The characteristics (foreshock-aftershock patterns, main shock to the largest aftershock magnitude ratio, and difference in magnitude) of two moderate injection-induced seismicity (IIS) earthquake sequences from Oklahoma, USA, namely, Prague (M 5.7, 2011) and Pawnee (M 5.8, 2016) earthquake sequences, are investigated to comprehend the shallow crustal heterogeneity.
The b-value in Oklahoma started increasing after 2009 because of pore pressure increase due to injection of fluids. A decrease was observed after the reduction of the injection rates in 2016 indicating the decrease in IIS at Oklahoma. A sharp fall in b-value usually precedes the main shocks of magnitude M ≥ 3.5. The foreshock b-values are lower than the regional b-value and aftershock b-values are higher than the regional b-value within the error limits. The observed high difference of 2 in the magnitudes of Pawnee earthquake and its largest aftershock could be a response to drastic reduction in injection rates in May 2016.
The characteristics of ISS observed at Oklahoma are similar to the observations from the reservoir-triggered seismicity (RTS). However, with multiple injection wells operating from time to time in the region with varying amount of fluids injected, the entire IIS at Oklahoma has an appearance of a swarm. Both the Prague and Pawnee earthquakes occurred in the regions with fewer mapped faults and in the vicinity of large paleo faults.
R. Rajesh & Harsh K. Gupta, J Seismol, https://doi.org/10.1007/s10950-020-09978-5