Temblors May Twitch First, Researchers Find : Seismology: Faint motion is found before main shocks in 30 earthquakes. Scientists hope discovery will someday help them learn how to predict quakes.

An earthquake may twitch before it rumbles, researchers say, a phenomenon that gives scientists hope of some day being able to predict temblors.

Government and Stanford University seismologists who analyzed and amplified the records of ground motion from 30 earthquakes said they found evidence of a pattern of very faint, irregular motion that immediately precedes a quake.

“An earthquake doesn’t start going rapidly from the very beginning,” Stanford scientist Gregory Beroza said Thursday. “There is a weak or tentative stage.”

Detecting this stage--from a split-second to five seconds before the actual quake--would not give time for a warning. But the discovery suggests that there may be an even earlier phase that signals the start of the earthquake process.

If such early signals exist, Beroza said, there is the theoretical possibility of predicting earthquakes.

Beroza and William Ellsworth of the U.S. Geological Survey report in the journal Science that the size and duration of the slow slippage phase also may signal the magnitude of the eventual quake.

“The fact that they see some systematic thing going on (before an earthquake) does hint that there is a preparation phase,” said James R. Rice, a Harvard University geophysicist. “It does give an optimism that there is something there to be discovered.”

The research, he said, shows that an earthquake is not abrupt, but that “there are starts and stops before it breaks out.”

In their study, Beroza and Ellsworth found cycles of slight and strengthening motion just before the main earthquake. Often this early motion is detected only faintly, and the study required that the data be amplified for the pattern to be found, they said.

Some of this pre-motion was a split second before the main movement, he said. For other quakes, the initial phase started and continued for up to five seconds.

Ellsworth said the finding “is a step toward answering the question about being able to predict earthquakes” by keeping alive the hope. “Whether we can ever realize it is still an open question.”