The rumble of a 3.8 magnitude earthquake in Dekalb, Ill., startled many Wisconsin residents awake around 4 a.m. Wednesday morning.
The quake was felt all the way in Madison, 134 miles away from the epicenter of the earthquake.
The earthquake occurred at 3:59 a.m. and was initially recorded as a magnitude 4.3 by the United States Geological Survey, but was modified to a 3.8 tremor later in the morning. In addition to Wisconsin, the earthquake was also felt in Iowa, Indiana and Michigan, according to USGS.
Seismologists measure the magnitude of an earthquake in terms of energy released, with smaller tremors measuring at a magnitude three or below.
The earthquake in Haiti Jan. 12, which the Haitian government estimates killed anywhere from 230,000 to 270,000 people, was a magnitude 7.0.
The Wednesday earthquake was caused by a fault in the bedrock 3.1 miles beneath the Earth’s surface, according to USGS.
“The fault is buried down beneath all the sediments — that accumulates stress and occasionally relieves the stress by popping out an earthquake,” said Charles DeMets, University of Wisconsin professor of tectonics.
Although earthquakes east of the Rocky Mountains are more rare than earthquakes on the West Coast, an Eastern U.S. quake can be felt over an area 10 times larger than an earthquake of a similar magnitude on the West Coast, according to the USGS.
“It’s kind of the difference between pounding a hammer on a sponge and pounding it on a steel plate,” DeMets said. “When you pound it on the plate, the force travels a lot farther, and with a sponge, it absorbs it. In the Midwest, the crust is efficient at transmitting energy over long distances. In the Western U.S., the crust is so fractured with so many faults, it’s less efficient at transmitting energy.”
The largest Illinois earthquake recorded by the USGS occurred Nov. 9, 1968, measuring in at a magnitude of 5.5, causing widespread damage to buildings and roads. Like Wednesday’s tremor, the 1968 earthquake was felt in Milwaukee, as well as all the way to Boston and Ontario.