Turkey was greatly affected by the earthquake disaster of Izmit, a 7.4–moment magnitude earthquake that occurred in the southeast Marmara region on August 17, 1999. A shallow seismic survey using a 0.65-lt air-gun was carried out in September 1999. The results of this investigation were compared with the conclusions obtained from seismic studies conducted along the same profiles in 1994 (using a 0.65-lt air-gun). The main objective of this research was to investigate structural and tectonic differences.
Izmit Bay is seismically under the control of the North Anatolian Fault. This regime developed within the former pull-apart structures. Seismic surveys reveal the presence of intensive gas formation in the Izmit Basin. The gas formation was overlaid by sediments in some areas of the basin and migrated from active fault fractures through the sea bottom. The penetration depth is rather shallow due to the acoustic masking effect of the gas formation. It is believed that the fractures caused by the 1999 Izmit earthquake will be reactivated along the existing tracks of the active fractures of the previous periods. It is accepted that the North Anatolian Fault was a single, deep, dextral strike-slip fault. This fault cuts the former pull-apart basins, which are the residuals of the former Thrace Basin. Occasionally, the Izmit segment of the North Anatolian Fault constitutes flower structures using some of the faults of the former pull-apart structures. It is suggested that this fracture, which was constituted with these alterations, continues toward the town of Ulaşli.