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.