As well as triggering tens of thousands of landslides on land,
the magnitude 7.8 KaikÅura earthquake 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.
This sediment core was
collected off East Cape on a recent research trip, and features several
dark turbidites, from previous turbidity currents generated by a massive
underwater landslide.
Photo: RNZ / Alison Ballance
The large canyon system on the
east coast of the South Island includes the KaikÅura Canyon, which flows
east into the Hikurangi Cahannel.
Photo: NIWA
The large canyon system on the east coast of the South Island
includes the KaikÅura Canyon, which flows east into the Hikurangi
Channel.
The KaikÅura Canyon and the Hikurangi Channel
The deep ocean comes very close to KaikÅura, in the form of the
KaikÅura Canyon, which is the reason sperm whales can be seen so close
to shore.
There are about 100 ocean canyons around New Zealand, and although
the KaikÅura 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 KaikÅura Canyon, which comes to within a kilometre of the
shore just south of Goose Bay.
A NIWA model of the KaikÅura Canyon.
Photo: NIWA
The continental shelf around the headwall of the KaikÅura Canyon is
just 30m deep, but the steep-sided 50km-long KaikÅura Canyon quickly
drops to 600m. It continues to deepen until it is about 2000m deep where
it joins the Hikurangi Channel.
This 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.
Large landslide near KaikÅura triggered by the magnitude 7.8 earthquake.
Photo: GNS Science
Turbidity currents
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 KaikÅura 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.
The sediment core on the left
was collected 300 km from Kaikoura, and shows fresh sediment settling
out from a large turbidity current. The core on the right shows previous
turbdites laid down hundreds of years ago.
Photo: NIWA
New research
When the KaikÅura 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
KaikÅura. 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 KaikÅura 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.”
Jamie Howarth from GNS Science explains one of the sediment cores collected off East Cape.
Photo: Dave Allen / NIWA
The impact on life in the canyon
The sediments on the floor of the KaikÅura 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.
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