Stress drop at the Kephalonia Transform Zone estimated from the 2014 seismic sequence

The Kephalonia Transform Zone (KTZ) is a seismically active dextral transform fault decoupling the extensional deformed area of the Ionian Abyssal Plain and the compressional deformed area of the Mediterranean Ridge. We estimate a prominent steady state strain rate of 225 +/- 20 nstrain/year across the KTZ from the mean velocities of permanent Global Navigation Satellite System (GNSS) stations in East Italy and West Greece, which confirms previous estimates. Based on the regional statistical seismicity and using the local Gutenberg-Richter relation we investigate the energetic balance between stress accumulated as a consequence of the continuous dextral shear deformation, and the average stress released by shallow seismicity. If the maximum expected magnitude is M-w = 7.4, the estimated a and b parameters of the local Gutenberg-Richter relation and the geodetically determined shear-strain rate set an upper limit to the regional stress drop Delta sigma <= 0.4 MPa. We verify these values by analyzing a seismic sequence on the KTZ between January and February 2014, which culminated in the M-w = 6.09 event of Jan. 26, and the M-w = 6.14 event of Feb. 3. The estimated epicenters lie within a few kilometers from each other, on the western side of the Kephalonia Island, at a hypocentral depth <= 10 km. The measured coseismic displacements of GNSS stations in this area fit the expected surface dislocation, which can be predicted for an elastic half-space using the measured fault-plane solutions as input. If the hypocenters of the aftershocks are taken as indicator of optimal dextral shear-stress orientation, then a low regional deviatoric stress of 0.4 MPa, that is comparable with the maximum regional stress drop estimated above, is required for the Coulomb stress to match the pattern of the aftershocks of the 2014 sequence. As a consequence, we conclude that the regional deviatoric stress and the seismically released shear stress are in close balance in the KTZ seismic province. (C) 2015 Elsevier B.V. All rights reserved.