Seismometers to give clearer image of subduction zone earthquakes

Scientists hope that the deployment of new ocean-bottom seismometers off the east coast of Wellington will help reveal the earthquake and tsunami potential for a ‘locked’ part of the Hikurangi Subduction Zone. But before then, plenty of work needs to be done.

Tomography helps geophysicists understand Earth’s interior. However, nearly all seismic networks are located on land, precluding seismologists from getting a complete picture of the planet’s internal churning.

Luckily for New Zealand, the Earthquake Commission (EQC) has funded research which will next year see 20 ocean-bottom seismometers deployed for 12 months to better understand how the Pacific Plate is slipping underneath the Australian Plate, off the east coast of Wellington.

The research is being conducted by Professor Martha Savage and her team from Victoria University of Wellington, CNS Science, as well as from Canada’s University of Ottawa and Dalhousie University.

“My colleagues in Canada are getting the instruments ready, being tested and doing follow up,” says Savage, who is a geology academic and Professor of Geophysics.

Permissions are also being readied from the likes of Fisheries New Zealand.

“The reason it’s going to be next year is it’s the first time the RV Tangaroa can help,” says Savage. “We have been on their list of projects – it is the NIWA ship that is going to help with deployment.”

And there is plenty of preparation to be done before the instruments are deployed – with no way for them to be accessed for data retrieval or maintenance during the 12-month period, it has to be ensured that everything is ready.

Once the seismometers have been shipped from Canada to New Zealand, and batteries are installed in each, they are then craned onto the RV Tangaroa.

“Then to get off the ship, they also get picked up by cranes and then dropped into the ocean. They are designed so that they will settle down onto the sea floor and be upright.”

While modern research seismometers are electronic and detect and record motions in all directions, the orientation remains unknown until the data is retrieved. After the 12 months of deployment, the anchors of each seismometer will be burned off and the seismometer will return to the surface.

“There are various issues every once in a while – people lose seismometers, hopefully that won’t happen this time,” says Savage.

Importance of the research

Being deployed off the east coast of Wellington, once the seismometers are back on land, the data will help scientists improve understanding of why the locked and slipping sections of the Hikurangi Subduction Zone behave differently.

“This is a big hazard for us because if it goes, it’s going to go fast. The result would be a large and sudden earthquake that could also trigger a tsunami,” says Savage.

“The GeoNet network does an excellent job of monitoring earthquakes on land. Yet, we are effectively blind to small earthquakes offshore. The behaviour of these more frequent small earthquakes can tell us more about the larger earthquakes that occur less often. We’re expecting to see 10 times more earthquakes than are currently reported.”

Ocean-bottom seismometers have previously been used to study earthquakes at the Hikurangi Margin further up the east coast and in short-duration subsurface imaging around New Zealand, but none have ever been deployed for time periods greater than a few weeks in the ‘locked’ area in the lower North Island.

“Our building standards are what keep people safe. We hope results will feed into the National Seismic Hazard Model and help to increase our overall resilience and preparedness as a nation.”

EQC research manager Dr Natalie Balfour says: “EQC has previously funded research on the Hikurangi Subduction Zone, but Professor Savage’s work will hopefully add another valuable piece to that puzzle.”

Did you know?

  • Seismometers used in earthquake studies are highly sensitive to ground movements, so that movements as small as 1/10,000,000 centimetres (distances almost as small as atomic spacing) can be detected at very quiet sites.
  • Modern research seismometers are electronic and detect and record motions in all directions.
  • Seismometers also commonly record ground motions caused by a wide variety of natural and man-made sources, such as trees blowing in the wind, cars and trucks on the highway, and ocean waves crashing on the beach.
  • The largest earthquakes create ground motions over the entire Earth that can be several centimetres high.