Like many northern Illinois residents Wednesday morning, Laura Ross of Grand Detour was shaken out of a deep sleep.
She and her husband, Joe, heard the windows rattling, and felt some type of vibration.
"I sat straight up and said, 'What the heck was that?'" Ross said.
What Ross and others were feeling was the 3.8-magnitude earthquake that shook northern Illinois.
The quake was centered about 1 mile south-southeast of Pingree Grove in rural Kane County, and in a farm field near Hampshire.
The Rosses' daughter, Dawnn Montague, was feeling the quake, too, from her room in a residence hall at Northern Illinois University.
"She said her building shook, as did her bed," Ross said. "She is on the fifth floor and top floor of the building. It woke her up, and she felt as if she was dreaming, but knew she was awake."
Wednesday's quake occurred on a previously unknown fault line, according to geologists.
That's typical for earthquakes in this region, said Paul Stoddard, an associate professor in the department of Geology and Environmental Geosciences at Northern Illinois University.
While there always are aftershocks from an earthquake, Stoddard said there shouldn't be any in the region large enough for anyone to feel. There were no reports of damage: Stoddard said a quake this small would result in little to no damage.
The Lee County Sheriff's Department received several calls from Amboy-area residents who felt the quake, Sheriff John Varga said. Whiteside County Sheriff Kelly Wilhelmi said his department did not receive calls.
Exelon said its nuclear plants in Illinois were not affected by the quake.
"Exelon plant teams conducted plant walk-downs and equipment checks to ensure there was no damage," said Paul Dempsey of the Byron Generating Station.
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. 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 the area: The Sandwich Fault runs from DeKalb west toward Oregon in Ogle County, 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."
The magnitude of an earthquake characterizes its relative size, and is based on the "measurement of the maximum motion recorded by a seismograph," according to the U.S. Geological Survey's Web site.
A 3.8 magnitude is considered a minor earthquake.
Rick Polad, an earth science instructor at Aurora University, and Rod Allen, a St. Charles-based geologist, said the intensity of Wednesday's quake is 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."
A 5.2-magnitude earthquake struck near West Salem in southern Illinois on April 18, 2008. That quake was the worst to hit the state since 1968, the USGS said.
By contrast, the earthquake that leveled Haiti on Jan. 12 was 7.0 in magnitude, meaning it was about 1,600 times the magnitude of the one Wednesday morning. Many of the numerous aftershocks to hit the island were approximately the same magnitude as Wednesday's.
Quakes of similar magnitude to that felt Wednesday pose little danger to people or property. Significant damage would not occur until an earthquake reaches a magnitude in the "high 5's or up above 6," Polad said.
And Allen said in areas in which building construction is regulated using building codes, the shaking would need to be very strong – likely well above 6, 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 Savanna area in Carroll County toward Byron in Ogle County.
There also are other anticlines, synclines and monoclines, Stoddard said.
Anticlines are layers of rock that folded into an arch, and synclines are the opposite – the layers of rock have folded into a U shape, Stoddard said. Monoclines are in the middle – they are rocks that go along, make curves and then flatten out again, he said.
Those are stable geological features, he added, and geologists typically don't worry about movement on them.
Kate Schott and Jonathan Bilyk of the DeKalb Daily Chronicle contributed to this story.
A history of area earthquakes
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