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A multiphase seismic investigation of the shallow subduction zone, southern North Island, New Zealand


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Reading, AM ORCID: 0000-0002-9316-7605, Gubbins, D and Mao, W 2001 , 'A multiphase seismic investigation of the shallow subduction zone, southern North Island, New Zealand' , Geophysical Journal International, vol. 147, no. 1 , pp. 215-226 .

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The shallow structure of the Hikurangi margin, in particular the interface between the Australian Plate and the subducting Pacific Plate, is investigated using the traveltimes of direct and converted seismic phases from local earthquakes. Mode conversions take place as upgoing energy from earthquakes in the subducted slab crosses the plate interface. These PS and SP converted arrivals are observed as intermediate phases between the direct P and S waves. They place an additional constraint on the depth of
the interface and enable the topography of the subducted plate to be mapped across the region.
301 suitable earthquakes were recorded by the Leeds (Tararua) broad-band seismic array, a temporary line of three-component short-period stations, and the permanent
stations of the New Zealand national network. This provided coverage across the land area of southern North Island, New Zealand, at a total of 17 stations. Rays are traced
through a structure parametrized using layered B-splines and the traveltime residuals inverted, simultaneously, for hypocentre relocation, interface depth and seismic velocity.
The results are consistent with sediment in the northeast of the study region and gentle topography on the subducting plate. This study and recent tectonic reconstructions of
the southwest Pacific suggest that the subducting plate consists of captured, oceanic crust. The anomalous nature of this crust partly accounts for the unusual features of the Hikurangi margin, e.g. the shallow trench, in comparison with the subducting margin further north.

Item Type: Article
Authors/Creators:Reading, AM and Gubbins, D and Mao, W
Keywords: converted phase, Hikurangi margin, New Zealand, plate topography, subduction zone.
Journal or Publication Title: Geophysical Journal International
ISSN: 0956-540X
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