Source Spectral Properties of Small to Moderate Earthquakes in Southern Kansas

The source spectral properties of injection‐induced earthquakes give insight into their nucleation, rupture processes, and influence on ground motion. Here we apply a spectral decomposition approach to analyze P wave spectra and estimate Brune‐type stress drop for more than 2,000 ML1.5–5.2 earthquakes occurring in southern Kansas from 2014 to 2016. We find that these earthquakes are characterized by low stress drop values (median ∼0.4 MPa) compared to natural seismicity in California. We observe a significant increase in stress drop as a function of depth, but the shallow depth distribution of these events is not by itself sufficient to explain their lower stress drop. Stress drop increases with magnitude from M1.5 to M3.5, but this scaling trend may weaken above M4 and also depends on the assumed source model. Although we observe a nonstationary, sequence‐specific temporal evolution in stress drop, we find no clear systematic relation with the activity of nearby injection wells. Plain Language Summary The rate of earthquake occurrence in regions of oil and gas production in the central and eastern United States has increased sharply over the last 8 years. In this study, we analyze the source spectra, or frequency content, of earthquakes occurring in one such prominent region of active oil and gas production: southern Kansas. This study is one of the first and the largest to date that provides a quantitative comparison between the spectral properties of these earthquakes, which are potentially induced by human activity, and those of earthquakes that occur in California due to natural tectonic processes. We find that earthquakes in southern Kansas are depleted in high‐frequency energy compared to natural earthquakes in California but that their relative frequency content increases significantly with depth and with magnitude. We also observe significant spatial and temporal variations in source spectral properties that may in part be driven by widespread wastewater disposal during oil and gas production. Characterizing the source spectral properties of these earthquakes is important because it lends insight into the physical processes causing these events and because the frequency content of the source has a strong influence on the intensity of shaking felt by the local population. Key Points We estimate moment, corner frequency, and stress drop for more than 2,000 ML1.5‐5.2 earthquakes in Kansas These events exhibit relatively low median values of stress dr