Shear-wave velocity structure beneath North Island, New Zealand, from Rayleigh-wave interstation phase velocities

SUMMARY

Fundamental-mode Rayleigh-wave dispersion velocities have been determined for interstation paths along the strike of the Hikurangi Margin in eastern North Island, New Zealand, for the period range 17 to 73 s. We invert the observed surface-wave dispersion velocities to show S-wave velocities of 4.2–4.4 km s⁻¹ (Vp∼ 7.5–7.8 km s⁻¹) within a subducting crust fixed at 12 km thickness, overlying an upper-mantle lid with S-wave velocities between 4.7 and 4.9 km s⁻¹ (Vp∼ 8.3–8.7 km s⁻¹), some 40 to 20 km, thick respectively. The inherent non-uniqueness in the inversion controls the surface-wave resolution at these depths to a minimum layer thickness and thickness increment of about 20 km. Earlier body-wave studies divide this upper-mantle lid into a 10 km thick layer of normal mantle velocities (Vp∼ 8.2 km s⁻¹) overlying a very high-velocity layer (Vp∼8.9 km s⁻¹). The implication from this surface-wave study is that this latter layer cannot be much thicker than about 10 km.

The lithospheric thickness of the subducting Pacific plate is of the order of 30 to 50 km, which is at the lower end of the thicknesses found elsewhere for >100 Ma oceanic lithosphere. The depth to the S-wave upper-mantle low-velocity zone of approximately 60 to 80 km correlates with the maximum depth of seismicity along the belt. The thickness and S-wave velocity of the low-velocity zone is controlled by the inversion to be 60–80 km thick with a velocity of 4.6± 0.1 km s⁻¹ (Vp∼ 8.2± 0.15 km s⁻¹).

Anisotropy of oriented olivine has recently been used to explain the presence of high velocities in the subducted slab's uppermost mantle in eastern North Island. However, large SKS splitting delays require the anisotropy to be more extensive in depth. We have carried out inversion of Rayleigh-wave interstation phase-velocity dispersion data assuming an anisotropic mantle with pseudo-hexagonal symmetry. Results indicate that mantle velocities derived from the isotropic inversion are overestimates of the S-wave velocity in the vertical plane of the anisotropic structure, perpendicular to the symmetry axis, by less than 3 per cent; this is within the error bounds of the isotropic inversion results. The LVZ is less well pronounced in anisotropic inversions.

anisotropy, lithosphere, Rayleigh waves, subduction, upper mantle