Refined earthquake locations for Oklahoma and Southern Kansas illuminate the regional fault network

The 2016 M5.8 Pawnee earthquake struck along the northeastern margin of the seismically active region of Oklahoma. This follows a pattern of several major earthquake sequences that occurred at the edges of the seismically active region including the M4.9 Milan, Kansas and M5.1 Fairview, Oklahoma earthquake sequences that occurred at the northern and western edges, respectively. For much of Oklahoma, augmentation of the seismic network with new stations in the activated areas has followed rather than preceded the spread of seismicity across the state, and consequently the network geometry is often unfavorable for resolving the underlying fault structures. With this study we augment the existing ANSS catalog with data from two industry operated networks for the period May 2013 to March 2016. These networks include 40 broad band seismic stations and cover seismically active north-central Oklahoma with a station spacing on the order of 25 km. Absolute locations with horizontal errors of about 300 m and relative locations better than 100 m reveal a striking pattern of seismicity illuminating many previously unmapped faults. Depths are usually well constrained to within 1 km. Relocated epicenters tend to cluster in linear trends of less than 1 km to more than 10 km in length. These trends are mostly in agreement with surface wave-derived moment tensors. To quantify our location precision, we obtain a high-resolution data set for the 2014/2015 earthquake sequence near Cushing, Oklahoma. Using cross-correlation and template matching techniques, we extend the regional catalog about 10-fold for the Cushing area and obtain precise relative relocations for about 600 events, complete to about M=1.0. Both data sets - the standard relative relocations and cross-correlation derived relocations - show at least two faults that were reactivated along about 1 km of their length and near their intersection point. Our high resolution data shows that seismicity near Pawnee, Oklahoma started in late 2013. The M5.8 epicenter occurred three years later at the edge of this seismically active region. Prior activity occurred in a scattered manner with a lack of major sequences such as those illuminating faults in other parts of the state e.g., Cushing.