Astronomy and Astrophysics – Astronomy
Scientific paper
Jun 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999geoji.137..675l&link_type=abstract
Geophysical Journal International, Volume 137, Issue 3, pp. 675-690.
Astronomy and Astrophysics
Astronomy
1
Dispersion, Havre Trough, Leaky Modes, New Zealand
Scientific paper
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 deg-25 deg) 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 (beta=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 beta=3.8 km s^-1. This is consistent with the continental crust of South Island, which lies along most of the event-station paths.
Gubbins David
Lodge Richard J. E.
Steblov Grigory M.
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