Ground Deformation Study of Broader Area of Patras Gulf (W. Greece) using PSI-WAP, DGPS and Seismicity Analyses
Sakkas, Vassilis1; Adam, Nico2; Voulgaris, Nikolaos1; Vassilopoulou, Spyridoula1; Lagios, Evangelos1; Stramondo, Salvatore3
1University of Athens, Dept. Geophysics & Geothermics, GREECE; 2DLR, GERMANY; 3INGV, ITALY
The majority of the seismic energy release of the Eastern Mediterranean is located in the Hellenic territory along the Hellenic Arc - Trench System (HATS) due to the subduction zone of the African Plate beneath the Aegean microplate. The broader area of Patras Gulf is an asymmetric rift being consisted of WNW-ESE trending normal faults of listric shape as a result of the tensional stress regime. The area is characterized by intense seismic activity with a large number of earthquakes of magnitude M>5 occurred in the vicinity of the area during the last 20 years (Patras Earthquake (iw= 5.4) in 1993, Aegion Earthquake (Mw=6.4) in 1995, Vartholomio Earthquake (Mw=5.6) in 2002, and the Andravida Earthquake (iw= 6.4) in 2008). The focal mechanisms of those last three major earthquakes have shown a NE-SW trending zone that crosses the area. Several others events have also occurred adjacent to the Patras Gulf and mainly along the Cephalonia Transform Fault and south of Zakynthos Island on the west. A prominent feature of the seismicity distribution is the absence of seismic activity in the marine area of Patras Gulf when compared to the broader area. The absence of seismicity may be indicative of the lack of major faulting systems in the area, or at least active faults in the area. It has to be noticed that the submarine area has not been fully mapped.
Long term combined ground deformation monitoring using the PSI-WAP technique and Differential GPS measurements together with the seismicity analysis could provide useful information about the tectonic regime of this complex area and lead to new insights on the development of this asymmetric rift. 56 descending ERS radar images were used to compile the PSI-WAP product, covering an area of about 11,200 km2 for the period November 1992 to October 2003. Initially, a local reference point was selected for the analysis and several distinctive clusters were processed. On a second processing attempt, the PSI-WAP data have been calibrated using GPS results from available stations in the area, and the absolute velocity field has been obtained. However, since the area exhibits a strong SW motion (with respect to Europe), a negative component (motion away from the descending satellite) has been added to the LOS velocity field causing a decrease in the final estimated LOS velocities.
The deformation of the southern part of Patras Gulf (as was deduced with respect to a local point) has been characterized by considerable subsidence varying from -0.5 to -2.0 mm/yr near the coastline, while locally exceeding -4 mm/yr. These PS points describe areas covered by alluvia deposits, as well as marine deposits. Significant subsidence has also been identified in areas along the down-throw side of possible faults, as well as areas where extensive water pumping is usually taking place for irrigation purpose. The City of Patras is characterized by a differentiated motion across and along an identified SW-NE trending fault, its northern part of which subsides, while its southern part appears to be stable (-0.5 to +0.5 mm/yr) or showing locally a slight uplift (up to +2 mm/yr). Since the local reference point lies on the Holocene deposits and the down-thrown side of the fault, the observed subsidence seems to be underestimated. Further to the south, where mainly limestone and flysch formations exist, stability has been observed while small alterations from uplift to subsidence is taking place across local tectonic zones clearly depicting several known and unknown local faults.
The northern coast of Patras Gulf is characterized by moderate subsidence (>-2 mm/yr) mainly taking place along the area opposite of Aegion, which might be have been affected by the Aegion Earthquake (M=6.4). Areas with alluvia and deposit formations appeared to subside with higher velocities compared to adjacent areas of limestone formations. The ground deformation in this part should rather be attributed to local geological and tectonic features, including locally some human activity (water pumping for irrigation). However, at the southern part of Patras Gulf, the strong seismicity that is taking place should rather be contributing to the observed ground deformation affecting mainly the coastal areas, where unconsolidated sediments usually prevail. This argument is supported by the damages caused in the aforementioned areas by the occurrence of the Andravida Earthquake, well after the spanning period of the WAP.
The analysis of the GPS data from the local network showed that the regional ground deformation that is taking place in the broader area had an ENE to NE direction, and a rate of 10.6-22.2 mm/yr at the northern part of the gulf (with respect to ITRF2000). The southern part exhibited an ESE to SE oriented displacement of 6.3-11.6 mm/yr, while uplift was generally observed relating to the vertical component. The uplift rate was about 3 mm/yr in the northern part of Patras Gulf, while in the southern part the uplift was higher ranging from 5.9-12.1 mm/yr. A distinct differential motion between the northern and southern part of Patras Gulf is observed from these results describing the “opening up” of the gulf. The absence of seismic activity in the marine area of Patras Gulf shows that the N-S extension that takes place in the area is of an aseismic/plastic character on a soft soil environment, and the regional uplift should likely be caused by the upper plate deformation due to plate convergence in the NW Peloponnese.
The horizontal strain-rate field of the area shows that the extension which is taking place in the western part of Patras Gulf gradually decreases toward its eastern part. The eastward decreasing extension of the Patras Gulf is consistent with previous research, and it should be attributed to the counter-clockwise rotation of the Peloponnese relative to the mainland of Greece, around a pole located in the Saronic Gulf south of Athens, as has been described by several researchers.