November 5, 2009

Modern Earthquakes May Be Aftershocks From Past Quakes

Scientists say many recent earthquakes might have been the aftershocks of large quakes that occurred hundreds of years ago, BBC News reported.

Researchers have described a new pattern in the frequency of aftershocks that could explain some major quakes.

A report published in the journal Nature found that, away from plate boundaries, echoes of past earthquakes could continue for several hundred years, since the earth takes longer to recover in the middle of a continent.

Seth Stein from Northwestern University in Illinois said it was something they had never spotted before.

"Most big earthquakes happen at plate boundaries -- like the San Andreas fault. There is a lot of movement there and aftershocks go on for about ten years after a big quake," he said.

Scientists monitor regular movement of the earth to gauge the likelihood of a future quake happening once the aftershocks of a current earthquake have stopped.

But Stein explained that small earthquakes also occur where there is no regular movement, so if the ground has not been storing up energy for future earthquakes, they are likely aftershocks.

Experts believe this could explain the disastrous earthquake in 2008 in China's Sichuan province. The discovery shocked many scientists in the field, since it was an area where there had been hardly any earthquakes in the past few centuries.

Meanwhile, earthquake experts say these "aftershock quakes" get smaller over time.

Stein said it even looks like they see small earthquakes today in the area along Canada's Saint Lawrence valley where a large earthquake occurred in 1663.

"If you look at where they are - they're on the fault plane of the big earthquake," he said.

Stein and colleague Mian Liu from the University of Missouri found the same pattern repeated in seismic data from faults around the world -"“ a discovery that could help scientists to foresee the location of big earthquakes.

Stein said predicting big quakes based on small quakes is like the whack-a-mole' game, you wait for the mole to come up where it went down. But they now know the big earthquakes can pop up somewhere else.

He suggested that instead of just focusing on the regions where small, regular earthquakes happen, scientists should use methods like GPS satellites and computer modeling to look for places where the earth is storing up energy for a large future earthquake.

Tom Parsons, a scientist from the U.S. Geological Survey (USGS) in California was not involved in this study, but said that with a more comprehensive approach to studying earthquakes, researchers would eventually be able to "arrive at a practical solution" - balancing the available resources with the need to protect areas that were at risk.


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