Increased risk of massive quake under central NZ

The chance of a massive earthquake under central New Zealand, as large as or larger than the magnitude 7.8 Kaikoura earthquake, has risen - however it still remains very unlikely.

GNS Science said the probability of a big quake in the coming years has increased to 5 per cent - from 3 per cent - due to increased slow-slip activity (or silent earthquakes) following last month's quake.

"This is approximately 6 times greater than it was prior to the Kaikoura Earthquake," said GNS.

Due to the large extent of slow-slips, the increased likelihood of a big quake now covers a larger region including the lower half of the North Island and the upper South Island.

The GNS blog said there were several faults in these areas capable of large quakes, including the subduction zone and crustal faults like those that ruptured during the Kaikoura earthquake on November 14.

"Our forecasts tell us what is likely (or unlikely) to happen in the future, but they can never definitively say if a large earthquake will occur or not."

The slow-slip events cannot be felt, but result in a similar amount of land movement as in an earthquake, only occurring over weeks or months.

Since the Kaikoura quake, slow slips have caused movements equivalent to a magnitude 7 quake in the Hawke's Bay-Gisborne region and magnitude 6.9 in the Manawatu-Kapiti region.

"We have observed many similar slow-slip events in these areas of this size, but this is the first time we've observed slow-slip occurring simultaneously in multiple areas around the North Island in the 15 years we've been detecting them," said GNS.

"This is also the first time we've been able to observe slow-slip in New Zealand after a magnitude 7.8 earthquake, so it's possible this is a normal pattern after such a large quake."

People evacuated to the top of Puketapu Rd hill in Taradale, Napier, after the 7.5 magnitude earthquake near Kaikoura triggered a tsunami warning in the middle of the night. Photo / File

The slow-slip event in the Hawke's Bay-Gisborne region was less than 15km deep and has almost finished. Those beneath the Kapiti and Manawatu regions have been ongoing at a relatively steady rate since last month's earthquake.

Experts from GNS Science and Victoria University evaluated many strands of evidence to determine the likelihood of an earthquake equal to or larger than the Kaikoura earthquake.

GNS have also consulted international experts to provide additional information on this phenomena.

"As scientists' understanding of this phenomena improves we hope to develop better ways to incorporate the mechanics of slow-slip events and their relationship with earthquakes into our forecast."

New Zealand has at least two to three slow-slip events each year.

GNS is urging New Zealanders to follow Civil Defence's advice and make sure they are prepared for earthquakes.