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Scientists get clearer picture of past earthquakes on the Hikurangi subduction zone - 17/04/2019

A fuller picture of earthquakes along New Zealand’s largest fault will help scientists more accurately assess hazards posed by the Hikurangi subduction zone.

In the most comprehensive study of its type, scientists have compiled geological evidence of 10 possible subduction quakes on the east coast of the North Island in the past 7,500 years.

The study by GNS Science in collaboration with Victoria University of Wellington is published in the journal Marine Geology - https://www.sciencedirect.com/science/article/pii/S0025322718304511

It collates and updates a large amount of existing research, and adds new information.

The 10 quakes occurred at various locations between Marlborough and Gisborne on the Hikurangi subduction zone, where the Pacific and Australian tectonic plates meet. Some may have impacted the whole area, while others appear to have been more localised.

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This study has identified areas for future research where significant advances can be made in clarifying the timing, size, and location of past subduction earthquakes and any associated tsunamis

Dr Kate Clark

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Data is too sparse for scientists to estimate the magnitude of the quakes, but at least four triggered tsunamis large enough to leave evidence of physical impact on land. 

Lead author, Kate Clark of GNS Science, said subduction earthquakes can be larger in magnitude than the more common ‘upper plate fault ruptures’, such as the magnitude 7.8 Hawke’s Bay Earthquake in 1931.

“They also affect a larger area and are more likely to trigger a tsunami,” Dr Clark said

The most recent subduction quake occurred between 520 and 470 years ago on the southern section of the subduction zone – between Marlborough and Wairarapa, and possibly up to Napier.

A second subduction quake occurred between 870 and 815 years ago, with the rupture potentially extending between Marlborough and Gisborne.

“We previously knew of evidence of these earthquakes at individual sites along the coast, but this is the first time we have compiled all the sites together.

“It gives us a much better idea of the total area impacted by these past earthquakes, which can be useful for forecasting the size of future earthquakes and how we plan for such an event.”

Dr Clark said identifying subduction earthquakes in the geological record of the Hikurangi subduction zone is particularly challenging due to the large number of earthquakes on upper plate faults which leave very similar signs in the landscape.

“This study has identified areas for future research where significant advances can be made in clarifying the timing, size, and location of past subduction earthquakes and any associated tsunamis.”

For an in-depth commentary on this research, go here - https://www.eastcoastlab.org.nz/news/hikurangi-subduction-earthquakes-stitching-the-evidence-together/
 

Earthquake geologists study the walls of a temporary trench on the coast north of Gisborne looking for evidence of past tsunamis and earthquakes.

Earthquake geologists study the walls of a temporary trench on the coast north of Gisborne looking for evidence of past tsunamis and earthquakes.