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Thursday January 25 2018

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need to care about them From ocean to lab Dr Carey said movement behaviour similar to that of slow-slip events had already been observed in some landslides. When analysed, material from these landslides had revealed how rapid and slow landslide movements could occur in response to changes in soil and rock properties caused by seismic shaking from regional earthquakes. “Given that the overall physics governing both landslides and faults are in some cases similar, there are great opportunities to combine approaches from these fields to bring a unique perspective to the understanding of shallow slow-slip event triggering.” At the same time, a large team of researchers working under the International Ocean Discovery Programme were probing the Hikurangi subduction zone from aboard the scientific drilling ship JOIDES Resolution, off the North Island’s East Coast. Along with installing high-tech instruments beneath the sea floor to monitor changes during slowslip events and earthquakes near Gisborne, the scientists were also recovering core samples of sediments, which Dr Carey and his team would analyse themselves. “To undertake experiments designed to suitably replicate the conditions in the shallow subduction zone requires very specialist laboratory testing equipment,” he said. “GNS Science is one of only a few institutions world-wide with the type of instrument that can simulate the conditions within a shallow subduction zone and subject the sediment to simulated earthquake shaking.” Lab tests will first determine the physical characteristics of the rocks in each subduction zone, before specialist experiments are undertaken to replicate stress conditions at each site and measure how and when slow-slip

RESEARCH: Sediments recovered along the Hikurangi subduction zone off the East Coast by scientific drilling vessel JOIDES Resolution will be used to study slow-slip events.

events are triggered by stress changes from passing seismic waves. “One hypothesis that we plan to test is whether or not the passing seismic waves cause a change in pressure from fluids in the fault zone, which might trigger the slow-slip.” All of the data would be combined with the movement records from what remain two of the world’s most rigorously investigated subduction zones known to undergo slip in shallow slow-slip events and generate large earthquakes and tsunamis. “We hope that this study will help to explain how distant earthquakes can trigger slow-slip episodes at subduction zones. “By determining how slow-slip events are triggered and their relationship to larger earthquakes at the Hikurangi subduction zone, we hope to greatly improve our understanding of the potential hazards subduction zone earthquakes and associated tsunamis pose to New Zealand,” said Dr Carey.

WHERE IT HAPPENS: A cross-section of the slow-slip zones at the boundary between the Australian and Pacific plates. Slow-slips in the south (Kapiti and Manawatu) happen deeper than those in the north (Hawkes Bay and Gisborne). IMAGES: GEONET

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