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Unlocking the secrets of the Hikurangi Subduction Zone

New Zealand’s largest and most active fault — the Hikurangi subduction zone — continues to be intensively researched to reveal the history of past earthquakes and modern-day fault slip behaviour. This zone is where the Pacific tectonic plate dives beneath the Australian plate, and it can produce our largest earthquakes and tsunamis.

Scientists standing in front of submersibles. Photo: John Callan

GNS Science leads an international team of experts working on the Hikurangi Subduction Zone. Photo: John Callan

GNS Science is leading several exciting projects looking at both the historical and present behaviour of the subduction zone. What we learn will help us understand and prepare for the next big earthquake. 

Much of this research is part of a five-year, $6 million MBIE-funded Endeavour project led by GNS Science, with major collaborations and contributions from national and international partners. It includes both onshore and offshore investigations. 

This year our scientists partnered with other science organisations to compile geological evidence of 10 possible subduction quakes on the North Island’s east coast in the past 7,000 years. We used evidence of uplifted terraces, coastal subsidence, and deposits from tsunamis at 22 locations along the east coast and Marlborough. Although considerable progress has been made toward understanding the seismic and tsunami potential of the Hikurangi subduction margin, gaps remain, and we are trying to fill them. 

Onshore, our scientists placed 600 seismic instruments across the Raukumara Peninsula on the east coast, in a collaboration with scientists from the United States and Victoria University of Wellington. These devices recorded seismic waves within the top 20-30km of the Earth’s crust. Thousands of kilometres of seismic images have also been acquired of the offshore Hikurangi subduction zone. 

Stuart Henrys with submersible equipment. Photo: Margaret Low

Specialised instruments allow scientists to build a 3D image of the Hikurangi fault. Photo: Margaret Low

All this information will help us build a 3D image of the subduction fault below the east coast. This will provide more understanding about the physical processes that control where earthquakes and slow-slip events occur. 

Offshore, we have been looking at the modern-day fault slip behaviour of the plate boundary. About 30 monitoring instruments were deployed under the sea off the coast of Gisborne, Hawke’s Bay and Wairarapa in October 2018. These instruments can detect ‘slow motion earthquakes’ offshore where slippage occurs across the fault, lasting weeks to months, rather than suddenly as in a typical earthquake. 

These frequent, large slow-motion events provide a window on how the plate boundary fault zone might behave in a large earthquake. A published study, led by our scientists, provided the first direct physical evidence of the way stresses change before, during and after slowslip events. This study might provide a way forward in forecasting future large earthquakes. 

In February 2019 we installed further seafloor instruments and downloaded data from earthquake borehole observatories installed beneath the seafloor in 2018. These instruments are giving us an unprecedented near-field view of what happens in the Earth’s crust during a slow-slip event. 

Findings from all this ongoing work on the Hikurangi subduction zone will help planners and emergency response agencies be better prepared for large earthquakes and tsunamis in New Zealand. 

Watch a video about slow slip events on the Hikurangi subduction zone: