Insights into Shallow Anisotropic Structure in the Forearc Hikurangi Subduction Zone, New Zealand via Splitting of Teleseisms
Abstract
We use a recent transect that consists of 10 broadband stations across the northeast of Wellington region to explore the anisotropic structure of the forearc of the Hikurangi subduction zone in the southern North Island (NI), New Zealand from shear-wave splitting of SKS, ScS and teleseismic S phases. These measurements are then integrated with the previous splitting measurements in northwest of the transect. Splitting parameters from teleseismic S-phases revealed an abrupt lateral variation in the anisotropic structure. The general trend of splitting agrees well with the previous studies around this area, with NE-SW trench-parallel fast direction (φ). The range of delay times ( 0.5 - 3.0 s) and slightly varying SKS φ across the southeast of NI suggest a laterally varying anisotropic structure. As inferred by splitting variations from long period (>7 s) phases across the profile, the upper-plate Wairarapa fault and basin area appear to be characterized by a distinct anisotropic structure that is possibly localized at crustal depths. The sharp change in delay time (δt) around this fault zone divides the region in to two distinct domains of eastern and western sides. The average δt on the eastern side (2.05 × 0.45 s) is ~0.6 s higher than that measured in the western side (1.44 × 0.24 s) of the Wairarapa fault. This change takes place between two stations that are separated by ~3 km. Clear frequency dependent splitting from ScS and teleseismic S-phases suggests that the anisotropic structure is either stratified or governed by more complex anisotropy. Multilayer models are unable to explain the observations adequately, suggesting a more complex structure. We think that this complex structure is governed in part by the laterally-varying crustal contribution of anisotropy and this lateral variation is likely associated with the multilayer anisotropy to form a more complex structure. We suggest that the subduction structure is dominated by the mantle flow in the subslab region and lithospheric shear beneath the upper-plate faulting. At crustal depths, there could be anisotropic contributions from fault structures and clay mineralization around them. To provide further constraints on the observed lateral variations, we will implement a finite-difference approach that enables us to verify whether the lateral variations in splitting parameters are due to discrepancies between anisotropic properties or the isotropic velocity variations of the different regimes in the subduction structure.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFMDI11A2179K
- Keywords:
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- 7240 SEISMOLOGY Subduction zones;
- 7208 SEISMOLOGY Mantle;
- 7218 SEISMOLOGY Lithosphere