This research is based on a multidisciplinary approach including geological, geodetic and geophysical analyses and provides the first evidence of active deformation in south-western Sicily, in an area that partially falls on the macroseismic zone of the Belice 1968 destructive earthquake sequence. Even though this is the strongest seismic event recorded in Western Sicily in historical times, the seismogenic source is still undefined.
The study area includes the westernmost frontal segment of the Sicilian Fold and Thrust Belt, a sector that was considered aseismic before the occurrence of the 1968 Belice earthquake. Seismic exploration has revealed that this frontal portion of the chain also extends in the offshore, in the northern sector of the Sicily Channel, at the eastern boundary of the Adventure Bank (ARGNANI et alii, 1986; CATALANO E D’ARGENIO, 1986).
The structural setting of western Sicily is characterized by a duplex geometry composed of two superposed thrust systems separated by a regional dècollement (CATALANO et alii, 1998; 2000). The shallower system consists of a 1-3 km thick stack of thrust sheets which originated from the deformation of the Neotethys and of the Africa continental palaeo-margin (CATALANO et alii, 2000). The deeper thrust system (Pelagian Sicilian Thrust Belt, FINETTI et alii, 2005) has structured since the Late Miocene-Early Pliocene, when collisional processes have involved in the thrust system the northernmost margin of the Pelagian Block. Compression mainly involved carbonate successions and produced a ~10 Km thick deep-seated imbricate thrusts, locally outcropping (e.g. Montagna Grande). The south-eastward deep thrust migration was accompanied by large antiformal folding of the tectonic units previously structured at the upper level and passively transported, including satellite sedimentary basins (e.g. the upper Miocene Castelvetrano Basin).
According to previous authors (MONACO et alii, 1996, CATALANO et alii, 1998) the deep-seated ramp thrusts of the Pelagian Sicilian Thrust Belt are part of a unique regional system which deforms the northernmost margin of the Pelagian block. At shallower crustal levels, ramp-thrusts splay into a series of minor contractional structures that exhibit a complicate geometries. This regional system has been incorporated by Lavecchia et al. (2007) in a geodynamic model that attributes to the active thrust belt (the Sicilian Basal Thrust, SBT) the role of single seismogenic source of western Sicily.
With the aim of better define the strain field that currently characterizes the study area, geodetic analysis, including interferometric (DInSAR and StaMPS) and GPS data, has been performed. The integration of these techniques allowed to define the ground motion vectors, in particular the vertical and horizontal components, respectively.
The DInSAR analysis has been conducted by using the "two steps" technique on a number of tracks (444 and 308 respectively) consisting of SAR images (Synthetic Aperture Radar) acquired between 2003 and 2010, both in ascending and descending orbit. The analysis was performed on short and long time intervals between two acquisitions and have allowed to detect a differential ground motion along a NNE - SSW alignment between the towns of Castelvetrano and Campobello di Mazara (CCA), just west of the macroseismic area of the 1968 seismic sequence. The StaMPS consists of a multi-interferogram analysis that was performed in order to obtain a more comprehensive map showing the average ground velocity. This technique, with millimeter accuracy, is based on the analysis of time series satellite images. The obtained results display a differential ground motion of 2 mm/yr along the Line Of Sight (LOS), in correspondence of the CCA.
In order to quantify the horizontal component of the geodetic deformation, a GPS measurement survey has been carried out by reoccupying seven IGM 1995 benchmarks (SEL1, SEL2, FGRA, BCMA, TLIP, MGRA e MGAI), two of which (SEL1 and BCMA) fall across the CCA. GPS data were acquired in a continuous modality (5-13 hours) by using Leica GX 1220 receivers and AR10 antennas. Measurements showed a ~ 0.6 mm/yr of geodetic contractional strain in a NW-SE direction which is tranversal to the CCA alignment. Although the confidence ellipses of the GPS vectors are relatively large, it is possible to compare magnitude and direction of the velocity vectors, that show compressional along the CCA.
