Distribution of seismic anisotropy in the subduction zone beneath the Wellington region, New Zealand

Shear wave splitting measurements from S arrivals of local earthquakes recorded at the Incorporated Research Institutions for Seismology (IRIS) broadband sensor SNZO are used to determine a basic anisotropic structure for the subduction zone in the Wellington region. With the use of high-frequency filters, fast anisotropic polarization (phi) and splitting time (delta t) measurements typical of crustal anisotropy are evident, but the larger splitting expected from the mantle is often not resolved. The small splitting seen agrees well with the results of previous studies concerning shallow crustal anisotropy. With the use of lower-frequency filters, measurements more consistent with mantle anisotropy are made. Anisotropy of 4.4 +/- 0.9 per cent with a fast polarization of 29 degrees +/- 38 degrees is calculated for the subducting slab, from 20 to 70 km depth. Using this result in addition to the results of previous studies, a model is proposed. The model requires a frequency-dependent anisotropy of less than 1.4 per cent when measured with a period of similar to 2 s to be present in the sub-slab mantle. Separate from this population, a band of events in northern Cook Strait with an 86 degrees +/- 10 degrees fast polarization is seen. This is at about 40 degrees from the strike of the Hikurangi margin, and suggests a source of shear strain 40 degrees removed from that found in the majority of the region. The cause of this is probably a deformation in the subducting slab in this region, as it moves towards a greater incline to the south.