Turkish Earthquake Records Analyzed
BUFFALO, N.Y., August 23,1999--Studying
the Turkish earthquake could be especially beneficial for U.S. earthquake engineers and
seismologists because, as MCEER engineering seismologist Apostolos Papageorgiou
points out, the quake occurred on a fault that is a "textbook example" of a
transform fault like California's San Andreas Fault. Moreover, the earthquake's
unusual signature-two ruptures of the fault at least 20 seconds apart may have made it
exceptionally destructive, Papageorgiou said.
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
"It's like a double earthquake," he said of the record he obtained of the
Turkish quake. "I can see from this record that there were two ruptures."
The record was obtained by earthquake engineer Gokhan Pekcan, postdoctoral researcher at
UB and MCEER, and a 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
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 subparallel 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.