Date: April 18, 2008
Location: Southern Illinois
Time: 4:37 am CDT
By Tom Held
Posted: April 19, 2008
The vibrations from a 5.2-magnitude earthquake that struck southern Illinois early Friday morning rattled windows and nerves in Milwaukee.
Workers in downtown office buildings also noticed the 4.6-magnitude aftershock that hit about 10:15 a.m., but the quake of 2008 will share nothing more than an anniversary date with the great California quake of 1906.
Even in the immediate area of the earthquake, which was centered six miles from West Salem, Ill., damage was minimal.
Brett Ketter, the seismologist in charge of monitoring quakes at the University of Wisconsin-Milwaukee, slept through the initial shock, which registered on equipment in the school's geosciences department.
Ketter said the solid bedrock under much of the Midwest allows even relatively minor movements in the earth to vibrate for hundreds of miles, spreading energy underneath the surface.
The tremors from the shift Friday, estimated to be seven miles below the surface, were felt from Georgia to Michigan's Upper Peninsula, according to the U.S. Geological Survey.
While Ketter slept, dozens of residents were awakened by unsettling, pre-dawn vibrations.
Many of them were alerted by dogs or cats that acted strangely and seemed to seek comfort shortly before 5 a.m.
Those who felt the temblor locally reported their windows rattled, doors creaked and beds shook.
Although largely a non-event in Wisconsin, the earthquake served as a lesson that the Midwest is vulnerable to geological events thought to be restricted to California and the San Andreas fault.
"It's not an uncommon event," Ketter said. "They typically happen once or twice every 10 years."
Digital record of this event (goes to earthquake pages)