New Zealand is at risk of a 'megaquake' and tsunami

This NZ lagoon gave up recent evidence of two huge quakes and a tsunami. Picture: GNS Science

Researchers have found alarming first evidence that a huge megathrust earthquake crossing fault boundaries near the New Zealand city of Wellington is overdue.

A study published in the Bulletin of the Seismological Society of America (BSSA) shows a subduction earthquake – where one tectonic plate is thrust under another – is a present risk along the Hikurangi margin to the east of New Zealand.

Geologists Jamie Howarth, William Ries, and Delia Strong at Big Lagoon in Marlborough. Picture: Kate Clark, GNS Science

According to a 2013 calculation by a different team, a worst-case M8.9 Hikurangi earthquake could cause about 3350 deaths and 7000 injuries, and lead to $13 billion in costs in New Zealand’s capital Wellington alone.

Current models indicate this particular earthquake happens every 500 to 1000 years and the last big quake occurred between 470 and 520 years ago

Another megaquake struck between 800 and 880 years ago. There’s evidence that quake created a 3.3m high tsunami that surged more than 360 metres inland at the study site.

Kate Clark and her colleagues at GNS Science examined geologic layers within a salt marsh at Big Lagoon in the southeastern Wairau River valley on South Island.

They analysed cores to look at differences in the kinds of sediment and the shells from tiny marine animals throughout a stretch of the lagoon’s history.

The data revealed the lagoon sank relatively suddenly twice during the past 1000 years, suggesting that the land was subsiding as a result of significant earthquakes.

Image: GNS Science

“Subduction earthquakes are not a new risk for New Zealand, as we have always assumed they can occur and they are accounted for in our seismic hazard models,” Clark said.

“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 [New Zealand] lower North Island and upper South Island, but there was previously no evidence of past subduction earthquakes on the southern Hikurangi margin.

“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.”

But local planners aren’t panicking, according to Stuff.co.nz. Wellington Civil Defence regional manager Bruce Pepperell was yet to read the study, but told Stuff that he did not think it would make much difference to disaster plans.

“In essence, one of the hazards that we face is these subduction zones just off to the east of the bottom part of the North Island,” he said.

“That has the potential to cause a tsunami. It’s a local source tsunami so the chances are that it would be upon you before you realised it.”

The only warning, Pepperell told Stuff, was likely to be an earthquake that was “long and strong”, lasting a minute, or more.

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