Observations of very high P-velocities in the subducted slab, New Zealand, and their relation with the slab geometry

Abstract

A set of high quality aftershock data recorded on a closely spaced network of seismographs at Wellington, New Zealand, has enabled the determination of apparent wavefront speeds and azimuths controlled by the subducting Pacific plate. The former were very high (8.7 ± 0.2 km s-1) and the latter were shifted by as much as 10d̀ eastward of the source-receiver line, which made an angle of about 12d̀ downdip with the strike of the subducted slab beneath the North Island.

The wavefront characteristics are modelled in terms of head-wave propagation along a curved refractor with a cylindrical geometry, using both a simple geometrical approach as well as a 3-D ray tracing program. With this structure, it is not possible to explain both the high apparent velocity as well as the large azimuthal anomaly, without the presence of an unusually high P-wave velocity of around 9.0 km s-1 within the upper mantle of the subducted Pacific plate. Such high velocities, although reported previously in the North Island, are very hard to reconcile with known upper mantle rock velocities. Other alternatives were sought to explain the apparent slowness. In particular, modifications in the slab curvature along strike, according to proposed models, were found to contribute to the high wavefront speeds, but very high intrinsic P-velocities (8.8 km s-1) are still required. From slowness vector data, it is not possible to constrain the depth at which such velocities have to exist; however, traveltime considerations limit them to depths between the base of the subducted oceanic crust to about 12 km deeper.