GeoNet picks up ‘unusual’ 5.5 earthquake off South Island coast

Scientists have picked up a “rare” magnitude 5.5 earthquake far off the east coast of the South Island yesterday afternoon. Image / GeoNet

Scientists have picked up an odd magnitude 5.5 earthquake, centred far off the east coast of the South Island in an area where few are recorded.

But, while yesterday afternoon’s quake was usual for its location, a seismologist says it poses no great cause for concern.

GeoNet recorded the quake striking about 340km east of Christchurch at 2.37pm.

It also appeared to have been relatively shallow.

With no stations closer than about 350km, calculating the quake’s depth was difficult: GeoNet was assuming it around 33km, while other agencies were estimating 10km.

It happened along the Chatham Rise, an area of ocean floor stretching about 1000km from near the South Island in the west, to the Chatham Islands in the east.

It’s considered part of the sprawling, sunken Zealandia continent that New Zealand sits upon.

“We don’t record many earthquakes in this spot, but there are a handful in the catalogue,” duty seismologist Dr Jen Andrews said.

Undeniably one of the most curious & unique earthquakes I've seen in the New Zealand area. The range in magnitudes (4.7-5.5) is curious, especially given a moment tensor solution gives the Mw at the lower end of the range. Felt 350km away in Christchurch but nowhere else! https://t.co/lZ6ADZq10u

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

GeoNet’s catalogue, spanning back to the 1940s, showed only 20 quakes larger than magnitude 4.5 in the area, along with just a few hundred of any size.

Most had been centred closer to the north and east – and much closer to the Hikurangi Subduction Zone, or on the Chatham Rise-Hikurangi Plateau margin.

“However, the earthquake recording capability over time will vary significantly in this offshore region, especially for smaller events,” Andrews said.

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

What triggered it?

M6.0 quake causing weak shaking near Christchurch https://t.co/coYDECG6gC

— GeoNet (@geonet) March 13, 2023

“The Chatham Rise has been studied as part of understanding greater Zealandia, and it is made up of continental crustal material,” Andrews explained.

“Imaging by seismic surveys show many fault structures, a testament to its active tectonic history over millions of years.”

Because they are offshore, the fault structures weren’t fully mapped, which made it hard to pin the quake to any specific rupture.

“This recent earthquake is likely just the result of the local tectonic stresses caused by the nearby plate boundary,” Andrews said.

“Similar to other regions of New Zealand that aren’t very close to the major fault zones, infrequent moderate-sized earthquakes are to be expected.”

She saw no reason for concern about tsunami risk, nor further investigation, which might be warranted if more significant activity was recorded there.