INFLUENCE OF PULL-APART BASINS ON RUPTURE MECHANICS OF THE AUGUST 17, 1999 KOCAELI EARTHQUAKE

The August 17, 1999 Kocaeli earthquake broke four distinct fault segments, from west to east, the Gölcük segment (29 km long), Sapanca segment (26 km), Sakarya segment (36 km), and Karadere segment (35 km) for a total fault rupture of 126 km. Fault rupture was nearly pure strike-slip with 1 to 1.5 m on the Karadere segment, up to 4 m on the Sapanca segment, and up to 5.5 m on the Sakarya and Gölcük segments. Minor displacements also were observed on the adjoining Yalova fault segment to the west and Düzce segment to the east. Each of the fault segments are bounded by distinct right, en-echelon stepovers and/or a rupture gap in the fault trace. From east to west, these stepover and rupture gaps include: Eften Lake stepover, Akayazi gap and restraining bend, Lake Sapanca stepover basin, Gölcük stepover, and Karamursel stepover. Distinct structural basins have formed within these right-releasing stepovers and the basins are infilled with thick Quaternary deposits, indicating that they are long-lived features in the displacement history of the fault. The Karamursel stepover is 5 km wide by 16 km long and arrested 4.5 to 5.5 m of rupture on the Gölcük segment. The Gölcük and Sapanca stepovers are each about 1 to 2 km wide and 6 to 9 km long. Basins typically are bounded on the ends by normal faults. Rupture of 3 to 5 m broke through each of these basins. The Eften Lake basin is about 2 to 4 km wide by up to 9 km along, and arrested 1 to 1.5 m of rupture on the Karadere fault segment. These observations provide important empirical data on the size of stepovers that can and cannot arrest fault rupture of different amounts. This information is valuable to develop and evaluate segmentation models for strike slip faults worldwide, and locally to forecast the location and size of future earthquakes along the North Anatolian fault.