Seismicity and source parameters for small-scale earthquakes along the splays of the North Anatolian Fault (NAF) in the Marmara Sea

Orgulu G.

GEOPHYSICAL JOURNAL INTERNATIONAL, vol.184, no.1, pp.385-404, 2011 (SCI-Expanded) identifier identifier


Major discrepancy between present-day fault models in the Marmara Sea is closely related to structural development of depression areas beneath the sea. Therefore, in this study, the Marmara Sea and especially its major basins are investigated by means of recent seismological observations consisting of microseismicity (1.2 <= M(d) <= 3.6; 2002 May-2006 May), focal mechanisms (2.4 <= M(d) <= 5.0; 1999-2006) and stress analysis based on the focal mechanisms. In the Cinarcik Basin where either pull-apart model or negative flower structure is proposed, northeastern margin is represented by strike-slip sources having a NW alignment, probably corresponding to a cross-basin strike-slip fault developing in a matured pull-apart basin. This suggestion is directly relevant to experiments of sand box analogue models. These experiments indicate that such a cross-basin strike-slip fault is produced after the pull-apart basins attain their mature stage, eventually resulting in less vertical displacements attributed to normal faults bounding the basin. This experimental decline may be corroborated by the paucity of normal mechanisms in the basin. Extensional motion in this region is more influential outside of the basin, especially to the area where an onshore seismic activity is monitored on the land. On the other hand, another strike-slip activity in the Cinarcik Basin is distributed on a possible strand, parallel to the northern coastline of the Marmara Sea in the Armutlu Peninsula. This may indicate that one or more short strands of the northern splay extend to the west by bounding the basin from the south. In the Central Basin, the existence of a right-lateral strike-slip faulting on an east-west trending fault is supported by similar strike-slip focal mechanisms located at both extremities of the basin. To the west, dominant style of deformation gradually shifts from strike-slip faulting to transpression along the western coastline of the Marmara Sea where the northern splay joins with the offshore continuation of the Ganos Fault. Finally, focal mechanisms in the Marmara Sea are inverted for the best fitting stress tensor, characterized by a subvertical sigma(2)-axis and two subhorizontal stress axes of sigma(1) and sigma(3), oriented NW-SE and NE-SW, respectively. Conclusively, this study infers that the current tectonics of the Marmara Sea is mainly governed by a strike-slip regime, ascribed to both submarine splays of the NAF, and partly dominated by extensional and compressional features indicating shallow processes at local scales.