Observations of free oscillation amplitude anomalies

In a frequency domain analysis of long‐period seismic data, observations of free oscillation excitation amplitudes are required in addition to frequency shifts to constrain odd‐degree aspherical earth structure. To determine the feasibility of incorporating such data into large‐scale inversions, we have undertaken a preliminary analysis of the observed amplitudes of fundamental spheroidal mode multiplets oSl with 26 ≤ l ≤ 43 using seismograms recorded by the IDA network. For many records, the amplitudes may be significantly better fit by allowing for a systematic shift of the theoretical spherical earth nodal lines either toward or away from the epicenter. In the great‐circular geometrical optics approximation, this spatial phase shift of the amplitude pattern can be asymptotically represented in terms of the minor arc integral of δωlocal — along the source‐receiver path. Results from common source‐receiver paths indicate consistent sampling of the structure underlying these paths. For a number of paths, the results differ significantly from the theoretical predictions of model M84A of Woodhouse and Dziewonski.