Several of the historical earthquakes in Kansas are believed to be associated with the Nemaha ridge, a buried mountain range that trends south from near Omaha, Nebraska, through Kansas to Oklahoma City, Oklahoma. The ridge is composed of Precambrian granitic rocks. Recent microearthquake locations and additional historical records indicate that the larger historical epicenters may be associated with the Humboldt fault zone, which forms the eastern boundary of the Nemaha ridge. New geological mapping and geophysical studies reported here indicate that the Humboldt fault breaks the surface through Permian rocks. Gravity and aeromagnetic data indicate that the Humboldt fault may be a complex zone of faulting rather than a single Precambrian structure as previously interpreted. A new interpretation of the Precambrian surface emphasizing the faulting complexity based on previously unavailable geophysical, remote sensing, and subsurface well data is presented here.