Long-wavelength<i>S</i>-wave velocity structure throughout the mantle

Abstract

Long-wavelength S-wave velocity anomalies are obtained for the entire mantle by inverting 6000 long-period (40–100s) SH body waves and 1000 long-period (100–500 s) Love waves. In the shallow mantle, the patterns of fast and slow velocity anomalies correlate well with age and tectonics. In the depth range of 400–1000 km, we find two antipodal fast velocity anomalies, one in the western Pacific and the other in South America. They form a fairly strong l = 2 anomaly. Moreover the l = 2 anomaly is the largest spherical harmonic component throughout the entire mantle except for in the layer of 1000–1300 km depth. A fast-velocity anomaly near the Caribbean, previously reported by several other investigators, is found between the depths of 1000 and 1300km. With diminishing amplitudes, it can be traced to Canada and the Aleutians and may be related to old subducted slabs. In the same depth interval, similar size anomalies can be found in the middle of the Pacific and in the Indian Ocean. These could be checked by more detailed studies to judge the reliability of this study. Heterogeneity power has a peak at the surface and a secondary peak at the bottom of the mantle (D″), consistent with the existence of chemical and/or thermal boundary layers. At the base of the mantle we also find a fast-velocity anomaly which surrounds the Pacific Ocean. This feature has been found by other researchers previously, but its interpretation is difficult due to the trade-off between velocity anomaly and (core-mantle) boundary deformation.