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.