Web edition: October 18, 2012
Print edition: November 17, 2012; Vol.182 #10 (p. 9)
Keeping an eye on geological faults can be useful even long after they convulse in a great earthquake. By watching a Turkish fault after a deadly 1999 quake, geologists have pieced together a detailed picture of creeping fault movement after a big shake-up.
The magnitude 7.4 Izmit earthquake broke part of the North Anatolian fault, killing more than 17,000 people. In the years since, scientists have found, the two sides of the fault have started to creep past each other again at rates up to about 27 millimeters annually, most likely building up stress to the west near Istanbul.
“We’re expecting a destructive earthquake in the near future in Istanbul,” says Ziyadin Çakir, a geologist at Istanbul Technical University. “The creeping makes additional stress on the fault.”
When one part of the fault ruptures in a quake, it releases stress in that area but transfers some of it to neighboring areas. Close to Istanbul, the fault has not broken in a major quake since 1766. Seismologists consider it ripe for an earthquake of magnitude 7.0 or greater.
Çakir and his colleagues combined satellite measurements of how the ground has shifted with field observations to show what the Izmit part of the fault has been doing. They describe the new work online October 2 in Geology.
Decades ago, scientists discovered that faults like the North Anatolian and California’s San Andreas — besides releasing energy in big jolts as quakes — can move slowly and steadily, or creep. But researchers haven’t been able to figure out what physical process within the rocks makes the creep happen, and how creep begins again after most of the accumulated stress is released in a big quake.
“There aren’t many places in the world where this kind of thing has been observed,” says Chris Marone, a geoscientist at Penn State who studies fault slip in the lab.
The Izmit earthquake broke the ground in a sudden and violent 45 seconds, but even after that, the fault continued to slide along a stretch about 60 kilometers long. Çakir’s team found that this creep began immediately after the 1999 quake and continued at least through June 2012 — the longest period ever observed after a major quake, he says.
Creep along other major faults may similarly have been kicked off by a big quake, the scientists say. Deep drilling at the San Andreas has shown that the fault zone contains weak minerals such as clays, which allow rocks to slide past each other more easily.
At Izmit, the creep rate is slowing down, Çakir says. It may eventually reach a steady state of very slow creep, or might even halt. For now, though, “the fault creep still seems to be progressing at a higher rate than before the quake,” says Elizabeth Hearn, a consulting geophysicist in Palo Alto, Calif. “This is an interesting result, but not entirely unexpected.”
Z. Çakir et al. Onset of aseismic creep on major strike-slip faults. Geology. Published online October 2, 2012. doi: 10.1130/G33522.1.
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