Optimal Fault Orientations within Oklahoma

One hundred and fifty-two focal mechanisms determined for earthquakes occurring in Oklahoma for about 30 months were used to determine the optimally oriented fault orientations within the region. A large number of these focal mechanisms were concentrated in central Oklahoma and can be associated with the ongoing Jones earthquake swarm and the 5 November 2011 Prague earthquake sequence. In addition, there were focal mechanisms determined throughout Oklahoma while the Earthscope Transportable Array was located in the region. Focal mechanisms used in this paper include Regional Moment Tensor solutions when available and first-motion focal mechanisms. The probability density functions (PDF) with a 10 degrees bin were calculated for both possible nodal planes associated with the observed focal mechanisms. Focal mechanisms within the Jones swarm are clearly different than those from the rest of Oklahoma, and appear to be primarily controlled by the orientation of existing natural fracture orientations. The results clearly demonstrate that strike-slip motion on steeply dipping faults dominates the focal mechanism distribution. Fault strike was restricted to the range of 0 degrees to 180 degrees. The optimal fault strike orientation ranges between 40 degrees-60 degrees and 130 degrees-150 degrees and represent fault orientations most likely to have an earthquake. From the PDF it is possible to define orientations of moderate likelihood of having earthquakes with strikes in the range of 20 degrees-40 degrees and 110 degrees-130 degrees with all other orientations of fault strike having a low likelihood. These results may help oil and gas operators in Oklahoma modify operations to reduce the likelihood of triggered seismicity.