SEISMIC AND ELECTRICAL ANISOTROPY IN THE MORNOS DELTA, GULF OF CORINTH, GREECE, AND ITS RELATIONSHIP WITH GPS STRAIN-MEASUREMENTS

Shear-wave splitting studies on seismograms from local earthquakes at the site of the Mornos delta (Gulf of Corinth, Greece) has revealed a clear seismic anisotropy, with a fast S polarization striking N55+/-25 degrees. Magneto-telluric soundings in the frequency range 0.01 to 10 0 Hz on the delta showed a clear electric anisotropy, with a N55 degre es+/-10 degrees direction for the highest frequency. This anisotropy s pans through the whole layer of sediments, about 1 km thick. Compariso n of the 1966-1972 triangulations and the 1991-1995 GPS positions of g eodetic points in and around the delta showed rapid extension strain ( 2x10(-6)/year) in the direction N340 degrees+/-30 degrees, perpendicul ar to the fast S and the highest conductivity directions, suggestive o f a causal relationship between these observations. We thus propose th at this strain controls the two reported anisotropies, by the formatio n and maintaining of fluid filled, steeply dipping antithetic faults a nd fractures, and fluid filled vertical cracks, all striking N55 degre es+/-20 degrees. The source of strain is likely to be active normal fa ulting near and under the delta, as independently evidenced by the rec ent discovery of nearby offshore faults, striking about N60 degrees, s ignificantly different from the dominant E-W strike of the major fault s of the Gulf.