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

Shear wave splitting measurements from S arrivals of local earthquakes reco rded at the Incorporated Research Institutions for Seismology (IRIS) broadb and sensor SNZO are used to determine a basic anisotropic structure for the subduction zone in the Wellington region. With the use of high-frequency f ilters, fast anisotropic polarization (phi) and splitting time (delta t) me asurements typical of crustal anisotropy are evident, but the larger splitt ing expected from the mantle is often not resolved. The small splitting see n agrees well with the results of previous studies concerning shallow crust al anisotropy. With the use of lower-frequency filters, measurements more c onsistent with mantle anisotropy are made. Anisotropy of 4.4 +/- 0.9 per ce nt 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 w ith 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 wit h 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 o f shear strain 40 degrees removed from that found in the majority of the re gion. 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.