Study shows past rumbles on NZ's big quake danger zone

Scientists have pulled together the most comprehensive picture yet of New Zealand's largest geological threat.

A major study by GNS Science and Victoria University lays out evidence of 10 possible large earthquakes along the high-risk Hikurangi Subduction Zone in the past 7500 years.

This zone is a sprawling offshore fault, east of the country, that marks where the Pacific Plate dives – or subducts – westward beneath the North Island.

Scientists believe the subduction zone has the potential to unleash "megathrust" earthquakes larger than magnitude 8, such as those which created tsunamis that devastated Indonesia in 2004 and Japan in 2011.

The new study, published in the journal Marine Geology, marked the first time that all evidence for past subduction earthquakes had been assessed together using a consistent and systematic method.

It combined information collected from 22 sites that had been studied over the past 25 years and added in several new sites.

The 10 quakes highlighted in the new study unfolded at various locations between Marlborough and Gisborne.

Some may have impacted the whole stretch of the subduction zone, the authors found, while others appeared to have been more localised.

Data was 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.

The study's lead author, Kate Clark of GNS Science, said subduction earthquakes could be larger in magnitude than the more common "upper plate fault ruptures", such as the magnitude 7.8 Hawke's Bay Earthquake in 1931.

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

"They also affect a larger area and are more likely to trigger a tsunami," 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.

Some past earthquakes along the Hikurangi Subduction Zone (in red) may have covered the entire area. Image / GNS Science

"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."

Clark said identifying subduction earthquakes in the geological record of the Hikurangi subduction zone was 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."

However, the new research didn't point to any increased risk, and instead served to fill in important gaps on the timing and location of past quakes and tsunamis.

The time gaps between large earthquakes in the zone varied from 350 to 1700 years, and the average recurrences also varied by location.

The longest record of past earthquakes was in Hawke's Bay, where the recurrence interval was between 800 and 1450 years.

While it still wasn't possible to accurately calculate the size of prehistoric earthquakes, comparisons with overseas subduction zone earthquakes suggested these were probably greater than magnitude 7.8.

It remained possible that a future event could be similar in size to Japan's 9.1 Tohoku in 2011 – but there were many other possibilities as well.

The last significant earthquakes in the zone were the 7.0 and 7.1 quakes in March and May 1947, which triggered localised tsunamis up to 11m high.

Over the past decade, the subduction zone has been the centre of a massive scientific collaboration between the world's leading earthquake experts.

Already, scientists have put in place two sub-seafloor earthquake observatories - making New Zealand only the fourth country in the world to have such capabilities.

The technology could help pave the way for offshore instrumentation needed for earthquake and tsunami early warning systems.

Some of the newly-installed instruments recently picked up one of the largest silent "slow slip" earthquakes yet observed in the country.