This week’s deadly earthquake was the latest in a string of major temblors that have struck northern Turkey progressively from east to west over the past 60 years.
Scientists had long considered Izmit, where the Tuesday’s quake was centered, to be a danger zone. “There is no question in the community that this was an area of stress buildup,” says Nafi Toksoz, a geophysicist at Massachusetts Institute of Technology.
In 1939, a magnitude-8.0 earthquake struck near the eastern city of Erzincan, rupturing 225 miles of the North Anatolian fault that cuts across northern Turkey. Then, from 1942 to 1967, five more major quakes struck, each rupturing a part of the fault to the west of the previous quake. There have also been a couple of earthquakes to the east of the 1939 quake.
“It follows up the trend of the other earthquakes,” Toksoz says of the latest earthquake. The fault rupture starts right where the 1967 rupture ended, he says.
“It’s a pattern that has long been recognized,” says Jim Dieterich, a senior scientist at the U.S. Geological Survey in Menlo Park, Calif. “There’s a progression of earthquakes that have now marched across much of northern Turkey.”
Dieterich was one of the authors of an August 1997 article in Geophysical Journal International that looked at whether one earthquake helped set off subsequent ones, and which areas might be susceptible to future earthquakes.
Their conclusion: The western end of the fault was “highly stressed” and there was a 12 percent chance of an earthquake of magnitude 6.7 or greater occurring to the south of Izmit during the next 30 years.
The latest earthquake “occurred right in the center of this region where stresses had increased during the previous earthquakes,” Dieterich says. “So this is now a continuation of that pattern.”
On the other hand, Tuesday’s earthquake is likely unrelated to others striking the Mediterranean in the past week, which were considerably farther away. Iran suffered a tremor measuring 5.1 and the island of Cyprus was rocked by a jolt hitting 5.8.
“The stress changes from those earthquakes are so infinitesmal they’re probably not a direct trigger,” says James Dolan, professor of earth sciences at University of Southern California.
Criss-crossed with fault lines and wedged between three large, constantly moving tectonic plates, Turkey and the wider eastern Mediterranean region is one of the world’s most earthquake-prone zones.
Some 96 percent of Turkey lies in a “high risk” area and 98 percent of the population lives in this area, according to Turkey’s Housing Ministry.
“A number of lines of crust weakness converge in this particular area,” says Iain Stewart, a lecturer in earth sciences at the British University of Brunel.
The African plate to the south and the Arabian plate to the east are moving north against the Eurasian plate, which moves south, resulting in an accumulation of pressure on the boundaries.
Scientists say the plates move relative to one another at speeds ranging from 0.5 to 8 inches a year. Large tremors usually occur at the boundaries of tectonic plates.
Similar to San Andreas
The North Anatolian fault is what geologists call a “strike-slip” fault, where two tectonic plates try to slide past each other but get stuck. These stresses build until the fault suddenly snaps, setting off an earthquake. The San Andreas fault that cuts down through California is also a strike-slip fault.
In an earthquake, stresses are relieved along the section where the fault slips, but increase in nearby areas that are still stuck.
Geologists see the North Anatolian fault almost like a buttoned shirt being pulled apart. When one button pops off, that transfers the stress to the next button, making it the one likely to pop off next.
Jim Dewey of the U.S. Geological Survey in Boulder, Colo., suggests a run in a stocking as another analogy. “Once it gets going, it keeps going.”
Where, Not When For a decade, Toksoz and his collaborators have been observing how the ground in the area has been shifting via once-a-year snapshots from global positioning system satellites. Those observations confirmed that stress was building up along the northern branch of the fault, the one that runs near Izmit.
A continuous monitoring system was planned for the region, which may have provided some warning. “Our hope was yes, most likely we would have seen something,” Toksoz says, “but I could not tell you what we may have been able to see.”
Geologists cannot explain why 32 years passed between an earthquake that struck the Mudurnu Valley to the east of Izmit in 1967 and Tuesday’s quake.
“There’s a certain randomness to the occurrence of earthquakes,” Dieterich says.
Unlike Other Faults
Perhaps surprisingly, this pattern of earthquakes traveling down a fault line is rare. Most earthquake faults are not of the strike-slip variety, but Dieterich also sees no pattern along the San Andreas. The great San Francisco earthquake of 1906 “may have just cleaned out [stresses on] the whole northern section of the San Andreas,” he says. A large earthquake in southern California in 1857 may have similarly relieved tectonic stress along the southern part of the fault.
“It’s possible the San Andreas at times will enter this type of pattern,” Dieterich says.
As with the earlier earthquakes, Tuesday’s relieved stress to other parts of the North Anatolian fault. Although too early to tell for sure where the highest-stress point is now, Toksoz says, “My guess is that it is further west” along the same fault branch, closer to Istanbul.
Reuters contributed to this report.