Constraining odd‐degree earth structure with coupled free‐oscillations

Several pairs of normal mode multiplets sensitive to long‐wavelength mantle structure are potential coupling partners. Geometric selection rules render such coupling the only means by which normal modes sample odd‐degree mantle structure. That overtone coupling can be strong and can significantly impact normal mode spectra is illustrated using synthetic seismograms for a realistic earth model. These observations motivate the generalization of normal mode spectral fitting to permit simultaneous estimation of even‐ and odd‐degree structural constraints. When generalized spectral fitting is applied to a data set of approximately 150 high signal‐to‐noise vertical records, dominated by data from the Northern Bolivian event (6/9/1994), it is found to reduce misfits appreciably for coupled multiplets 1S5–2S4 and 2S5–1S6, to yield odd‐degree constraints generally consistent with recent mantle models, and to improve agreement between estimated and predicted even‐degree constraints.