A magnitude 8.1 quake is very large and could trigger considerable aftershocks and each one depending on its location, depth and faulting characteristics could be a tsunami threat as well, says a geophysics professor.

Professor of Geophysics at Victoria University John Townend told Nine to Noon it’s been a busy morning but the experts were now confident they had “a good handle” on the location of all three of the major earthquakes.

He said there was still some uncertainty about the depth and faulting characteristics of the 7.1 quake that occurred off the East Coast of New Zealand but scientists were more focused on the threat posed by the 8.1 quake off the Kermadecs just before 9am.

“The biggest earthquake – the one a couple of hours ago – is the current focus now and I think we’ve got pretty good evidence from the global seismic network about what type of earthquake it was, where it was and that is what’s being fed into by the tsunami experts into the tsunami hazard models.”

He said it had been a complicated series of earthquakes and the link between the first and second, which happened about 1000km further north, was a bit unclear.

However, the connection was clear between the second and third, which happened in close proximity, with the second one triggering “the big one” about two hours later.

Each one has had to be evaluated as a potential tsunami threat.

“The biggest one in particular has posed a substantial hazard to much of the North Island.”

He said it was unusual for later quakes to be bigger than the first one.

“We have to be vigilant and think about the hazard each one poses as well as the ongoing sequence of earthquakes that we might expect.”

A major quake had a complicated effect on the seabed and the tsunami could travel in directions and speeds that seemed counter-intuitive. So it was important that people respected the instructions from Civil Defence who were guided by the modelling done at the likes of GNS Science and elsewhere.

“An earthquake of this size produces a very complicated pattern of waves. As we’ve heard they have travelled around the country already and that just reflects the size and complexity of the earthquake to start with.”

The 8.1 quake was similar to the one experienced in Japan in 2011 and other major quakes in the past. When a major quake occurred a big column of water was disrupted at the seabed and then it flowed out in other directions, he said.

“So our concern right now is for tsunami waves coming from the Kermadecs towards New Zealand but equally there are waves in principle that could be shooting out across the Pacific in different directions.”

Professor Townend said countries in the Pacific were always on alert for the risk posed by earthquakes.