Scientists find seabed changes after quake

A freight train sits where it came to a halt beside State Highway 1, on the coast north of...
A freight train sits where it came to a halt beside State Highway 1, on the coast north of Kaikoura, during the 7.8 magnitude earthquake. Photo: Getty Images

Scientists have revealed astonishing new insights into how the 7.8-magnitude quake affected New Zealand's seabed, finding that an underwater offshoot of the Kekerengu Fault ruptured over a distance of 34km.

Researchers on board 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.

The survey area covered the offshore Hikurangi Subduction Margin - potentially New Zealand's largest source of earthquake and tsunami hazard and risk.

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.

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 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.

The core samples also showed the quake stirred up a huge amount of seafloor sediment in the Hikurangi Trough offshore from 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 could not pinpoint the location of those landslips 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.

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, Niwa scientists said  a possibly severe tsunami off the Kaikoura coast could be triggered by either a submarine earthquake or underwater landslips 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 Prof 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, Prof Pettinga and Dr 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 found 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.

``Spectacular fault displacements were observed across Leslie Hills Road at the eastern end of the Emu Plain,'' Dr Fenton said.

``As well as fault rupture, this area has suffered from widespread liquefaction, causing extensive damage to local roads.''

The next phase of these investigations began this week with a number of teams dispatched to carry out detailed surveys of the fault traces.

This information was vital for understanding the nature of this earthquake and for quantifying seismic hazard, Dr Fenton said.

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