Small 'Foreshock' Rumbles May Precede Big Quakes

Charles Q. Choi, OurAmazingPlanet Contributor

Date: 17 February 2011 Time: 01:35 PM ET

Bursts of energy rocked the earth in the hour right before an earthquake devastated Turkey in 1999 a new finding that might one day help researchers predict major quakes.

Many large earthquakes are preceded by smaller rumbles known as foreshocks. However, there is apparently no way to distinguish these tremors from other small quakes that don't portend a larger temblor. At the same time, many large earthquakes do not seem to have any foreshocks.

The magnitude 7.6 earthquake that hit near Izmit in northwest Turkey in 1999, killing at least 17,000 people and leaving nearly 500,000 homeless, is now helping to shed light on the anatomy of such catastrophes. The calamity was one of the best-recorded large earthquakes to date, since researchers had seismic recording stations very close to the fault, said seismologist Michel Bouchon of the University of Joseph Fourier in Grenoble, France.

Now, after analyzing the deluge of information from before, during and after the earthquake, Bouchon and his colleagues have detected repeated, accelerating blips of seismic activity before the Izmit quake, near the point where the rupture began.

Theoretical and laboratory studies of earthquakes suggest they should be preceded by slow slipping along a fault in the earth. Before the Izmit quake began, the research suggests the fault slipped irregularly but near-continuously for 44 minutes, generating the bursts the scientists detected, as well as increasing low-frequency seismic noise.

"This will motivate seismologists to look more for these types of signals before earthquakes," Bouchon told OurAmazingPlanet. "Seismologists have kind of given up finding such signals in the early nucleation phase before earthquakes."

Future research can analyze details of other large, well-recorded earthquakes for similar signals.

"What we found is encouraging, but we don't know how common these signals will be," Bouchon said.

The scientists detailed their findings in the Feb. 18 issue of the journal Science.