Clues to when the next big tsunami-causing quake might happen have been uncovered on the sea floor off the coast of New Zealand.
The magnitude 7.8 quake which hit Kaikoura in 2016 has been one of the most-studied in history, and it's still giving up secrets. The latest is proof underwater landslides can be used to accurately reconstruct past quakes, perhaps helping pick when future ones might occur.
"It's probably not a surprise... that subduction zones - of which we have the Hikurangi margin off the east coast of the North Island of New Zealand - can produce earthquakes of a magnitude up to eight," lead author, Jamie Howarth of Victoria University of Wellington, told Newshub.
"These are really damaging to society, so being able to forecast when they are likely to occur in the future is important."
Scientists have long thought layers of sediment known as turbidites could be evidence of past destructive quakes, but until now their theories hadn't been confirmed.
"These are formed when really strong shaking from earthquakes destabilises slopes on the sea floor, generating flows of sediment and water kind of like floods and rivers," said Dr Howarth. "These lay down these deposits we call turbidites. Globally these have been used to generate records of subduction zone earthquakes, but many geologists are dubious about how reliable those records are."
In a new paper, he and other scientists from NIWA, GNS Science and the University of Auckland show how the massive Kaikoura quake "triggered landslides that evolved into bottom-hugging mud slurries that ran out far beyond the original landslide".
Turbidites were found in 10 canyons covering 200km along the Hikurangi subduction margin - from Marlborough in the south to the coast of Wairarapa.
"Turbidites may act as 'natural seismometers'," Dr Howart said in a statement accompanying the research, which was published in journal Nature Geoscience on Friday.
"Turbidites preserved in sediment cores [could] determine the direction of rupture and spatial variability of ground motions during prehistoric earthquakes. Both are essential elements for reliable seismic hazard forecasting but up to now have been difficult or impossible to infer from the geological record."
Subduction zones - where one tectonic plate is being submerged beneath another - are where the world's strongest earthquakes happen. Friday's quake further north off the East Cape happened where the Pacific and Australian plates meet, for example. The plates also meet at the Hikurangi subduction zone, and are responsible for the Southern Alps which straddle the South Island.
"Turbidites in marine sediment cores produce arguably the longest and most complete records of subduction-zone earthquakes around the globe, but the use of them in forecasting has been vigorously debated by earthquake scientists, as there are few examples where the relationship between the fault(s) that rupture in an earthquake, the spatial extent of strong shaking, and the deposition of turbidites has been observed," said Dr Howarth.
"The work has global relevance and is also particularly relevant for New Zealand because it shows that turbidites are reliable recorders of past earthquakes on the Hikurangi margin, New Zealand’s largest potentially hazardous source of large quakes."
If a megathrust quake hit the Hikurangi margin, it could be as strong as the one which hit Japan in 2011, letting off 11,000 times as much energy as the 2011 Christchurch quake, Martha Savage - professor of geophysics at Victoria University - warned in 2019.