MECHANICS AND POTENTIAL EARTHQUAKE HAZARDS FOR THE HUMBOLDT FAULT IN POTTAWATOMIE COUNTY, KANSAS

Detailed structure mapping in Pottawatomie County, Kansas has revealed a suite of structures comprising an uplifted block (sq. km) and four boundary faults. Two right-lateral faults trend northeast and bound the northwest and southeast sides of the uplift. A reverse fault oriented N 5º W 70º SW is located on the east flank of the uplift, and a vertical fault occurs along the west flank. Field evidence indicates that the reverse fault curves to join the northeast-trending fault southeast of the uplift. Several epicenters of microseismic events are located within 5 miles of the structural suite. One epicenter was within a hundred meters of the northeast-trending fault on the northwest of the uplift. The structural suite results from regional stresses. The preferred kinematic model is a contractional stepover (push-up) between echelon strike-slip fault segments. Mechanical analysis using the Displacement Discontinuity Boundary Element Method supports the model of a contractional stepover along northeast-trending right-lateral faults and indicates that an east-northeast maximum compressive stress trajectory is responsible for the formation of the structural suite. The structural suite overlies a known basement fault that is part of the Humboldt fault zone marking the east side of the Midcontinent Rift System. The trend of the basement fault is north-northeast, and the strike-slip fault traces have a clockwise rotation with respect to the basement faults. This rotation is consistent with the expected rotation if the strike-slip faults formed as twist segments above a preexisting basement fault in the model-determined regional stress field. An east-northeast maximum compressive stress trajectory suggests potential activity during the Laramide Orogeny, which agrees with the age of kimberlite emplacement in neighboring Riley County. The current stress field in Kansas has an east-west maximum compressive stress trajectory that could potentially produce earthquakes along these faults. Similarities between this structural suite and structural interpretations for the New Madrid Seismic Zone support the potential for seismicity along the Pottawatomie County faults.