As well as triggering tens of thousands of landslides on land, the magnitude 7.8 Kaikōura earthquake, that shook New Zealand in the early hours of Monday 14 November, also caused a massive underwater landslide.
It moved down the deep canyon system that lies just offshore, generating a turbulent turbidity current of mud, sand and water that was detected more than 300km away, off the coast of Hawke’s Bay.
For geologists aboard NIWA’s research vessel Tangaroa, it was a once-in-a lifetime opportunity to see the immediate aftermath of a turbidity current as it was still settling on the floor of the ocean.
A large turbidity current can be hundreds of metres thick, large enough to burst over the deep channel in which it flows. NIWA marine geologist Phil Barnes says that while they have plenty of evidence of past turbidites, this was a rare chance to see fresh evidence.
The Kaikōura Canyon and the Hikurangi Channel
There are about 100 ocean canyons around New Zealand, and although the Kaikōura Canyon is special, NIWA geologist Joshu Mountjoy says it’s just part of a more massive, very active canyon complex on the north east coast of the South Island.
North of Banks Peninsula, the Pegasus and Okains canyons flow north to join the Kaikōura Canyon, which comes to within a kilometre of the coast, just south of Goose Bay. This deep water is the reason sperm whales can be seen so close to shore.
The continental shelf around the headwall of the Kaikōura Canyon is just 30m deep, but the steep-sided 50km-long Kaikōura Canyon quickly drops to 600m. It continues to deepen until it is about 2000m deep where it joins the Hikurangi Channel.
The Hikurangi Channel, which is also fed by the Cook Strait Canyon, is a long meandering abyssal river. It flows for several thousand kilometres, up the east coast of the North Island, eventually emptying all the sediment into a large fan in the South Pacific Basin.
This massive underwater canyon and river system is like several Grand Canyons flowing into a river like the Mississippi.
The Hikurangi Channel only flows every couple of centuries during catastrophic underwater flash floods or turbidity currents.
Turbidity currents are a kind of density flow that starts when mud and sand on the continental shelf are loosened by something like an earthquake. As the landslide rushes down the slope and mixes with water, it forms the sediment-laden flow known as a turbidity current. The avalanche of turbid water pours along the canyon floor like a river in flood, picking up more sediment and increasing in size and speed as it flows.
Phil Barnes from NIWA says these flows move at about 5 to 10m a second, or about 20-30km/h. The resulting layer of sediment on the sea floor is very distinctive and known as a turbidite.
There is no shortage of sediment in this east coast canyon system - scientists estimate that about 1.5 million cubic metres of nearshore sediment enters Kaikōura Canyon each year.
And it is the scouring action of all this sand and gravel, sourced from the rapidly uplifting and eroding mountains, that has actually created the canyons, eroding them out of the continental shelf. This process has gone on for about two million years, and was particularly active during the glacial periods, when the continental shelf was exposed during periods of low water level.
When the Kaikōura earthquake occurred, the NIWA research vessel Tangaroa was already at sea, studying the Hikurangi subduction margin off East Cape. This is where the Pacific tectonic plate dives beneath the North Island, which lies on the Australian Plate. It’s considered to be New Zealand’s largest earthquake and tsunami hazard.
Barnes was part of a team collecting cores from the sea floor to look for evidence of the turbidites. They plan to date these so they can work out how often large earthquakes have happened, where they occurred, how big they were, and so on.
Barnes says they’d already collected 61 cores, each containing plentiful evidence of past turbidites, when they diverted the ship to Kaikōura. They were keen see if they could find evidence of a new turbidite, one that might have been triggered by the recent shaking.
En route to Kaikōura they stopped a couple of times to take cores, using a device called a multi-corer. Several of the cores were in the Hikurangi Channel itself and one was out of the channel on the nearby seafloor, on what you might think of as the riverbank. Barnes says they were stunned by what they found.
“Lo and behold there was a sea of mud down there. There is still mud and clay falling out of the water column and it’ll probably go on for days or weeks or even months. We’ve got about 10 centimetres of sand and silty sediment already on the seafloor now. That is about 300 kilometres away from where it must have been sourced from.”
It is possible that the earthquake triggered simultaneous underwater landslides in a number of canyons.
“We can’t say where the landslides occurred exactly,” says Barnes, “but we certainly know that large sediment failures occurred.”
The impact on life in the canyon
The sediments on the floor of the Kaikōura canyon are remarkably rich in life. Earlier research by NIWA identified it as a hotspot of benthic biomass, and “one of the most productive habitats described so far in the deep sea".
The biodiversity is not great, but it includes plentiful burrowing sea cucumbers, spoon worms, bristle worms, irregular urchins and very high numbers of large nematode worms. The biologists also recorded high numbers of fish known as rattails, which they think are feeding on this high level of biomass in the canyon's sediments.
Scientists suspect the gently sloping canyon floor traps organic matter and sediments coming from the nearby coast, as well as organic debris from the productive waters above.
While no one has yet investigated the impact of the underwater landslides and turbidity flow on the floor of the canyon, NIWA marine biologists Ashley Rowden, David Bowden and Daniel le Duc say there will certainly have been widespread disturbance.
Larger invertebrates would have been buried by a thick sediment layer, although it’s possible that some of the stronger burrowers may have been able to pull themselves back to the surface if they weren’t buried too deeply.
Le Duc thinks that many nematodes may have been swept along in the sediment and may have survived to be deposited tens or even hundreds of kilometres away.
David Bowden points out that the richest site they sampled in the canyon was a more sheltered area away from the main canyon. He wonders if it might have escaped the main turbidity current but still been affected by the 1m to 6m of uplift along the coast that shunted the intertidal zone above the high-water mark.
There will be chemical as well as physical changes in the canyon, and several weeks of high silt levels in the surrounding waters are probably having an impact on species far beyond the canyon floor.