Scientists are keeping a closer eye on a large North Island fault zone that appears to be "locked" and bottling up stress that could one day cause it to snap, resulting in a huge earthquake.
The plate boundary fault, known as the Hikurangi subduction zone, underlies the North Island and northern South Island, and continues offshore of the North Island's east coast.
GNS Science geophysicists have been watching it for many years, with highly sensitive GPS measurements of land movement in the North Island.
These measurements suggest that the fault beneath the lower North Island is currently "stuck".
They also believe it may have been loaded up with more with pressure from November's 7.8 Kaikoura Earthquake, along with stress from a pair of silent, unfelt, "slow-slip" quakes that took place off the coasts of Kapiti and Hawke's Bay just after the Kaikoura earthquake.
The Hawke's Bay slow slip event lasted only two weeks, while the Kapiti coast slow slip is continuing.
When the fault region does finally become un-stuck, scientists expect to see a massive amount of pent-up energy suddenly released.
GNS geodetic scientist Dr Laura Wallace said the region had likely become locked up due to friction at the boundary between the Australian and Pacific tectonic plates, which intersect New Zealand.
"It's basically like having two pieces of sandpaper and rubbing them together: you find that it's very hard for them to move past each other.
"That's what typically happens on faults during the time between earthquakes: two pieces of the Earth's crust get stuck together along a fault, even though the plates and the crust around it are still moving.
"When enough energy finally builds up to overcome the strength of the fault, then you get a sudden rupture and accompanying seismic energy release."
In 2011, the same process unfolded with catastrophic consequences when built-up stress on a subduction zone off the coast of Japan unleashed a 9.0 megathrust earthquake and tsunami that killed nearly 16,000 people.
Wallace said the section of the Hikurangi Subduction Zone plate boundary that ran beneath the lower North Island had been locked for at least 20 years, and probably much longer.
"We know that this type of locking can last for hundreds of years - but we don't know how far into the future that it's going to rupture.
"It's possible that it could relieve build-up stress in series of moderate to large quakes - so several magnitude 7 events - and it's possible that it could go in something as big as a magnitude 8 or larger.
"Ultimately, we really don't know how and when this stress will get relieved because we haven't yet seen examples of this in New Zealand's historical record.
"The last big earthquake on the subduction zone beneath the lower North Island was probably around 500 to 600 years ago."
There were a number of possible explanations to why that specific part of the plate boundary had became locked: notably rock types, or the influence of temperatures or water creeping in.
"This is something that we are really trying to understand as well."
She said that faults in the northern South Island had also been locked before being released by the power of November's earthquake.
The Kaikoura earthquake and the slow slip events that followed has caused Wallace and her colleagues to watch the locked North Island area even more closely, via GeoNet's network of seismometers and GPS instruments.
"We do know that both the Kaikoura event and the slow-slip events that followed it have increased stress in that locked area, and we've been trying to account for that in the earthquake forecasts that GeoNet have been issuing."
Even before the Kaikoura event, Wallace and a large team of researchers had begun investigating the hazard posed by the Hikurangi subduction zone in a $6.5 million, five-year study, supported by the Government's Endeavour Fund.
That work is also looking at the little-understood impact that slow-slip earthquakes are having on the Hikurangi subduction zone, which has potential to be the country's single biggest geological hazard.
"It's an incredibly important thing to be studying, because in a few cases slow-slip events have been observed to precede large earthquakes, including the magnitude 9 event in Japan and other big events in South America.
"However, it is important to remember that there have also many hundreds more slow slip events that have not led to large earthquakes."
Scientists hope that better understanding of the linkage between slow slip events and earthquakes may lead to improved earthquake forecasting in the future.