A strong quake that hit off the eastern coast of the North Island overnight simply wasn't big enough to cause a devastating tsunami wave, a seismologist says.
The magnitude 7.1 quake hit a spot in the Pacific Ocean 105km east of Te Araroa at 2:27am, waking up many Kiwis and prompting tsunami warnings for the East Cape, Bay of Plenty, Coromandel, Great Barrier Island and the Chathams.
Just after 6am the National Emergency Management Agency said the threat had passed.
"People can go about their day as usual," acting director Roger Ball told The AM Show.
The quake happened in almost the exact same spot as a magnitude 7.1 quake in 2016.
"Offshore of the North Island you have the Pacific tectonic plate, and then the North Island sits on the Australian tectonic plate, and the Pacific plate is pushing - or what we call subducting - underneath the Australian plate," GNS Science seismologist John Ristau told The AM Show.
"This earthquake appears to have happened within that Pacific plate as it was pushing down."
Many Kiwis on social media reported the quake going on for an extremely long time - up to a minute
"I felt that quake very faintly for about half a minute before it really started to kick in in welly," one person tweeted.
"That quake lasted at least a minute, started like a normal one but then got pretty intense," another said.
Ristau said that's because the quake happened quite deep - 90km below the surface - and quite far away, so Kiwis only felt the "surface waves", not the initial shock.
"Those can be these really long, rolling kind of motions. They can go on for 30 seconds, a minute, for a quake of this size."
Though it was a big quake, it failed to create a tsunami of the likes seen in Japan in 2011 or Asia in 2004 because it wasn't strong enough, and was the wrong type, Ristau said.
"To get a tsunami that big, it has to be a much bigger earthquake than the one we had - we're talking at least magnitude 8, high 8s, if not magnitude 9," said Ristau.
"It has to be one that's occurring right on the boundary of these two plates as one is pushing down beneath the other one causing this stress to build up, then that whole thing just kind of snaps and causes the entire water column in the ocean to move, from top to bottom... [Friday's quake] was not that type of earthquake."
The quake which caused the tsunami in 2004, killing 227,000 people, measured between magnitude 9.1 and 9.3; and the 2011 quake in Japan which killed nearly 16,000, measured 9.1. Both were megathrust earthquakes. Friday's quake is yet to be categorised, but the 2016 quake in the same spot was an oblique-slip fault.
Because the magnitude scale is logarithmic, a one-point rise equals 10 times the strength of the seismic wave and 31 times the energy released - meaning the 2004 and 2011 quakes had waves 100 times stronger than Friday morning's quake off the North Island and released 1000 times more energy.
Ristau said there's still the chance of "stronger, unusual currents" off the East Cape today, with dozens of aftershocks measuring at least 3.5 recorded already.
He said there have been an unusual number of quakes in recent years.
"New Zealand has gone through cycles over the course of the last 150 years or so - you get a lot more of these large earthquakes, then it quietens down for a few decades, like the second half of the 20th century, there weren't too many large earthquakes in New Zealand.
"But ever since the beginning of the 21 century - starting, going back to about 2009 with the 7.8 in Fiordland, we've had a number of major earthquakes."
The biggest were the 2009 Fiordland and 2016 Kaikoura quakes, both measuring 7.8.
"It didn't generate that really kind of high-frequency, really strong shaking - it was more this long rolling motion, so a lot of people didn't really take too much note of it," Ristau said of the Fiordland quake.
The most devastating 21 century quake in terms of loss of life in New Zealand was the Christchurch quake of 2011, which killed 185 people. The strongest in New Zealand recorded history hit Wairarapa in 1855, and measured 8.2.
