New 3D maps reveal hundreds of ancient landslides south of Auckland

A new mapping exercise has revealed hundreds of historic landslides around Pukekohe, while offering a multi-dimensional view of the region’s active faults and volcanic features. Image / GNS Science

Just-released maps have revealed hundreds of ancient landslides around Pukekohe, while offering a multi-dimensional view of the region’s active faults and volcanic features.

Capturing more than 830 square kilometres, from Manukau Harbour’s southern shoreline to the lower Waikato River, the new GNS Science-produced maps show Auckland’s rolling southern landscapes as they’ve never been seen.

It was when scientists were building the maps, using light detecting and ranging (Lidar) data capturing detailed information on local land surfaces, that they uncovered evidence of some 800 previously-unidentified landslide sites.

Found mainly around the foothills of the Hunua Ranges, but also in the downlands around Tuakau, Pōkeno and Pukekohe, the prehistoric landslides were shown as hummocky, sloping ground, often with steep scarps at the top.

“The sheer number of large landslides that captured in the Lidar data was interesting,” GNS senior geologist Dr Kyle Bland said.

The freshly-defined landslides would be added to local and national databases used to inform planning, he said.

The maps have similarly shed fresh light on three active local faults - the Pukekohe (Waiuku), Paerata and Aka Aka, along with another dozen others thought to be possibly active.

Those three faults were believed to have ruptured in relatively recent times, given their position at the surface - but scientists hadn’t been able to pinpoint just when.

The new GNS Science maps reveal some 800 landslide sites south of Auckland - shown here in turqoise. Image / GNS Science

“We currently do not have any good information on their ages of last movement,” GNS geologist Dr Dougal Townsend said.

Although faults in this area weren’t new to scientists, most had been only inferred until now - and this was the first time they’d been mapped at the modern land surface, in high detail.

While Auckland’s seismic hazard had been raised in New Zealand’s latest hazard models, the region’s earthquake risk was nonetheless considered low.

“The fact that we don’t have large hills and mountain ranges associated with these faults shows that their activity is low and that earthquakes on them are infrequent,” Townsend said.

Now that the faults had been mapped at the surface, he said more research was needed to understand how often earthquakes occurred along them - and how large those quakes could be.

Discussions about a new study into the faults were now under way with the Auckland Council and the University of Auckland.

The maps have also revealed much more about Pukekohe’s long, fiery volcanic history, including what appeared to be newly-identified eruptive centres within the South Auckland Volcanic Field.

Among the features were ancient explosion craters and their surrounding tuff rings, lava flows, and scoria cones.

“The new mapping allows us to better characterise the general history of volcanic eruptions in the south Auckland area, which provides insights into how these intraplate volcanic fields evolve through time,” Bland said.

Unlike the active field that central Auckland is built upon, the South Auckland Volcanic Field is considered extinct, with the last eruption occurring some half a million years ago.

The geological maps - the first produced for the region in three decades - come after GNS recently released two sophisticated seismic hazard models for the entire country.

The updated National Seismic Hazard Model released found New Zealand’s “shaking hazard” had grown, on average, by 50 per cent or more in most locations.

But that didn’t mean the risk of earthquakes themselves had risen, with experts putting the increased calculations down to what they’ve learned from a decade of quakes and new science.

Meanwhile, the New Zealand Community Fault Model - described in a recently-published study - mapped 900 faults capable of generating moderate to large quakes.

Among the active faults included in the model were some of our best-known - notably the 600-kilometre-long Alpine Fault - but also those smaller, lower-risk faults covered in the new Pukekohe maps.