NZ quakes, tsunami warnings

New Zealanders woke this morning to a series of earthquakes. The largest of them, an 8.1 magnitude quake centred near the Kermadec Islands, triggered a rush of tsunami warnings for both the east and west coasts in the country’s north.

The 8.1 quake occurred at a depth of 20km, relatively shallow for earthquakes, and conditions created could have caused a large tsunami. Fortunately, ocean conditions stayed calm.

“There are a number of complex criteria that have to come together to determine whether there will be significant tsunami, such as the speed of propagation, and whether material was displaced on the ocean floor,” said Tim Stern, a geophysicist at Victoria University, Wellington.

The earlier 7.4 quake also happened in the Kermadec region, about 1000km north-east of NZ’s North Island, and it’s extremely likely it triggered the 8.1 quake. “They both occurred on the subduction zone interface, at the same location, with the same mechanism,” GNS Science, Te Pū Ao, seismologist John Ristau told the New Zealand Herald.

The first 7.3 quake, however, occurred further south, closer to East Cape, about 350 km south east of Auckland. Jose Borrero, of Raglan-based marine consultancy eCoast Ltd, told the Herald he believed it was too early to tell if there’s a link between the New Zealand and Kermadec earthquakes.

“The only link that we can say right now is that they’re around the same subduction zone interface – the Tonga-Kermadec Trench,” said Borrero. “Everything beyond that is speculation.”

The subduction zone is part of the Pacific and Australian tectonic plate boundary, which extends from the North Island northwards for thousands of kilometres.

GeoNet’s network of seismic instruments monitor seismic activity in New Zealand, the Chatham Islands, and Raoul Island in the Kermadecs. Open ocean DART (Deep-ocean Assessment and Reporting of Tsunamis) buoys send back wave height and wave-length data to help warn against potential tsunamis. Additionally, scientists use seismic and GPS data, along with tide gauges, to model potential tsunamis that could occur in the aftermath of a large earthquake.

At 3:43pm NZ time, tsunami threat levels were downgraded, and evacuated people were told they could return to their homes, but people were still being warned to stay away from the shore in at-risk places.

“Strong and unusual currents and unpredictable surges will continue for up to another 24 hours,” NZ’s National Emergency Management Agency, Te Rākau Whakamarumaru, reported. “People should remain vigilant and take extra precautions with regards to beach and ocean activities.”