The Virtual Receiver Approach: Probing the Internal Structure of Large Low-Seismic-Velocity Provinces

Understanding the internal structure of the Large Low-Seismic-Velocity Provinces (LLSVPs) is crucial for deciphering the convection system of the lower mantle. However, the exact origin, nature, and composition of the LLSVPs remain a subject of ongoing debate. Existing techniques, such as seismic tomography, have been highly successful in imaging large-scale structures of the Earth's mantle, including long-wave shear wave velocity anomalies, the geometries of subducting slabs, and the global extent of the LLSVPs. However, these techniques are less sensitive to small-scale variations and sharp lateral transitions, especially in the lower mantle, which makes it challenging to explore fine-scale heterogeneities and internal complexities. The newly developed Virtual Receiver Approach (VRA) provides a complementary framework to address these limitations by sampling velocity fields at depth using teleseismic data, thereby enabling the detection of subtle and spatially confined anomalies. VRA is a technique that leverages travel-time differences between closely spaced seismic stations to directly estimate local absolute velocities, independent of assumed Earth models. This independence from pre-assumed velocity structures provides a unique opportunity to investigate deep Earth features with minimal bias.

 

This study develops the mathematical framework for VRA and provides theoretical validation. Synthetic tests confirm the robustness of this approach. To test its application to real-world data, VRA was applied to teleseismic travel times of transversely polarised SH waves, while focusing on events with turning points located in the circum-Pacific region. A scatter of local velocity measurements beneath the Pacific Ocean, sampling the lower mantle, was obtained. Overlaying these on existing tomographic maps allowed for identification of significant features, such as the LLSVP boundaries, the Galapagos plume, and low- and high-velocity anomalies within the LLSVP. Though the majority of results agreed with the well-known slower nature of S-wave velocities inside the LLSVPs, distinct high-velocity anomalies were also observed. Results from petrological modelling suggest a correlation between these high-velocity anomalies and low FeO content, which potentially indicates the inclusion of post-perovskite material into the LLSVP through mantle convection. On the other hand, these could also be remnants of old subducted oceanic crust. High-velocity anomalies thus observed indicate lateral compositional variation within the LLSVP, making them more complex and heterogeneous than previously thought. 

 

The study demonstrates the potential of VRA as a high-resolution imaging tool. Ongoing studies aim to extend the current isotropic medium used in VRA to incorporate anisotropic properties, thereby enhancing its reliability and accuracy. The method's sensitivity to various properties is also under study. Such developments of newer and higher-resolution methods are crucial for furthering our understanding of deep Earth processes.