Developing Tools for Submarine Earthquake Geology Along the North Anatolia Fault Zone in the Marmara Sea, Turkey

The late Holocene behavior of the North Anatolia Fault beneath the Marmara Sea is being investigated by using two complimentary approaches. First, earthquake ruptures are resolved in time and space along small fault basins (equivalent to sag-ponds on land) and larger transform basins. Second, Holocene deformation and slip rates are quantified by using datable horizontal makers, such as shorelines and turbidites, and piercing points, such as offset channels and slumps. High-resolution subbottom profiling (CHIRP) and multibeam bathymetry acquired on the shelf highlight the linear trace of the Ganos segment of the NAF connecting `sag pond'-like basins. Such small basins along a fault trace are choice localities for earthquake geology because they subside and fill rapidly and can thus accumulate a complete and decipherable record of local fault ruptures. Their drainages are often very restricted and they are thus isolated from other distal submarine ruptures, as well as input from fluvial and alluvial fan that may include weather-related events. Sedimentation on the floor of the larger, deeper basins is punctuated by erosional events that removed thousands of years of sedimentation. Cores from these basins preserve homogenites linked by 14-C dating to the 740AD earthquake, whose epicenter has been located in Cinarcik Basin [Ambrassey and Finkel, 1995]. Homogenites are thick deposits of reworked mud that overlay finely laminated sands and were probably deposited by earthquake-induced seiche-like currents. A complete record of sedimentation of the larger transform basins can be extracted. However, it is critical to sample the precise depocenters because rapid tilting can drastically reduce the area of active deposition during high sea level and low turbidite flux. Depocenters may be eccentric because tilting oblique to the border fault is typical of transform basins. Vertical displacement has been documented along the Imrali Shelf and Izmit Gulf. In Imrali the horizontal marker is an undeformed sedimentary layer containing fresh water mollusks, diatoms and foraminiferal assemblages that document the lacustrine to marine transition of Marmara Sea at 12ky BP. A vertical displacement of 40m offsets this horizon. We are evaluating if the displacement resulted from motion along the Imrali Fault and slumping, or if the level of the Marmara Lake was much lower than the 85m Dardanelle sill at the time of marine flooding. These two alternate explanations could be distinguished with additional coring Initial findings demonstrate that submarine earthquake geology methods can characterize the seismic and neotectonic activity along fault segments and has the potential for extending the earthquake records further back in time than in paleoseimic land investigations. Critical data include very high resolution bathymetry, closely spaced grids of seismic profiles (less than50m apart), high precision coring, long cores in the deepest part of a basin, closely-spaced transects of cores, and very good chronology.