SEISMIC REACTIVATION OF BASEMENT STRUCTURES IN OKLAHOMA

Increases in both the frequency and magnitude of seismicity (2009-2018) in the Central and Eastern US (CEUS) have generated renewed interest in characterizing the structure and material properties of the midcontinent crystalline basement. In the Oklahoma/Kansas region, widespread seismicity is attributed to the reactivation of pre-existing, critically-stressed, and seismically unstable faults due to decades of wastewater injection. Here, we explore the major ingredients needed for induced earthquakes by characterizing the pre-existing basement faults, and demonstrating that their geometry, structure and mechanical properties make them critically susceptible to seismic reactivation. We accomplish this through a multi-disciplinary approach that includes: outcrop and satellite mapping, 3D seismic analysis, basement core investigation, experimental determination of basement strength, stability and petrophysical properties, and the geological/geophysical characterization of a previously unmapped fault exposed in the Precambrian Troy Granite (Johnston County, OK). To date, our work shows that: 1) exposed Precambrian basement contains widespread fracture/fault systems with trends that display a marked similarity to the trends of recent earthquake lineaments, 2) faults visible in 3D seismic analyses are steeply-dipping, basement-rooted, and in some cases, penetrate the overlying sedimentary sequences, 3) the top of the basement is heavily fractured and there is widespread evidence of hydrothermal alteration in the basement cores, and 4) frictionally unstable behavior and pronounced water-weakening occurs in laboratory experiments designed to mimic in situ conditions. While many of these studies remain ongoing, our preliminary results show that Oklahoma’s Precambrian basement is prone to widespread seismicity by reactivation of preexisting basement structures facilitated by the key properties of these structures.