Combined PSI And Differential GPS Study Of Zakynthos Island (W. Greece) For The Period 1992-2012

Ground deformation studies based on Differential GPS (DGPS) measurements and Permanent Scatterers InSAR (PSI) analysis using ERS and ENVISAT radar data have been conducted on the island of Zakynthos (Western Greece) covering the period 1992 to 2012. These results were compared, validated and integrated with geological, geotectonic and seismological data aiming to better understanding the pre-earthquake deformation process, as well as the behavior of the present tectonic regime, in order to lead to a more effective and standardized approach to monitor the hazard and risk in this seismically very active region of Western Greece. PSI results for Zakynthos Island indicate that a slight uplift (1-1.5 mm/yr) had occurred along the extreme western part of the island, while a moderate subsidence (about -2mm/yr) took place at its southwestern part. Ground differential motion along faulting zones was apparent in the PSI-derived velocity field, verifying the multi-fragmented character of the western part. The occurrence of the seismic outbreak that took place offshore to the south of Zakynthos during 2005-2006 may have contributed to the different deformational pattern as revealed by the ERS and ENVISAT PSI products covering the periods 1992-2000 and 20032010, respectively, as well as the strong opening of the southern part of the island as deduced by the DGPS measurements. 1. BRIEF GEOΤΕCTONIC SETTING The area of Western Greece, and in particular the central Ionian islands, plays an important role in the kinematic processes of the eastern Mediterranean, since it lies within a seismotectonically very active and complex region that is undergoing rapid and intense ground deformation. The highest seismic activity in Europe currently takes place in that region, constituting part of the Eastern Mediterranean lithosphere that is subducting beneath the Aegean lithosphere along the Hellenic Arc. The subduction zone terminates against the Cephallonia Transform Fault (CTF) – a major strike-slip fault that links the subduction boundary to the continental collision between the Apulian microplate and the Hellenic foreland [1], and plays a key role in the regions geodynamic complexity [2], [3], [4]. The CTF lies offshore to the west of Cephallonia in an area with a deep bathymetric trough (water depths in excess of 3000 m) that strikes N20E. Its slip-rate varies from 7 to 30 mm/yr based on DGPS measurements [5], [6] which is consistent with seismological data [7]. The phenomena of active subduction and continental collision are responsible for earthquakes produced by compressional stress along the western coasts of Greece and Albania, as well as the convex side of the Hellenic arc. Zakynthos Island consists of rocks of two isopic zones, the Pre-Apulian or Paxos Zone and the Ionian Zone (Fig. 1). Both of these zones lie to the west of the Pindos thrust and the boundary between them is the Ionian thrust which is generally considered to represent the most external Hellenide structure [8]. This thrust is not well exposed in Zakynthos, because the absence of Mesozoic carbonates and the occurrence of a thick Pliocene cover make the boundary between these zones difficult to define. The Pre-Apulian or Paxos zone is characterized by Upper Cretaceous to Miocene sediments (limestones, marly limestones etc.), whereas the Ionian zone, which appears in the southeastern part of the island, consists of limestones and evaporitic rocks of Triassic age. The post Alpine deposits are marls of Pliocene age and Quaternary marine and continental formations [2],[8]. Zakynthos Island is separated from mainland Greece by an extensional basin, which is referred to in the literature as the Zakynthos Valley/Canyon system and is located between the coastline of Western Peloponnesus and the island, in the vicinity where many earthquake epicenters have occurred over the past decades. This system is characterized as a structural basin trending parallel to the local tectonic zones [9]. The Zakynthos Valley/Canyon system is thought to have been formed during the middle Pleistocene – Quaternary and is controlled by salt tectonics and compressional tectonics that are associated with the subduction along the Hellenic Arc [9]. In response to these regional tectonic activities, subsidence or uplift has affected large parts of Zakynthos’ coastline during historical times. Considering all the above mentioned phenomena it can be clearly concluded that the broader region of Zakynthos is geologically and seismotectonically very complex and is key for a better understanding of the processes related to the collision between the African and Eurasian plates. Figure 1. Detailed Geological Map of Zakynthos Island, showing also the main faulting features (IGME, Geological Map of Greece, 1983) 2. THE GPS MEASUREMENTS The GPS network in Zakynthos was first installed in 2005 [10] (and re-measured in 2006 and 2012). Fourteen (14) stations were distributed around the island focusing mainly on the northern and the southern parts, which are the most fractured zones of the island. The data were analyzed with the Bernese GPS software v.5. 2 [11]. Final IGS (International GNSS Service) products (e.g. satellite orbits and clocks) were used to calculate daily coordinates and tropospheric parameters. Overall rms errors of about 1.0-5.3 mm and 2.0-8.1 mm for the horizontal and vertical components of displacement, respectively, were achieved for the majority of the stations (at a 90% confidence level). Station No. 73 was chosen as a local reference station because of its location on the center of the island in an alpine basement of the Paxos unit and thus, its anticipated better geological and tectonic stability as compared to other parts of the island. The station was operating continuously during all campaigns and tied to the ITRF2008 using the observational data from a number of IGS Reference Frame Stations in Europe (http://www.epncb.oma.be/). This station has a horizontal motion to the SSE direction with respect to ITRF2008 (VEast=9.1±1.4 mm/yr and VNorth=-9.2±1.3 mm/yr), while the vertical component is rather stable (VUp=-1.9±1.8 mm/yr). The overall behaviour of station No. 73 which is consistent with the anticipated regional motion [6], [10], [12] has to be particularly considered, since it represents the local reference point not only for the DGPS measurements, but also for the interferometric analysis. Fig. 2a presents the motion vectors with respect the ITRF2008 for the first re-measurement period 20052006. It is evident that a horizontal extension of the southern part of the island had occurred in the area around Laganas Bay which seems to be “opening”. This deformational pattern may be attributed to an increase of the seismicity in the area during that period. An earthquake Mw=5.6 occurred offshore to the south of the island on October 18, 2005; thereafter, a significant sequence of at least four earthquakes (Mw=5.5 to 5.7) occurred in the same region between April 4-12, 2006 [13], [14]. Analytically, in the southern part of the island the western part exhibited a southwestern motion with magnitude ranging from 15-20 mm, while its eastern part had magnitude of about 30 mm but towards the SSE. The central part of the island appears rather stable. The northern part, however, presents an inconsistent pattern with a westward motion that prevailed for most of the stations. The vertical deformation is expressed with uplift mostly in the southern part bounding the area of Laganas Bay, with values up to 60 mm in the western part. The extreme northern part had generally subsided by 12-30 mm. For the second re-measurement period (2006-2012) the horizontal deformation is consistent with the anticipated southwestern motion of the area with respect to the ITRF (Fig. 2b). However, in the northern half of the island the vectors show a westward motion (about 80 mm) while to the south a southwestern motion prevails (of the same amplitude). The vertical deformation shows significant subsidence for the whole of the island, with the higher amplitudes taking place to the south (more than -50mm), in the areas where the strongest uplift took place in the previous period (2005-2006). The later may indicate a rebound of the whole area to the state that existed before the outbreak of the seismic activity during 2006. The strong differentiation in the motion between the northern and the southern parts of the island, observed mainly in the first re-measuremnt period (2005-2006) but also in the second period (2006-2012), highlights the different kinematic status of the two areas as has been described in previous works [6], [10].