The magnitude 7.4 earthquake that struck northwestern Turkey in the early hours of 17 August, killing over 10,000 people, caught almost everybody by surprise--except seismologists. Although they made no predictions of when the earthquake would strike, they had long expected it. This week's tragedy was just the latest in a series of massive earthquakes that have marched along 1000 kilometers of the North Anatolian fault.
The sequence began in eastern Turkey with a magnitude 7.9 earthquake that killed more than 20,000 people in 1939, then moved westward in nine more events of varying degrees of destructiveness. "This is probably the most spectacular example of earthquake progression known," says Ross Stein of the U.S. Geological Survey in Menlo Park, California. The most recent quake in the series had been a magnitude 7.0 that struck in 1967. The quiet stretch of fault to the west, passing near the port of Izmit, "was the obvious candidate for the next earthquake," says Nicholas Ambraseys of Imperial College London.
The ultimate impetus for Turkey's earthquakes is the collision of the tectonic plates carrying the Arabian Peninsula and Eurasia. Caught in between, Turkey is being squeezed to the west, sliding along two major faults, the North Anatolian and the Southeast Anatolian. Historical records show that both faults have experienced periodic clusters of large earthquakes, says Ambraseys, which seem to flip-flop between the faults every century or two. The last major earthquake cluster on the North Anatolian fault struck in the 1700s, flattening Izmit at least once.
As rescuers tried to free people trapped in the wreckage this week, geologists took to the field to map the surface rupture. By late in the week, they had traced the break for more than 65 kilometers and measured up to 2.6 meters of ground displacement along the fault. The eastern end of the rupture overlaps the site of the 1967 earthquake, says Ambraseys. "It extends the earthquake sequence to 2000."
The fault is known to continue west from the ruptured area, diving under the Sea of Marmara and skirting 30 kilometers from Istanbul. That path leads some researchers to believe that the next quake may strike near that metropolis. "It's very scary," says Massachusetts Institute of Technology seismologist M. Nafi Toksöz.