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.