Quantitative Geomorphic Analyses of Neotectonic Activity in the Central U.S. and Implications for Current Increased Seismicity

Abstract. This regional reconnaissance compares vector and scalar geomorphic indicators (transverse drainage basin asymmetry (TI), hypsometric integral (HI), and stream sinuosity index (SI)) in Kansas and Oklahoma to assess neotectonic geomorphology and explore the spatial association between geomorphic indices and basement structure. The study is motivated by both an observed increase in seismicity in the area between 2009 and 2019 and the gap in studies of crustal deformation in this region. In Kansas and Oklahoma, 1,697 and 950 4th-order basins, respectively, were analysed to assess TI to generate spatially averaged mean vectors. Geographic ‘domains’ with the preferred azimuthal direction of mean vectors were defined. In Kansas, six azimuthal domains were defined, whereas in Oklahoma, five azimuthal domains were defined. Most of the basin asymmetry can be attributed to regional climate conditions; however, an azimuthal domain containing increased seismicity in Oklahoma is consistent with neotectonic activity on faults of the Ozark Uplift. HI and SI pattern maps for both Kansas and Oklahoma were generated from hotspot analysis (LISA method) and the Total Sinuosity method combined with Empirical Bayesian Kriging (EBK). The distribution of HI hotspots and SI anomalies suggests ongoing tectonism. In eastern Kansas, active crustal deformation is indicated for an area containing increased seismicity within the Bourbon Arch and Nemaha Ridge fault zones. Another HI hotspot may relate to neotectonism of the Central Kansas Uplift fault zone, while a HI cold spot and low SI anomaly in Gray and Finney counties may indicate a subsiding neotectonic block. In Oklahoma, two possible neotectonic corridors coincide with high earthquake activity. A north-south corridor of high HI and SI values aligns with the Nemaha Ridge, extending into Kansas along the Humboldt Fault Zone. A high SI anomaly across the Cherokee Shelf and Arkoma Basin fault zones corresponds to an area of anomalous drainage basin asymmetry, consistent with neotectonism. These results are consistent with slow active tectonism in Kansas and Oklahoma, suggesting that the significant increase in earthquakes over the past two decades may have a natural source of seismicity.