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David B. Nelson/MEDILL

Geophysicist Seth Stein of Northwestern University helps communities decide how to prepare for earthquakes.  Monitors on campus keep tabs on earthquake activity.

Baby rattle: Epicenter of 2.4 temblor was in McHenry

by David B. Nelson
Jan 31, 2012

A 2.4 magnitude earthquake struck along the border of Illinois and Wisconsin late Monday night. Scientists pinpointed the epicenter near the far north suburb of McHenry, though the small quake rattled through buildings as far south as Chicago.

“Most reports that came in from the area talked about hearing an explosion,” said David Christiensen, director of emergency management in McHenry County. Christiensen said there was no sign of damage, adding that he didn’t even feel the tremor. “The funny thing is, I get a page if it’s anything over 2.5, so I didn’t even get one for this.”

“It seems to have been recorded by eight or so seismographs in the area,” said Timothy Larson, a geophysicist with Illinois State Geological Survey. The epicenter originally was traced to Lake Shangrila in nearby Wisconsin, but later moved to McHenry, specifically along Chapel Hill Road.

“The area is not entirely rural, but it’s well spread out with several farms,” Christiensen said.

The nearest culprit would seem to be the New Madrid Seismic Zone, a fault that crosses into several states including Missouri, Tennessee, Kentucky and Illinois. But experts say the fault was not to blame.

Larson suggested a possible theory for earthquake activity in a usually seismically static place such as Illinois.

“Some of these earthquakes are related to the glaciers some 15,000 to 18,000 years ago,” he said. “There was enough ice [then] that it weighed down the crust of the earth. When it left, the crust sort of bounced back up.”

“The ground in Illinois is still adjusting to the effects of the glaciers,” said geophysicist Seth Stein, a professor of Earth sciences at Northwestern University. “We don’t know if that’s a primary factor, there’s no question we still see motions here due to the motions of the glaciers, but it could also be forces associated with plates moving as they move along the earth’s surface. It could have been a number of things.”

The Richter Scale labels 2.0 to 2.9 magnitude earthquakes as “generally not felt, but recorded" in comparison to 9.0 and higher earthquakes referred to as "devastating n areas several thousand kilometers across." The 9.0 quake off the coast of Japan last year resulted in over 15,000 deaths and $15 to $35 billion in damage after also causing a tsunami and a near-meltdown at the Fukushima Nuclear Plant.

While yesterday’s earthquake left no damage, and little geophysical evidence, researchers are still looking into the future for Illinois seismology.

“That’s the question: how big the next one is likely to be,” said Larson. Larson said that a 1909 earthquake with a magnitude near 5.0 did cause minor damage in the Chicago area during a time when seismology was a fledgling discipline. “If we had another one of those, which is not completely unrealistic, it could cause damage especially to old brick buildings.”

But experts said that there is not much to worry about.

Stein, who researches earthquakes, also helps communities decide how to prepare for them. “One question is how much money you should spend putting steel in schools in comparison to hiring teachers. In this area, it would not be worth spending the money.”

Christiensen found strange a coincidence in last night’s minor event, especially with earthquake preparedness programs gearing up next week through the Illinois Emergency Agency. “I don’t know if it was just coincidence, or a wake up call.”