The earthquake that hit northwest Illinois early Wednesday morning was mild compared with those in California, experts say, but it did register as one of three the upper Midwest has experienced in the past 21 years.
The US Geological Survey rated the quake as a 3.8-magnitude about 5.5 miles east of Sycamore and 3.1 miles underground, which Northern Illinois University geologist Phil Carpenter says “is not really significant” compared with the West Coast, where quakes routinely are rated 5.0 and higher. Because the Midwest represents one of the most stable geological regions of North America, tremors are infrequent. And when they do happen, geologists are hard-pressed to come up with a reason why.
“We don’t quite understand why this happens,” says Mr. Carpenter. “They seem to move around from one location to another. Obviously, there is some release of stress due to zones that are fractured in the sub-surface, but we don’t know about stress released by slippage in these zones. It’s a good research problem.”
Carpenter says he and other geologists plan to examine records at seismic stations throughout the area and in Wisconsin and Minnesota to determine the level of aftershocks, which can still be recorded even if they are below the level of human perception. “Aftershocks might be occurring, but we might not be able to feel it,” he says.
Wednesday’s earthquake joins two others in 2004 and 1999, which were both in the 3.5-4.0-magnitude range.
Earthquakes in the lower Midwest are more frequent, especially in the Wabash Valley and New Madrid seismic zones, which are considered more fragile and susceptible to quakes and more dangerous. In 2008, a 5.2-magnitude earthquake struck West Salem, in downstate Illinois, making it the worst to hit Illinois since 1968.
The most devastating earthquakes are those reaching 7.0-magnitude, such as the quake that hit Haiti Jan. 12, which was approximately 1,600 times the magnitude that hit Illinois Wednesday.
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