Structure of the D″ layer inferred from the Earth's free oscillations

Abstract

We detected, by applying the Sompi method of spectral analysis to the long-period records of the International Deployment of Accelerometers (IDA), the spheroidal modes of the Earth's free oscillations 1 S 8, 1 S 9, 1 S 10 and 2 S 16, which are primarily sensitive to the structure adjacent to the core-mantle boundary (CMB). The observed central frequencies are systematically lower than those predicted from the standard Preliminary Reference Earth Model (PREM). These frequencies and those of several other lower-mantle modes were inverted into a spherical structure of the D″ layer. A global feature resolvable from the data is a decrease in P velocity gradient in the D″ layer. A possibility of anomalous density increase in the D″ layer is suggested but a trade-off between density and S velocity hampers a definitive conclusion. There is no indication of a strong decrease in Q in the D″ layer as might be expected from a simple thermal boundary layer model. We retrieved the splitting functions up to spherical harmonic degree four from the frequencies of apparently split peaks of 0 S 7, 1 S 8 and 1 S 10. The splitting functions of these modes consistently show negative anomalies in the Pacific and in Africa and positive anomalies in the circum-Pacific region, in good agreement with body wave tomographic models. Their intensities increase significantly with the increasing sensitivity to the structure near the CMB, indicating enhancement of lateral heterogeneity in the lowermost mantle, a result difficult to explain by a simple thermal boundary layer model, in which the isothermal CMB tends to minimize the thermally induced heterogeneity of the D″ layer.