Lawrence Berkeley National Laboratory

Large-scale wastewater disposal has led to a fast-paced reawakening of faults in the Oklahoma/Kansas region. High-resolution earthquake relocations show that the inventory of ancient basement faults in the study region differs from results of seismic surveys and geologic mapping focused on the sedimentary cover. We analyze the evolution of seismic activity in the Guthrie-Langston sequence in central Oklahoma in greater detail. Here, seismic activity has reactivated a network of at least 12 subvertical faults in an area less than 10 km across. Recorded activity began in late 2013, peaked about six months later, and includes two M 4 earthquakes. These earthquakes characteristically occur at about 4 km depth below the top of the basement and do not reach the sedimentary cover. The sequence shows a radial growth pattern despite being no closer than 10 km to significant wastewater disposal activity. Hydrologic modeling suggests that activity initiated with a time lag of several years relative to early injection activity. Once initiated, earthquake interactions contribute to the propagation of seismicity along the reactivated faults. As a result, the spatiotemporal evolution of the seismicity mimics a diffusive pattern that is typically thought to be associated with injection activity. Analysis of the fault slip potential shows that most faults are critically stressed in the contemporary stress field. Activity on some faults, for which we find low slip probability, suggests a significant contribution of geomechanical heterogeneities to the reawakening of these ancient basement faults.