Pinar, A., K. Kuge, and Y. Honkura, Moment tensor inversion of recent small to moderate sized earthquakes: Implications for seismic hazard and active tectonics beneath the Sea of Marmara, Geophys. J. Int., 153, 133-145, 2003.

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 broadband seismic stations. The near-field displacement records are inverted to their sources utilizing the method of Kuge (1999) 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 s1 in the western part is rotated 16 degrees in counter-clockwise compared to s1 in the eastern part. The s2 axis is close to vertical (shear tectonic regime) in the east and the plunge of s2 axis in the west is 36 degrees (transpressive tectonic regime). Changes in the s3 axis are also observed, that is, it is close to horizontal in the east and dips 49 degrees 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 to the western part, and we identify five distinct subsidiary faults. (1) a WNW-ESE-striking, right-lateral strike- slip fault located a few km SW of the Princes¡Ç 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 C?narc?k, (4) a NNW-SSE- striking, left-lateral strike-slip fault located NE of the Princes¡Ç 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, s1, in the NW-SE direction and minimum compression, s3, 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, s1, oriented ESE and minimum principal stress axis, s3, oriented NNE.