New Zealand was lucky to avoid a tsunami following a 7.4 quake that struck south of the Kermadec Islands early this morning, a scientist says.
Despite striking at about 12.50am – and being centred 700km to the northeast of New Zealand – the quake was felt widely across the East Coast, with more than 8000 people reporting it to GeoNet.
GNS Science experts, along with the National Emergency Management Agency (NEMA), quickly analysed the data and determined the quake was unlikely to cause a tsunami that would have posed a threat to the country.
But some people were left annoyed that 50 minutes passed before they were officially told there was no tsunami threat.
While the quake measured over 7.0, events were common in the area it struck – and GeoNet classified it as "weak" because of its far distance from the country.
"Our shaking intensity scale gives the impact of an earthquake on people living on the earth's surface, instead of magnitude which is the energy released during an earthquake, and so it can be more useful as an indicator of the earthquake's significance to the community," explained GNS scientists Dr David Burbidge and Dr William Power.
"This earthquake was felt widely along the East Coast of New Zealand as a large, shallow earthquake like this produces strong low-frequency surface waves which should travel nicely through the crust along the plate boundary."
Dr Jose Borrero, of Raglan-based marine consultancy eCoast Ltd, said the event was just on the small side and didn't have the right sense of motion to generate a tsunami – meaning the country had effectively dodged a bullet.
But it was just on the north end of what he called New Zealand's "hot zone" for tsunami effects, from the Tonga-Kermadec Trench.
From that point south, tsunami energy would be aimed directly at sections of the New Zealand coast – but north of it, most of the energy was sent into the western South Pacific, passing the country.
"That is why the Samoa 2009 tsunami was not so big down here, the tsunami energy is mostly radiated perpendicular to the trench," Borrero said.
"But once the source region slips south of the latitude of North Cape, the east coasts of Northland, Auckland, Waikato and Bay of Plenty are in the direct line of fire.
"This morning's quake was also a 'strike slip' quake - meaning the motion was side-to-side and not up and down, so the amount of seafloor displacement was small and so was the subsequent tsunami."
A tsunami from the Kermadecs Arc area above New Zealand – called a "regional" tsunami – could offer one to three hours' warning for evacuation.
We have more information on this mornings M7.4 Kermadec Earthquake and How GeoNet Monitors Tsunami. Read all about it here: https://t.co/60zIlmIoTQ pic.twitter.com/8iJ4EsEuyI
— GeoNet (@geonet) June 18, 2020
That was unlike New Zealand's most hazardous form of tsunami - a violent near-shore event that could leave people just minutes to evacuate.
Headline projections in an EQC-commissioned report estimated worst-case scenario impacts from a one-in-500 year event could include 33,000 fatalities, 27,000 injuries and $45 billion of property loss.
One recent simulation suggested tsunami waves - up to 12m high in places - could inundate the coastline within an hour if a "megathrust" earthquake struck here.
One of the most deadly scenarios for Wellington was a two-punch 8.9 earthquake and tsunami in the Hikurangi Subduction Zone, assuming a rupture extending across Cook Strait.
One GNS Science report found this catastrophic event could cause 40 deaths due to earthquake shaking - but a further 3200 in a tsunami that followed.
Although scientists were uncertain whether a rupture could extend from the subduction zone to the Cook Strait, such an eventuality could send tsunami waves 5m to 10m high barrelling toward the capital.
It would bring "extremely damaging and deadly inundation" in low-lying southern suburbs of Wellington, along with damaging and moderately deadly inundation in busy areas bordering Lambton harbour.
In Napier, a tsunami up to 8m high could plough through the low-lying city within minutes of an earthquake, reaching as far as 5km inland – and more waves could come over following hours.
Gisborne was also vulnerable to a quick-fire tsunami, with the city's beachfront, and north of Muriwai at the end of Poverty Bay, most at risk.
Christchurch would also be hit particularly hard.
For earthquakes on the other side of the Pacific, a tsunami could take up to about 16 hours to reach New Zealand, depending on exactly where it was.