The 3-D shear wave structure in the mantle by overtone waveform inversion-II. Inversion of X-waves, R-waves and G-waves

Abstract

SUMMARY The 3-D upper-mantle structure from the surface of the Earth to a depth of about 1000 km is obtained by inverting waveforms of fundamental and higher-mode surface waves. Minor- and major-arc waves of R-waves, G-waves and X-waves (higher-mode wave packets) are used as basic data. Two features of the solution seem fairly robust; one is the shallow structure in the upper few hundred kilometres which is strongly correlated with the age of the crust. The second feature is the deep fast-velocity anomaly near the western Pacific (Indonesia, Philippines and Mariana) region and its antipodal region of South America, both of which are subduction zones, which forms a strong l= 2 pattern. Simple slab models with 100 km thicknesses of 10 per cent faster-velocity material do not explain such large-scale fast-velocity anomalies. This may indicate large-scale deformation of the 670 km discontinuity, piling up of slabs in the upper mantle or thickening lithosphere in the lower mantle or large horizontal motions of subducted material. These results are stable whether an earth model 1066A or PREM is used as a starting model.