The morphostructural analysis was carried out on field and in laboratory and allowed to associate the geodetic data to a topographic offset of Lower Pleistocene marine sediments. Morphometric analysis, performed on a 2x2 cell size DEM, and field data showed the occurrence of a morphological ridge (parallel to the CCA), characterized by a scarp dipping at 10-15° to the SE on its southeastern flank. Morphostructural analyses evidenced the occurrence of an anticline related to the Pleistocene activity of the Bank Adventure thrust system in the on-land sector here named Granitola-Castelvetrano Thrust System (GCTS)
Along the GCTS, three morphological segments have been distinguished: CCA (Castelvetrano-Campobello di Mazara), S (Straglio) and SPG (Straglio-Punta Granitola) respectively. They show variable lengths (between 2 and 12km) and trends (NNE-SSW the CCA and SPG and ~E-W the S). The execution of a series of transversal topographic profiles across these segments allowed to detect an irregular trend of the scarp. Except for the more regular S segment, here interpreted as a relay zone, the variation in height of the scarp along the other segments has been related to the growth of distinct folds bounding fault segments.
The discovery of dislocated archaeological remains on the hanging-wall of GCTS (CCA segment) provided the first evidence of probable coseismic deformation, related to a seismogenic source in the area. An ancient road of Early Bronze to Greek age, is offset by a contractional structure. This structure (named CTF, "Carraia Thrust Fault") is a N30E trending, slightly oblique (dextral) reverse fault, dipping at 50-60° to the SE and displacing the road by about 5cm; inversion of meso-structural data collected on the CTF allowed to estimate a N110°E orientation for the compression axis. Partially dissimilar results were obtained by the analysis of fractures found in the archaeological site, which suggested a compression axis oriented N170° E (normal to the transverse fractures). The deformation along the CTF also involves the asphalt of the modern road that runs alongside the archaeological remains. It is manifested by fractures oriented NNE-SSW showing a slight reverse component of motion, similar to the faults of the substratum, making it plausible that the deformation continues today, likely as creep.
Field data were integrated by high resolution marine surveys consisting of single-channel Sparker profiles acquired transversally to the possible offshore prosecution of the GCTS, the northernmost part of the ATF (Adventure Thrust Front, ARGNANI et alii, 1986). A preliminary interpretation of seismic data, calibrated by logs of exploratory wells, evidenced contractional deformation in lower - middle Pleistocene sedimentary units, caused by the propagation of SE-verging reverse faults that affect also the sea floor. Along the offshore prosecution of the morphological high, a structural high involving Late Pleistocene- Holocene sediments has been detected. These transgressive deposits, formed during the highstand of the last sea level rise, rapidly deepen towards the southeast, while they become very thin towards the northwest along a continental shelf, suggesting an uplift of this sector during the late Quaternary. The ascent of fluids and the presence of bioconstructions, arranged in groups oriented N45°E, on the continental shelf, also suggests their close relationship with active faults.
In order to better characterize the GCTS in the seismotectonic framework of south- western Sicily, the analysis was extended north and east of the GCTS, within the epicentral area of the Belice earthquake sequence. A morphometric analysis was carried out in the sector located north of the GCTS, on 19 drainage basins which flow into a main river (Fiume Freddo). Some drainage basins showed a convex-shaped hypsometric curve testifying a possible morphological variation controlled by recent tectonics. In addition, some geometric anomalies in stream pattern have been observed on the NW flank of the Belice syncline where an interferometry anomaly (StaMPS) also occurs. Here, the morphometric anomaly depicts a curved-shaped lineament that extends for about 10 km from the sector to the south of Partanna village as far as the ruins of Gibellina (Partanna-Gibellina, PG lineament). It is characterized by a SW-NE direction along the Belice river valley and a NNW-SSE orientation on the northwest flank of the Belice syncline. Although this anomaly shows a minor continuity with respect to the CCA, it matches with a region where a significant accumulation of strain occurs. In addition, the proximity to the epicentral zone of the 1968 earthquake suggests that it could be the expression of recent deformation related to the seimogenic structure.
