Faults and Stress: Investigating Sources of Seismic Hazard in Southwest British Columbia.

The Cascadia subduction zone in Southwest British Columbia experiences both megathrust earthquakes at the subduction interface as well as large events from faults in the overlying crust. These crustal events, with recorded magnitudes of up to 7.3, pose significant hazard to major population centres in the area. Previous studies have shown this is a complex region of deformation located above a bend in the subducting Juan de Fuca plate. By investigating seismicity patterns and the state of stress, we aim to gain a better understanding of the forces driving earthquake activity and the resulting seismic hazard. We present evidence for new active structures in the region by using the double difference technique for earthquake relocation. These structures do not correlate with mapped surface faults, but have a similar strike to faults in the area. We also examine crustal anisotropy from shear-wave splitting measurements at both permanent and temporary stations in the vicinity of Vancouver Island. Past observations of crustal anisotropy in various regions around the globe have been interpreted as the fast direction being related to the orientation of the maximum horizontal compressive stress. However, anisotropy can be complicated by crustal structure, as indicated by variations of the fast direction between nearby stations. Sheared and foliated rocks due to faulting and metamorphism can influence crustal anisotropy. This appears to be the case for some of the Vancouver Island stations. To use seismic anisotropy as a stress indicator requires identifying which stations are influenced by stress and which by structure. This can be done for most stations, as the fault strikes differ from the maximum horizontal compressive stress direction. Preliminary results indicate that most stations show margin-parallel fast directions related to the crustal stress; however, stations located within a few kilometres of major faults have fault-parallel fast directions. The stations that show stress-related anisotropy tell us about the orientation of the maximum compressive stress and could be used to monitor temporal changes in the stress over extended time periods. Stations with structure-related anisotropy are used to investigate how crustal faults effect shear-wave splitting results.