Fine-scale fault structures revealed by earthquake relocations near the Leech River Fault, southern Vancouver Island

We present results of microseismic relocations that delineate potentially seismogenic structures along the Leech River Fault (LRF), southern Vancouver Island. The potential seismic activity of the LRF in particular has been the focus of many recent investigations, given its proximity to Victoria, the capitol of British Columbia. We investigate the seismic evidence of subsurface structures near the LRF by relocating 1528 earthquakes reported by Canadian National Seismograph Network (CNSN) catalog from 1985 to 2015 using the HypoDD [Waldhauser, F., 2001] method. This relocation procedure reveals two general northeast dipping structures representing the subsurface fractured deformation zone beneath the LRF and the San Juan Islands area, respectively. The subsurface structure underlying the LRF extends about 30 km along the LRF strike and has a dip of about 45o, which is generally consistent with fault dip inferred from seismic imaging [Green et al., 1987]. In addition, we use the k-mean++ clustering algorithm [Lloyd, 1982; Arthur et al., 2007] to select 5-subclusters for individual relocations in order to investigate possible finer faulting structures. The relocated sub-cluster seismicity, combined with earthquake focal mechanism solutions from previous studies, delineates the subsurface geometry of minor faults near the LRF. Relocation results also show the eastern segment is seismically active while the western segment lacks seismicity. We suggest two potential explanations for this marked variation in seismic behavior. First, seismicity might reflect variations in the regionals tress field [Balfour et al., 2011]. The orientation of the maximum horizontal compressive stress near the LRF is 10-30° clockwise from the strike of eastern segment, which would promote right lateral slip, but is nearly orthogonal to the strike of the western segment, which would inhibit slip. Second, foliations and mylonitic fabric are generally stronger, more developed, and exist over a wider zone ( 200-500m) in the west than in the east ( 10m). The strong anisotropy generated by the well-developed foliations in the west may inhibit microseismicity. Regardless of interpretation, our relocation results suggest that the LRF east and west segments needs to be treated separately in earthquake hazard assessment.