VARIATIONS AND SOURCES OF CRUSTAL ANISOTROPY IN THE CASCADIA FOREARC, SOUTHWEST BRITISH COLUMBIA

This paper aims to identify sources and variations of crustal anisotropy using shear-wave splitting measurements in Southwest British Columbia. Crustal anisotropy is often attributed to stress and interpreted with the fast direction related to the orientation of the maximum horizontal compressive stress. However, anisotropy can be complicated by crustal structure. Stations that show stress-related anisotropy can indicate the orientation of the maximum compressive stress and could be used to monitor temporal changes in stress over extended time periods which may be related to subduction-zone processes. Stations with structure-related anisotropy can be used to investigate how shear-wave splitting is influenced by sheared and foliated rocks caused by faulting and metamorphism. To use seismic anisotropy as a stress indicator requires identifying which stations are influenced (predominately) by stress and which by structure. This can be done if the strike of the structure and the orientation of maximum compressive stress are different. Over 3000 events recorded at 20 permanent stations and 15 temporary stations were available for shear-wave splitting analysis in the Cascadia Forearc near the bend in the subducting Juan de Fuca plate. This is a complex region of crustal deformation, and some of the stations are located near large, ancient faults. In general, results from shear-wave splitting measurements in Cascadia show margin-parallel fast directions. However, some stations close to old terrane-bounding faults show fault-parallel fast directions. Other stations show variations of fast direction with back-azimuth that might also indicate an influence from crustal structure. Determining the sources of variability in anisotropy can lead to a better understanding of the crustal stress and structure, and in the future could be used as a stress-monitoring and mapping tool.