Thuany Costa de Lima is a third year PhD student in the Seismology & Mathematical Geophysics group at RSES. Her PhD focuses on investigating the physical properties of the deep Earth structure in the light of seismological tools, from the inner core to the mantle.

Below 40°S there is no law, and below 50°S there is no God.”

I recently heard this old sailors’ saying for the first time after stepping on-board the Marine National Facility (MNF) Research Vessel (RV) Investigator. I was far away from the Australian mainland, and close to accomplishing a ground-breaking milestone for scientists: using seismology to investigate the 3D structure of the Macquarie Ridge Complex!

After multiple rescheduling, the time for our long-anticipated research voyage to the Southern Ocean had finally come! I was thrilled when we received the confirmation that we were set to sail in early October 2020. Given the current COVID pandemic, being able to gather a crew and making this fieldwork happen was an extraordinary feat. Before I narrate my fantastic experience of living on a research vessel and sailing on the Southern Ocean, I think I should mention when and how the idea for this research project was born.

This project was planned long ago in 2016 when my advisor, Hrvoje Tkalčić, and Caroline Eakin joined forces with Mike Coffin from the University of Tasmania, Nick Rawlinson from the University of Cambridge and Joann Stock from Caltech, united in their expertise and thirst for solving scientific puzzles. They wrote an ARC Discovery proposal to explore Earth structure under the Macquarie Ridge Complex and shed light on the largest underwater earthquakes not associated with active subduction, as well as on unique geology underneath Macquarie Island. They then wrote another proposal to the Marine National Facility for the ship time on RV Investigator. After several years, all their efforts led to what we now call: “A voyage to the furious fifties”, which happens to be my first marine research experience ever!

On 8th October 2020 at 2 pm UTC, we departed from Hobart heading south. We were a team of 11 researchers (including myself) from ANU and IMAS (Institute for Marine and Antarctic Studies, University of Tasmania). After sailing over waves of a very angry sea, we reached the destination only a few days later. Macquarie Island was there, right before our eyes.

Macquarie Island sunset. Photo credit: Andrew Latimore.

Macquarie Island, or just Macca, is the surface manifestation of the dynamic environment in place at the Macquarie Ridge Complex. This particular place on Earth is very special for a vast number of reasons which led it to be assigned as a UNESCO World Heritage Site. Located halfway between New Zealand and Antarctica, Macca is a wildlife paradise, being home to multiple species of penguin colonies, seabirds, and seals. It is also a site of outstanding geological features, being the only island in the world composed entirely of oceanic crust and rocks from the mantle. In a tectonic context, Macquarie Island stands right at the contact of the plate boundary between the Australasian plate and the Pacific plate. Because of the tectonic stress, the seafloor uplifted resulting in the narrow 5 km wide and 34 km long Macquarie Island.

In the last few years, major earthquakes (Mw > 8) have occurred in the Macquarie Ridge Complex, but no consensus has been established to explain such big seismic events in that area. Earthquakes of those magnitudes are expected in subduction zones around the world, such as the ones we observe in Indonesia or Chile, but not in a strike-slip plate boundary contact. This is the reason we came here, to understand why the plate boundary is generating such large earthquakes, with chances of a tsunami that could impact Australia, New Zealand, and islands nearby.

Myself standing beside some of the Australian Geophysical Observing System (AGOS) deployed during this voyage. Photo credit: Dr. Xiaolong Ma.

To provide the answers for such questions, we need to better understand the geological architecture underlying the Macquarie Ridge Complex, and for that, we rely on seismological tools. This is why we install seismometers on the Earth’s surface (in this case, the ocean floor). To detect ground motions that are caused by earthquakes or ambient noise, and use that information to constrain properties of the structure of the Earth beneath the surface.

There are multiple techniques the science team is going to use to image the 3D structure of the Macquarie Ridge Complex, and hopefully we will be able to answer in the future why those earthquakes are happening there, and what we can do to mitigate seismic hazards that could affect the countries nearby.

We are still a few days from heading back to Hobart, and I can truly say I am taking many good memories with me!

Demonstrating the immersion suit. By my side is Astrid Wilson, the master 2 of this voyage. Photo credit: Dr. Caroline Eakin

On the next blog, I will share a bit more about some of the challenges we are facing during this fieldwork, and the fun we are having during our downtime. Stay tuned!