On 17 August 1999, a catastrophic magnitude 7.6 earthquake struck the Kocaeli Province of Turkey, causing monumental damage and between 17,127 and 18,373 deaths. Named for the quake's proximity to the northwestern city of İzmit, the earthquake is also commonly referred to as the 17 August Earthquake or the 1999 Gölcük Earthquake. The earthquake occurred at 03:01 local time (00:01 UTC) at a shallow depth of 15 km. A maximum Mercalli intensity of X (Extreme) was observed. The earthquake lasted for 37 seconds, causing seismic damage and is widely remembered as one of the deadliest natural disasters in modern Turkish history.
The 1999 earthquake was part of a seismic sequence along the North Anatolian Fault that started in 1939, causing large earthquakes that moved progressively from east to west over a period of 60 years. The earthquake encouraged the establishment of a so-called earthquake tax aimed at providing assistance to the ones affected by the earthquake.
Tectonic setting
The North Anatolian Fault Zone, where the earthquake occurred, is a right-lateral strike-slip fault zone. It extends from the Gulf of Saros to Karlıova. It formed around 13—11 million years ago in the eastern part of Anatolia and developed westwards. The fault eventually developed at the Marmara Sea around 200,000 years ago despite the shear-related movement in a rather broad zone which had already started in late Miocene.
The fault zone has a diverse geomorphological structure and is seismically active. It produced a series of earthquakes as large as 8.0 on the moment magnitude scale. Since the 17th century, it has shown cyclical behavior, with century-long large earthquake cycles beginning in the east and continuing westward. Although the record is less clear for earlier times, active seismicity could still inferred in that timespan. The 20th century earthquake record has been interpreted as where every earthquake concentrates the stress at the western tips of the ruptured areas leading to westward migration of larger earthquakes. The İzmit and November 12, 1999 events increased stress on the Marmara segment of the fault. An earthquake of up to magnitude 7.6 event was expected between 2005 and 2055 with a probability of 50 percent on this segment. Presently, the deformation of rocks by stress in the Marmara Sea region is asymmetric. This is conditioned by the regional geology and is believed to be such the case for most of the NAFZ.
Earthquake
The 17 August 1999 earthquake was the 7th in a sequence of westward-migrating seismic sequence along the NAFZ. 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 0.45 g. The earthquake lasted 35–45 seconds according to various sources. The closest cities affected 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 earthquake caused a surface rupture comprising four segments; the Hersek/Karamürsel–Gölcük, İzmit–Lake Sapanca, Lake Sapanca–Akyazı, Akyazı–Gölyaka and Gölyaka–Düzce segments. These segments altogether measured over 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.