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Rapid induced seismicity mitigation and its impact on aftershock productivity in Oklahoma

Thomas H. Goebel, Zach Rosson, Emily E. Brodsky, & Jake I. Walter

Published August 13, 2018, SCEC Contribution #8416, 2018 SCEC Annual Meeting Poster #089

Induced earthquakes provide a rare opportunity to study the connection between stress perturbations and resulting seismic activity. Fluid-injection can directly trigger seismic events, leading to elevated background rates, but may also result in main-aftershock sequences and event clustering similar to tectonic sequences. We examine the influence of targeted injection rate reductions after notable earthquakes on ensuing aftershock sequences in Oklahoma. We identify aftershock sequences by separating the population of clustered from background events within a space-time-magnitude domain. In comparing aftershock productivity between California and Oklahoma up to magnitude ~6, we find similar power-law scaling between mainshock magnitude and average number of aftershocks with an exponent close to 1. However, several events above M4.4 in Oklahoma appear significantly deficient in number of aftershocks compared to the overall trend. Sequences with fewer aftershocks experienced rapid mitigation and reduced injection rates, whereas two events with M4.8 and M5 without mitigation exhibit normal aftershock productivity. The timing of when aftershock activity is reduced or ceases correlates with drops in injection rates within about a week. We quantify the expected poroelastic stress perturbations due to injection rate changes within a layered axisymmetric model and find stresses to be lowered by 10s to 100s kPa within the injection zone. The reduction in induced stresses is about an order of magnitude smaller at the depth of the earthquakes. The observations and modeling results suggest that targeted injection-rate-decrease can lower fault stresses below triggering stresses from preceding mainshocks. Induced seismicity mitigation may explain rapid drops in seismicity rates and low-productivity aftershock sequences.

Goebel, T. H., Rosson, Z., Brodsky, E. E., & Walter, J. I. (2018, 08). Rapid induced seismicity mitigation and its impact on aftershock productivity in Oklahoma. Poster Presentation at 2018 SCEC Annual Meeting.

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