ScSp observed on North Island, New Zealand: implications for subducting plate structure

Abstract

Seismic phase conversions provide important constraints on the layered nature of subduction zone structures. Recordings from digital stations in North Island, New Zealand, have been examined for converted ScS-to-p (ScSp) arrivals from deep (>150 km) Tonga-Kermadec earthquakes to image layering in the underlying Hikurangi subduction zone. Consistent P-wave energy prior to ScS has been identified from stations in eastern and southern North Island, where the subducted plate interface is at a depth of between 15 and 30 km. Two ScS precursors are observed. Ray tracing indicates that the initial precursor (ScSp1) corresponds to conversion from the base of an 11-14 km thick subducting Pacific crust. The second precursor is interpreted as a conversion from the top of the subducting plate. The amplitude ratio, ScSp1:ScS, increases from 0.10 to 0.19 from northern to southern North Island. This is within the range expected from a simple first-order velocity discontinuity at an oceanic Moho. A 1-2 km thick layer of low-velocity sediment at the top of the subducting plate is required to explain the remaining ScSp waveform. Our results imply that the abnormally thick Hikurangi-Chatham Plateau has been subducting beneath New Zealand for at least 2.9 Myr, thus explaining the high uplift rates observed across eastern North Island.