The multidisciplinary approach has provided interesting results which also contributes to improve the knowledge about the area which was the most damaged by the 1968 earthquake sequence. Accordingly, another possible active segment (the Castelvetrano-Garcia Thrust System, CGTS), similar to GCTS, is inferred to occur here.
The CGTS shows a N88°E direction and a total length of about 25km. Similarly to the GCTS, it can be subdivided into two segments (CP, Castelvetrano-Partanna and GLG, Gibellina- Garcia Lake), on the basis of different morphological features.
The discovery of deformed ancient and modern artifacts along the CP and GLG segments allowed to define contractional kinematics and boundaries of the CGTS. Along the CP segment, northeast of Castelvetrano, an ancient aqueduct is slightly deformed. Along the GLG segment, a concrete side-wall of the road by-passing the Garcia lake (about 5 km northeast of the 1968 main shock) is aseismically deformed by a pop-up structure. These findings testify the ongoing compressive regime during the inter-seismic or post-seismic period of the Belice sequence. Further, SAR and fluvial anomalies are concentrated between the CP and GLG segments, in the vicinity of Mt. Porcello where, according to Michetti et al. (1995), coseismic ruptures occurred during the 1968 earthquake. This sector, here named MPRZ (Monte Porcello relay zone), and the zone of plastic deformation near the Garcia Lake delimit, as a whole, the sector of strong seismic release of the 1968 earthquake, also representing the outer boundaries of the areas where interferometry data indicate the largest strain accumulation.
The integration of the whole dataset confirms the hypothesis that oblique thrusting along blind structures and surface folding currently occur in response to a NW-SE oriented compression. The morphostructural and morphometric analyses suggest, in particular for the GCTS and for the Castelvetrano ridge, a model of thrust propagation fold. Slip movements could occur along a deep-seated crustal ramp that at shallow crustal levels assumes a complex geometry, splaying into a series of minor reverse faults responsible for most superficial deformation and for growth of fold structures.
The presented results have tentatively allowed the preliminary parameterization of the GCTS and, approximately, of the CGTS (1968 epicentral area). As a whole, the thrust system, here named SOST (Sud-Ovest Sicilia Thrust) is a first-order structure in the deformation context of western Sicily. It corresponds to the basal thrust of Lavecchia et al. (2007), with its ENE-WSW trending eastern sector corresponding to the thrust system inferred by Monaco et al. (1996) as responsible for the 1968 earthquake.
The SOST appears as a segmented structure and the overall activation during a single seismic event seems to be mechanically difficult because of the strong orientation change between the GCTS and the CGTS. Using the empirical relationships of Wells and Coppersmith (1994), values between 6 and 6.3 of moment magnitude equivalent (Meq) have been estimated in the case that single segments were activated. If the eastern part of CGTS (GLG and east part of CP segments) was activated during the Belice event, we estimate that about half (~ 5km) of the segments could have generated coseismic rupture. The estimated Meq (Meq = 5.9) is, in fact, compatible with the Mw registered during the Belice main shock (Mw = 5.9). If the GCTS and CGTS were completely activated, Meq values of 6.5 and 6.7, respectively, could be estimated. These observation, together with the evidence of continuous creep deformation both in the post-seismic period of the 1968 event and during the intersismic activity of the GCTS, the occurrence of an earthquakes with M> 6-6.3 appears unlikely.
Even though it is not immediate the relation between the GCTS and the historical earthquakes that have destroyed the Greek city of Selinunte (Guidoboni et al, 2002; BOTTARI et al, 2009) and between the CGTS and the Belice earthquake sequence, the results of this study must be considered in evaluation of the seismic hazard of this densely populated area of Sicily
