Why would you drop 27 seismometers on the ocean floor?

Near Macquarie Island in the Southern Ocean, a research vessel is currently retrieving 27 seismometers from the sea floor to find out what drives earthquakes and tsunamis.

The research is part of a long-term project to learn about the undersea earthquakes and landslides that occur around the Macquarie Ridge – an undersea subduction zone halfway between New Zealand and Antarctica – and how these tremors might affect coastal populations in Australia and New Zealand.

“In order to do that, we first need to understand the underground,” says Professor Hrvoje Tkalčić, of the Research School of Earth Sciences at the Australian National University.

“The continuing waveforms that these instruments record will help us do that.”

In October 2020, Tkalčić and colleagues journeyed to the remote island on the CSIRO research vessel Investigator. They dropped 27 seismometers on the ocean floor, some to depths below 5000 metres, and deployed a further five on the island.

The RV Tangaroa, a New Zealand research vessel, has been contracted by the CSIRO for the Southern Ocean voyage. Credit: Rebekah Parsons-King / NIWA.

Now, after 12 months of continuous data recording, the New Zealand research vessel Tangaroa set out on 10 November to collect the instruments.

Border restrictions prevented Tkalčić from going on this trip, but he’s keenly watching and advising on the retrieval from Canberra.

“[We] determined their location already when we deployed them last year. So we know exactly where they are.”

The seismometers were connected to concrete ballasts when they were dropped, but they’re fitted with a release mechanism. When instructed – usually by an acoustic signal, but with other messages if that fails – the seismometer will disconnect from the concrete and float to the ocean surface.

“Then, when they are spotted by the crew, at the deck, they get picked up by a long pole with a hook,” says Tkalčić.

“If that doesn’t work, then we have [something like] a fishing net to basically bring these instruments on the deck.”

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The mission has been fiendishly complicated to design. The Macquarie Ridge has extremely steep slopes, for instance – and above the water, the weather and waves are wild and unpredictable.

“We often like to compare it with deploying instruments on other planets,” says Tkalčić.

“It’s a really challenging environment, and we don’t know how many of these instruments’ release mechanisms will work.

“They all landed and they communicated with us when we deployed them. But how long will they survive, and will we be able to recover them? That’s another story.”

Scientists Ian Smith, Ollie Twigge and Sam Harrison retrieving one of seismometers from the ocean floor. Credit: Joshu Mountjoy / NIWA.

The ship is due to return on 2 December, at which point the researchers will be able to collect data from the instruments and learn how successful their mission was.

Tkalčić is confident that, despite the uncertainties, they’ve deployed enough instruments on enough channels to draw some valuable conclusions. The researchers will jump into analysis in 2022.

“I’m pretty sure they will have useful data.”