Fast P-wave precursors in New Zealand: high velocity material associated with the subducted Hikurangi Plateau

• Published in: Geophysical journal international 2015-08, Vol.202 (2), p.1223-1240
• Main Authors: Love, H., LeGood, M., Stuart, G., Reyners, M., Eberhart-Phillips, D.E., Gubbins, D.
• Format: Article
• Language: English
• ISSN: 0956-540X; 1365-246X
• DOI: 10.1093/gji/ggv205

Seismic tomography has revealed very high P-wave velocities, over 8.5 km s−1, at shallow depths, 30–100 km, beneath New Zealand. Here we study fast, high-frequency arrivals at North and South Island stations that contain additional information about the crust and mantle structure. These arrivals, which are from earthquakes within or close to the land mass, have a characteristic high-frequency precursor followed by a lower frequency, larger amplitude, main phase. Precursors were seen on at least one station from 262 of 306 candidate events; the best-recorded 76 events were analysed for wave speed, frequency content and polarization. Time–distance plots are consistent with two phases travelling at 8.38 ± 0.03 and 6.93 ± 0.05 km s−1. The precursor has typical frequencies 4–9 Hz, the second arrival 2–4 Hz. Polarizations are off-azimuth by 30° and steeper than predicted by ray tracing through a smooth 3-D tomographic model. These results are explained by propagation through a dipping layer of order 10 km thick with seismic velocity around 8.5 km s−1; it is too thin to propagate frequencies below 4 Hz and waves refract from it at a steep, out-of-plane angle, explaining the anomalous polarization. Ray paths cover a region coinciding with the subducted Hikurangi Plateau; the fast layer is interpreted as the lowest section of the plateau that has transformed to eclogite, which has the same fast seismic velocity that we observe. Unlike the fast, eclogitic layers identified in subduction zones such as the Kermadecs, this layer is shallower, at 30 km, than the eclogite transformation; we therefore propose that it formed at the base of the thick plateau prior to subduction.