Clustering of induced earthquakes in Oklahoma and the relation to subsurface geologic features

Abstract:

Recent research shows that a significant amount of seismic activity in Oklahoma since 2009 is likely related to fluid injection. Seismicity has been observed to generally increase in areas where injection volume and rates are highest but there is a significant amount of variation, with nearby areas experiencing notably different amounts of seismicity. Likely contributors to these differences are the presence of optimally oriented faults and differences in rock permeability structure since rocks with higher permeability are more likely to experience pore fluid pressure changes that can bring faults closer to failure. Most earthquakes occur at depths of ~4-8 km, within crystalline rock of the mid-Proterozoic western granite-rhyolite terrane. Within this terrane, drill-hole data show instances of mafic intrusive rocks that may in part be related to the mid-continent rift. To better understand relations between subsurface geology and seismicity, we reprocessed 1970’s NURE airborne magnetic data. The 5-8-km line spacing of the surveys is coarse, but the processed data show subtle, ~20-35-km long lineaments trending mostly NNW but also NW and NE. Some of these correspond to mapped faults; others are interpreted to represent faults or folds. In some areas the lineaments correspond to earthquake locations, but the magnetic data are often too coarse to establish a clear correspondence. The magnetic maps also show rounded highs that are located near sites where drilling has encountered mafic rock, strongly suggesting that the highs represent mafic intrusions. We find that many earthquakes cluster around or near the edges of the magnetic highs but much less frequently over them, even when they are located close to high-volume injection wells. We hypothesize that the rounded magnetic highs represent mafic intrusions with decreased permeability. In contrast, contacts at the edges of these intrusions may have higher permeability and are thus more likely to experience seismicity.