GeoNet captures an unusual 5.5-magnitude earthquake off the coast of the South Island

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Scientists detected a “rare” earthquake measuring 5.5 on the Richter scale off the east coast of the South Island yesterday afternoon. Image / GeoNet

Scientists have detected an “unusual” earthquake with a magnitude of 5.5 on the Richter scale off the east coast of the South Island.

But while yesterday afternoon’s quake was unusual for its location, the seismologist says it doesn’t pose any major cause for concern.

GeoNet recorded the quake occurring about 340 kilometers east of Christchurch at 2.37pm.

It also appears to have been relatively shallow.

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With no stations closer than about 350 kilometers away, calculating the depth of the quake was challenging: GeoNet had assumed the quake was about 33 kilometers, while other agencies estimated 10 kilometers.

It happened along the Chatham Heights, a region of ocean floor that stretches about 1,000 kilometers from near the South Island in the west, to the Chatham Islands in the east.

It is considered part of the sprawling sunken continent of Zealandia on which New Zealand is located.

“We don’t record many earthquakes in this spot, but there are a few in the catalog,” said on-duty seismologist Dr. Jane Andrews.

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Undeniably one of the most curious and unique earthquakes I’ve seen in the New Zealand region. The range in magnitudes (4.7–5.5) is intriguing, especially given that the instantaneous tensor solution gives Mw at the lower end of the range. It felt like 350km away in Christchurch but nowhere else! https://t.co/lZ6ADZq10u

— Jamie Gurney 🇬🇧🇳🇿 (@UKEQ_Bulletin) March 13, 2023

A GeoNet catalog, going back to the 1940s, showed only 20 earthquakes larger than magnitude 4.5 in the area, along with only a few hundred of any size.

Most of them were centered near the north and east – and much closer to the Hikurangi Subduction Zone, or on the fringes of the Chatham Rise Hikurangi Plateau.

“However, the ability to record earthquakes over time will vary greatly in this marine area, especially for smaller events,” Andrews said.

“It’s fair to say we don’t record many events like this.”

What triggered that?

M6.0 earthquake causes weak shaking near Christchurch https://t.co/coYDECG6gC

– GeoNet (@geonet) March 13, 2023

“The Chatham Rise has been studied as part of understanding the larger Zealandia, and is made up of continental crust material,” Andrews explained.

“Imagination by seismic surveys shows many of the fault structures, a testament to their active tectonic history over millions of years.”

Being located outside, the fault structures were not fully mapped, which made it difficult to pin the earthquake to any specific rupture.

“This latest earthquake is likely the result of local tectonic stresses from nearby plate boundaries,” Andrews said.

“Similar to other areas of New Zealand that are not very close to major fault zones, rare moderate-magnitude earthquakes are expected.”

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She saw no cause for concern about tsunami risks, and no further investigation, which might be warranted if greater activity had been recorded there.

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