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 10years 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.