GNGTS 2013 - Atti del 32° Convegno Nazionale

Fault System, along the main direction striking about N-S and the conjugated structures NE- SW. Moving eastward, events are concentrated in the area of the Hyblean Plateau. They become sparse or lacking beyond its edges, namely to the south, to the north (in the Scordia-Lentini Graben area) and between the Scicli-Ragusa Fault System and the Tellaro Line (Fig. 1b). Several earthquakes occurred instead along the coastal area, matching quite well the faults that border the Augusta and Floridia basins (Fig. 1b). In the plateau, the geometry of several clusters seems indicate the NW-SE direction as the main direction of the seismogenic structures. In the nearby Ionian offshore, earthquakes lie along the HMEFS, between Catania Gulf and Siracusa, while a cluster is detectable at the Alfeo Seamount (AF). Eastward, other events are more scattered, although some NW-SE lineaments could be traced. Here the seismogenic depths is between 20 and 40 km, but this last finding must be read with caution as the geometrical gap of the network could create artefacts. The following step was to analyze the stress pattern acting on southeastern Sicily. We calculated the fault-plane solutions (FPSs) of a selected subset of earthquakes by using the FPFIT algorithm (Reasenberg and Oppenheimer, 1985), with rays traced through the computed 3D velocity model. From the initial dataset 195 earthquakes with M L >1.0 and at least 8 clear first-motion picks (71% of the 195 shocks had 10 or more first-motion readings, 15 on average) have been selected. We discarded FPSs if they met any one of the following criteria: i) ratio NDisc/NPol (NDisc is the number of discrepant observations and NPol is the number of first readings used in the solution) greater than 0.2; ii) large uncertainty in P- and T-axes orientation and regions overlapped; iii) averaged uncertainties in strike, dip and rake greater than 20° and iv) the number of multiple solutions greater than 2. These selection criteria yielded 165 well- constrained FPSs (Fig. 2). Uncertainties in fault parameters (strike, dip, and rake) range mostly between 10° and 15°. All types of mechanisms are represented in this dataset although strike-slip solutions outweigh normal and reverse ones. A plot of all P and T axes obtained (inset (a) of Fig. 2) shows that the tensional axes have an average trend that is nearly horizontal and in the NE-SW direction, while the average compressive stress axis is nearly horizontal and in the NW-SE direction. Although some vertical compressive axes were observed, they are less numerous in comparison to the horizontal ones. To determine stress directions from the 165 selected focal mechanisms we applied the focal mechanism stress inversion (FMSI) computer program developed by Gephart and Forsyth (1984). The parameters obtained from the inversion algorithm are the directions of the maximum (σ 1 ), intermediate (σ 2 ), and minimum (σ 3 ) principal stress axes, and a measure of their relative magnitudes R = (σ 2 – σ 1 )/(σ 3 – σ 1 ). Moreover, a variable misfit (F) is introduced in order to define discrepancies between the stress tensor and the observed fault plane solutions. The main findings for the whole dataset (165 fault-plane solutions) can be summarized as follows: F= 6.598°; R=0.6; s1= N320° dip 0°; s2=N203°E dip 89°; s3=N50°E dip 1° (inset (b) of Fig. 2). The average misfit of 6.598° suggests that the dataset may be affected by some heterogeneity. GPS data. Geodetic GPS-based monitoring of the Hyblean area is currently carried out by INGV through the set-up of a permanent GPS network in the framework of the “Rete Integrata Nazionale GPS (http://ring.gm.ingv.it) ” project, since 2005. In addition, a network comprising a local trilateration network and a levelling route managed by the International Institute of Volcanology (merged into INGV in 2001) and the Istituto Geografico Militare Italiano (www. igmi.org) respectively, was surveyed in 1998, 2000, 2005 and 2006. All available GPS data were processed by using the GAMIT/GLOBK software (Herring et al. , 2010). To adequately show the crustal deformation pattern over the investigated area, estimated GPS velocities (Fig. 3a) were aligned to a fixed Eurasian reference frame (Palano et al. , 2012). By taking into account the observed horizontal velocity field and associated covariance information we derived a continuous velocity gradient tensor on a regular 0.1° x 0.1° grid (whose nodes do not coincide with any of the GPS stations) using a “spline in tension” technique 218 GNGTS 2013 S essione 1.2