Pent-up fault under watch
‘Stuck’ plate boundary could one day give and generate severe quake, says scientist
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 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 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 finally does become unstuck, 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.
“That’s what typically happens on faults . . . 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 Active plate boundary
It could go in something as big as a magnitude 8 or larger. Dr Laura Wallace, GNS
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 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 could relieve build-up stress in a series of moderate to large quakes — 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 why that specific part of the plate boundary had become locked: notably rock types, or the influence of temperatures or water creeping in.
Scientists hope that better understanding of the link between slow slip events and earthquakes may lead to improved earthquake forecasting in the future.