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The Gulf of Corinth graben, is at present, the most active part of the Corinth rift. (Ambraseys and Jackson, 1990; Papazachos and Papazachou 1997; Papadopoulos 2000) All its seismic activity is concentrated in the underwater part along its main axis. Nine shallow earthquakes with M>=6,2R have been recorded since 1861. (Fig. 1.1) Geodetical surveys show that the average rate of extension is between 4 and 14mm/year during the last 100 years (Billiris et al 1991; Clarke et al 1997; Briole et al 2000).
High resolution seismic surveys have revealed that the gulf is a complex asymmetric graben whose geometry varies significantly along its length. (Fig. 3.1) (Stefatos et al 2002)
A detailed map of the offshore faults in the gulf shows that a major fault system of nine distinct faults limits the basin to the south, contributing to the uplifting of the southern margin of the gulf. (Stefatos et al 2002) (Fig. 1.1) The northern Gulf appears to be undergoing subsidence and is affected by an antithetic major fault system of eight faults. (Fig. 1.1) The faults along the southern margin dip to the north at an angle of about 50o, whilst the faults on the northern margin dip to the south at an angle of 50o. These major faults at the northern and southern margin usually define the limit between the upper slope and the base of slope, with the actual fault plane acting as part of the slope.
All these major faults have been active during the Quaternary. Growth fault patterns and thickening of the strata towards fault planes indicate that some of these faults on both sides of the graben are active even at present. (Fig. 3.2)
The faults lengths do not exceed 26km implying that the maximum magnitude of earthquakes that can occur in an individual fault is of 6,7R. Seismotectonic studies indicate that the overall morphology of the western and eastern part of the Gulf of Corinth graben, is mainly due to repeated earthquake on 30 to 50o north dipping normal faults along the southern border of the graben (Goldsworthy and Jackson 2001). Recent studies however, in the western part, have shown that the steeply north dipping normal faults root at a depth of about 10 to 12km at a low angle dipping (15o) detachment zone. (Doutsos and Poulimenos 1992, Sorel 2000 and Rigo et al 1996)
Fig. 3.1: Series of structural cross-sections in the Gulf of Corinth. The cross-sections are based on the interpretation of the seismic reflection profiles. Cross-sections A, B and C to the east exhibit the typical half graben geometry with the major fault lying to the south. Cross-section D and E show a more symmetric geometry while cross section F and G to the west suggest a shift in the polarity of the graben dip to the north.
Fig. 3.2: Airgun profiles and interpretative section across the western part of the Corinth Gulf showing that the northern and southern limits of the basin floor are fault controlled. The wedge shape of the basin fill sedimentary layers and the progressive decrease in stretal dip upwards at the hangingwall of both faults indicate continuous activity.
Ambraseys, N. and Jackson J. (1990). Seismicity and associated strain of central Greece between 1980-1988. Geophys. J. Intern. 101:663-708
Billiris H., Paradissis D., Veis G., England P., Featherstone W., Parsons B., Cross P., Rands P., Rayson M., Sellers P., Ashkenazi V., Davison M., Jackson J., Ambrseys N., 1991. Geodetic determination of tectonic deformation in central Greece from 1900 to 1988. Nature 350:124-129.
Briole P., Rigo A., Lyon-Caen H., Ruegg J.C., Papazissi K., Mitsakaki C., Balodimou A., Veis G., Hatzfeld D., Deschamps A. 2000. Active deformation of the Corinth rift, Greece: Result from repeated Global Positioning System surveys between 1990-1995. J. Geophys. Res. 105:605-625.
Clarke P., Davies R.R., England P.C., Parsons B.E., Billiris H., Paradissis D., Veis G., Denys P.H., Cross P.A., Ashkenazi V., Bingley R., 1997. Geodetic estimate of seismic hazard in the Gulf of Corinth. Geophys. Res. Letters 24:1303-1306
Goldsworthy M. and Jackson M. 2001. Migration of activity within normal fault systems. Examples from Quaternary of mainland Greece. J. of Structural Geology 23:489-506.
Papadopoulos G., 2000. Historical Earthquakes and Tsunamis in the Corinth rift, central Greece, publ. No 12 National Observatory of Athens. Institute of Geodynamics.
Papazachos B., and Papazachou, 1997. Earthquake in Greece. Publication Zitis, Thessaloniki
Rigo A., Lyon-Caen H., Armijo R., Deschamps A., Hatzfeld D., Makropoulos K., Papadimitriou P., Kassaras I., 1996. A microseismic study in the western part of the Gulf of Corinth (Greece): implications for large scale normal faulting mechanisms Geophys. Journal Intern. 126:663-688.
Stefatos A., Papatheodorou G., Ferentinos G., Leeder M. and Collier R. 2002. Seismic reflection imaging of active offshore faults in the Gulf of Corinth: their seismotectonic significance.Basin Research 14:487-502