Tuesday’s magnitude 7.0 earthquake 420km northeast of Gisborne was New Zealand’s biggest jolt since a magnitude 7.1
earthquake struck in a similar location on 6 February 1995.
The Institute of Geological & Nuclear Sciences Limited (GNS) said there had been more than 130 aftershocks, mostly between magnitude 3 and 5, since
the quake struck at 6.52pm on Tuesday. The focal point of the main shock was 33km deep.
The earthquake was centred at the southern end of the Tonga-Kermadec subduction zone – the world’s busiest area for deep
earthquakes, GNS seismologist Terry Webb said.
“ It’s fortunate that it was centred so far from land. If it had been closer to shore, or near a population centre, we
would have been talking about serious damage and possibly casualties,” Dr Webb said.
The most likely cause of the earthquake was the bending of the Pacific plate as it sinks beneath the Australian plate to
the east of New Zealand.
“ We would expect an earthquake of this magnitude to occur on-land in New Zealand once every decade, so it’s quite an
Dr Webb said the earthquake released about 30 times more energy than the atomic bomb dropped on Hiroshima at the end of
World War 2.
“ As with all large earthquakes, it was detected by seismic instruments globally.”
It was unlikely that the earthquake had relieved the stress on any of New Zealand’s major active faults as it was too
Dr Webb noted that amateur meteorologist Harry Alcock of Hamilton had last week predicted New Zealand would be affected
by a large earthquake because of the Moon’s proximity to the Earth.
“ We’re not aware of any scientific literature confirming that the Moon is able to trigger earthquakes, although
scientists have investigated this. It would be really exciting if a breakthrough occurred in an area such as this.”
Dr Webb suggested that Mr Alcock should document his observations and have them reviewed by scientists.
“ Testing ideas is how science advances. The science community would welcome the opportunity to evaluate such a
hypothesis if the facts were laid out before us.
“ Over the past 50 years many people have tried unsuccessfully to predict earthquakes. Any prediction technique needs to
have a solid track record of successes before it can be taken seriously.”
Dr Webb said false predictions had the potential to cause panic and could end up causing as much disruption as an
He said that earthquake activity in New Zealand had been “quieter than the long-term average” during the past 50 years.
“ The number of big on-land earthquakes since the early 1940s gives a misleading impression about the earthquake hazard
we face. New Zealand won’t stay in this relatively quiet state indefinitely, so it makes sense to be prepared for a
“ Tuesday’s earthquake is a reminder that we live in one of the world’s more seismically active countries.”