Stress tensor inversion in Western Greece using earthquake focal mechanisms from the Kozani-Grevena 1995 seismic sequence

Abstract

Stress tensor inversion has been applied to estimate principal stress axes orientations in Western Greece, from 178 earthquake fault plane solutions from the Kozani-Grevena May 13, 1995 sequence. All focal mechanisms were previously determined through the deployment of a dense portable array. The magnitude range is 2.7-6.5 and the depth range is 4.0-15 km. A single stress tensor with an average misfit of 6.5°, small enough to support the assumption of stress homogeneity, can describe the stress field. The maximum compressive stress, s1, has a NNE-SSW trend (N26°E) and a nearly vertical plunge (80°) while the minimum compressive stress, s3, has a NNW-SSE orientation (N159°E) and a shallow plunge (7°) southwards. The scalar quantity, R (stress ratio) was found equal to 0.4 suggesting a transtensional regime (normal faulting with strike-slip motions) in which s2 is compressional. The identification of the fault plane from the auxiliary plane was achieved for 99 fault plane solutions out of 178 in total (56%). Vertical cross sections support previous results concerning the north dipping main fault segments and the south dipping antithetic faulting. The strike-slip motion is mainly dextral, along NNE-SSW structures, which follow the direction of the main neotectonic faults while the scarce sinistral strike-slip motion is connected to NW-SE trending zones of weakness pre-existing the old phase of compression in the Aegean. The strong strike slip motion that supports the transtensional regime probably reflects the effect of the motions of the North Anatolian Fault, taken up by normal faulting in the area of Western Greece.