New fault lines found off the West Coast are likely to be strong enough to cause a tsunami, but it's not clear yet how damaging they could be.
Niwa scientists found 10 marine faults parallel to the coast about 30km offshore between Farewell Spit and Hokitika while carrying out a mapping project for the West Coast Regional Council as part of the council's tsunami risk assessment.
Niwa marine geologist Philip Barnes says the faults range between 10km and 120km in length and could cause earthquakes between magnitudes of 6.4 and 7.8.
Dr Barnes says national tsunami modelling for the West Coast doesn't take these fault lines into account. At present they only consider tsunamis generated from much further away.
"Although the work hasn't yet been done and modelled, I would be surprised if these faults don't generate tsunamis that would at least impact on the local coastal area along the Westland region there," he told NZ Newswire.
"You can't really make a statement about the likely damage, but certainly low-lying areas of the coastal region, along those townships or at rivermouths and things would be more vulnerable and any infrastructure and development in the way would be susceptible to whatever's thrown at it."
Dr Barnes said the faults had low slip rates, meaning quakes of this magnitude were only likely every 7500 to 30,000 years, though this doesn't mean there would not be significant aftershocks.
"Should an earthquake occur on one of them, as has been experienced in the Canterbury earthquakes, faults can then interact," he said.
"There is very likely to be some level of interaction between these faults should an earthquake sequence propagate."
He said the discovery of the fault lines wasn't entirely new or unexpected. But it's the first time data had been gathered to determine whether they were active faults, and the potential earthquake hazard they posed.