Kaikōura quake not the big one, scientists warn

Christchurch, Kaikōura and Fiordland - geologists say these quakes would pale in comparison to one that could strike the Hikurangi margin.

Running from Gisborne in the north to the top of the South Island, it's the southern part of the Kermadec-Tonga subduction zone and believed to be capable of generating quakes up to magnitude 9.0 - and a perhaps even more devastating tsunami.

"We need to think Japan 2011 basically, because if our whole plate boundary ruptured it would be a magnitude 9.0 earthquake," GNS scientist Ursula Cochran tells Fairfax.

"One thing about reflecting on from the Kaikōura earthquake is we don't want people to think this is the big one."

The 2016 Kaikōura quake measured 7.8. That might not sound like much less than 9.0, but earthquakes are measured on a logarithmic scale - and each point you go up the scale equals 32 times the energy released.

So while the Kaikōura quake at 7.8 released 177 times as much energy as the 2011 Christchurch quake (6.3), a megathrust quake measuring 9.0 would be in excess of 11,000 times as powerful.

Megathrust quakes happen when one tectonic plate is suddenly forced up over another. The Hikurangi margin marks where the Indo-Australian and Pacific plates collide.

Because it runs along the eastern coast of the North Island just offshore, a megathrust quake there could create a massive tsunami like that which swamped Sendai in Japan in 2011. People in Wairarapa would have perhaps only seven minutes to prepare once the quake hits, GNS modelling shows.

Civil Defence on Sunday tested its new mobile phone alert system, with mixed results - many phones aren't equipped to receive the alerts, and some who did receive an alert took to social media to ask what it was - completely unaware of the initiative.

GNS and international researchers are presently studying the Hikurangi margin, and told Fairfax it may have had a part to play in the Kaikōura quake.

The Sendai quake of 2011 was a megathrust quake measuring about 9.1. It and the subsequent tsunami killed nearly 16,000 people. The 2004 Boxing Day quake, also a megathrust, measured 9.3, killed as many as 280,000 people and literally shook the planet.

Other megathrust quakes include the one which hit Chile in 2010 measuring 8.8 and the biggest quake ever recorded - a 9.5 which shook the South American country in 1960.