SEISMIC OBSERVATIONS AT AN OVERSTEP OF THE WESTERN NORTH ANATOLIAN FAULT (ABANT-SAPANCA REGION, TURKEY)

The course of the active North Anatolian Fault system from Lake Abant to Lake Sapanca was traced by its high micro-earthquake activity. If a pproaching from the east this section includes a broad south to north overstep (fault offset) of the main fault. Local seismicity has been r ecorded in this area by a semi-permanent network of 8 stations since 1 985 within the frame of the Turkish-German Joint Project for Earthquak e Research. The effect of the overstep and its complex fracture kinema tics are reflected by the seismicity distribution, the variations of c omposite fault-plane solutions, and by the spatial coda-Q distribution . Areas of different stress orientation can be distinguished and assig ned to different groups of faults. The stresses and the tectonic patte rn only in part correspond to a simple model of an extensional overste p and its correlative pull-apart basin. Other types of deformation inv olved are characterized by normal faulting on faults parallel to the g eneral course of the main strike-slip fault and by synthetic strike-sl ip faults oriented similar to Riedel shears. Shear deformation by this fault group widely distributed in an area north and east of the main fault line may play an important role in the evolution of the overstep . The development of a pull-apart basin is inhibited along the eastern half of the overstep and compatibility of both strands of the main fa ult (Bolu-Lake Abant and Lake Sapanca-Izmit-Marmara Sea) seems to be a chieved with the aid of the fault systems mentioned. The extension of the missing part of the pull-apart basin seems to be displaced to posi tions remote from the Lake Abant-Lake Sapanca main fault line, i.e. to the Akyazi-Duzce basin tract. Highest Q-values (lowest attenuation of seismic waves) were found in the zone of highest seismicity north and west of the overstep which is the zone of strongest horizontal tensio n. If high coda-Q is an indicator for strong scattering of seismic wav es it might be related to extensional opening of fractures.