Seismotectonics of western Anatolia: Regional stress orientation from geophysical and geological data

Available fault plane solutions of shallow earthquakes occurring in the central part of western Anatolia were reviewed in order to determine the stress directions active in this region. Data on 66 shocks were used for numerical analysis with the P- and T-dihedra method, some of these data being used in further determination of stress tensors through numerical methods. Despite uncertainties on depth determinations, results were found consistent, and in agreement with those of geological analyses independently carried out on recent and active faults. Selections from the data were adopted on the basis of areal distribution, magnitude and depth of earthquakes. Variations in stress axes orientation with depth seem significant and related to increasing influence of crustal anisotropy near the surface. The dominant trend of the present day extension in this region, as indicated by the analysis of available earthquake mechanisms, is NNE-SSW with a corresponding azimuth of the σ3 axis N25°E. The stress regime is extensional with dominant normal fault motions and increasing importance of strike-slip faults from south to north. This pattern of stress in western Turkey is consistent with kinematic reconstructions indicating a westward and southwestward motion of Anatolia and Aegea relative to Eurasia. © 1993.