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Locations of Injection‐Induced Earthquakes in Oklahoma Controlled by Crustal Structures

In recent years, many small‐ to moderate‐size earthquakes occurred in central northern Oklahoma, likely associated with wastewater injection. The most recent one is the M5.8 Pawnee earthquake occurred on 3 September 2016. It is still not clear what controls the locations of these injection‐induced earthquakes. Here we conduct 2‐D Pg wave tomography with anisotropy to image seismogenic structures in this region, using more than 10 years of Pg arrivals recorded by 81 seismic stations. Our high‐resolution Pg wave tomography shows two high‐velocity zones with northwest fast direction alternated by low‐velocity zones with northeast fast direction. The two dominant anisotropy directions are consistent with conjugated microfractures from surface faults and two nodal planes from focal mechanisms of moderate‐size earthquakes. Most moderate‐size (M > 4) earthquakes occurred either close to the boundaries between high‐ and low‐seismic velocity zones or within the high‐velocity zones, suggesting that they are associated with geological boundaries of different basement rock properties or with strong material properties in the upper crust. We suggest that although these moderate‐size earthquakes were induced by fluid injection, their spatial locations were likely controlled by local geologic structures.