Geological structure of the Paradox Valley Region, Colorado, and relationship to seismicity induced by deep well injection

The Bureau of Reclamation (Reclamation) operates a deep injection well at Paradox Valley in western Colorado. Seismicity has been recorded since 1985, with more than 5900 likely induced earthquakes detected since injection operations began in 1991, making this project a unique and valuable case study in the long-term development of injection-induced seismicity. Likely induced earthquakes occur in a complex pattern with clusters separated by aseismic gaps of 2 km or more, and have been observed up to 16 km from the injection well. To evaluate how the subsurface geologic structure may control the occurrence of induced seismicity, we compare the earthquake locations to a set of geologic models that Reclamation commissioned prior to drilling the injection well. Our analysis indicates that many aspects of the observed seismicity pattern can be attributed to the complex geologic structure. The earthquake depths and locations are generally consistent with the structural trends of the primary injection target formation, within the uncertainty of the models, and some of the mapped basement faults appear to serve as boundaries in the earthquake locations, suggesting that they are barriers to flow. Additionally, the majority of events follow the predicted fluid flow pattern, which trends northwest-southeast and wraps around Paradox Valley. Some features of the seismicity pattern, such as the early onset of seismicity to the northwest of the injection well and the distinct clustering, are not explained by the current geologic models and may be related to unmapped geologic features or local variations in the stress field.