We retrieve the moment tensors of 64 small to moderate sized events that occurred mostly beneath the Sea of Marmara using near-field data recorded at strong-motion and broad-band seismic stations. The near-field displacement records are inverted to their sources utilizing Kuge's method where the best fit between the synthetic and observed seismograms is achieved through searching a centroid moment tensor (CMT) point in a grid scheme. We also analyse the stress fields acting in the eastern and western parts of the Sea of Marmara by inverting the P - and T -axes of the focal mechanisms obtained. Significant biases in the stress tensors are obtained. The nearly horizontal maximum compressive axis sigma(1) in the western part is rotated 16degrees counter-clockwise compared with sigma(1) in the eastern part. The sigma(2) -axis is close to vertical (shear tectonic regime) in the east and the plunge of sigma(2)-axis in the west is 36degrees (transpressive tectonic regime). Changes in the sigma(3) -axis are also observed, that is, it is close to horizontal in the east and dips 49degrees in the west.
The spatial distribution of the focal mechanisms suggests that the stress field in the eastern part of the Sea of Marmara is homogenous compared with the western part, and we identify five distinct subsidiary faults. (1) a WNW-ESE-striking, right-lateral strike-slip fault located a few kilometres SW of the Princes' Islands, (2) a WSW-ENE-striking, right-lateral strike-slip fault named the Yalova-Hersek fault, (3) an E-W-striking normal fault located onshore between Yalova and Cinarcik, (4) a NNW-SSE-striking, left-lateral strike-slip fault located NE of the Princes' Islands and (5) minor thrust faults located in the Central High of the Sea of Marmara and in the vicinity of the Hersek Delta. The locations and the sense of motion of these five shear zones are explained by a very simple deformation model that requires a major E-W-striking right-lateral strike-slip fault, namely the North Anatolian Fault (NAF), within a stress field with maximum compression, sigma(1), in the NW-SE direction and minimum compression, sigma(3), in the NE-SW direction, as was derived from the stress tensor analysis. The mechanisms of the events occurring in the western part of the Sea of Marmara reveal a heterogeneous stress field that may result from the change in the strike of NAF from nearly E-W to WSW. The western Marmara Sea events are consistent with a deformation model that requires a major right-lateral strike-slip fault striking ENE-WSW with a stress field with maximum principal stress axis, sigma(1) , oriented ESE and minimum principal stress axis, sigma(3) , oriented NNE.