Researchers at the University of Grenoble, France, studying seismograms recorded before the Izmit earthquake in Turkey that killed some 17,000 people, say the records showed foreshocks rippling away from the source of the rupture in the 45 minutes before the quake, NewScientist.com reported Thursday.

Theoretical models have suggested Earth's crust is unstable in the hours leading up to a major earthquake, but a way to detect that instability has proven elusive.

In the Izmit recording, researchers saw five small shocks in the final 20 minutes before the event, each a sequence of two types of waves, called P-waves and S-waves. In each of the five small shocks, a P-wave was followed 2.4 seconds later by a higher-amplitude S-wave.

"The difference in time between these two arrivals was always the same, implying that they came from the same spot on the fault," says researcher Michel Bouchon.

A pattern in the foreshocks, becoming progressively stronger and more closely spaced leading up to the quake itself, is encouraging for the development of early warning systems, researchers say.

However, one scientist says, such an early warning system might be limited in its usefulness.

"Earthquake early warning is not the same as earthquake prediction," Ian Main, a seismologist at the University of Edinburgh, says.

An early warning system could only give hours notice, not the days that would be needed to prepare an evacuation, he said.

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