Journal Article

Fundamental leaking mode (<i>PL</i>) propagation along the Tonga—Kermadec—Hikurangi—Macquarie margin

Richard J.E. Lodge, Grigory M. Steblov and David Gubbins

in Geophysical Journal International

Volume 137, issue 3, pages 675-690
Published in print June 1999 | ISSN: 0956-540X
Published online June 1999 | e-ISSN: 1365-246X | DOI: http://dx.doi.org/10.1046/j.1365-246x.1999.00816.x
Fundamental leaking mode (PL) propagation along the Tonga—Kermadec—Hikurangi—Macquarie margin

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Summary

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.

Keywords: dispersion; Havre Trough; leaky modes; New Zealand

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Subjects: Geophysics

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