Seismic illumination of small-throw seismogenic faults, Anadarko Basin, Oklahoma

Substantial increase in the occurrence of injection-induced seismicity across Central and Eastern United States in the past decade, has highlighted a need for novel approaches to geophysical subsurface imaging of potentially seismogenic faults. Active clusters of seismicity illuminate linear fault segments within the sedimentary cover and crystalline basement that were unknown until seismicity began. Such surprises are due to the limited availability of 3D seismic reflection surveys and the difficulty of imaging relatively shallow earthquake events from sparse seismic monitoring arrays. The Sooner Trend Anadarko Basin Canadian Kingfisher Counties (STACK) play of the Anadarko Basin, Oklahoma, provides an opportunity to investigate these earthquake-prone basement faults. Modern high-quality seismic data acquired to map unconventional resource plays in the STACK enable us to assess the detailed subsurface structure. Furthermore, because of increased earthquake risk from anthropogenic activities in the past decade, state regulatory agencies have deployed a dense array of seismic monitoring stations, which allows us to integrate earthquake data with seismic reflection data for analysis of active faulting. We have mapped structural deformation using a suite of seismic attributes, including multispectral coherence, volumetric curvature, and aberrancy, in a 3D seismic reflection data set covering 625 sq mi of the STACK area. To unravel the relationship between the structures and seismicity, we use relocated locally recorded earthquakes and compute the focal mechanism solution for the events. Our results reveal previously unmapped small-throw (<120 m) fault segments with dominant north–south, northwest, and northeast trends, most of which extend from the basement up into the shallower sedimentary Hunton and Woodford Formations. Because of the small offset, we find that aberrancy and the curvature attribute best illuminate the basement-rooted faults in the sedimentary cover. Fault segments with significant vertical offset are better illuminated by band-limited multispectral coherence. We argue that the inherited structure of these faults makes them mostly illuminable by flexure-related seismic attributes, especially within the sedimentary cover. The integration of the illuminated faults with relocated earthquakes and focal mechanism solutions shows that the illuminated faults that have hosted intrasedimentary and/or basement seismicity are reactivated by strike-slip faulting. We hypothesize that careful attribute mapping of faults and related flexures, integrated with recent seismic activity data, and an understanding of the local stress and geomechanical properties, can help mitigate seismic hazards in similar intraplate geological settings where small-throw faults predominate.

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