The August 17 earthquake that killed thousands in northwestern Turkey increased the likelihood of a future earthquake near the metropolis of Istanbul by increasing the forces on the fault just south of the city, say MIT researchers who have been studying earthquakes and tectonic deformation in Turkey since 1971.
Turkey's North Anatolian fault runs east-west for nearly 1,000 kilometers and is very similar to the San Andreas fault in California. Since 1939, earthquakes have ruptured successive segments of the fault. The segment of fault that ruptured last week is next to an unruptured segment extending westward under the Sea of Marmara. Istanbul is about 10 miles north of that area.
As a result, if that particular segment ruptures in another earthquake, it could directly affect the metropolis of 12 million, said research team leader Nafi Toksoz, professor of geophysics and seismology at MIT and a native of Turkey.
The recent earthquake "increased the forces on the westward extension of the fault, increasing the likelihood of a future earthquake in this vulnerable area," he said. The last big earthquake on the North Anatolian fault occurred in 1967, just east of last week's temblor.
The MIT team, in cooperation with universities and research centers in Turkey, is monitoring the ground deformation with great precision in and around the Sea of Marmara, using the Global Positioning System with accuracy better than 0.1 inch.
These measurements showed the deformation along the fault segment that ruptured. A continuous network of monitoring stations is in the process of being established. The first of these was installed by MIT near the fault 18 months ago as part of a major research project monitoring seismicity and tectonic deformations in Turkey and surrounding areas, with a very intense effort around the Sea of Marmara, including the Izmit (site of last week's quake) and Istanbul areas.
The MIT research team includes Drs. Robert Reilinger, Robert King, Simon McClusky and other members of the Department of Earth, Atmospheric and Planetary Sciences.
DATA ON QUAKE
A strong earthquake occurred in northwestern Turkey at the eastern extension of the Sea of Marmara at 3:01:48 local time (8:01:48 pm EDT) on August 17. The earthquake's epicenter was located 65 miles east of Istanbul, at 40.8 degrees N latitude and 30.1 degrees E longitude, near the town of Izmit on the North Anatolian fault. The earthquake measured 7.5 (moment magnitude) on the modified Richter scale. This was the largest earthquake in Turkey since 1976. The earthquake was followed by hundreds of aftershocks.
The earthquake did extensive damage to residential and industrial buildings in Izmit and surrounding areas. The heavy casualties include at least 12,000 killed, several thousand injured and missing, and as many as 200,000 homeless. Property damage and some deaths occurred in Istanbul, 65 miles to the west, and in the city of Bursa, about 40 miles to the southwest of the epicenter. The most damage and casualties were in the towns on both shores of the Gulf of Izmit. This region is a highly industrialized area, and in addition to human casualties, the economic impact on the industrial facilities is likely to be in the billions of dollars.
The largest earthquakes in Turkey have occurred on the North Anatolian fault. The latest episodes of earthquakes started in 1939 with a magnitude 8 earthquake near Erzincan. Earthquake activity migrated westward, rupturing the fault zone in a series of earthquakes with magnitudes 7 and greater, in 1942, 1943, 1944, 1955 and 1967.
A version of this article appeared in MIT Tech Talk on September 25, 1999.
