Fundamental leaking mode (PL) propagation along the Tonga-Kermadec-Hikurangi-Macquarie margin

High-quality digital data from the temporary Leeds Tararua broad-band array , North Island, New Zealand and the station SNZO at Wellington often record long-period oscillations within the body wave train from large, shallow ev ents in the Tonga-Kermadec and Macquarie seismic zones at regional distance s (8 degrees-25 degrees) along the Australasian/Pacific plate boundary. The se arrivals are dispersed typically from 40 s period to 25 s and exhibit pr ograde elliptical polarization, which is diagnostic of the regional leaking mode FL. Theoretical dispersion curves are generated with the simplest str ucture of a slow layer overlying a faster half-space. We have analysed the group velocity dispersion characteristics of the recorded waveforms and suc cessfully modelled it as purely fundamental mode propagation in a low-veloc ity waveguide. Our best-fitting structure north of New Zealand consists of a low-velocity layer within the mantle (beta = 4.1-4.3 km s(-1)) with a thi ckness of 32-34 km overlying a typical mantle structure. A seismicity study for the period 1976-1992 at SNZO shows that those events generating PL are shallow (10-60 km), with the highest concentration along the Kermadec are, suggesting that the low-velocity layer is connected with this feature. We suggest that a small degree of partial melt within the uppermost mantle is responsible for creating a low-velocity channel within the region of backar c spreading in the Havre Trough. Dispersion curves derived from waveforms t ravelling northwards from events on the Macquarie Ridge give a similar stru cture with a slow layer 31 km thick with beta = 3.8 km s(-1). This is consi stent with the continental crust of South Island, which lies along most of the event-station paths.