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Carolyn Boulton

2016: Dr Carolyn Boulton , Victoria University of Wellington, has been awarded Rutherford Foundation postdoctoral fellowship, for research entitled: “Slipping fast and slow: How sediments influence Hikurangi subduction zone seismic hazard”

Subduction zones are formed where one tectonic plate plunges beneath another. These zones produce the largest earthquakes and tsunami on Earth. The North Island of New Zealand overlies the Hikurangi subduction zone (HSZ), an enormous plate-boundary fault that accommodates westward subduction of the Pacific Plate. New geodetic and seismological data have highlighted profound variations in the HSZ’s seismic behaviour and provided evidence for complex interplay between seismic earthquakes and aseismic creep, that is, between fast and slow slip on the plate boundary.

The HSZ fault occurs in Late Cretaceous-Paleogene (70-32 million-year-old) marine sediments that were deposited on both the Australian and Pacific Plates prior to the initiation of subduction. Equivalent marine sediments are exposed across the eastern North and South Islands, providing an ideal opportunity to document variations in sediment composition and collect samples for detailed analyses. This sequence of sedimentary rocks has never been investigated with the aim of understanding Hikurangi subduction zone seismic behaviour and potential hazards. Indeed, there is an intriguing spatial correlation between changing sediment composition and the transition from predominantly aseismic-to-seismic HSZ fault behaviour near Hawke’s Bay.

In this project, Dr Boulton will measure – for the first time – how the frictional strength and stability of Late Cretaceous-Paleogene marine sediments varies from North to South along the HSZ. Since pressurized fluids also affect fault strength, she will determine the conditions required to generate and sustain high fluid pressures in these same fault-forming sediments. Laboratory data will be incorporated into numerical models that simulate faulting in order to understand how sediment composition influences subduction zone mechanics, fluid flow, and potential earthquake rupture scenarios. Ultimately, it is hoped that the proposed research will provide a better understanding of earthquake rupture mechanisms and the hazard that the Hikurangi subduction zone poses to New Zealand society.