Natural Forces at work

A front-row seat to the internal workings of New Zealand’s largest fault

Natural Hazards and Risks

Data gathered from seafloor observatories east of Gisborne earlier this year are giving scientists an unprecedented insight into the internal workings of the Hikurangi subduction zone.

The two observatories were installed below the seafloor in 2018 to enable scientists to learn more about the internal workings of the giant fault, particularly the enigmatic slow-slip earthquakes it produces.

New Zealand is one of only four places in the world where these hi-tech sub-seafloor observatories are keeping tabs on offshore plate boundaries.

Since they were installed, they have been continuously recording a range of physical and chemical changes inside the fault. This was the first time that data has been downloaded from the observatories in two years and it’s been an anxious wait for scientists.

“It’s been tremendously exciting to get a first look at the data. It is telling us there’s a lot going on inside the fault that we didn’t know about,”
says Dr Laura Wallace of GNS Science, who led the voyage that serviced the observatories and downloaded their data.

“It has opened our eyes to things we are unable to see in data recorded by onshore GPS instruments operated by GeoNet, which is what we’ve relied on until now.

“The observatories are sitting right on top of the fault and the data they are collecting is very high resolution. We can clearly see two large slow-slip events plus at least half a dozen smaller ones since the last data was collected two years ago.”

Only two of these events had been clearly visible in the data recorded by the onshore GPS instruments.

Image caption: Scientists in RV Tangaroa’s control room watch live footage from ROPOS exploring the sea floor. Credit: Jess Hillman, GNS Science

“The data has enabled us to observe how Hikurangi slow-slip earthquakes evolve in much greater detail than any other types of data we have used before.”

The data also showed there was a change in pressure under the seafloor after the three magnitude 7-plus earthquakes off East Cape and at Raoul Island on 5 March 2021.

“This is a direct measure of the amount of strain in the Earth’s crust during slow slip-events and earthquakes – it’s a whole new view into the subduction zone that we haven’t had before.”

Scientists used the Canadian-built ROPOS Remotely Operated Vehicle to download data from the seafloor observatories.

They also used ROPOS to explore several other important features along the Hikurangi subduction zone, including Boo Boo Fault - the most active fault in Cook Strait.

Dr Wallace says the information acquired during the voyage will add to scientists' growing understanding of the Hikurangi subduction zone and the earthquake and tsunami risk it poses to New Zealand.

The three-week voyage took place on NIWA’s research ship Tangaroa and included scientists and engineers from GNS Science, NIWA, the University of Washington, the Canadian Scientific Submersible Facility, Te Herenga Waka—Victoria University of Wellington, and The University of Auckland.

The voyage was supported by funding from MBIE, and by the United States National Science Foundation.

Image caption: ROPOS being launched from the RV Tangaroa. Credit: Jess Hillman, GNS Science

Image caption (top): ROPOS being launched from the RV Tangaroa. Credit: Jess Hillman, GNS Science

New Alpine Fault insights

The Alpine Fault represents a major earthquake threat to the South Island and recent scientific findings have helped to refine the nature of that threat. One of those findings puts the chance of a major quake on the fault at 75 percent in the next 50 years, compared to the previously estimated figure of 29 percent.

Read Article

Celebrating 20 years of GeoNet

In July GeoNet will hit a ground-breaking milestone as it turns 20 years old.

Read Article

Monitoring March tsunami

New Zealand’s network of deep ocean tsunami sensors demonstrated its value during the series of earthquakes and tsunamis off East Cape and at Raoul Island in early March.

Read Article