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S U M M A R YThe major strike-slip faults in the greater Marlborough region, central New Zealand, are of both scientific and societal interest as they accommodate relative plate motion in the upper plate of an oblique subduction zone and pose a high seismic risk to central New Zealand. Studies in California suggest that some plate-bounding strike-slip faults are frictionally weak and that crustal anisotropy is controlled by the ambient stress. Whether these observations are more generally applicable to major strike-slip faults has yet to be determined. We have used inversions of focal mechanism and first motion data to calculate the principal stress directions in Marlborough and related them to the geometry of the major faults. The average angle between the axis of maximum horizontal compressive stress (S Hmax ) and the average strike of the major faults is 60 • ; this is substantially higher than the ∼30 • expected for reactivation of a vertical strike-slip fault given Byerlee friction and hydrostatic fluid pressure. This geometry can be explained, however, by the faults having a moderately low friction coefficient (∼0.35), a moderately high fluid pressure (∼0.7 × lithostatic) or some combination of the two. This observation substantiates the hypothesis that the San Andreas fault is not unique in being frictionally weak. We have also conducted shear-wave splitting analysis on local S phases to determine the directions of crustal anisotropy and investigated their orientations with respect to the geological fabric and the principal stress directions. The anisotropy determined using shallow earthquakes reveals that the fast direction is 65 ± 50 • and is generally aligned with the NE-SW-striking faults, and we therefore conclude that crustal anisotropy in Marlborough is controlled more by the geological structures than by the prevailing stress field.

[1] This paper investigates the orientation and sources of stress in the forearc of the Cascadia subduction zone in southwest British Columbia, using Bayesian inversion results from focal mechanism data and comparing results with GPS derived short-term strain rates. The subduction margin in this region includes a change in orientation from N-S in Washington State to NW-SE in British Columbia. Over 1000 focal mechanisms from North American crustal earthquakes have been calculated to identify the dominant style of faulting, and ∼600 were inverted to estimate the principal stress orientations and the stress ratio. Our results indicate the maximum horizontal compressive stress orientation changes with distance to the trench, from approximately margin-normal along the coast to approximately margin-parallel 100-150 km inland from the coast. Comparing stress orientations with GPS data, we relate the margin-normal stress direction to subductionrelated strain rates due to the locked interface between the North American and Juan de Fuca plates just west of Vancouver Island. Further from the margin the plates are coupled less strongly, and the margin-parallel maximum horizontal compressive stress in the North American Plate relates to the northward push of the Oregon Block, which is also observed in the horizontal shortening direction of the residual strain rates, after the subduction component is removed.

S U M M A R YThis paper aims to identify sources and variations of crustal anisotropy from shear-wave splitting measurements in the forearc of the Northern Cascadia Subduction Zone of southwest British Columbia. Over 20 permanent stations and 15 temporary stations were available for shear-wave splitting analysis on ∼4500 event-station pairs for local crustal earthquakes. Results from 1100 useable shear-wave splitting measurements show spatial variations in fast directions, with margin-parallel fast directions at most stations and margin-perpendicular fast directions at stations in the northeast of the region. Crustal anisotropy is often attributed to stress and has been interpreted as the fast direction being related to the orientation of the maximum horizontal compressive stress. However, studies have also shown anisotropy can be complicated by crustal structure. Southwest British Columbia is a complex region of crustal deformation and some of the stations are located near large ancient faults. To use seismic anisotropy as a stress indicator requires identifying which stations are influenced by stress and which by structure. We determine the source of anisotropy at each station by comparing fast directions from shear-wave splitting results to the maximum horizontal compressive stress orientation determined from earthquake focal mechanism inversion. Most stations show agreement between the fast direction and the maximum horizontal compressive stress. This suggests that anisotropy is related to stress-aligned fluid-filled microcracks based on extensive dilatancy anisotropy. These stations are further analysed for temporal variations to lay groundwork for monitoring temporal changes in the stress over extended time periods. Determining the sources of variability in anisotropy can lead to a better understanding of the crustal structure and stress, and in the future may be used as a monitoring and mapping tool.

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