Shear-wave splitting measurements are made using teleseismic S, ScS and SKS waveforms recorded at the GDSN broadband station SNZO, situated in South Karori, New Zealand. The average S and SKS delay times are around 2 to 3 s, among the highest in the world. The average ScS delay time is significantly smaller, around 1 s. This discrepancy appears to be due to differences in dominant frequency. The most likely cause of frequency-dependent anisotropy is oriented heterogeneities with a scale-length much smaller than the wavelength. The fast polarizations range between 21ø and 79 ø, with an average NE-SW direction which is sub-parallel to the trend of the local geologic structure and the strike of the Hikurangi subduction zone. Azimuthal variations in delay time, which cannot be explained by differences in period, may be due to a dipping axis of symmetry, or laterally varying anisotropy, or a more complicated symmetry system.
Abstract. Teleseismic ScS and SKS events recorded on nine broadband seismograph stations have been used to investigate seismic anisotropy beneath the lower half of the North Island, New Zealand. This area lies above the Hikurangi subduction zone, and the array provides ray paths which sample the mantle both above and below the slab. Shear wave splitting measurements give similar fast polarizations and delay times at each station. The average SKS fast polarization is approximately NE-SW, subparallel to the strike of subduction and the major geological features, with an average SKS delay time of 1.6 + 0.1 s. This lack of.variation in splitting parameters suggests that similar fast polarizations are found in both the mantle wedge and the subslab mantle. The anisotropy in the lithospheric portion of the mantle wedge is most likely caused by the preferred orientation of olivine due to the shear deformation associated with oblique convergence. Any anisotropy in the slab is probably due to fossil mineral alignment. Anisotropy in the asthenosphere is most likely caused by the preferred orientation of olivine due to asthenospheric flow. The similar NE-SW fast polarizations found in the asthenosphere both above and below the slab suggest that the mantle flow is in a trench-parallel direction in both regions. IntroductionShear wave splitting of teleseismic phases such as ScS and SKS is now a common method used to investigate mantle anisotropy, which can in turn be related to deformation of the upper mantle [e.g., Silver, 1996]. This phenomenon occurs when a shear wave enters an anisotropic medium, upon which it is split into two orthogonally polarized waves which travel with different velocities [e.g., Crampin, 1981 ]. Shear wave splitting measurements can be characterized by two parameters; the polarization direction of the fast shear wave, q•, can be related to the symmetry of the anisotropic system, and the time separation between the two waves, fit, can be related to the strength of anisotropy and the path length through the anisotropic material. Thus, if the cause of the anisotropy is known, the splitting parameters can be related to the deformation and tectonic structure of a region. For example, mantle anisotropy is usually assumed to be due to straininduced lattice-preferred orientation of olivine. The polarization direction of the fast shear wave is then assumed to align parallel to the mantle flow direction, if it is in the form of
S U M M A R YFrequency-dependent anisotropy was previously observed at the permanent broad-band station SNZO, South Karori, Wellington, New Zealand. This has important implications for the interpretation of measurements in other subduction zones and hence for our understanding of mantle flow. This motivated us to make further splitting measurements using events recorded since the previous study and to develop a new modelling technique. Thus, in this study we have made 67 high-quality shear wave splitting measurements using events recorded at the SNZO station spanning a 10-yr period. This station is the only one operating in New Zealand for longer than 2 yr. Using a combination of teleseismic SKS and S phases and regional ScS phases provides good azimuthal coverage, allowing us to undertake detailed modelling. The splitting measurements indicate that in addition to the frequency dependence observed previously at this station, there are also variations with propagation and initial polarization directions. The fast polarization directions range between 2 • and 103 • , and the delay times range between 0.75 s and 3.05 s. These ranges are much larger than observed previously at SNZO or elsewhere in New Zealand. Because of the observed frequency dependence we measure the dominant frequency of the phase used to make the splitting measurement, and take this into account in the modelling. We fit the fast polarization directions fairly well with a two-layer anisotropic model with horizontal axes of symmetry. However, such a model does not fit the delay times or explain the frequency dependence. We have developed a new inversion method which allows for an inclined axis of symmetry in each of the two layers. However, applying this method to SNZO does not significantly improve the fit over a two-layer model with horizontal symmetry axes. We are therefore unable to explain the frequency dependence or large variation in delay time values with multiple horizontal layers of anisotropy, even allowing for dipping axes of symmetry. For the upper layer of anisotropy, the modelled fast polarization directions lie in the range 30 • to 110 • , subparallel to the major geological features as well as the relative direction of plate motion. We suggest that the upper layer of anisotropy consists of crust and subducting slab, and that the lower layer consists of subslab mantle. For the lower layer of anisotropy, the modelled fast polarization directions lie in the range −50 • to 30 • , which are intermediate between the values expected for trench-parallel flow and subduction entrained flow, suggesting a combination of both effects.
We performed nonlinear waveform inversion for source depth, time function, and mechanism, by modeling direct P and S waves and corresponding surface reflections at teleseismic distances. This technique was applied to moderate size events, and so we make use of short period or broadband records, and utilize SV waveforms in addition to P and SH. For the inversion we used a direct search method called the neighborhood algorithm (NA), which requires just two control parameters to guide the search in a conceptually simple manner, and is based on the rank of a user-defined misfit measure. We use a simple generalized ray scheme to calculate synthetic seismograms for comparison with observations, and show that the use of a derivative-free method such as the NA allows us to easily substitute more complex synthetics if necessary. The source mechanism is represented in two different ways; the superposition of a double-couple component with an isotropic component, and a general moment tensor with six independent components. Good results are obtained with both synthetic input data and real data. We achieve good depth resolution and obtain useful constraints on the source-time function and source mechanism, including an isotropic component estimate. Such estimates provide important discriminants between man-made events and earthquakes. We illustrate inversion with real data using two earthquakes, and in both cases the source parameter estimates compare well with the corresponding centroid moment tensor solutions. We also apply our technique to a known nuclear explosion and obtain a very shallow depth estimate and a large isotropic component.
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