TARARUA BROAD-BAND ARRAY, NORTH-ISLAND, NEW-ZEALAND

The Tararua broadband array, consisting of nine three-component seismo meters, was deployed in southern North Island, New Zealand, from Febru ary 1991 to September 1992. This L-shaped array had an approximately 4 0 km N-S arm and 30 km E-W arm. Its primary purpose was to record wave forms from Tonga-Kermadec earthquakes whose propagation paths have spe nt the majority of their time in the Pacific Plate that has been subdu cted beneath the Australasian Plate along the Tonga-Kermadec-Hikurangi Margin; the continuous recording allowed simultaneous acquisition of an excellent teleseismic and regional broadband seismogram dataset. Fo r the main experiment, broadband three-component waveforms with P-wave precursive slab phases, up to 15 sec early relative to the Jeffreys-B ullen travel times, have been recorded for 71 events. Multiple frequen cy analysis of the P waveform shows that the high frequencies (>5 Hz) arrive first, with coherent dispersion below about 4.5 Hz. This behavi or can be attributed to a high-velocity layer, 8 to 10-km thick, lying above the cold, fast lithosphere of the subducted Pacific Plate. Pola rization analysis of the P-wave precursors show them to arrive at stee p incidence angles of 20 degrees to 30 degrees, suggesting the phase r efracts off the subducted plate's surface to the array. We observe exc eptionally large PL leaking modes from shallow events in the Tonga-Ker madec region to the north.