On This Date in Minnesota History: July 9

July 9, 1975 - The 1975 Morris earthquake occurred in Morris, Minn. at 9:54 a.m. “The strongest instrumentally recorded rupture in the history of the state, it registered at magnitude 4.6 on the Richter scale. It was the first earthquake to be recorded on any seismic instrument in the state since 1917. Tremors were felt over much of Minnesota, northern Iowa, and the eastern Dakotas.”
http://en.wikipedia.org/wiki/1975_Morris_earthquake

http://www.encyclopedia.com/topic/Richter_scale.aspx