Long-wavelengthS-wave velocity structure throughout the mantle

Abstract

Long-wavelength S-wave velocity anomalies are obtained for the entire mantle by inverting 6OOO long-period (40-100s) SH body waves and lo00 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 I = 2 anomaly, Moreover the 1 = 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. A t 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.

Generation of long wavelength heterogeneity in the mantle by the dynamic interaction between plates and convection

“…The resolution of lateral seismic velocity variations through the deep mantle has revealed significant power at the gravest spherical harmonics, including degrees one through six [Dziewonski, et al, 1977 Tanimoto, 1990]. The lower mantle body wave tomographic inversions [Dziewonski, 1984;Hager and Clayton, 1989;Inoue, et al, 1990] give comparable results at degrees two and three, but are inconsistent at shorter wavelengths [Inoue, et al, 1990].…”

Predominance of large-scale heterogeneity and the shift of velocity anomalies between the upper and lower mantle.

“…The results for the lower mantle are also shared by other models; Fig. 3 shows RMS variation of two models, CC by Clayton and Corner (1983) and DZ by Dziewonski (1984, model L02.56) both obtained from ISC P-wave data, in addition to TT (Tanimoto, 1990;MDLSH) and GDC (Gudmundsson et al, 1990;MDSTP). More recent results by Morelli and Dziewonski (1987) are very similar to Dziewonski (1984).…”

“…Good correlation exists beyond 670 km, almost down to 1,000 km. Because of the finite resolution of our seismic data (Tanimoto, 1990), it is hard to claim that this is a real feature. However, note that even Fig.…”

“…2 are from Gudmundsson et al (1990, MDSTP1 hereafter MDSTP) and Tanimoto (1990, MDLSH), which were derived from different data sets using entirely different approaches. Gudmundsson et al (1990) analysed direct P-wave arrival time from ISC data using a new stochastic approach, while Tanimoto (1990) inverted SH (transverse) component waveform data from digital network data. Since MDSTP is for P-wave velocity heterogeneity and MDLSH is for S-wave velocity heterogeneity, it is not surprising to see a difference between the two of almost a factor of two.…”

“…2. RMS slowness variations for the two mantle models, MDSTP1 by Gudmundsson et al (1990) and MDLSH by Tanimoto (1990). MDSTP1 was derived from ISC P-wave data, while MDLSH was derived from SH waveforms of digital seismograph network data.…”