Investigation of Induced Seismicity in Southern Kansas

Abstract

The US midcontinent has experienced an unprecedented increase in seismic activity over the last decade attributed to increased wastewater injection into the Arbuckle Group, directly above the Precambrian igneous and metamorphic basement. This dissertation presents local earthquake monitoring from 2015 to 2020 at Wellington field in Sumner County, south-central Kansas, which generated a catalog of approximately 4,000 events with a magnitude of completeness of Mw 1.6. This catalog is particularly unique in that it shows the entire cycle of induced seismicity from increasing earthquake frequency to returning towards baseline. Seismicity is shown to progress northward over time, in response to a presumed northward advancing pressure front from distant large-volume wastewater injections to the south of the monitoring area. Cluster analysis on the entire earthquake catalog identified previously unknown reactivating faults trending NW and NE. Those faults align with known basement structures along the Nemaha Ridge and this work is the first direct evidence of their existence. Individual spatial earthquake clusters revealed swarm temporal behavior of induced seismicity, with earthquake activity along a fault segment lasting from several hours to a few weeks. An S to P direct converted wave was identified for the first time in local induced earthquakes and a new method was developed to employ S to P direct converted waves to constrain earthquake depth, improving accuracy of local networks. This is particularly useful in temporary networks that are established to monitor industrial activities. Novel automated shear wave anisotropy analysis codes were developed and used to evaluate very local induced earthquake shear wave splitting parameters, resulting in a deeper understanding of sources of anisotropy in south-central Kansas. This work reveals observations of temporal change in anisotropy and it examines mechanisms of how changes in pore fluid pressure can impact anisotropy. Lastly, undertaking this research involved all aspects of local earthquake monitoring: from planning a network, to field installation, to compiling the earthquake catalog, and finally analyzing the observations. This work resulted in the development of practical guidelines for establishing and maintaining local earthquake monitoring networks. The methods and analyses presented in this dissertation advance the understanding of monitoring and evaluating induced local earthquakes.