Called the Puysegur Trench, the boundary stretches for more than 800km south from the South Island, to a point in the wild and windy Southern Ocean, around 400km west of the Auckland Islands.
“This area is actually very poorly understood,” said seismologist Dr Finn Illsley-Kemp, whose Victoria University colleagues have been carrying out work in the region.
“There’s been far less research done here compared to other regions, leaving many unknowns about the exact tectonics – but it’s hosted some of our biggest earthquakes.”
Among them was 2009’s 7.8 Fiordland earthquake, which struck close to the northern end of the trench – releasing 25,000 times more energy than the atomic bomb dropped on Nagasaki in 1945.
Its force was enough to twist the South Island, moving Puysegur Pt on the southwestern tip of the island 30cm closer to Australia.
But its remote location, near Dusky Sound, meant there was relatively little damage to buildings.
A magnitude 7.2 quake hit the trench itself in November 2004, while a 6.4 event was recorded in July 2017.
In the follow-up on today’s event, Illsley-Kemp said GNS Science experts would be focusing on the earthquake’s “focal mechanism”, telling them how the fault moved during the earthquake.
That information helped assess potential tsunami risks, which are an ever-present danger facing Southland.
In one recent study, scientists reported how, over an average “return period” of 2500 years, much of the region’s southern coast sat exposed to tsunamis 8m to 12m high – and waves of 4m to 8m over 500-year periods.
That put at risk communities including Invercargill, Bluff and Riverton, along with major transport hubs such as Southland’s main port and airport.
The Hikurangi Subduction Zone, off the North Island’s East Coast, ultimately posed the greater threat to people.
Along this subduction zone, scientists have estimated a 26 per cent chance of an event with a magnitude of 8.0 or larger striking beneath the lower North Island within the next 50 years.
Jamie Morton is a specialist in science and environmental reporting. He joined the Herald in 2011 and writes about everything from conservation and climate change to natural hazards and new technology.
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