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 100 Hz on the delta showed a clear electric anisotropy, with a N55 degrees+/-10 degrees direction for the highest frequency. This anisotropy spans through the whole layer of sediments, about 1 km thick. Comparison of the 1966-1972 triangulations and the 1991-1995 GPS positions of geodetic points in and around the delta showed rapid extension strain (2x10(-6)/year) in the direction N340 degrees+/-30 degrees, perpendicular to the fast S and the highest conductivity directions, suggestive of a causal relationship between these observations. We thus propose that this strain controls the two reported anisotropies, by the formation and maintaining of fluid filled, steeply dipping antithetic faults and fractures, and fluid filled vertical cracks, all striking N55 degrees+/-20 degrees. The source of strain is likely to be active normal faulting near and under the delta, as independently evidenced by the recent discovery of nearby offshore faults, striking about N60 degrees, significantly different from the dominant E-W strike of the major faults of the Gulf.