Revolutionizing seismology on Macquarie Island on July 6, 2021

Macquarie Island, and the steep mountain range that rises from it, has become a seismic observatory for the Southern Ocean to better understand earthquakes and tsunamis caused by collision of continental plates.

The set of 32 seismometers, spread across the tectonically active region known as the Macquarie Ridge Complex (MRC), is part of a project led by the Australian National University (ANU)* — with help from the Australian Antarctic Program missions — to study the mechanisms that cause undersea earthquakes. in the region.

earthquake hot spot

The head of geophysics at ANU, Professor Hrvoy Tkalić, said MRC had experienced two major undersea earthquakes in the past 32 years, with magnitudes 8.2 in 1989 and 8.1 in 2004, both of which were caused mostly by ‘slip’ events – where Horizontal rock masses slide past each other parallel to the fracture or “blow” between them.

Violent earthquakes shook the Macquarie Island research station, and tremors were felt as far away as Tasmania and the South Island of New Zealand.

“This slip mechanism has baffled the global seismological community, as slips of this size typically occur only in dense continental crust, not thin oceanic crust,” said Professor Tkalić.

“As the MRC sits on the boundary between the Australian Pacific and Indo plates, it is likely that we are seeing the formation of a new ‘subduction zone’, where plates collide and slide one under the other.

This project aims to collect data that will help us better understand the mechanism of earthquakes in the region, and create maps of the subsurface structure.

“The maps will help us predict how seismic energy will spread from hypothetical future earthquakes, and simulate different wave propagation scenarios, earthquakes and tsunamis.”

deep sea sensors

In late 2020, Professor Tkalči and colleagues from ANU and the University of Tasmania, aboard the CSIRO (RV) Investigator research vessel, published 27 seafloor ocean-floor seismometers around Macquarie Island, covering approximately 25,000 square kilometers (described in this EOS article).

Seismometers measure ground motion caused by earthquakes locally, regionally, and over 1,000 kilometers (“teleseismic” distances), as well as seismic and ocean noise.

The team deployed the tools in “smooth areas” among the submarine’s steep, rocky and dangerous terrain, at depths between 520 and 5,517 meters, and in certain patterns.

“The southern part of the seismometer array has three spiral arms, while the northern part has an X shape,” Professor Tkal said.

“This configuration is designed to achieve the best possible amplification of signals from distant earthquakes, in the same way an antenna or telescope would.”

Walk the shaky line

In June this year, the final components of the array – five seismometers – were installed on Macquarie Island by an Australian Antarctic Program expedition.

Professor Tkalic said: “Seismometers are evenly distributed along the main axis of the island to study the structure of the crust and upper mantle below the island, along with ocean floor instruments.

Macquarie Island station doctor, Rob Dixon, and a small team of volunteers traversed the island’s main walking path to install equipment.

“We carried about 25 kilograms of equipment in addition to all our personal field equipment and safety equipment,” said Dr. Dixon.

“Most of the people at the station helped in some way, professions included – with their shovel tickets, our communications technician, the Tasmanian park rangers who drove the southern installations, because they fall that way regularly.

“The station commander and I also consider carrying a heavy backpack for her a ‘recreational activity’.”

Once at the installation site, the group dug a 50cm-deep hole, connected the seismometer, battery, and solar panel to a regulator, protected the seismometer with plastic bags and covered it in dirt, and attached the solar panels against the wind, then synced the device through Bluetooth to a tablet.

“We also put electrical black tape on any red cords, as skuas can mistake red wires for tasty gut,” said Dr. Dixon.

Solve tectonic puzzles

Ocean seismometers will be retrieved in November 2021 and terrestrial instruments at least a year after publication.

While the thought of living on a shaky island may deter some, the team at the Macquarie Island research station is well prepared for any damage from the resulting earthquake or tsunami, with shelter 100 meters above the station on Wireless Hill.

The shelter contains communication equipment, clothing and food to support the group until help arrives – by ship or airdrop.

Professor Tkalić said the insights gained into the physical mechanism that generates marine earthquakes in the region will help scientists better understand the risks.

“This knowledge should allow us to provide more accurate assessments of the potential for earthquakes and tsunamis in the region, which we hope will benefit at-risk communities along the Pacific and Indian Ocean coasts,” he said.

“This work could also be critical to helping us answer one of the biggest scientific puzzles in plate tectonics – how subduction begins.”

*The research is led by the Australian National University in partnership with Geoscience Australia, the University of Tasmania, the California Institute of Technology, and the University of Cambridge. It is supported by a sea time grant on an RV Investigator from the CSIRO Marine National Facility.

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