Historic Earthquakes

Magnitude 7.6 TURKEY
1999 August 17 00:01:39 UTC

World Location
Regional Location

The Izmit earthquake occurred at 00:01:39 UTC (3:01 a.m. local time), and was centered at 40.74 N., 29.86 E., which places the epicenter about 11 kilometers, or seven miles, southeast of the city of Izmit. This location indicates that the earthquake occurred on the northernmost strand of the North Anatolian fault system. The earthquake originated at a depth of 17 kilometers, or about 10.5 miles, and caused right-lateral strike-slip movement on the fault. Preliminary field reports confirm this type of motion on the fault, and initial field observations indicate that the earthquake produced at least 60 kilometers (37 miles) of surface rupture and right-lateral offsets as large as 2.7 meters, or almost nine feet.

On 19 August at 14:16 and 15:18 UTC (5:16 and 6:18 PM local time), two light-to-moderate earthquakes with magnitudes of 4.8 and 5.0 occurred approximately 80 km to the west of the mainshock. Previous aftershocks of this size were centered to the east of the mainshock, further from the cities of Bursa and Istanbul. While this suggests the possibility of a larger earthquake occurring in this new location, it is also probable that the mainshock rupture extended into this region, into the Gulf of Izmit, making these events aftershocks. We have no means of predicting the likelihood of a larger event in this new location.

At least 17,118 people killed, nearly 50,000 injured, thousands missing, about 500,000 people homeless and estimated 3 to 6.5 billion U.S. dollars damage in Istanbul, Kocaeli and Sakarya Provinces. Felt as far east as Ankara. Felt (III) at Anapa, Russia; Chisinau, Moldova; Simferopol and on the south coast of Crimea, Ukraine. As much as 5 meters of right-lateral strike-slip displacement occurred along a 120-km zone of the North Anatolian Fault between Karamursel and Golyaka. Rupture proceeded from west to east in two subevents. Duration of strong shaking was 37 seconds with maximum acceleration 0.3-0.4g.

Aftershocks
From experience with earthquakes around the world, we know that there are likely to be future aftershocks to the August and November earthquakes. Some of the aftershocks may be large enough to cause additional damage. Buildings that have already been damaged by an earlier earthquake are at highest risk from damage due to a future aftershock. We also know from past experience that the number of aftershocks will decrease over the next few months. There might be short episodes of higher activity, but the overall trend will be for fewer aftershocks as time goes by. Seismologists are not able to predict the timing and sizes of individual aftershocks.

Istanbul has experienced damaging earthquakes every century or two. Seismologists are concerned that the recent large earthquakes may have increased the risk of an earthquake closer to Istanbul. U.S. Geological Survey scientists are collaborating with scientists in Turkey to calculate how the probability of an earthquake nearer to Istanbul has changed as a result of the August and November earthquakes. The factors that control the timing of earthquakes are very poorly understood, however. Therefore, the studies won't be able to state the time of the next damaging earthquake in Istanbul, not even approximately to within several decades.