Preliminary Oklahoma Optimal Fault Orientations Determined by Focal Mechanism Solutions

Abstract:

Six hundred and eighty-eight focal mechanisms were calculated between 2010 and 2015 and used to determine optimally oriented fault orientations within the contemporary stress field in Oklahoma. The contemporary stress, maximum horizontal stress (s_hmax) orientation of N85°E, is determined from the orientation of the P- and T-axes of the 688 focal mechanisms used. The majority of the focal mechanism solutions were computed using earthquakes occurring in central and north-central Oklahoma, a region where the greatest numbers of recent earthquakes have occurred. The focal mechanisms used in this compilation include Regional Moment Tensor solutions and first-motion focal mechanisms. We determined a mean s_hmax of 83.2° with a standard deviation of 21.3° azimuth. The median s_hmax is 84.8° with 633 observations. From the probability density functions, it is possible to define orientations of optimal, moderately optimal and sub-optimal fault strikes. The focal mechanism distribution is dominated by strike-slip motion on steeply dipping faults and thus fault strike is restricted to the range of 0° to 180°. Optimal fault strike orientation ranges between 45°-60°, 105°-120° and 135°-150° and represent fault orientations most likely for failure. Moderately optimal fault strike orientation ranges between 15°-45°, 60°-75°, 90°-105° and 120°-135° and represent fault orientations moderately likely for failure. All other orientations of fault strike are sub-optimal orientations and have a low likelihood of failure. These results do not indicate that failure cannot occur on sub-optimal fault strikes, but suggest that they are less likely. We apply these results to the most comprehensive Oklahoma fault map to date. Identifying optimal fault orientations is important for determining the potential earthquake hazard of both naturally occurring and triggered seismicity.