High-resolution investigation of the crustal structure of the Sicilian region (southern Italy) by local earthquake tomography

Abstract:

We present a new seismic velocity model for the crust and uppermost mantle beneath Sicily (Southern Italy) and surrounding areas, obtained by local earthquake tomography. 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). Four major seismogenic domains characterize the region: (I) the Southern Tyrrhenian E–W striking domain 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 (southeastern Sicily) with primarily strike-slip deformation. To perform the tomographic inversion, we selected ca. 7100 earthquakes that have occurred between 1990 and 2012 in the depth range 0-100km. Data and recordings have been derived by the Italian recording networks (www.ingv.it). By using the LOTOS code (Koulakov, 2009), we computed the distribution of Vp, Vs, and the Vp/Vs ratio in the study area. In order to increase the detail of our analysis with respect to previous study carried out in the same sector, we tested several spacing for the inversion grid and performed synthetic tests to estimate a possible effect of noise on the resolution as well as the optimal values of inversion parameters. The obtained velocity models, jointly evaluated with the hypocenter distribution and geological information, give us new constraints to geodynamical and structural knowledge of the study area. Main results evidence (i) the presence of a clear discontinuity in the P-wave velocity pattern 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; (ii) high velocity anomalies in the Iblean area with respect to the adjacent sector of central Sicily in the depth range 3 - 15 km; (iii) low P-wave velocity anomalies in correspondence with the Mt. Etna and the Eolian Island volcanos in the upper crust layers.