Marlborough Express - Weekend Express
How dangerous is our biggest faultline?
Researchers are lowering sensors onto the seafloor off the Wairarapa coast to learn more about the chances of a massive earthquake that could shake much of the country, and send a large tsunami surging onshore.
The study is the first aimed at learning about the “locked” southern part of the Hikurangi Subduction Zone – Aotearoa’s largest and most active fault, which runs the length of the North Island east coast. The locked section starts roughly offshore from Porongahau and runs south to the top of the South Island.
Previous studies have investigated areas of the zone further north, where slow-slip events are thought to be relieving some of the stress on the plate. Even if that stress is being relieved, there are concerns slow-slip events could trigger large earthquakes in neighbouring parts of a fault.
The zone is a plate boundary fault, where the Pacific tectonic plate dives westward beneath the Australian tectonic plate.
“These places are where the biggest earthquakes happen in the world,” team leader Professor Martha Savage, from Te Herenga Waka-Victoria University of Wellington, said.
Not many earthquakes had been felt from the locked part of the plate boundary at the southern end of the subduction zone. “It just gets stuck. It gets stuck, and the plate keeps pushing along. Then it will go boom. It will break in a big earthquake, which could be from magnitude 7 to magnitude 9, depending on what other parts of it get involved,” Savage said.
It was expected the ocean floor sensors would pick up 10 times more earthquakes on the locked zone than were reported now, Savage said. “The behaviour of these more frequent small earthquakes can tell us more about the larger earthquakes that occur less often.”
Dr Natalie Balfour, head of research at Toka Tū Ake EQC – which is helping funding the research – said a better understanding of the locked portion of the fault and the smaller earthquakes that happened there, would help preparations for a large earthquake and potential tsunami should the fault come unstuck.
It would also be significant if very few earthquake signals were picked up in the locked zone, GNS Science Te Pū Ao researcher Dr Emily Warren-Smith said.
“It helps confirm that our land-based observations have been right and that there is significant stress build-up occurring offshore.
Twenty sensors would be placed on the seafloor offshore from central Hawke’s Bay down to Wellington, and from as close to shore as possible out to about 100km east of the Hikurangi Trench.
The sensors would be in place for a year, then they would be retrieved and the data they had collected would be analysed.
The sensors would then be moved to an area offshore from Marlborough and in Cook Strait for a new project that would be trying to understand the change in earthquake risk following the Kaikōura Earthquake.
Part of that involved looking at where the subduction zone terminated beneath the top of the South Island, where it fed into faults that ran through Marlborough.