the region and it is believed that this is the case for most of the North Anatolian Fault Zone. Earthquake The 17 August 1999 earthquake was the 7th in a sequence of westward-migrating quake sequence on the North Anatolian Fault. This earthquake sequence began in 1939 and ruptured along a 1000-km part of the fault zone, with horizontal displacements of up to 7.5 m. The maximum observed ground motion was 0.45g. The earthquake lasted 35–45 seconds according to various sources. The closest cities damaged or affected by the earthquake were İzmit, Gölcük, Yalova, and Adapazarı, all located near the eastern end of the Marmara Sea, within the Gulf of İzmit. The earthquake also caused serious damage in Istanbul, especially in the district of Avcılar which is located in the western part of the city, around 70 km away from the epicenter. Despite the distance, it killed about 1,000 people in the district. The preliminary magnitude of the earthquake was 7.4 by the U.S. Geological Survey (USGS) and by the Kandilli Observatory, which operate in this country. The earthquake caused a surface rupture which consisted of four segments; Hersek/Karamürsel–Gölcük, İzmit–Lake Sapanca, Lake Sapanca–Akyazı, Akyazı–Gölyaka and Gölyaka–Düzce segments which in total, had length of more than 125 km. All the segments are separated by pull-apart stepovers of 1 to 4 km in width. The maximum offset throughout the rupture was measured on the Sapanca–Akyazı segment where the surface break displaced a road and a tree line by 5.2 m. It also showed pure strike-slip, and the fault plane is almost vertical in most of the places where a surface break was observed. Most of the major aftershocks (M>4) were located near Düzce, south of Adapazarı, in Sapanca, in İzmit, and the Çınarcık area. At Değirmendere, a small coastal town west of Gölcük, the rupture cut the edge of a fan delta where the center of the town was located, which caused a slump measuring 300 m long and 100 m wide, as a result a part of the town center slid under the water, including a hotel and several shops and restaurants. At another fan delta east of Gölcük, which is within the step-over area of the ruptures, the fault produced a 2 m-high normal fault scarp. Data was used from seven broadband stations as well as some other short-period stations across the area to calculate the regional moment tensor of the