Full-waveform inversion reveals diverse origins of lower mantle positive wave speed anomalies
Scientific Reports volume 14, Article number: 26708 (2024 ) Cite this article
Determining Earth’s structure is paramount to unravel its interior dynamics. Seismic tomography reveals positive wave speed anomalies throughout the mantle that spatially correlate with the expected locations of subducted slabs. This correlation has been widely applied in plate reconstructions and geodynamic modelling. However, global travel-time tomography typically incorporates only a limited number of easily identifiable body wave phases and is therefore strongly dependent on the source-receiver geometry. Here, we show how global full-waveform inversion is less sensitive to source-receiver geometry and reveals numerous previously undetected positive wave speed anomalies in the lower mantle. Many of these previously undetected anomalies are situated below major oceans and continental interiors, with no geologic record of subduction, such as beneath the western Pacific Ocean. Moreover, we find no statistically significant correlation positive anomalies as imaged using full-waveform inversion and past subduction. These findings suggest more diverse origins for these anomalies in Earth’s lower mantle, unlocking full-waveform inversion as an indispensable tool for mantle exploration.
Solid-state convection of the rocky, 2,890-km deep mantle has shaped the evolution of Earth’s interior and surface over billions of years. Uncovering Earth’s internal structure and the distribution of thermal and compositional heterogeneity, however, remains a scientific challenge that requires cross-disciplinary efforts. Seismic tomography represents the primary method to image the Earth’s interior, leveraging anomalies in wave speed stemming from heterogeneities in its thermal and chemical structure. Classical travel-time tomography reveals numerous regions of large, positive seismic wave speed anomalies throughout Earth’s mantle1,2,3 that are robust features across models4 (Suppl. text S1). Because seismic wave speed is a non-unique expression of a combination of material parameters5, however, interpreting the nature of any imaged anomaly is inherently challenging. Commonly, positive wave speed anomalies in the mantle are attributed to the presence of a cold6 and/or chemical anomaly (e.g. Fe-, Mg- or Si- enrichment)7 (Suppl. text S2). There is abundant geochemical and geophysical evidence suggesting that Earth’s mantle hosts chemical heterogeneity at a variety of scales8,9,10,11,12,13,14, which is corroborated by geodynamic simulations15,16,17,18,19. However, the positioning of positive seismic wave speed anomalies in the lower mantle directly below - or proximal to - locations of modern and ancient subduction zones have prompted their interpretation as (remnants of) cold subducted plates, or ”slabs”6,20,21 (Suppl. text S2). The advent of quantitative global plate reconstructions22 revealed a statistically significant correlation (\(p \le 0.01\) ) between these positive anomalies and past subduction23. A corollary to this interpretation is that the seismically imaged mantle structure reflects dominantly thermal as opposed to thermochemical heterogeneity24,25,26.
