UB Engineer Says Turkey's 'Twin Quake' Occurred Along A Fault Similar To San Andreas Fault

BUFFALO, N.Y. - The "twin quake" that struck western Turkey last week occurred on a fault that is a "textbook example" of a transform fault like California's San Andreas Fault, according to an engineering seismologist at the University at Buffalo.

Apostolos Papageorgiou, Ph.D., UB professor of civil engineering and an investigator with UB's Multidisciplinary Center for Earthquake Engineering Research, added that the earthquake's unusual signature -- two ruptures of the fault at least 20 seconds apart -- may have made it exceptionally destructive.

It is possible, he added, that a similar type of earthquake also could occur along the San Andreas Fault.

As an engineering seismologist, Papageorgiou develops mathematical models for faults in order to simulate the kinds of ruptures generated by certain types of ground motions. These models, developed to simulate specific earthquake events, are used to program UB's shake-table, upon which UB and MCEER engineers test the earthquake-resistance of models of structures.

"It's like a double earthquake," he said of the record he obtained of Tuesday's quake. "I can see from this record that there were two ruptures."

The record was obtained by earthquake engineer Gokhan Pekcan, Ph.D., postdoctoral researcher at UB and MCEER, and native of Turkey. The information, available on the Web at http://www.koeri.boun.edu.tr, was retrieved from an accelerograph that was operated by the Kandilli Observatory and Earthquake Research Institute in Istanbul and located at the surface of the ground literally at the earthquake's epicenter, west of Golcuk. The accelerograph records the ground motions of the point in the earth where it is located; from these measurements, researchers easily can determine other measurements, such as velocity and displacement.

While it is not unusual for an earthquake to have two so-called subevents, they usually occur with a very small time lapse between them.

"We know that large events are often composed of small subevents," said Papageorgiou, "but these subevents usually occur very close in time. Here, there is a very long pause between the two subevents, at least 20 seconds. That's what makes it weird.

"One can speculate that it started as a smaller rupture, perhaps a 6.5, and because it didn't reach equilibrium, it was followed by the second subevent," he said.

He explained that what usually happens on such occasions is that the rupture temporarily is arrested for some reason by material of the earth's crust that is more difficult to fracture. It also is possible that it jumps to a sub-parellel fault plane, and that is what he thinks happened in Turkey.

He noted that such a "twin quake" could have been responsible for wreaking even more destruction in Turkey than would have occurred with a single one.

"It is possible that there may have been some structures that survived the first event and that would not have failed if the second event had not happened," he said.

"I have seen the accelerograms from a lot of earthquakes and it is rare to see subevents with such a long delay between them," he said. "I have not seen strong ground motions from previous events that were so well-identified as distinct subevents as the ones from this earthquake," he said.