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Distribution of Fine-Scale Mantle Heterogeneity from Observations of P_diff Coda

U.S. Geological Survey
MS 966 Box 25046 DFC
Denver, Colorado 80225
(P.S.E.)

Scripps Institution of Oceanography, SIO-0225
La Jolla, California 92093-0225
(P.M.S.)

We present stacked record sections of Global Seismic Network data that image the average amplitude and polarization of the high-frequency P_diff coda and investigate their implications on the depth extent of fine-scale (10 km) mantle heterogeneity. The extended 1-Hz coda lasts for at least 150 sec and is observed to a distance of 130°. The coda's polarization angle is about the same as the main P_diff arrival (4.4 sec/deg) and is nearly constant with time. Previous studies show that multiple scattering from heterogeneity restricted to the lowermost mantle generates an extended P_diff coda with a constant polarization. Here we present an alternative model that satisfies our P_diff observations. The model consists of single scattering from weak (1%) fine-scale (2 km) structures distributed throughout the mantle. Although this model is nonunique, it demonstrates that P_diff coda observations do not preclude the existence of scattering contributions from the entire mantle.