Combined PSI and Differential GPS Study of Zakynthos Island (W. Greece) for the Period 1992-2012
Sakkas, Vassilis¹; Novali, Fabrizio²; Voulgaris, Nikolaos¹; Vassilopoulou, Spyridoula¹; Fumagalli, Alfio²; Allievi, Jacopo²; Lagios, Evangelos^1
1University of Athens, Dept of Geophysics & Geothermics, GREECE; ²Tele-Rilevamento Europa - T.R.E. slr., ITALY

The Central Ionian islands in Western Greece play an important role in the kinematic processes of the Eastern Mediterranean. They are situated within a very active seismotectonically complex area which is being 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 subducted beneath the Aegean Lithosphere along the Hellenic Arc. The subduction zone terminates against the Cephallonia Transform Fault, a major strike-slip fault that links the subduction boundary to the continental collision between the Apulian microplate and the Hellenic foreland, and plays a key role in the region's geodynamic complexity. Monitoring of the ground deformation is such an active zone helps determining a probable change in the tectonic stress field of the area. A combined study using Differential GPS and PSI techniques on Zakynthos (for the period 1992-2012) contributed to the understanding of the tectonic behavior and regime of the broader area, and revealed important information about the tectonic characteristics of the island.

29 ascending and 39 descending images from Zakynthos were used for the ERS PSI products for the period 1995-1999 and 1992-2000, respectively. The ENVISAT PSI product was based on 21 ASAR images covering the period 2003-2010, where more than 52,000 PS points were determined. The PSI velocity maps revealed a linear velocity history with small discrepancies from linearity only in the southern part of the island. The PSI results indicated that a slight uplift (1-1.5 mm/yr) was observed along the extreme western part of the island, while a moderate subsidence (about -2 mm/yr) was noticed at its southwestern part. Ground differential motion across faulting zones was apparent in the PSI velocity field, verifying the multi-fragmented character of the western part of Zakynthos.

Considering the descending ENVISAT velocity image, the PSI points were an order of magnitude lesser compared to the ERS images, mainly due to the smaller number of images used for the compilation of this product. In areas where a slight subsidence was taking place at the half western part of the island during the period of the ERS descending velocity field, the motion has changed to a slight prominent uplift (about 1.5 mm/yr). That change in the ground motion is more evident at the SW part of the island. The eastern part seems to maintain the same deformation pattern for both periods of the ERS and ENVISAT descending images. However, a stronger uplift is observed at the southern part (Laganas Bay), most probably due the seismicity outbreak offshore of Zakynthos during 2005-2006. The latter is supported also by the DGPS results in the area during the same period. It is evident from the Differential GPS measurements that a horizontal extension of the southern part of the island has occurred in the area around Laganas Bay which seems to be "opening". Its western part showed generally a westerly motion with magnitude ranging from 15-20 mm, while its eastern part had magnitude of 26 mm but toward the NNE. The central part of the island appears to be stable. The northern part, however, presents an inconsistent pattern emphasizing the highly fractured environment prevailing in this part of the island. The vertical deformation is expressed with uplift mostly in the southern part bounding the area of Laganas Bay and associated with the seismic unrest period of 2005-2006. The horizontal strain-rate field that was calculated has shown a clear differentiation between the northern and southern part of the island. Extension predominantly takes place in the south, while a slight compression occurs in the north, marking the different tectonic characteristics between the northern and southern part of the island. The former may be related to a possible large extensional deformation mechanism that is taken place along a N-S striking axis crossing the island. A recent remeasurement of the GPS network in 2012 has revealed a deformation image of smaller amplitude, and has highlighted some of the kinematic characteristics of the area that had been initially defined from the previous remeasurement period.