Seismicity Relocation and Fault Structure Near the Leech River Fault Zone, Southern Vancouver Island

Relatively low rates of seismicity and fault loading have made it challenging to correlate microseismicity to mapped surface faults on the forearc of southern Vancouver Island. Here we use precise relocations of microseismicity integrated with existing geologic data to present the first identification of subsurface seismogenic structures associated with the Leech River fault zone on southern Vancouver Island. We used the HypoDD double-difference relocation method to relocate 1,253 earthquakes reported by the Canadian National Seismograph Network catalog from 1992 to 2015. Our results reveal an ~8–10 km wide, NNE dipping zone of seismicity representing a subsurface structure along the eastern 30 km of the terrestrial Leech River fault zone and extending 20 km farther eastward offshore, where the fault bifurcates beneath the Juan de Fuca Strait. Using a clustering analysis, we identify secondary structures within the NNE dipping fault zone, many of which are subvertical and exhibit right-lateral strike-slip focal mechanisms. We suggest that the arrangement of these near-vertical dextral secondary structures within a more general NE dipping fault zone, located 10–15 km beneath the Leech River fault as imaged by LITHOPROBE, may be a consequence of the reactivation of this fault system as a right-lateral structure in crust with a preexisting NNE dipping structural fabric. Our results provide the first confirmation of active terrestrial crustal faults on Vancouver Island using a relocation method. We suggest that slowly slipping active crustal faults, especially in regions with preexisting foliations, may result in microseismicity along fracture arrays rather than along single planar structures.