The largest recorded was estimated at 5.5 in Wamego, Riley county in 1867. There is an area known as the Humboldt fault zone It roughly follows the Flint hills (the Nemaha uplift.)
There was also a small 2.8 earthquake today near Prescott not far off 69 highway.
http://earthquake.usgs.gov/earthquakes/map/
5.5 in 1867 in Wamego, KS? That would have been Dorothy landing on the Wicked Witch of the East. Wamego is the home of the Wizard of Oz museum. (and also of several of my cousins.)