a predefined value (3 km in our case). In order to increase the detail of our analysis with respect to previous study carried out in the same sector (Barberi et al., 2004; Chiarabba et al., 2008 ; Orecchio et al., 2011; Calò et al., 2013), we tested several spacing for the inversion grid. We also performed synthetic tests to estimate a possible effect of noise on the resolution as well as the optimal values of inversion parameters. Main results show that a clear discontinuity in the P-wave velocity pattern is detectable between the Tyrrhenian off-shore of Sicily, characterized by high velocity anomalies, and the northern Sicilian on-shore, where a large almost E-W low velocity anomaly is evident at all depths. At the upper crust layers, low P-wave velocity anomalies are detectable in correspondence with the Mt. Etna and the Aeolian Island Volcanoes. In the depth range 3 - 15 km the Hyblean area (southern Sicily) shows higher velocity anomalies with respect to the adjacent sector of central Sicily. The obtained velocity model, jointly evaluated with results of synthetic modeling, as well as with the hypocenter distribution and geological information, furnishes new constraints to geodynamical and structural knowledge of the study area.