SES-10. Induced Seismicity Mitigation Effects Captured in Earthquake Location Time Lapse

Abstract
We present results from a multi-year seismic observation study, conducted in close proximity to wastewater disposal wells. We present an updated catalog of a resulting induced earthquake sequence near Greeley, CO, between 2014 and 2019. High-precision relocations of the microearthquakes through time reveal a pronounced shift in the orientation of active structures, with events in the 2014-mid-2016 time period activating faults in both north-northeast and north-northwest orientations. Events in the mid-2016-2019 time period highlight fault activity only striking in the north-northwest direction. These changes in active fault orientations correspond to a number of injection rate decreases and bottom-well cementations implemented at the two wastewater wells nearest to the seismicity. Our results suggest that the combined mitigation at nearby wells was successful in restricting pore-pressure build up in basement faults beneath the injection sites. To support our observations, we consider theoretical pore pressure ranges needed to activate slip on specific faults, in the context of three-dimensional Mohr-Coulomb stress theory and failure criteria. To support our observation of fault orientation from seismicity cloud shapes, we perform arrival-based focal mechanism analysis for as many of the larger events as possible. Fault-plane orientations were less-constrained in the earliest parts of the sequence, given to a smaller number of seismic stations initially in the observation network. Finally, we combine our observations with secondary stress indicators to infer the local crustal stress state near Greeley, CO.