In Situ Stress and Active Faulting in Oklahoma

Appreciable injection-induced seismicity has been occurring in northcentral Oklahoma since 2009. To better understand these earthquakes, we have compiled new information on the state of stress in the state to compare it with both mapped faults and faults inferred from earthquake epicenters and focal plane mechanisms. Seventy-five new in situ stress orientations are available from wellbores throughout the state. In the north-central part of the state where the induced seismicity is occurring, stress orientation and relative magnitude from focal mechanism inversions show excellent agreement with the wellbore stress orientations. All of the data show remarkably uniform stress directions. The azimuth of SHmax, the maximum horizontal stress, is about N85° 5° E. Strike-slip faulting is occurring in central Oklahoma, with strike-slip/normal and normal faulting observed in northern Oklahoma and southern Kansas. As very few of the thousands of M ≥2:5 earthquakes that have recently occurred are located on, or near, already mapped faults, we utilize the stress information to interpret the likely fault planes associated with over 300 well-constrained focal plane mechanisms. In the vicinity of the January 2016 sequence of magnitude 4 and 5 earthquakes in the Fairview, Oklahoma, region, we illustrate how knowledge of the stress field can be used to identify the faults responsible for the seismicity and better evaluate the potential hazard associated with possible future earthquakes in the area. The September 2016 Mw 5.8 Pawnee event occurred on an unmapped N70°W trending strike-slip fault expected to be active in the local stress field.