A magnitude 7.1 quake that prompted an emergency tsunami risk assessment today struck in a familiar area for big shakes – including an 8.1 event that sparked widespread warnings here two years ago. Image / Niwa Weather
A magnitude 7.1 quake that prompted an emergency tsunami risk assessment today struck in a familiar area for big shakes – including an 8.1 event that sparked widespread warnings here two years ago. Image / Niwa Weather
A magnitude 7.1 quake that prompted an emergency tsunami risk assessment today struck in a familiar area for big shakes – including an 8.1 event that sparked widespread warnings here two years ago.
Shortly after the quake hit near the Kermadec Islands at 12.42pm, officials from the National Emergency Management Agency (NEMA) and GNS Science assessed the jolt was unlikely to have caused any tsunami threat to New Zealand.
The quake reportedly caused a small tsunami measuring 20cm, occurred at a depth of 50km and was a result of reverse-faulting – in which a compressed area of the Earth’s crust is pushed up relative to rock on the other side.
Mw 7.1 earthquake near Raoul Island today. Reverse faulting at 50 km depth. Small tsunami (~20 cm peak-to-trough) recorded at Raoul Island. The earthquake was well recorded across the New Zealand seismic network. pic.twitter.com/gJfahJ6KAK
Dr Jose Borrero, a member of the tsunami expert panel that reviewed the hazard, estimated today’s quake released about 30 times less energy than the 8.1 event on March 5, 2021 – which happened to follow a 7.4 quake in the same area a few hours earlier.
Unlike that big rupture, this afternoon’s quake was considered too small and too far away from New Zealand to have posed any risk – but the region nonetheless remained a key area for “regional-source” tsunamis.
Other large recent events included two in 2011 - a 7.6 quake and a 7.4 jolt that occurred several months apart – and another “doublet” of quakes measuring 8.2 and 7.8 on January 15, 1976.
Borrero, an international tsunami scientist and director of Raglan-based marine consultancy eCoast Ltd, explained these quakes occurred along a well-known interface - the Tonga-Kermadec Trench.
The severe 7.1 quake struck in the Kermadec Islands region north of New Zealand this lunch hour. Image / Geonet
That trench was part of a subduction zone that itself marked a convergent boundary between the Pacific and Australian tectonic plates, and stretched from the North Island of New Zealand northward for thousands of kilometres.
The subduction zone interface was where the Pacific plate dived, or subducted, beneath the Australian plate, as part of an ongoing geological scrum.
“The Pacific and Australian plates are moving closer to each other at a rate of a few centimetres each year – roughly the rate that your fingernails grow – and that strain gets built up over time,” he said.
“Once that strain exceeds the strength of the rocks, then it cracks and results in an earthquake.”
Subduction processes were happening all the way along the margin, spanning from New Zealand up to Samoa.
You can see the seismic action northeast of NZ, around the Kermadec Islands.
“But the spot [where today’s earthquake occurred] right near Raoul Island is where we seem to see a lot of activity happening more frequently – which may mean there’s a lot more slippage, or energy being released there,” he said.
“The thing that is perhaps more worrisome are those areas of the margin where this is not happening – the plates are still converging, but they’re not releasing any slip.”
To monitor offshore quakes, scientists rely on GeoNet’s network of seismic instruments which are placed all over New Zealand, Chatham Islands, and Raoul Island.
Tsunami modelling utilises seismic and GPS data, along with data from a network of tide gauges to model tsunamis.
More information comes from DART buoys - international network of instruments deployed in the open ocean which measure wave heights and wave lengths, and are used to detect tsunamis in the open ocean.
A tsunami from the Kermadecs Arc area above New Zealand could offer one to three hours’ warning for evacuation.
That was unlike New Zealand’s most hazardous form of tsunami - a violent near-shore event that could leave people just minutes to evacuate.
In 2021, scientists published a study finding three out of the last four earthquakes on the Hikurangi Subduction Zone beneath the Wellington and Wairarapa regions generated large tsunamis, and the recurrence interval – or time between events – had been about 500 years.
From this, the study team was able to calculate a 26 per cent likelihood of a large subduction earthquake occurring within the next 50 years.
