Researchers use coring machine to take sediment deposits at the Hikurangi Subduction Margin. Source: NIWA
Scientists have revealed astonishing new insights into how the 7.8-magnitude quake affected New Zealand's seabed, finding that an underwater off-shoot of the Kekerengu Fault ruptured over a distance of 34km.
Researchers onboard the NIWA research vessel Tangaroa have been surveying the sea floor around the Cook Strait, Marlborough and Kaikoura regions to observe any major changes since the earthquake, particularly on the Needles and Chancet faults.
They also discovered sediment displaced in submarine landslides - likely resulting from last week's earthquake - moved up to 300km away from where the landslides happened.
The researchers had just begun a five-year project funded by the Ministry of Business Innovation and Employment focusing on studying the offshore Hikurangi Subduction Margin - potentially New Zealand's largest source of earthquake and tsunami hazard and risk.
Their aim was to build an earthquake history for the area by looking at sediment cores. While they were on their expedition, the 7.8 earthquake hit, giving them a unique opportunity to study the seafloor directly after a quake.
As part of the expedition, the researchers collected 61 sediment cores - each up to 5.5m long - to identity any sediment flows as a result of submarine landslides.
The scientists also carried out multibeam surveys - using echosounders to map the seabed in detail - on the Marlborough coast in search of potential fault surface ruptures.
Initial results revealed that the submarine Needles Fault ruptured in the quake, forming newly discovered scarps on the seafloor along the northeast Marlborough coast south of Cape Campbell.
An image of the fault shows where the Needles fault ruptured after the recent earthquake. Photo/supplied
The fault is an offshore extension of the Kekerengu Fault, on which earthquake displacements of 5m to 10m occurred on land.
The offshore rupture may have extended up to 34km offshore, suggesting that the length of the Kekerengu-Needles fault rupture may now extend for around 70km.
The new data would help inform understanding of the entire fault network activated during the earthquake.
NIWA principal marine geology scientist Dr Phil Barnes said they discovered sediment had flown in a newly created current up to 300km from where landslides occurred post-earthquake, though the team had not found out where the landslides happened.
Dr Barnes believed an earthquake would need to be "quite sizable" to create a landslide large enough to move sediment that distance.
He could not say much on how the stirred up sediment would have affected marine life, though some burrowing organisms might be able to make their way back up through the layer if it was thin enough.
The earthquake is likely to have relieved stress on the Needles fault but added stress to faults in other areas.
Dr Barnes hoped the public could "sense the excitement" he and his team had over their findings.
The image shows the fault rupturing that took place on land and off shore in the 7.8 quake. The Needles and Chancet faults are highlighted in red. Image / NIWA
The core samples also showed the quake stirred up a huge amount of seafloor sediment in the Hikurangi Trough offshore of Marlborough and Wairarapa, likely a result of submarine landslides.
Turbidite, which had only been recently placed, measured about 10cm to 20cm over a large region, extending at least 300km from Kaikoura.
This material was still settling on the seabed and it could take many more days before the deposits filtered through the water column.
Scientists couldn't pin-point the location of those landslides that had stirred up the material.
The voyage's leader, National Institute of Water and Atmosphere (Niwa) marine geologist Dr Phil Barnes, said it also remained unclear whether the quake had increased the risk of landslide in the submarine Kaikoura Canyon.
"We haven't mapped it yet, so we've got new information and I can't comment on that."
Recent modelling has suggested the Kaikoura Canyon - which comes up to within 500m of the coast south of Kaikoura and feeds sediment into the 1500km-long Hikurangi Channel that runs east of New Zealand - is capable of generating a wave with a crest 13m above sea level.
The image shows the sites where NIWA scientists collected sediment cores. Photo / NIWA
It posed a unique threat due to its combination of factors, including very shallow water next to the canyon edge; steep slopes of the canyon which drops off down to 1000m; large areas where there might be weak sediment; large earthquake sources nearby; and all of it very close to the highway and populated areas.
In 2006, scientists from the Niwa said that a possibly severe tsunami off the Kaikoura coast could be triggered by either a submarine earthquake, or underwater landslides that occurred at the canyon's head every 200 years on average.
Researchers make new finds on the ground
Meanwhile, on the ground, Canterbury University researchers have been observing new evidence of faulting in Marlborough's freshly-scarred landscape.
A reconnaissance team comprising academics Professor Jarg Pettinga, Dr Clark Fenton, Dr Anekant Wandres, and geology PhD students Alan Bischoff and Andrea Barrier have discovered many previously unmapped fault traces.
One prominent displacement, across SH70 north of Waiau, was found to have push the earth 2.2m sideways and 1.5m upwards.
While walking along one of the fault traces, the team also rescued a lamb from a 1.5m deep fissure.
They found a broad, complex zone of faulting extending from the Mason River, about 6km northeast of Waiau, across the Waiau River flood plain to Emu Plain, about 9km west of Waiau.
The total zone of faulting was approximately 3km across - most of it happening on fault traces that had not been previously mapped.
In a second expedition, Pettinga and Fenton were joined by Dr Kate Pedley and Dr Narges Khajavi for a survey of an area stretching from the western end of the Amuri Range, back across the Emu Plain and continued as far as the junction for Mt Lyford Village.
All roads beyond this point were closed and only passable by military vehicles.
Further fault traces were located across the Emu Plain, highlighting further complexity in the fault structure.
Although no clear evidence for faulting was discovered to the east of the Mason River, this was identified as an area for further detailed study.
Much of the Mason River valley was affected by large scale landsliding, particularly in the area around Mt Cookson.
