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We investigated the seismological structure beneath the equatorial Melanesian region, where is tectonically unique because an immense oceanic plateau, a volcanic chain and subduction zones meet. We conducted a multi-frequency P-wave tomography using data collected from an approximately 2-year-long seismic experiment around the Ontong Java Plateau (OJP). High-velocity anomalies were revealed beneath the center of the OJP at a depth of ~ 150 km, the middle-eastern edge of the OJP at depths of 200–300 km, and in the mantle transition zone beneath and around the OJP; low-velocity anomalies were observed along the Caroline volcanic island chain above 450 km depth. These anomalies are considered to be associated with the thick lithosphere of the OJP, remnant dipping Pacific slab, stagnant Pacific slab, and a sheet-like upwelling. The broad stagnant slab was formed due to rapid trench retreat from 48 to 25 Ma until when the OJP with thick lithosphere collided with a subduction boundary of the Pacific and Australian plates. This collision triggered slab breakoff beneath the arc where the dipping slab remained. The stagnant Pacific slab in the mantle transition zone restricted the plume upwelling from the lower mantle causing sheet-like deformed upwelling in the upper mantle.

The Ontong Java Plateau in the western Pacific Ocean is the world’s largest oceanic plateau. It was formed 122 million years ago by a massive volcanic event that significantly affected Earth’s environment. The cause of the magmatic event remains controversial because the upper mantle structure beneath the plateau is poorly known. Here we use passive seismic data obtained through seafloor observations, alongside existing seismic data, to determine the three-dimensional radially anisotropic shear wave velocity to depths of up to 300 km. We find that the lithosphere–asthenosphere boundary is approximately 40 km deeper beneath the centre of the Ontong Java Plateau than beneath the surrounding seafloor. Based on our results and petrological and rheological constraints, we propose that the lithosphere–asthenosphere boundary has deepened as a result of underplating of dehydrated residual material beneath the pre-existing lithosphere during formation of the Ontong Java Plateau by a thermochemical mantle plume. The lithosphere beneath the Ontong Java Plateau is thickened by up to 40 km relative to the surrounding ocean floor which suggests it formed through the ascent of a mantle plume, according to analyses of seafloor seismic observations.

Determining damping of our plates For plate tectonics to operate, a weaker layer called the asthenosphere must underlie the rigid lithospheric plates. Quantifying the difference in strength comes down to how much each layer attenuates energy. Takeuchi et al. exploited an ocean-bottom seismic network and seismic energy from the 2011 Japanese Tohoku-oki earthquake to quantify the attenuation in each layer (see the Perspective by Dalton). The attenuation of energy in the asthenosphere lined up with previous estimates, but the lithospheric attenuation was roughly one-fifth as strong as that predicted by some previous models. Science , this issue p. 1593 ; see also p. 1536

[1] We analyze seismic waveforms from deep-focus earthquakes occurring in the subducting slab beneath Japan, recorded by broadband ocean bottom seismometers (BBOBSs) installed on the northwestern Pacific Ocean seafloor. The data reveal waveforms with a low-frequency direct P onset, followed by large-amplitude, high-frequency, long-duration Po and So waves. From the analysis of the BBOBS records and a numerical finite-difference method simulation of seismic wave propagation, we elucidate the generation and propagation processes of such guided waves. We demonstrate that the low-frequency direct P and S waves propagate in the asthenosphere and that the following high-frequency, long-duration Po and So waves are developed by multiple forward scattering of P and S waves. The scattering occurs due to laterally elongated heterogeneities in both the subducting and horizontal parts of the oceanic lithosphere, with the apparent velocities (Vp = 8.1 km/s, Vs = 4.6 km/s) being close to the velocities of oceanic lithosphere.

We determined the three-dimensional shear wave velocity structure beneath the South Pacific superswell down to a depth of 200 km by analyzing fundamental Rayleigh wave records from permanent and temporary land-based and seafloor seismometers in the Pacific Ocean. Data from the Tomographic Investigation by seafloor ARray Experiment for the Society hotspot (TIARES) project yield excellent spatial resolution of velocity anomalies in the central part of the superswell, near the Society hotspot. Localized slow anomalies are found near hotspots in the upper mantle, but the vertical profiles of the anomalies vary from location to location: Slow anomalies near the Samoa, Macdonald, Pitcairn, and Society hotspots extend to at least 200 km depth, while a slow anomaly near the Marquesas hotspot extends only to ~150 km depth. Owing to the recently deployed seafloor array, horizontal resolutions of slow anomalies near the Society hotspot are substantially improved: The slow anomalies are about 300 km in lateral extent and have velocity anomalies as low as −6 %. The lithosphere thickness is estimated to be ~70 km in the vicinity of all hotspots, which may indicate thermal erosion by mantle plumes.

Seismic anisotropy can provide fundamental information on past and present-day deformation processes in the upper mantle. Using Rayleigh and Love waves recorded by land and seafloor broadband seismometers, we analyzed the isotropic and anisotropic shear-wave velocity structures in the northern Philippine Sea region. We found that the fast directions of azimuthal anisotropy are parallel to the directions of ancient seafloor spreading in the lithosphere of the Shikoku and West Philippine Basins and Pacific Ocean, whereas they are parallel to the direction of the present-day absolute plate motion (APM) in the asthenosphere of the Shikoku Basin, and oblique to the direction of the APM in the Pacific Ocean (by ∱/430°) and in the northern part of the West Philippine Basin (by ∱/455°). In the subduction zones around the Philippine Sea plate, the fast direction of azimuthal anisotropy is trench-parallel in the Ryukyu arc, and oriented NW•SE in the Izu•Ogasawara island arc. The Philippine Sea plate, which is a single plate, shows very large lateral variations in azimuthal and radial anisotropies compared with the Pacific plate.