Foreshock seismicity suggests gradual differential stress increase in the months prior to the 3 September 2016 M_w 5.8 Pawnee Earthquake

The 3 September 2016 M_w 5.8 Pawnee earthquake was the largest earthquake in Oklahoma history. To determine the relationship with nearby seismic activity and whether there were any precursory foreshocks to the event, we analyze a dataset comprising regional seismometers throughout Oklahoma, including the Oklahoma Geological Survey statewide network. We use a matched-filter technique to identify potential missing earthquakes before and after the mainshock. This technique utilizes cataloged waveforms as templates that, when correlated against continuous waveforms, are able to identify repeating or nearby earthquakes. As shown in other studies, the aftershocks roughly fall along an east-southeast-trending linear belt that illuminates a previously unknown fault. We utilize the augmented seismicity catalog to draw two main conclusions. From standard statistical seismological methods, we deduce that the b-value decreased prior to the Pawnee mainshock; previous work suggests that b-value and differential stress are anticorrelated. The b-value result suggests the differential stress along the fault that eventually ruptured was gradually increasing in the few months leading up to the mainshock. During the aftershock period, we infer an Omori-Utsu p - value close to 1, typical for most tectonic events. Although the prolonged seismicity in the months leading up to the event may be a hallmark of induced earthquakes and may help decipher induced earthquakes from natural earthquakes, the rate of aftershocks is indistinguishable from natural tectonic earthquakes though deficient in larger aftershocks.