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

Seismic phase conversions provide important constraints on the layered natu re of subduction zone structures. Recordings from digital stations in North Island, New Zealand, have been examined for converted ScS-to-p (ScSp) arri vals from deep (> 150 km) Tonga-Kermadec earthquakes to image layering in t he 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 pr ecursor (ScSp(1)) corresponds to conversion from the base of an 11-14 km th ick subducting Pacific crust. The second precursor is interpreted as a conv ersion from the top of the subducting plate. The amplitude ratio, ScSp(1):S cS, increases from 0.10 to 0.19 from northern to southern North Island. Thi s is within the range expected from a simple first-order velocity discontin uity at an oceanic Moho. A 1-2 km thick layer of low-velocity sediment at t he top of the subducting plate is required to explain the remaining ScSp wa veform. Our results imply that the abnormally thick Hikurangi-Chatham Plate au has been subducting beneath New Zealand for at least 2.9 Myr, thus expla ining the high uplift rates observed across eastern North Island.