Vertical profiles of temperature anomalies inferred from anomalies in shear-wave speed (VS) and Q at two different location (left: Northern Pacific; right: Western Pacific). © Deschamps et al. (2019).
A dominant feature of the Earth’s mantle (the rocky layer that extends from depths of 50 to 2890 km) is the presence of large regions, called LLSVPs, where shear-wave velocity is reduced by a few percent compared to its horizontal average. The exact nature of these regions is still debated, but several hints point to a combination of thermal and compositional changes. Because seismic velocities alone cannot separate thermal and compositional contributions, other data are needed to infer the nature of LLSVPs. Seismic attenuation strongly depends on temperature and may thus be used as a proxy to infer temperature changes. Together with collaborators, Dr. Frédéric Deschamps (IES research fellow) performed studies to recover variations in shear-wave velocity (VS) and quality factor (Q) in the lowermost mantle, using inversions of seismic waveform data (Deschamps et al., EPSL, 2019). Doing so, they obtained radial models of VS and Q in the depth range 2000-2890 km at two different locations, beneath the Northern and Western Pacific. At the Western Pacific (WP) location, sampling the western tip of the Pacific LLSVP, both VS and Q are substantially lower than their mantle average. From the core-mantle boundary (2890 km) up to a depth of 2600 km, observed anomalies in VS and Q cannot be explained by thermal anomalies alone, even if changes in the stability field of the post-perovskite phase are accounted for. Compositional changes are also needed, and an excess in iron oxide by 3.5 to 4.5 % provides a good explanation to the observations.
Deschamps, Frederic, Kensuke Konishi, Nobuaki Fuji, Laura Cobden, (2019), Radial thermo-chemical structure beneath Western and Northern Pacific from seismic waveform inversion, EARTH AND PLANETARY SCIENCE LETTERS 520, 153-163,