Kinematics, mechanics, and potential earthquake hazards for faults in Pottawatomie County, Kansas, USA

Abstract

Many stable continental regions have subregions with poorly defined earthquake hazards. Analysis of minor structures (folds and faults) in these subregions can improve our understanding of the tectonics and earthquake hazards. Detailed structural mapping in Pottawatomie County has revealed a suite consisting of two uplifted blocks aligned along a northeast trend and surrounded by faults. The first uplift is located southwest of the second. The northwest and southeast sides of these uplifts are bounded by northeast-trending right-lateral faults. To the east, both uplifts are bounded by north-trending reverse faults, and the first uplift is bounded by a north-trending high-angle fault to the west. The structural suite occurs above a basement fault that is part of a series of north–northeast-trending faults that delineate the Humboldt Fault Zone of eastern Kansas, an integral part of the Midcontinent Rift System. The favored kinematic model is a contractional stepover (push-up) between echelon strike-slip faults. Mechanical modeling using the boundary element method supports the interpretation of the uplifts as contractional stepovers and indicates that an approximately east–northeast maximum compressive stress trajectory is responsible for the formation of the structural suite. This stress trajectory suggests potential activity during the Laramide Orogeny, which agrees with the age of kimberlite emplacement in adjacent Riley County. The current stress field in Kansas has a N85°W maximum compressive stress trajectory that could potentially produce earthquakes along the basement faults. Several epicenters of seismic events (<M2.0) are located within 10 km of the structural suite. One epicenter is coincident with the northwest boundary of the uplift. This structural suite, a contractional stepover between echelon northeast-trending right-lateral faults, is similar to that mapped in the New Madrid Seismic Zone, and both areas currently feature roughly east–west maximum compressive stress trajectory. Based on these similarities, the faults in Pottawatomie County have the potential for seismicity. The results demonstrate that mechanical analysis of minor structural features can improve our knowledge of local earthquake hazards.