NIU Geologist Says Northern Illinois Earthquake a "Total Surprise"

DeKalb, Ill. — NIU Geologist Philip Carpenter, an expert in seismology, was writing a proposal Tuesday night on how to study natural ground vibrations to identify the location of northern Illinois geologic faults, which occasionally generate small earthquakes.

Scientists don’t know much about the fault locations, but Carpenter found evidence of one early Wednesday morning.

At 3:59 a.m., a quake with a magnitude of 3.8 on the Richter Scale awoke thousands of residents across the region.

“I heard a booming sound followed by shaking, like you’d feel from an explosion, that lasted 10 seconds or so,” Carpenter says. He and colleague Paul Stoddard man NIU’s seismic center, located in the Department of Geology and Environmental Geosciences.

According to the U.S. Geological Survey, the earthquake was centered about one mile south-southeast of Pingree Grove, or about 15 to 20 miles east of Sycamore. So scientists now know there must be a fault there.

“It’s a total surprise,” Carpenter says. “No one knew there was a fault there. We’ve never had an earthquake centered in this particular area, and we have 150 years of records.”

That’s not to say an earthquake has never been centered in northern Illinois. Carpenter says there have been two other mild quakes in the last two decades, one centered near Amboy and the other near LaSalle-Peru.

“No faults have been mapped for the three earthquakes in the last 20 years,” Carpenter says. “We want to pinpoint the locations of the faults causing the small sporadic quakes.”

In addition to studying natural ground vibrations, scientists also could use portable seismometers that can detect tremors that can’t be felt by people, Carpenter says.

He adds that there are two major identified faults in northern Illinois—the Sandwich fault and the Plum River fault—but both have been dormant for the last 150 years.

The Sandwich fault runs through Sandwich and extends to Oregon, and the Plum River fault runs from the Savanna area toward Byron. Scientists have visibly identified fracture zones for those faults in outcrops, quarries and wells, Carpenter says.