The 1999 Izmit earthquake was an approximately 7.4 magnitude earthquake that struck northwestern Turkey on August 17 , 1999 , at about 3:02 am local time . The 1999 Ä zmit earthquake ( also known as the Kocaeli earthquake or the GÃ lcÃ 1\/4 k earthquake ) was a 7.6 magnitude earthquake that struck northwestern Turkey on August 17 , 1999 , at about 3:02 am local time .