Variation in apparent attenuation of the Earth's normal modes due to lateral heterogeneity

Study of 3242 full first order synthetic seismograms of isolated normal mode multiplets shows that constructive and destructive interference of the singlets comprising a multiplet can account for the noise‐corrected variance of free oscillation attenuation measurements in the frequency band 3.0‐4.0 mHz. This interference gives rise to a distribution of synthetic Q measurements which is sensitive to the short wavelength velocity features of a model. While individual attenuation measurements depend also on the moment tensor, the overall variance of Q fluctuations is insensitive to the characteristics of the source employed and depends little on the length of the time series processed. The Woodhouse‐Dziewonski model M84A of upper mantle velocity structure causes a scatter in the synthetics large enough to interfere significantly with any signal from lateral Q structure. Correlation of individual synthetics with data is poor, however, which suggests that either there are remaining uncertainties in the velocity model, that mode‐mode coupling occurs in the data in a way that is poorly understood, or that lateral attenuative structure contributes to the observed scatter. Resolution of any such Q structure will be difficult given the present data set.