A small earthquake in northern Illinois set off car alarms, knocked books off the shelves and jolted scores of people awake at 4 a.m. Wednesday, but otherwise caused no serious damage, officials said.

The United States Geological Survey reported that the earthquake, which had an estimated magnitude of 4.3, was centered near Virgil, about 50 miles northwest of Chicago, at a depth of about three miles. There were no immediate reports of aftershocks

Sarah Evans, 29, of the northwest Chicago suburb of Elgin, said she was awakened at about 4 a.m. when her house started trembling.

“I popped out of bed,” she said. “I felt extreme shaking. I shook my husband, and said, ‘Oh my God, I think it was an earthquake.” She said the dogs didn’t bark but it seemed as if everything in her 100-year-old single-family home was shaking, as if a train was roaring by.

Ms. Evans, who is the owner of Sevans Strategy, a public relations strategy firm that guides companies on how to use Twitter and other social media, said she then reached for her iPhone and posted a message on Twitter. “Seriously weird,” she wrote. “Something that felt like a minor quake just woke us up. Can anyone else in the CHI area/burbs confirm?”

Soon, she said, many people were trading information about the quake through the forum.

Pat Gengler, a spokesman for the Kane County sheriff, said the switchboards had been lighting up with calls, but he said there were no immediate reports of any damage.

“Right now the only reports we were getting were some people a little startled at 4 in the morning when their house started shaking,” he said. “We got a lot of snow out here and we had a little earthquake, so it’s been an interesting 12 hours.”

Amy Vaughan, a geophysicist with the agency, said that the tremor was only the second notable earthquake in more than 30 years to rattle the area. Even though the quake was relatively small, it was felt by residents as far away as southern Wisconsin, according to news reports.

“This is not a very seismically active area,” Ms. Vaughan said. “We don’t know what fault system this is, but there are faults everywhere throughout the world that can slip at any given time.”