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Step up in quake research

A UK-led scientific study off the coast of Gisborne is resulting in a “step-change” in the level of understanding about the region’s earthquake and tsunami threats.

Scientists from Imperial College London and GNS Science in New Zealand, have managed to plug current sound wave information into an imaging technique called full waveform inversion.

This method helped them paint a picture of the Hikurangi fault zone in unprecedented detail, a statement from Imperial College said.

Their technique combines traditional acoustic mapping with a newer method called full waveform inversion. They found their new method enhanced their view of rocks along a fault line — a break in the Earth’s crust — off the east coast of New Zealand’s North Island.

“The researchers hope that their clearer view of the rocks around these fault lines — whose movements can trigger earthquakes and subsequent tsunamis — will help them better understand why such events happen,” said lead author Melissa Gray, from Imperial College London’s department of earth science and engineering.

“We can now scan underwater rocks to see their properties in greater detail. Hopefully this will help us to better work out how earthquakes and tsunamis happen.”

They also captured the shallow faults which were responsible for the large Gisborne tsunami in 1947 — an example of a large tsunami caused by a relatively small slow-slip earthquake.

The paper has been published by AGU Journals.

Speaking to the Gisborne Herald, GNS Science geophysicist Dr Stuart Henrys said the study used new techniques to re-process data gathered off the Gisborne coast in 2005.

“We have applied a new technique to the data, something we couldn’t achieve in 2005, when we didn’t have the computing resource available. So, this is a step-change in the way we analyse data.

“It’s a really stunning result.”

The Hikurangi subduction zone (where the Pacific Plate dives under the Australian Plate) runs the length of the North Island’s East Coast.

Subduction zones are associated with the world’s most powerful earthquakes, as well as slow-slip events, also known as silent earthquakes.

Slow-slip events happen like regular earthquakes, only over a much longer period of time, and are detectable by sensitive continuously recording GPS sensors operated by GeoNet.

Dr Henrys said the single recorded 2D profile of the subduction zone came as close as 20km from the coast and stretches another 150km.

“The fact that this technique has been tested on this line is fantastic. We have now a better resolution of the structures of the rock layers. Hopefully, that will tell us more about slow-slip and earthquakes because we can focus the image of the subduction zone and improve information about the physical properties of the rocks at depth where slow-slip and earthquakes occur.

“Previously it’s like a blurred photograph and now we are getting to sharpen the image.”

Dr Henrys said the pace of discovery around the subduction zone science was increasing and was expected to reveal more details in the coming years.

“We’ve thrown everything at it. We have observatories in bore holes now, we have rolling ocean-bottom instruments where we are collecting data and have new imaging tools. One of the really cool things, is we have collected a 3D dataset in 2018. The other reason why we applied this new waveform inversion technique on this older profile was to refine our techniques which we will now apply to the 3D dataset in coming years. 3D is like having full CAT scan, so we can understand that margin offshore from Gisborne in even greater detail.

Dr Henrys said the reason why Gisborne was such a focus for studies was due to the fact the subduction was at a comparatively shallow depth and scientists can apply knowledge learnt in New Zealand to subduction zones globally.

MODERN WAYS: A map detailing the mapped Hikurangi subduction zone. New techniques have helped map Gisborne’s position in reference to two tsunami-triggering events. Image by Imperial College London