Small earthquakes in US are actually aftershocks of 19th century quakes
5 Nov
2009, 1319 Hrs
Washington, November 5 A new research has shown that most small earthquakes occurring in the US are actually aftershocks of big earthquakes that happened in the 19th century.
Researchers from Northwestern University and the University of Missouri-Columbia conducted the study.
When small earthquakes shake the central US, citizens often fear the rumbles are signs a big earthquake is coming.
Now, new research has shown that most of these earthquakes are aftershocks of big earthquakes (magnitude 7) in the New Madrid seismic zone that struck the Midwest almost 200 years ago.
"This sounds strange at first. On the San Andreas fault in California, aftershocks only continue for about 10 years. But in the middle of a continent, they go on much longer," said the study's lead author, Seth Stein, the William Deering Professor of Geological Sciences in the Weinberg College of Arts and Sciences at Northwestern.
There is a good reason, according to co-investigator Mian Liu, professor of geological sciences at Missouri.
"Aftershocks happen after a big earthquake because the movement on the fault changed the forces in the earth that act on the fault itself and nearby. Aftershocks go on until the fault recovers, which takes much longer in the middle of a continent," he added.
The difference, according to Stein, is that the two sides of the San Andreas fault move past each other at a speed of about one and a half inches in a year - which is fast on a geologic time scale.
This motion "reloads" the fault by swamping the small changes caused by the last big earthquake, so aftershocks are suppressed after about 10 years.
The New Madrid faults, however, move more than 100 times more slowly, so it takes hundreds of years to swamp the effects of a big earthquake.
"A number of us had suspected this because many of the earthquakes we see today in the Midwest have patterns that look like aftershocks. They happen on the faults we think caused the big earthquakes in 1811 and 1812, and they've been getting smaller with time," Liu said.
To test this idea, Stein and Liu used results from lab experiments on how faults in rocks work to predict that aftershocks would extend much longer on slower moving faults.
"Instead of just focusing on where small earthquakes happen, we need to use methods like GPS satellites and computer modeling to look for places where the earth is storing up energy for a large future earthquake," said Stein. (ANI)
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