131218 - OGS.Atti.32 Vol.1.30.Indd 215 01/11/13 08:41 GNGTS 2013 Sessione 1.2
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GNGTS 2013 SEssIONE 1.2 ACTIVE TECTONICS OF SOUTH-EASTERN SICILY AS DEPICTED BY SEISMOLOGICAL AND GEODETIC OBSERvatIONS C. Musumeci, M. Palano, D. Patanè, L. Scarfì Istituto Nazionale di Geofisica e Vulcanologia, Osservatorio Etneo - Sezione di Catania, Italy Introduction. Southeastern Sicily, one of the most seismically hazardous zones in central Mediterranean area, experienced several historical destructive events such as the 1169 and 1693 earthquakes (MCS intensities of XI with estimated magnitudes of about 7 or higher; Boschi et al., 2000), and more recently, a ML = 5.4 earthquake occurred on December 13, 1990 about 10 km offshore (Amato et al., 1995). From a geological point of view, southeastern Sicily must be considered in the frame of the complex tectonic features of the Mediterranean basin, which is dominated by the ~N20°W Neogene-Quaternary convergence between Nubian plate (the African plate west of the East African Rift) and Eurasia, occurred in the last 100 million years (Faccenna et al., 2001). It is mostly formed by the foreland domain (the Hyblean Plateau; Fig. 1a), a crustal unit located at the northeastern end of the mostly submerged Pelagian Block extending up to Sicily Channel, the Maltese islands and Tunisia (e.g. Malinverno and Ryan, 1986; Boccaletti et al., 1990; Patacca et al., 1990). The Sicilian foreland consists of a thick Meso-Cenozoic sequence of carbonate sediments which are underlain by a continental basement of unknown age (Yellin-Dror et al., 1997). Eastward, the foreland is bounded by the NNW-SSE-striking Hybleo-Maltese Escarpment Fault System (HMEFS; see Fig. 1a), a Mesozoic lithospheric boundary separating the Ionian oceanic basin from the thick Pelagian Block continental crust (Nicolich et al., 2000). The northern and western margins of the Hyblean Plateau are characterized by an older extensional belt (named Gela-Catania Foredeep), that is downbent by a NE-SW fault system under the front of the northern imbricate mountain chain (Appennine-Maghrebian Chain or Maghrebian Thrust Belt; Yellin-Dror et al., 1997). The frontal thrust belt was considered locked since the Middle Pleistocene (Butler et al., 1992; Lickorish et al., 1999; Tortorici et al., 2001), but recent studies, based on geological (Bousquet and Lanzafame, 2004; Catalano et al., 2008), seismolological (Lavecchia et al., 2007; Visini et al., 2009) and geodetic (see Palano et al., 2012 for an overview) observations have detected the occurrence of active contraction. Another main tectonic alignment is a 70 km long fault system (named as Scicli line or Scicli- Ragusa Fault System, SRFS in Fig. 1b: Ghisetti and Vezzani, 1980, Grasso and Reuther, 1988) extending along the main direction striking about N-S and the conjugated structures NE-SW from northeast to southwest of the western district of the foreland. Despite the recent advances achieved through geological, seismological and geodetic studies (among others Scarfì et al., 2007; Catalano et al., 2008; Palano et al., 2012), the geodynamic processes and the tectonic regime of southern Sicily are far from being unanimously described. Several aspects related to the geometry, type and contributions of each fault and its seismogenic role have still not been satisfactorily explored and there is little consensus regarding the location and geometry of the faults causing major earthquakes. In this work, exploiting local monitoring network high-quality data, we performed an in- depth analysis of the ongoing tectonics of southeastern Sicily, between the outermost front of 215 131218 - OGS.Atti.32_vol.1.30.indd 215 01/11/13 08:41 GNGTS 2013 SEssIONE 1.2 the chain and the Ionian offshore, through the integration of seismic and GPS-based geodetic observations, collected in nearly two decades. In particular, we carried out a simultaneous inversion of both the three-dimensional velocity structure and the distribution of seismic foci to define the geometry of seismogenic structures responsible of this seismicity. The computation of fault plane solution for the major and best recorded earthquakes was of great importance for the characterization of seismogenic sources and the orientation of the crustal stress field acting in the area. The comparison of these findings with the GPS-based ground deformation will help to improve the knowledge of the investigated area. Seismic data. About 1020 small-to moderate-magnitude earthquakes (1.0 ≤ M L ≤ 4.6), re- corded between 1994 and 2013 by a local network, were selected from the “Catalogo dei ter- remoti della Sicilia Orientale - Ca- labria Meridionale. INGV, Catania” (Gruppo Analisi Dati Sismici, 2013). In particular, the events have been employed as data source for simultaneous inversion of both a 3-D velocity structure and the hy- pocentre parameters by the code to- moDDPS (Zhang et al., 2009). With respect to more simple algorithms, this code has the ability to improve the accuracy of event locations and to sharpen the velocity images near the source region because of the combination of absolute and differ- ential arrival times. For the tomography, we filtered the initial dataset according to the quality criteria of location. In par- ticular, we selected only well lo- cated events, i.e. those with at least nine observations (P- and S-phases), root-mean-square (RMS) residuals smaller than 0.35 s and horizontal and vertical location errors lower than 2.5 km and 4.5 km, respectively. Finally, about 880 earthquakes with Fig. 1 – a) Simplified tectonic map of Sicily. b) Relocated seismicity (see text for further details). c) Vertical sections through the VP model. The traces of sections are report- ed in the map (b) with blue lines. Contour lines are at an interval of 0.25 km/s and the relocated earthquakes, within ±10 km from the sections are plotted as grey cir- cles. HMEFS, Hybleo-Maltese Escarpment Fault System; SLG, Scordia-Lentini Gra- ben; AB, Augusta basin; FB, Floridia ba- sin; SRFS, Scicli-Ragusa Fault System; AF, Alfeo seamount. 216 131218 - OGS.Atti.32_vol.1.30.indd 216 01/11/13 08:41 GNGTS 2013 SEssIONE 1.2 a total of 6131 P and 3668 S absolute arrival times and 52980 P and 30138 S catalogue-derived differential times. We performed the data inversion with an horizontal grid of 7x7 km node spacing, covering an area of 110 x 110 km, and a vertical step varying between 3 and 6 km, from the surface to 28 km of depth, following the one-dimensional reference velocity model of Musumeci et al. (2003). This mesh configuration allows to achieve a better spatial resolu - tion with respect the other tomographic studies carried out in the area, also ensuring a good over-determination factor (i.e. the ratio between known and unknown parameters). The finely tuned velocity distribution was used to relocate by TomoDDPS the whole dataset of events re- corded in the studied region. This provided an improvement in the quality of the final locations and an higher clustering of the earthquakes, which emphasizes some lineaments (Fig. 1b). As we can observe, tomographic images (Fig. 1c) reveal several sharp lateral velocity perturbations pointing out meaningful discontinuities in the crust which can be related to major tectonic structures. In particular, a low velocity anomaly was detected in the north- western sector (profile A), about corresponding with the Gela-Catania Foredeep. Location and geometry of this feature suggest that it may be related with the Hyblean Foreland crust bending beneath the outermost edifice of the Maghrebian Chain. Coastal and offshore zones (from Catania to Siracusa) and the western sector are also accompanied by a considerable velocity contrasts. These anomalies likely mark the known fault systems, striking about NW- SE, such as the HMEFS and those bounding the Augusta basin and the Scicli-Ragusa Fault System, respectively. Current results, compared to the previous tomographic inversion of Scarfì et al. (2007), obtained with a similar calculation code (TomoADD: Zhang and Thurber, 2005), maintain the fundamental characteristics but at the same time show a greater detail, this thanks to the larger amount of available data, which results in an higher density of seismic rays sampling the spatial mesh. Looking at the 3D event locations, to the north and northwest, earthquakes are distributed along the edge between the foreland and the chain, with depth increasing towards NW, down to about 40 km, to outline the flexure of the Hyblean crust below the thin-skinned outermost chain. Towards south, in the western district, the epicentres finely overlap to the Scicli-Ragusa Fig. 2 – Computed focal mechanisms. Red, strike-slip fault; blue, normal fault; black, inverse fault. Insets show the stereonets of the P and T-axes (a) and the orientations (stars) of the principal stress axes (b), with the 95% confidence limits (grey areas). 217 131218 - OGS.Atti.32_vol.1.30.indd 217 01/11/13 08:41 GNGTS 2013 SEssIONE 1.2 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.