Mixed Rayleigh-Stoneley modes: Analysis of seismic waveguide coupling and sensitivity to lower-mantle structures

Abstract

A better understanding of Earth's core-mantle boundary (CMB) region is required to address major questions about our planet's internal dynamics, magnetic field, and thermal evolution. Valuable constraints have come from observations of (CMB-) Stoneley modes, a class of seismic free oscillation whose displacement decreases away from the solid-fluid boundary. The high-frequency modes that are most sensitive to the CMB region are too localized there to be observed at Earth's surface. Here we clarify why some higher-frequency Stoneley modes can be detected: via 'mixing' with surface-localized Rayleigh-type modes of similar frequency. We examine the concept of mixed Rayleigh-Stoneley modes analytically and with a finite-element method. Our calculations show that mixed modes are a sensitive probe of radial and lateral variations in material properties near the CMB. More generally, 'seismic waveguide coupling' could help to characterize systems ranging from cell membranes to Pluto's lithosphere.