Scientists listen to rocks to better understand damage caused by quakes

August 18th, 2009 - 1:00 pm ICT by ANI  

Washington, August 18 (ANI): Scientists are using a technique called “ambient noise correlation,” to observe significant changes in the behavior of parts of Earth’s crust that were disturbed by three major earthquakes.

University of Illinois seismologist Xiaodong Song and graduate student Zhen J. Xu are using the technique.

“The observations are important for understanding the aftermath of a major earthquake at depth, and for understanding how the rock recovers from it and begins again to accumulate stress and strain for future earthquakes,” Song said.

Researchers have used ambient noise to image Earth’s interior and to monitor changes in seismic velocity near active volcanoes.

Xu and Song used the technique to examine how surface waves (extracted from ambient noise) between seismic stations change with time, because of earthquake-induced changes in the surrounding rock.

Xu and Song were not measuring the time it took for earthquake waves to travel from the epicenter to a seismic station.

Rather, they were measuring the time it took for surface waves to travel from one station to another.

Because the distance between stations is fixed, the technique allowed researchers to detect very tiny changes in seismic velocity.

“The observations allow us to see not just what happened at the surface, but what happened at depth, and how it affects not just the rupture area, but also the surrounding area,” Xu said.

In their study, the researchers examined the three largest and most recent earthquakes in Sumatra, Indonesia.

The earthquakes took place on Dec. 26, 2004; March 28, 2005; and Sept. 12, 2007.

The earthquakes occurred along the Sumatra subduction zone, where a portion of the Indian tectonic plate dives beneath the Eurasian plate.

Fault rupture lengths ranged from 450 kilometers for the 2007 earthquake to 1,200 kilometers for the 2004 earthquake.

“We observed a clear change in surface wave velocity over a large area after each of the earthquakes,” Xu said.

Using ambient noise correlation, the researchers can observe changes in stress several hundreds of kilometers from the source region.

The researchers also observed an unusual time shift that took place a month before the 2004 earthquake.

More data is needed, however, to draw a conclusion and to determine whether it was a precursory signal to a major earthquake.

An abundance of data was recorded at nearly 300 seismic stations in the source region by seismologists in China.

The analysis of respective time shifts will help the researchers better understand how the fault and surrounding behaved before and after the earthquake. (ANI)

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