This paper applies double‐difference earthquake relocation techniques to investigate sources of seismicity in southwest British Columbia, Canada, and the San Juan Islands, Washington. The study area is a complex region of deformation and has the potential for large earthquakes in the North Americancrust. Double‐difference earthquake relocation techniques are applied to identify otherwise‐hidden active structures that may pose a hazard to nearby population and infrastructure. We present evidence for previously unrecognized active structures using precise relative earthquake relocations obtained using both catalog and waveform cross‐correlation data. Results have significantly reduced errors over routine catalog locations and show lineations in areas of clustered seismicity. In southwest British Columbia, these lineations or streaks appear to be hidden structures that do not disrupt near‐surface sediments; however, in the San Juan Islands the identified lineation could be related to recently mapped surface expressions of faults identified from seismic reflection and multibeam bathymetric surveys. We use a variety of velocity models for the relocations and find that inappropriate models lead to artifacts at layer boundaries and increased vertical errors.

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