Mapping the Core-Mantle Boundary Using Sdiff Postcursors

• Main Author: Martin, Carl
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: University of Cambridge 2023
• Subjects: Core-Mantle Boundary; Deep Earth; Earth Sciences; Lower Mantle; Seismology; ULVZs; Iceland
• Online Access: https://www.repository.cam.ac.uk/handle/1810/369951; https://doi.org/10.17863/CAM.109540

The core-mantle boundary – the interface between the rocky mantle and the fluid iron outer core – is host to a diverse collection of phenomena across all length scales. At the largest length scales, continent-sized structures characterised by low velocity anomalies lie opposite to each other on the core-mantle boundary. These structures, known as large low shear velocity provinces (LLSVPs), are predominantly situated beneath the Pacific Ocean and the African continent. Smaller, more extreme phenomena (10s km thick, up to 100s km wide, dVs -10–50%, dVp -5–25%) have been identified on top of the core-mantle boundary, called ultra-low velocity zones (ULVZs). The largest of these anomalies have been associated with whole-mantle plumes at the base of major hotspots (Hawaii, Iceland, Samoa, and Galapagos), and have been suggested to act as a plume root and geochemical reservoir. In this thesis, we use the S core-diffracted phase (Sdiff) to detect ULVZs. Sdiff wavefronts propagating across the core-mantle boundary refract through low velocity anomalies and create additional wavefronts, called ‘postcursors’, whose move-out pattern is determined by the size, velocity, and shape of the ULVZ. These structures can then be detected by searching for the move-out of postcursory energy across a large seismic array sequenced by azimuth and inferences made about the ULVZ. We perform an extensive global search for Sdiff postcursors from earthquakes between 1990–2022 with magnitudes ≥5.7 at any depth in the IRIS earthquake catalogue. This results in the identification of 100 events sampling the Hawaiian ULVZ, and 100s more sampling the Iceland, Galapagos, and several newly discovered ULVZs. By pushing the dataset to higher frequencies than previously used, we find evidence for a thinner ULVZ beneath the mid-Pacific (less than 10 km) – which we refer to as the ‘ultra-thin ULVZ’. Modelling of these structures using Sdiff postcursors has, so far, required computing 3D full waveform synthetics to the periods corresponding to the length ...