Abstract

The Tararua broadband array, consisting of nine three-component seismometers, was deployed in southern North Island, New Zealand, from February 1991 to September 1992. This L-shaped array had an approximately 40 km N-S arm and 30 km E-W arm. Its primary purpose was to record waveforms from Tonga-Kermadec earthquakes whose propagation paths have spent the majority of their time in the Pacific Plate that has been subducted 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.

For the main experiment, broadband three-component waveforms with P-wave precursive slab phases, up to 15 sec early relative to the Jeffreys-Bullen travel times, have been recorded for 71 events. Multiple frequency analysis of the P waveform shows that the high frequencies (>5 Hz) arrive first, with coherent dispersion below about 4.5 Hz. This behavior can be attributed to a high-velocity layer, 8 to 10-km thick, lying above the cold, fast lithosphere of the subducted Pacific Plate. Polarization analysis of the P-wave precursors show them to arrive at steep incidence angles of 20° to 30°, suggesting the phase refracts off the subducted plate's surface to the array. We observe exceptionally large PL leaking modes from shallow events in the Tonga-Kermadec region to the north.

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