Why do earthquakes occur in Michigan?

There have been many earthquakes in Michigan dating back to at least 1872. The most notable of these was the August 10, 1947 quake centered southeast of Battle Creek which registered at 4.6 on the Richter. According to the USGS, was a 3.5 centered about 1.5 miles north of Capital City Airport in Lansing, September 2, 1994. There was also one in 1998.