Kansas
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Most Kansans have never felt so much as a tremor, but at least 25 earthquakes rumbled through the state between 1867 and 1976 (Fig. 6) and more than 100 were measured between 1977 and 1989 (Fig. 7). Most of these were microearthquakes, which are defined as earthquakes that are too small to feel. The largest recorded Kansas earthquake hit the Manhattan area in 1867. It toppled chimneys and cracked foundations and was felt as far away as Dubuque, Iowa.
Some Kansas earthquakes are associated with the Nemaha Ridge, a buried granite mountain range that extends from roughly Omaha, Nebraska, to Oklahoma City (Fig. 8). This mountain range was formed about 300 million years ago, and the faults that bound it are still slightly active today, especially the Humboldt fault zone that forms the eastern boundary of the Nemaha Ridge, passing near Wamego, east of Manhattan, and near El Dorado, east of Wichita. About 50 miles (80 km) west of the Nemaha Ridge is the Midcontinent rift, a zone of the earth's continental crust that was ripped apart and filled with oceanic-type crust (basaltic rocks) about 1100 million years ago. This zone of rifting extended from central Kansas near Salina, northeast-ward across Nebraska, Iowa, and Minnesota and into the Lake Superior region. For unknown reasons the rifting stopped after only spreading about 30 to 50 miles (50-80 km); if it had not stopped, eastern and western Kansas would likely be on different continents today.
To better understand these earthquakes in Kansas, seismologists at the Kansas Geological Survey monitored seismic activity throughout the state from 1977 to 1989. Information from this research will help to refine building codes and design dams and power plants.

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