The Hikurangi Subduction Zone - where the Pacific and Australian plates meet offshore and beneath the North Island - poses New Zealand’s single largest earthquake and tsunami threat. Image / GNS Science
The Hikurangi Subduction Zone - where the Pacific and Australian plates meet offshore and beneath the North Island - poses New Zealand’s single largest earthquake and tsunami threat. Image / GNS Science
Fresh geological evidence indicates both “slow” and large, sudden quakes can occur in a poorly understood part of the zone off Hawke’s Bay.
The most recent event 600 years ago was big enough to have forced down the coastline near Napier.
Ancient traces of massive undersea landslides has transformed scientists’ understanding of our biggest fault zone - and its ability to unleash tsunami-generating mega-quakes off the Hawke’s Bay coast.
The Hikurangi Subduction Zone marks the boundary where the Pacific plate plunges below the Australian plate. If we drained theocean, it’d appear as a vast mountain range rising up from the sea floor off the North Island’s east coast.
This endless tectonic scrum produces an enormous amount of pent-up energy, and modelling has estimated one-in-four odds of southern part of the zone triggering a magnitude-8.0-plus earthquake within the next 50 years.
The Hikurangi Subduction Zone is where the Pacific plate dives beneath the Australian plate on the east coast of the North Island. Images / East Coast Labs
In May, scientists warned a major Hikurangi tsunami could kill more than 22,000 and injure nearly 26,000 - even assuming three-quarters of people could evacuate in time.
Now, a just-published study, researchers have shed crucial new light on the poorly understood central part of the zone, located offshore and underneath Hawke’s Bay.
Yet it hasn’t been clear whether those ruptures struck at the subduction interface - an area capable of generating quakes big enough to cause large tsunamis - or on smaller local faults.
To build a clearer picture, the scientists turned to ancient deposits collected from the sea floor in sediment cores during a 2016 expedition.
Within these geological time capsules, spanning the last 10,000 years, were “turbidites” - traces of underwater landslides and sediment flows that could be triggered by huge earthquakes.
“The advantage of using turbidites to reconstruct past earthquakes is that they produce complete, continuous and long earthquake records compared to those from onshore environments,” explained study co-author Dr Jamie Howarth of Victoria University.
The deposits indicated over the last 5000 years, a number of big quakes had indeed erupted from within the subduction interface.
The most recent event, around 600 years ago, was big enough to have sunk the coastline near Napier, while causing shaking across the entire 175km stretch of the central subduction zone.
The new insights carried major implications for ongoing efforts to understand enigmatic “slow-slip” earthquakes, which are thought to act like pressure valves by releasing energy in silent, weeks-long episodes deep underground.
Slow-slip events - silent earthquakes that can last for days, weeks, or months - occur in an area where the Hikurangi Subduction Zone is transitioning from being "stuck" beneath the southern North Island, to an area where the subduction zone is "creeping" further north, around Gisborne and Hawke's Bay. Image / GeoNet
Last year, scientists observed how slow-slip quakes deep beneath the North Island at the time had dissipated the equivalent energy of a single magnitude-7.2 shake.
The new study’s lead author, Dr Charlotte Pizer of Victoria University, said it now appeared the central part of the zone could host both slow-slip and those sudden and violent “seismic slip” quakes - making it “conditionally stable”.
“This means that although its current slip behaviour is quiet and creeping slip, conditions could suddenly change to allow a large earthquake to occur,” she explained.
“The cause and timing of this sudden change are difficult to pin down, making forecasting earthquake hazards challenging.”
Still, there was evidence to show the region could respond to large quakes further south in the subduction zone.
“We can’t rule out the possibility that the next large earthquake on the southern Hikurangi Subduction Zone will extend up into Hawke’s Bay, as it appears to have done in the past,” GNS Science earthquake geologist Dr Kate Clark said.
“The devastating 2011 Tōhoku-oki earthquake and tsunami in Japan is a recent example of this sort of behaviour.”
Clark said the more evidence that could be gathered from both land and sea, the better scientists were able to reconstruct past quakes.
“It’s a bit of a detective story to put all these different bits of earthquake evidence together, but the records together are much more powerful than being used in isolation.”
Pizer said for Hawke’s Bay residents, a key message was the next big event was unlikely to be like the 1931 quake - still New Zealand’s deadliest natural disaster - which hauled up swathes of coastline without causing a major tsunami.
Four men try to refloat their boat in what was Napier's Ahuriri lagoon after the 1931 Hawke’s Bay earthquake emptied it of most of its water. Scientists say the next big event in Hawke's Bay is unlikely to cause the same effects. Image / Keith Winks
“Our research shows that past earthquakes have caused subsidence, some have been accompanied by tsunamis, and we need to be prepared for all types of earthquakes and their impacts,” she said.
“It’s a timely reminder to prepare your grab-and-go bags, know your personal tsunami evacuation plan, and remember: ‘When shaking is long and strong, get gone’.”
Jamie Morton is a specialist in science and environmental reporting. He joined the Herald in 2011 and writes about everything from conservation and climate change to natural hazards and new technology.
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