Aftershock Deficiency of Induced Earthquake Sequences During Rapid Mitigation Efforts in Oklahoma

Session: Earthquake Source Parameters: Theory, Observations and Interpretations
Type: Oral
Date: 4/30/2020
Time: 08:30 AM
Room: 120 + 130
Description: 

Induced earthquakes provide a rare opportunity to study the connection between stress perturbations and resulting seismic activity. Fluid-injection may directly trigger events leading to elevated background rates but may also cause main-aftershock sequences and event clustering commonly observed in tectonic sequences. We examine the influence of targeted injection rate reduction after notable mainshocks on ensuing aftershock sequences in Oklahoma. In comparing aftershock productivity between California and Oklahoma up to magnitude ~6, we find similar power-law scaling-behavior 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. Such sequences with deficient aftershocks also 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 roughly correlates with drops in injection rates within less than 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 in Oklahoma.

Presenting Author: Thomas H. W. Goebel

Authors