19 May 2015

New Zealand's megathrust earthquakes

10:03 am on 19 May 2015

For the first time, geologists have found direct evidence for large 'megathrust' subduction earthquakes beneath central New Zealand.

The Big Lagoon, in Marlborough.

The Big Lagoon, in Marlborough. Photo: Supplied

They identified the geological signatures of two subduction earthquakes that ruptured in the southern part of the Hikurangi Margin in the past 1000 years.

The margin marks the collision zone between the Pacific and Australian tectonic plates: this is the first evidence that the southern part of this boundary, under the Cook Strait-Marlborough area, could rupture in large earthquakes.

These earthquakes differ from other quakes, in that they occur on the underside of the upper plate, where the two plates meet, rather than along faultlines within the upper plate.

They are responsible for some of the biggest quakes - and tsunamis - in the world, including the recent magnitude 9.0 Tohoku earthquake and tsunami in Japan in March 2011, and the magnitude 9.3 Indian Ocean earthquake and tsunami in December 2004.

"Subduction earthquakes are not a new risk for New Zealand," said lead author and GNS science geologist Kate Clark.

"We have always assumed they can occur, and they are accounted for in our seismic hazard models. This study is significant in that it confirms that risk."

"We have a record of three to five past earthquakes on most of the major upper plate faults in the lower North Island and upper South Island, but there was previously no evidence of past subduction earthquakes on the southern Hikurangi margin," she explained.

"Subduction earthquakes have the potential to be significantly larger in magnitude than upper plate fault ruptures, affect a much larger spatial area, and are much more likely to trigger tsunami."

A diagram showing the Hikurangi Margin.

A diagram showing the Hikurangi Margin. Photo: Supplied

The team identified the quakes from sediment cores extracted from Big Lagoon, a large coastal lake east of Blenheim.

Radiocarbon dates of organic material from different levels of the cores show evidence of two sudden subsidence events, first at 880 to 800 years ago, and again at 520 to 470 years ago, when the land dropped by up to half a metre.

Sudden drops like this can only be explained by moderate-to-large earthquakes, and these two events do not match any known significant earthquakes on nearby faults in the upper Australian plate.

Dr Clark said the geological evidence pointed towards the two quakes happening at about 10km to 30km beneath the seabed in Cook Strait.

"The findings are significant in terms of understanding earthquake and tsunami hazards in the lower North Island and upper South Island," she said.

Judging by the sedimentary debris found at the time of the first earthquake, this event was accompanied by a three-metre tsunami that swept more than 360 meters inland at the study site.

There is no evidence of a tsunami with the more recent of the two quakes.

She said the more recent quake possibly correlated with a quake already identified further north on the central section of the Hikurangi margin, under Hawke Bay, which raised the possibility that both central and southern sections of the margin may have ruptured in the same quake.

From the evidence available, the team couldn't estimate the size of the two quakes, but Dr Clark said quakes with similar impacts in comparable geological settings were larger than magnitude 7.5.

The earthquakes took place roughly 350 years apart, which may mean that the time between large earthquakes in this region is shorter than scientists have thought.

"We would like to go further back in time and find evidence of older subduction earthquakes," Dr Clark added.

"With a longer record of past subduction earthquakes we can get a better constraint on the recurrence of such earthquakes, which will help to forecast future subduction earthquakes."

The research was published today in the Bulletin of the Seismological Society of America.