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

Abstract The Macquarie Ridge Complex (MRC), located at the boundary between the Australian, Macquarie, and Pacific plates south of New Zealand, is currently recognized as a dominantly transform plate boundary that evolved from a mid-ocean spreading ridge. We deployed five land stations and 27 ocean-bottom seismometers (OBSs) on and around Macquarie Island from 2020 to 2021 along the MRC in the Southern Ocean. From the waveforms recorded on successfully recovered OBSs and island stations, including permanent station MCQ, we generated a 3D S-wave velocity model of the crust and uppermost mantle using an adjoint waveform tomography method after five iterations based on surface waves (5–20 s) extracted from ambient seismic noise. The initial 3D model is constructed using real bathymetry, a water layer, and an optimal 1D model. During the inversion, we use the spectral element method to perform forward and adjoint seismic wavefield simulations with Specfem3D_Cartesian. The shortest resolvable period is about 1.35 s. The new S-wave velocity model reveals a pronounced increase in velocity across expected crustal and uppermost mantle depths between 7 and 12 km. Relatively high S-wave velocities (>3.8 km/s) in the uppermost lithosphere are consistent with the presence of upper mantle rocks at relatively shallow depths distributed along the ridge. Widespread high-velocity material may indicate that the uppermost lithosphere is not substantially deformed during obduction.