Double-pair double-difference relocation improves depth precision and highlights detailed 3D fault geometry for induced seismicity in Alberta, Canada

Precise earthquake locations with well-constrained uncertainties can improve our understanding of faulting. Double-difference relocation methods, particularly event-pair double-difference relocations, are well established and have been applied to large earthquake catalogs to provide fault geometries. Previous adaptations of the event-pair double-difference method include data space extensions to use additional information from station pairs, referred to as double-pair double-difference relocation. We apply double-pair double-difference relocation to data from a dense network of borehole geophones for induced seismicity monitoring. This experiment was acquired in an area with strong lithological variation and sharp velocity contrasts, and most previous studies using this dataset are subject to poorly constrained focal depths. We compare the double-pair double-difference to event-pair double-difference relocations and study the effectiveness and uncertainties of both methods. Although double-pair double-difference relocation does not improve absolute locations, substantially improved relative locations and reduced uncertainties are obtained. The method reduces the impact of path effects in the source region, which is essential for applications where reservoir units in the source region can exhibit strong velocity contrasts, anisotropy, and fractures. From the improved relocation, we produce a detailed 3D fault interpretation of the dataset that constrains the geological interpretation. The improved catalog shows excellent depth constraints with seismicity that is restricted to specific geological units. We interpret that seismicity activated pre-existing faults in the reservoir layer and adjacent units. Notably, the results show no evidence of induced seismicity activating basement structures.