The Kaikōura Canyon links into the southern end of the Hikurangi Channel system.

The under sea "flushing" that barrelled through the Kaikōura Canyon after the November 2016 earthquake moved 850 million tonnes of sediment and deepened the canyon by 50m, scientists say.

The Kaikōura Canyon, which is cut into bedrock and comes within 1km of the coast south of the Kaikōura Peninsula, feeds into the southern end of the Hikurangi Channel system. More than 1500km long, the channel system runs under the ocean east of the North Island.

Researchers estimate high-energy currents after the 7.8 magnitude Kaikōura earthquake flushed 850 million tonnes of sediment through the canyon out to the deep sea. It is one of the largest canyon flushing events ever documented.

NIWA The rim of the Kaikōura Canyon before the earthquake.

It also deepened the floor of the two main reaches of the canyon by up to 50m, while in the middle-canyon region, depressions in the bedrock floor deepened by 20m to 30m.

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"The event has completely changed much of the canyon floor, eroding into rock and moving dunes of gravel through the lower canyon," Niwa marine geologist Dr Joshu Mountjoy, the study's lead author, said.

NIWA Rim of the Kaikōura Canyon after the earthquake, showing widespread landslides and a new fault scarp.

Samples of sediment were taken after the quake at distances up to 680km from the Kaikōura Canyon mouth. At the most distant sample site the layer of sediment deposited by the flushing event was 65cm thick.

At that point, evidence shows the sediment flow reached to at least 180m high above the channel floor, indicating the flow likely went well beyond that point.

When the head of the canyon was studied in 2013, everything was smooth and draped in mud. But investigations two months after the 2016 earthquake showed the mud from almost every part of the upper slope had been stripped away, from a length of about 30km along the canyon rim.

NIWA The middle canyon floor before the earthquake.

"This study unequivocally demonstrates that earthquake-triggered canyon flushing is the primary process that carves out submarine canyons and delivers coastal sediment to the deep ocean," Mountjoy said.

It is thought flushing events could nourish deep-sea benthic ecosystems, which are those that live in and on the bottom of he ocean floor, and include worms, crabs and sponges. In the Kaikōura case, an estimated 7m tonnes of carbon was among the material transported to the deep ocean.

The study is part of a collaboration, which includes Niwa, Victoria University and GNS Science. It was published in the journal Science Advances.

NIWA The middle canyon floor after the earthquake, showing significant scouring.

The observations from Kaikōura were the first to quantify the extent of seafloor landscape change and large-scale sediment transport associated with an earthquake-triggered full canyon flushing event, the paper said.

Researchers calculated that in the 11 sq km middle reach of the canyon, where bedrock was exposed before the earthquake, erosion deepened the canyon by an average 5.6m.

Sediment waves in the lower 35km of the canyon, with amplitudes - distance from the centre line of the wave to the top of the crest - of 10-20m and average crest spacing of 250m, were substantially modified.

NIWA Erosion and deposition on the middle canyon floor.

Pre-existing sediment waves were found to have moved down-canyon by up to 560m but maintained the same configuration.

Based on the estimated 140 + or - 30-year interval between earthquake-triggered flushing in the Kaikōura Canyon, researchers calculated the mid-canyon area is being deepened by an average of 40mm + or - 11mm a year.

That was three to 30 times higher than the long term river incision rates measured in the dynamic landscape of Taiwan, and comparable to localised short term rates in steep upland rivers where debris flows were the main erosion mechanism.

NIWA Lower canyon gravel waves before the earthquake.

The canyon flushing event also completely removed the benthic community from the canyon system. Before the quake, that community had one of the highest biomasses measured in the deep sea, as a result of the canyon's position at the confluence of nutrient inputs from coastal and oceanic upwelling sources.

A Niwa study last September found signs of recovery, with evidence of juvenile animals that once dominated the head of the canyon beginning to colonise the seafloor.

NIWA After the earthquake, gravel waves have migrated down the canyon.