One of New Zealand’s biggest earthquakes of the year has struck the Boisegur Trench – but no one felt it

A 5.9-magnitude earthquake struck the south of the South Island this afternoon. Image / GeoNet

One of the largest earthquakes in New Zealand this year struck at 2.21pm today, in an area with proven tsunami-generating potential.

But such was the distance of the 5.9-magnitude quake from the mainland – it struck 450km south of the South Island, and 285km southwest of the remote Snares Islands – that only one person reported feeling it on GeoNet.

It happened to strike near an incomprehensible seismic zone which produced some of the most powerful earthquakes in modern New Zealand history.

This was the Puysegur Trench: the plate boundary where the Australian plate sinks below the Pacific plate—in the opposite way to how the two plates sink on the North Island, in the well-known Hikurangi subduction zone.

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The border extends more than 800km south from the South Island, to a point in the wild and windswept Southern Ocean, about 400km west of the Auckland Islands.

Noticed shallow regional earthquake M6.2 330 km southwest of Stewart Island on the Puysegur Trench along the plate boundary. https://t.co/euG0LLwirp pic.twitter.com/NZfotyTCpK

– Gulf Hauraki weather (@GulfHauraki) May 31, 2023

In 2009, a magnitude 7.8 earthquake struck the Fjordland near the northern end of the trench, releasing 25,000 times more energy than the atomic bomb dropped on Nagasaki in 1945 and twisting the South Island, moving Puysegur Pt on the southwestern tip of the island by close to 30 centimeters. Australia.

Its remote location, near Dusky Sound, meant that there was relatively little damage.

A magnitude 7.2 earthquake hit the trench itself in November 2004 – a month before a magnitude 8.1 earthquake near the trench sent shaking felt as far as Tasmania.

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GeoNet’s alternate seismologist, Dr. Jonathan Hanson, said we can be reasonably confident that the earthquake this afternoon is related to subduction zone processes in the area.

“The fact that we’re seeing a lot of events located in a really similar area reinforces that it’s likely related to some of the processes going on there.”

In the past 20 years, scientists have identified about 100 events in the region.

This was actually a M6.0 earthquake, 450 kilometers south of Stewart Island, near the Boisejour subduction zone. Read more about why we get “ghost quakes” here: https://t.co/8ECm6t1svL https://t.co/CfOx6VCgnW

— GeoNet (@geonet) May 31, 2023

“The actual amount is likely to be much higher than that, but since we’re so far away from our instruments, it’s hard to get a full sense of how active the seismic activity is there — other than we need an event above magnitude five, to be relatively confident in it.”

Hanson said events like today have also been instrumental in shedding new light on a hard-to-study seismic region.

“One of the things that caught my eye about this event was that it was a slip event, whereas in that area, we tend to see a mixture of reverse slip and slip.”

In a counter-slip fault, each side of the crustal mass slides past the other without being lifted or pushed down, while in a reverse fault, the mass above the fault moves up relative to the mass below the crack.

“This is an interesting phenomenon in itself, as we can see two distinct types of seismic activity in this region,” he said.

“But the main thing to take away from this, is that it’s a reminder that we have a subduction zone off the South Island that is productive and capable of generating large tsunamis.”

At noon today, no tsunami risk has been identified for New Zealand.

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