EARTHQUAKE FOCAL MECHANISM AND STRESS TENSOR INVERSION ALONG THE CENTRAL SEGMENT OF THE NORTH ANATOLIAN FAULT

In this study, over hundred events with magnitude ≥ 3, which are recorded during the North Anatolian Passive Seismic Experiment (2005-2008), are relocated and focal mechanism determination, along with stress tensor inversion is carried out. Purpose of this study is to focus to the seismicity associated with relatively less-known central segment of the North Anatolian Fault (NAF).

In the region all relocated earthquakes have shallow focal depth, most occurred in the the uppermost 20 km and widely distributed suggesting a broad zone of deformation in the southern block. Focal mechanism determination is done by FOCMEC program, in which first motions of P- and S- waves are used along with SH/P wave amplitude ratios to obtain more unique solutions. Resultant 109 well-constrained focal mechanisms suggest a dominant strike-slip mechanism in the southern block that correlates with the splays of NAF. Conversely, normal and thrust mechanism solutions can be related to the secondary fault occurrences in the area. Focal mechanism solutions are further used for the determination of P- and T- axes orientations. Although overall stress filed is highly variable across the region, the orientations of P- and T- axes are in good agreement with the tectonic regime of NAF’s right lateral strike slip motion. The trends of the P- T- axes show noticeable variations along the main strand of NAF, which is consistent with its’ arc-shape geometry, whereas variations in the southern block is mainly characterized by NAF’s major splays.

In order to have better understating of the seismicity and its driving forces across the region, stress inversion is performed using well-constrained focal mechanisms. Two different inversion methods are applied, assuming that the stress in the study area is uniform and invariant in space and time. Both inversion methods resulted in same stress orientations: maximum principal stress, σ₁, is found to be subhorizontal striking NW-SE, the intermediate principle stress, σ₂, is vertically orientated and the minimum principal stress, σ₃, is found to be NE –SW striking. Substantial heterogeneity is observed due to the large misfit values, but calculated faulting style represents the characteristic strike-slip motion.