Underwater slips and slides

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Underwater landslides: out of sight, but not out of mind 

One million years ago something triggered an underwater landslide in the Tasman Sea off the coast of Taranaki. A massive amount of sediment, 32 times the volume of Mount Ruapehu, thundered down a slope.  

But if the land slides underwater and there’s nobody there to see it, does it have an impact?  

Early modelling suggests yes – it’s likely to have caused a large tsunami that would have hit the west coast of Aotearoa.  

So today, with our populated coastlines and our underwater communication cables, what’s the likelihood of it happening again?  

That’s what GNS senior marine geohazard researcher Dr Suzanne Bull is keen to answer.  

Figuring out the where and why 

Just like on land, any sloped area under the sea has the potential to slide – but underwater landslides tend to be far more massive.  

Understanding what triggers them is tricky, says Suzanne, because you can’t easily survey after an event.  

For example, in the aftermath of Cyclone Gabrielle more than 140,000 landslides have been documented from on-the-ground reports and aerial imagery, but doing a similar assessment underwater is far more expensive and time consuming.  

Earthquakes could be another potential trigger. One natural disaster that woke scientists up to the potential hazard of such landslides was when a 7.0 magnitude earthquake off Papua New Guinea in 1998 resulted in an unexpected tsunami. Investigations indicated that it was caused not by the earthquake itself, but by a large underwater landslide.   

Listen to the episode to hear Suzanne explain how she and her colleagues have been investigating a group of large landslides in the Tasman Sea in the hopes of learning more about the potential risks to Aotearoa.   

Homing in on the zone 

Of course, Aoteaoroa is also at risk from tsunamis generated in the more ‘classic’ way: large earthquakes occurring underwater.  

Off the North Island’s East Coast, the Pacific Plate dives under the Australian Plate in an area known as the Hikurangi subduction zone.  

Such subduction zones have been responsible for extremely large earthquakes around the Pacific Ring of Fire, says GNS scientist Dr Stuart Henrys, so he and others are keen to understand the Hikurangi area in as much detail as possible.  

GPS sensors revealed that in part of this zone, to the east of Gisborne, slow-slip earthquakes are occurring regularly – every two years or so. In slow-slip earthquakes, the movement is similar to a traditional earthquake, except it takes weeks to occur rather than seconds. The slip is so slow that no damaging shaking occurs.  

Internationally there’s been a lot of scientific interest in slow-slip earthquakes, in part because they occurred in advance of the extremely destructive 9.1 magnitude Tohoku earthquake in Japan in 2011.  

The slow-slip earthquakes observed off Gisborne are the shallowest in the world, so researchers from New Zealand, the US, the UK and Japan pooled resources to create a 3D map of part of this region in incredible detail.  

Having crunched the data for many years, Stuart tells Claire Concannon about the surprising findings he and his colleagues have uncovered.  

To learn more:  

• The National Geohazard Monitoring Centre is where geohazard analysts are on alert 24/7. This is where all the hazard research is applied in a practical way to assess risks and alert civil defence when something happens.  

• A giant underwater landslide was also identified in the Kaikōura canyon after the November 2016 earthquake.  

• Learn more about the ‘biopsies’ taken at the Hikurangi subduction zone and about two underwater observatories that were placed in the area in Understanding New Zealand’s largest fault.  

• Our Changing World has an earthquake episode collection where you can learn a lot more about the geohazards that affect Aotearoa.