Seismological Observations: Kocaeli, Turkey Earthquake, August 17, 1999
by Apostolos Papageorgiou, University at Buffalo
The earthquake that struck northwestern Turkey on August 17,1999 (origin time 0 h 1 min 38.6 sec GMT) had a moment magnitude Mw 7.4 and caused over 14,000 deaths and destruction of immense proportions. The earthquake was caused by slippage of a segment of the North Anatolian Fault which filled a 100 to 150 km long seismic gap between an event that occurred in 1967 on the east of the gap and two events that occurred in 1963 and 1964 on the west of the gap. The seismic gap was first pointed out by Toksoz, Shakal and Michael (1979) and was later further investigated by Stein, Barka and Dieterich (1997). The latter publication forecasted that an earthquake, such as the one that occurred on August 17,1999, had a 12% probability of occurrence in the above-mentioned seismic gap over the period of 30 years from 1996 to 2026. It is thus evident that the earthquake should not have come as a surprise.
The causative fault zone, the North Anatolian Fault Zone (NAFZ), is a textbook example of a right lateral strike-slip fault that has been extensively studied in the past. It is considered a close analogue of the San Andreas Fault Zone in that it exhibits similar slip rates and similar total length. Preliminary reports by a team of USGS field investigators (http://quake.wr.usgs.gov/study/turkey/) reveal a complex pattern of faulting for the 1999 Izmit earthquake that exceeds 110 km in length. As the USGS team points out in their report,"...understanding how the Izmit earthquake linked together these various fault segments may provide a new understanding of the range of possible earthquakes that can occur near San Francisco and will ultimately help us more accurately estimate hazards in many regions."
There are several obvious reasons why the 1999 Izmit earthquake was so destructive -- besides the quality of construction of the infrastructure --one of them being that the fault segments that ruptured causing this event crossed the most densely populated region of Turkey. This was compounded by the fact that the infrastructure was no match for the very energetic near-source pulses in the immediate vicinity (within 3 km) of the fault. Furthermore, the right-stepping en-echelon strike-slip segments of the fault in the vicinity of the cities of Golcuk and Izmit gave rise to elongated sedimentary basins, which may have trapped and amplified the incident seismic waves, thus enhancing the destructive power of this event. Finally, the accelerograms recorded in the vicinity of the fault reveal at least two major subevents, probably located about 30 km apart from each other.
M. N. Toksoz, A. F. Shakal, and A. J. Michael (1979), "Space-Time Migration of Earthquakes Along the North Anatolian Fault Zone and Seismic Gaps," Pageoph, Vol. 117, pp. 1258-1270.
R. S. Stein, A. A. Barka and J. H.Dieterich (1997). "Progressive failure on the North Anatolian fault since 1939 by earthquake stress triggering," Geophysical Journal International, Vol. 128, pp. 594-604.