An earthquake of moment magnitude 7.6 struck Kocaeli Province, Turkey, on 17 August 1999. According to official figures, at least 18,373 people died and 48,901 people were injured during the earthquake, and 5,840 people were missing. At least 155 deaths were associated with the tsunami. The damage was estimated at between $12 billion and $20 billion (in 1999 U.S. dollars) according to various sources such as the World Bank. The earthquake was named for the epicenter's proximity to the northwestern city of İzmit. It occurred at 03:01 local time (00:01 UTC) at a shallow depth of 15 km (9.3 mi). 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 those affected by the earthquake, and the creation of National Disaster Insurance Institution for providing compulsory earthquake insurance.
The North Anatolian Fault Zone (NAFZ), where the earthquake occurred, is a 1,200 km (750 mi) 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 is seismically active and has a diverse geomorphological structure. 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 was likely to have occurred. The 20th century earthquake record has shown that the earthquakes concentrate 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 representative of most of the NAFZ.
The 17 August 1999 earthquake was the seventh 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 (25 ft).
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 of which are 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 (43 mi) 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 1–4 km (0.62–2.49 mi) 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 (17 ft). 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 (980 ft) long and 100 m (330 ft) wide. As a result, 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 mainshocks and larger aftershocks. As a result, most of the earthquakes were found to be split in segments, with the moment tensor's focal mechanism reading either a strike-slip on the fault which is west–east striking, or normal faulting which is between rupture segments. This also proves that the main characteristic of the quake is dextral strike-slip.
From the timing of P wave and S wave arrivals at seismometers, there is strong evidence that the rupture propagated eastwards from the epicenter at speeds in excess of the S-wave velocity, making this a supershear earthquake.
Ten provinces were affected with deaths and collapsed buildings. An official Turkish estimate dated 19 October 1999 reported casualties of 17,127 killed and 43,953 injured, but many sources suggest the actual figure may have been closer to 45,000 dead and a similar number injured. Reports from September 1999 stated 127,251 buildings were damaged to varying extents and at least 60,434 others collapsed, while an American Red Cross report from 2003 estimated that 320,000 homes and businesses were destroyed. More than 250,000 people became homeless. About 60 km (37 mi) of the Istanbul-Ankara highway, almost 500 km (310 mi) electricity cables and over 3,000 electricity distribution towers were damaged.
Over 9,500 people were killed in İzmit. In Gölcük, at least 4,556 people died, 5,064 were injured, thousands more were left missing and at least 500 buildings collapsed, trapping about 20,000 families; overall, up to 80% of the city's buildings were damaged or destroyed. About 200 sailors went missing after a naval base collapsed. There was also destruction in Yalova, where 2,501 people died, 4,472 were injured and 10,134 buildings collapsed. The cause of most damage in Yalova was suspected to be liquefaction-induced. Since the area mostly comprised Quaternary alluvial soil, it was prone to liquefaction. The approximately 200 drilling sites and boreholes, and many streams and rivers, factored in the severe liquefaction.
In Istanbul, at least 978 people were killed and 3,547 others sustained injuries. Severe damage in the city was concentrated in Avcılar district. Avcılar was built on relatively weak ground, mainly composed of poorly consolidated Cenozoic sedimentary rocks, which made the district vulnerable to earthquakes. In Eskişehir, there were 86 deaths and 70 buildings collapsed. At least 263 people died and 333 others were injured in Bursa. Three deaths and 26 injuries were reported in Zonguldak. At least 2,627 people were also killed and 5,084 others were hurt in Sakarya Province.
Private contractors faced backlash for using cheap materials in their construction of residential buildings. Many of these contractors were prosecuted but few were found guilty. Government officials also faced backlash for not properly enforcing earthquake resistant building codes. Direct cost of damage is estimated at US$6.5 billion, but secondary costs exceeded US$20 billion. In 2010, the research branch of the Grand National Assembly of Turkey stated there were 18,373 deaths as a result. In the same report, it stated there were 48,901 injured, 505 permanently injured, 96,796 homes heavily damaged or destroyed, 15,939 businesses heavily damaged or destroyed, 107,315 homes moderately damaged, 16,316 businesses moderately damaged, 113,382 homes slightly damaged, 14,657 businesses slightly damaged, 40,786 prefabricated homes distributed and 147,120 people rehoused into these homes.