Strong ground motion characteristics from the 17 August 1999 Kocaeli, Turkey earthquake

The 17 August 1999 Kocaeli, Turkey earthquake (M_w=7.4, USGS) occurred in the western part of the North Anatolian Fault Zone about 80 km east of Istanbul. The mechanism of the main event was almost a pure right-lateral strike-slip, and the aftershock distribution indicates that the rupture was located toward the western end of the North Anatolian Fault Zone. The earthquake affected a wide area in the Marmara region, as well as the city of Istanbul. Most of the damage and fatalities occurred in towns located on the narrow, flat shoreline of the Sea of Marmara. Since the broken fault segment traversed the densely populated and industrialized east Marmara region, damage was enormously high. Widespread liquefactions caused bearing capacity losses and consequent foundation failures in the Adapazari region, as well as extensive subsidence along the shoreline in Gölcük (Gulf of Izmit) and Sapanca. The earthquake struck also the western suburbs of Istanbul, the Avcilar region, causing severe damage on buildings even though the distance from the epicenter was about 80 km. In this study, we discuss the ground motion characteristics, as well as directivity and soil effects of recorded ground acceleration of the Kocaeli earthquake. Strong-motion data were obtained from the networks managed by the Bogaziçi University, Kandilli Observatory and Earthquake Research Institute and by the General director of Disaster Affairs, Earthquake Research Department. Although the distribution of the accelerometers deployed in the epicentral area seems sparse, the Izmit earthquake generated approximately twenty strong-motion records within 200 km of the fault. Maximum peak ground acceleration reached 0.41g in Adapazari, 40 km east from the epicenter and 3 km from the fault rupture. This value is rather small, only half of the value observed in various large earthquakes, e.g., 0.8 g in the 1995 Kobe, Japan earthquake and 0.9 g in the 1999 Chi-Chi, Taiwan, earthquake, while the maximum ground velocity was about 0.8 m/s, that is comparable to the typical value observed in large earthquakes.