The Central Macquarie Ridge Complex in 3D from Adjoint Waveform Tomography Using Ambient Seismic Noise

The Macquarie Ridge Complex (MRC), situated at the the boundary between the Australian, Macquarie, and Pacific plates south of New Zealand, is presently understood to be a primarily transform boundary that originated as a mid-ocean ridge. Although the northern (Puysegur) and southern (Hjort) sections of the MRC are considered to be initiating subduction, the tectonic activity along its central portions (Macquarie and McDougall segments) remains ambiguous.

Macquarie Island is situated on the central segments of the MRC. The region is characterized by exceptionally rugged topography, a feature that can indicate the incipient subduction. Furthermore, the MRC has produced some of the most powerful intra-oceanic strike-slip earthquakes on record. Such significant seismic events in this area pose a potential tsunami hazard. Consequently, despite its isolation, detailed geophysical studies are warranted, which led to our deployment of an integrated network of ocean bottom seismometers (OBSs) and seismometers on the island (Tkalčić et al., 2020; 2021). The primary aim of our research is therefore to employ seismological methods to advance the comprehension of the tectonic development of the Australian-Macquarie-Pacific plate boundary.

From 2020 to 2021, we installed a network comprising five land-based stations and 27 ocean bottom seismometers on and around Macquarie Island along the MRC in the Southern Ocean. Utilizing data from the successfully retrieved OBSs and island stations, including the permanent station MCQ, we applied an adjoint waveform tomography technique to surface waves (5-20 s period) extracted from ambient seismic noise. This process, conducted over five iterations, yielded a 3-D model of S-wave velocity for the crust and uppermost mantle. Our starting 3-D model incorporated accurate bathymetry, a water layer, and an optimized 1-D velocity structure. For the seismic wavefield simulations required in the inversion, we employed the spectral element method with Specfem3D_Cartesian (Komatitsch and Tromp, 1999). The resulting S-wave velocity model shows a marked velocity increase at crustal and uppermost mantle depths, between 7 and 12 kilometers. The presence of relatively high S-wave velocities (>3.8 km/s) in the shallow lithosphere aligns with upper mantle rocks being located at unusually shallow depths along the ridge. The extensive distribution of this high-velocity material suggests that the uppermost lithosphere has not undergone significant deformation during the process of obduction.

Reference
Tkalčić, H., Eakin, C., Rawlinson, N., Coffin, M. F., & Stock, J. (2020) Macquarie Ridge [Data set]. AusPass: The Australian Passive Seismic Server. https://doi.org/10.7914/SN/3F_2020

Tkalčić, H., Eakin, C., Coffin, M. F., Rawlinson, N. & Stock, J. (2021) Deploying a submarine seismic observatory in the Furious Fifties, Eos, 102, https://doi.org/10.1029/2021EO159537 

Komatitsch, D., & Tromp, J. (1999). Introduction to the spectral element method for three-dimensional seismic wave propagation. Geophysical Journal International, 139(3), 806-822. https://doi.org/10.1046/j.1365-246x.1999.00967.x