The 3.8-magnitude earthquake that shook northern Illinois early Wednesday morning occurred on a previously unknown fault line, according to geologists.
And that’s typical for earthquakes in this region, said Paul Stoddard, an associate professor in the department of Geology & Environmental Geosciences at Northern Illinois University.
Earthquakes are caused by a shifting of rocks, Stoddard said. A fault is a crack in a rock where stress begins to build up on either side, he said. Eventually, enough stress is built up, and the rocks slip along that crack, usually fairly quickly, causing an earthquake.
There is at least one known fault line in DeKalb County: The Sandwich Fault runs from that city west toward Oregon, Ill., but there never have been any earthquakes recorded along it, Stoddard said.
“We know it from the geology, but not any activity,” Stoddard said of the Sandwich Fault. “That’s not unusual for this part of the world. Most of the earthquakes we get are on previously unknown faults.”
Wednesday’s quake was centered about 1 mile south-southeast of Pingree Grove, or near Switzer Road between Plank Road and Route 20 in rural Kane County.
A 3.8 magnitude is considered a minor earthquake, according to the U.S. Geological Survey. The magnitude of an earthquake characterizes the relative size of an earthquake, according to the USGS. It is based on the “measurement of the maximum motion recorded by a seismograph,” according to the USGS Web site.
Rick Polad, an earth science instructor at Aurora University, and Rod Allen, a St. Charles-based geologist, said the intensity of Wednesday’s quake was about as strong as northeastern Illinois should expect to experience.
“Since 1909, there have only been five earthquakes of this magnitude in Illinois, period,” Polad said. “We have a number of small fracture zones extending throughout Illinois, and this may be a new one we didn’t know about.”
Quakes of similar magnitude to that felt Wednesday pose little danger to people or property. Significant damage would not occur until an earthquake reaches the “high-5’s or up above 6” on the Richter scale.
And Allen said in areas in which building construction was regulated using building codes, the shaking would need to be very strong – likely well above 6 on the Richter scale, approaching 7 – to cause any damage.
The main fault line in Illinois is the New Madrid Fault Zone, which is the most seismically active area in the Midwest, Stoddard said. That is in southern Illinois and runs down the Mississippi River Valley toward Memphis, Tenn., he said. That area gets several small earthquakes, he said, occasionally reaching a 5.0 magnitude.
About 200 years ago, there were three earthquakes along the New Madrid Fault with a magnitude of 8.0, he said.
“Those were huge earthquakes,” he said. “They rang church bells in Boston. They changed the course of the Mississippi River.”
There is another fault zone in northern Illinois, the Plum River Fault Zone, which runs from the the Savanna area toward Byron.
Earthquakes in northern Illinois
The U.S. Geological Survey placed the epicenter of Wednesday’s 3.8-magnitude earthquake at 1 mile south-southeast of Pingree Grove in northern Kane County (42.053°N, 88.412°W).
A USGS analysis said the earthquake’s depth was 3.1 miles below ground.
Other notable earthquakes from the surrounding area, according to the Illinois State Geological Survey, include:
• 2004: LaSalle County, with a magnitude of 4.2
• 1999: Lee County, with a magnitude of 3.5
• 1985: DuPage County, with a magnitude of 3.0
• 1972: Lee County, with a magnitude of 4.5
• 1947: Kane County, with a magnitude of 3.1
• 1944: Kane County, with a magnitude of 2.7
• 1912: Kendall County, with a magnitude of 4.7
• 1909: Will County with a magnitude of 5.1