When small earthquakes shake the ground there is often a fear the rumbles are signs that a big earthquake is coming. However, new research shows that most of these earthquakes are aftershocks of big earthquakes that occurred 200 or even 400 years ago.
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The study is conducted by researchers from Northwestern University and the University of Missouri-Columbia. "This sounds strange at first," said the study's lead author, Seth Stein, professor of geological sciences at Northwestern. "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."
Most earthquakes occur along faults, fractures in earth's crust. As the crust moves, the rocks adjacent to the fault are deformed out of shape and the deformation is spread across many kilometres.
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.
The difference 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.
To test this idea, Stein and Mian Liu, professor of geological sciences at Missouri, used results from lab experiments on how faults in rocks work to predict that aftershocks would extend much longer on slower moving faults. They then looked at data from faults around the world and found the expected pattern. For example, aftershocks continue today from the magnitude 7.2 Hebgen Lake earthquake that shook Montana, Idaho and Wyoming 50 years ago.
"This makes sense because the Hebgen Lake fault moves faster than the New Madrid faults but slower than the San Andreas," Stein noted. Aftershocks go on for long times in other places inside continents, Stein said. It even looks like we see small earthquakes today in the area along Canada's Saint Lawrence valley where a large earthquake occurred in 1663.
"Now know the big earthquakes can pop up somewhere else. 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," Stein said. ■