When Hikurangi subduction zone megathrust triggers tsunami 'think Japan 2011'
Scientists thought the southern end of the country's largest fault was dormant before the Kaikōura earthquake because it had not moved in recorded history.
And while some still debate how big a role the southern Hikurangi subduction zone played in the 7.8-magnitude earthquake, they agree it is now moving.
The Hikurangi subduction zone runs offshore from the east of Gisborne to the top of the south and would be capable of a devastating earthquake and tsunami.
Subduction zones are where one tectonic plate dives (subducts) under another plate, causing a 'megathrust'.
*Giant quake risk off east coast of North Island
*Kaikōura an example of how large quakes trigger slow-slip events at a distance
*A grid of special seismometers will be lowered into the sea to study the Hikurangi subduction zone
Past earthquakes on subduction zones included Sumatra 2004, Chile 2010 and Japan 2011.
GNS scientist Ursula Cochran said the effects of a quake on the Hikurangi subduction zone could be devastating.
"We need to think Japan 2011 basically, because if our whole plate boundary ruptured it would be a magnitude-9 earthquake," Cochran said.
"One thing about reflecting on from the Kaikōura earthquake is we don't want people to think this is the big one," Cochran said.
Cochran said some scientists believed the Hikurangi subduction zone played a bigger role in the Kaikōura earthquake than previously thought.
"A couple of people from overseas think that the Hikurangi subduction zone was quite heavily involved, whereas a lot of the local scientists think it played quite a minor role," Cochran said.
Cochran said the very southern end of the subduction zone was previously thought to be "pretty stuck" and not likely to move that much.
"Of course we did see it move after the Kaikōura earthquake in those slow slip events and some slips under Marlborough, but there are some theories out there that say it moved more than what we think it moved," Cochran said.
Cochran said scientists wanted to put their focus on understanding the subduction zone as well as the Alpine Fault.
A current international research effort planned until 2021 would drill boreholes into the subduction zone and place monitoring equipment down the boreholes, Cochran said.
The impacted area from a quake on the subduction zone would be much greater than the Kaikōura earthquake, Cochran said.
"It's the whole of the east coast of the North Island as well as the northern South Island impacted.
"One of the biggest hazards of that kind of earthquake is the tsunami that is triggered by a fault rupture offshore," Cochran said.
Tsunami modelling of a magnitude-9 rupture on the Hikurangi subduction zone showed people living in coastal communities would not have much time to escape.
"We know from tsunami modelling from a hypothetical earthquake from the Hikurangi subduction zone that the travel times could be very short to the coast, so seven minutes for some of the south Wairarapa coast," Cochran said.
Cochran said it would take between 10 to 30 minutes to reach Cloudy Bay in Marlborough but Blenheim was not at risk due to being too far inland.
Cochran was speaking at an international earthquake conference in Blenheim earlier this month, on the anniversary of the Kaikōura quake.
Visiting San Diego State University professor Tom Rockwell said the first moments following a powerful quake on the subduction zone would be vital to survive a tsunami.
"People normally are just stunned by the fact that they went through a magnitude-9, and you're not thinking about it when you're on the beach that there might be something coming at them," Rockwell said.
University of Otago scientist Caroline Orchiston said the science was all about providing a public service.
"There's so much incredible science going on in this space and for me my interest is in how to translate that into a way that has public good.
"I would encourage people to say, 'look I'm going to measure how much time it takes me to get from my place or my work place up to higher ground," Orchiston said.
Rockwell said his work highlighted that earthquakes came in clusters.
"The concern there [for New Zealand] would be from an outside perspective is that you've had a series of significant earthquakes, and you're expecting your plate margin fault to go someday.
"This would be a time for extra preparedness," Rockwell said.
The Marlborough Express