Anisotropy Studies in Central Greece

The Gulf of Corinth, located in Central Greece, is a tectonic graben characterized by high seismicity level. GPS measurements indicate extension of the Gulf in an approximately N-S direction, with a rate of 10 to 15 mm/year. The southern part of the Gulf is dominated by the presence of large active normal faults in an almost E-W direction, dipping north, resulting to the subsidence of the central part of the graben. Analysis of data recorded by the Cornet network, which is the permanent network of the University of Athens, revealed the existence of an anisotropic upper crust at the eastern part of the Gulf of Corinth. Anisotropy was also observed at the western part of Corinth Gulf, using data recorded by a temporary seismological network installed in the area. Furthermore, shear wave splitting analysis was performed in the region of Attica (to the NE of the Gulf) that hosts Athens, the capital of Greece, using aftershocks of the 1999 Athens earthquake (Mw=6.0) that caused 143 fatalities. The methods used for the determination of the splitting parameters are the polarization vector as a function of time (polarigram) and the hodogram. For each selected event the direction of polarization of the fast shear wave, the delay between the two split shear waves and the polarization of the source were measured. Concerning both parts of the Gulf of Corinth, the obtained mean values of anisotropy vary between N90° and N142°. In the region of Attica the mean values of the anisotropy direction of all stations vary between N95° and N100°, almost parallel to the azimuth of the Parnitha fault. The time delay between the split shear waves vary between 0.020s and 0.130s. The obtained anisotropy measurements are in agreement with the extensive dilatancy anisotropy (EDA) model, since the direction of anisotropy is independent from the event-station azimuth and perpendicular to the direction of extension. ACKNOWLEDGMENTS The present study was co-funded by the European Social Fund and National Resources - (EPEAEK II) PYTHAGORAS, contract No. 70/3/7306.