Morning quake-up call?
“I thought, ‘Am I feeling this?’” the Janesville woman said.
“And then it happened again.”
Karen Murdy was jolted out of her Janesville bed when she felt the room shake.
A 5.4-magnitude earthquake centered about 290 miles south of Janesville near West Salem, Ill., rocked people up to 450 miles away just before 4:37 a.m. today, surprising residents unaccustomed to such a powerful Midwest temblor.
The sheriff’s departments in Rock, Walworth and Green counties all received reports from people who felt something this morning.
“It’s just a strange sensation because everything is just kind of shaking,” Murdy said. “You feel like you’re in the Twilight Zone.
“You don’t think of living in Wisconsin and having an earthquake.”
Jessica Sigala, a geophysicist at the U.S. Geological Survey, said earthquakes are infrequent in this area but not unknown.
She said this earthquake was felt so far away from the epicenter because of where it’s centered.
“There’s such a big difference between western and eastern,” Sigala said, referring to earthquakes that happen on either side of the Rocky Mountains.
If an earthquake of the same magnitude had happened in California, it wouldn’t have been felt as far away, she said, because the mountains keep the earthquake’s energy contained in a smaller area.
“But in the eastern part (of the country), you don’t have anything,” Sigala said. “The energy just goes everywhere.”
Although the magnitude of an earthquake measured by the Richter scale doesn’t change based on the distance from its epicenter, intensity does.
“People close to it are going to feel a higher intensity compared to people farther away,” Sigala said.
Intensity is measured on the modified Mercalli scale.
Near the epicenter of this earthquake, the intensity was about 6, Sigala said, “which means it was felt by pretty much everybody.”
People feel shaking and have difficulty standing. Windows and dishes break. Damage is slight to moderate in most buildings.
But in southern Wisconsin, the intensity was between 3 and 4, Sigala said, which means it was felt indoors. Hanging object swing back and forth. Windows and dishes rattle.
“It’s like a truck is passing by,” Sigala said.
The earthquake occurred in the Illinois basin-Ozark dome region, which covers parts of Indiana, Kentucky, Illinois, Missouri and Arkansas and stretches from Indianapolis and St. Louis to Memphis, according to the U.S. Geological Survey.
The USGS Web site indicates earthquakes occur irregularly in the area, and that the largest historical earthquake in the region—also a magnitude 5.4—caused damage in southern Illinois in 1968.
The Illinois basin-Ozark dome region borders the much more seismically active New Madrid seismic zone, according to the USGS Web site. In 1811 and 1812, the New Madrid fault produced a series of earthquakes estimated at magnitude 7.0 or greater.
Experts say another major New Madrid quake could destroy buildings, bridges, roads and other infrastructure and disrupt communications.
Information from the Associated Press was used in this story.
Last updated: 8:56 pm Thursday, December 13, 2012