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

Abstract

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 events in the Tonga-Kermadec and Macquarie seismic zones at regional distances (8°-25°) along the Australasian/Pacific plate boundary. These arrivals are dispersed typically from 40 s period to 25 s and exhibit prograde elliptical polarization, which is diagnostic of the regional leaking mode PL. Theoretical dispersion curves are generated with the simplest structure of a slow layer overlying a faster half-space. We have analysed the group velocity dispersion characteristics of the recorded waveforms and successfully modelled it as purely fundamental mode propagation in a low-velocity waveguide. Our best-fitting structure north of New Zealand consists of a low-velocity layer within the mantle (β=4.1-4.3 km s-1) with a thickness 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 arc, 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 backarc spreading in the Havre Trough. Dispersion curves derived from waveforms travelling northwards from events on the Macquarie Ridge give a similar structure with a slow layer 31 km thick with β=3.8 km s-1. This is consistent with the continental crust of South Island, which lies along most of the event-station paths.