Teleseismic Delay Times In A 3‐D Earth and A New Look At the S Discrepancy

Abstract

When the earliest measurements of the Earth's normal modes became available, it was soon noticed that Vs models satisfying this new data set were generally slower than models based on S traveltimes. This S discrepancy has since then been attributed to the influence of dispersion that accompanies attenuation, but it ignores a competing process that may have the same effect: in a heterogeneous earth, seismic rays will bend to take advantage of local high‐velocity anomalies. This effect is difficult to quantify because of the computational problems associated with the determination of minimum traveltime paths in heterogeneous earth models in three dimensions. We have developed a new method to find absolute minimum traveltimes even in very complicated earth models and applied this to a large number of rays generated in different random earth models. the S discrepancy is a function of correlation length and amplitude of Vs heterogeneity. A standard deviation s̀ of 0.15 km s−1, with a correlation length of 200 km in the whole mantle, would give a discrepancy even larger than observed at Δ>60° and must be discarded. A s̀ of 0.15 km s−1 in the upper mantle and 0.05 km s−1 elsewhere explains a discrepancy of 1‐2s, depending on the correlation length. Since the total discrepancy is estimated to be about 4s, this still allows for effects of attenuation. We conclude that the S‐wave discrepancy can be used to set limits to the heterogeneity in the lower mantle at short correlation lengths. With our present‐day knowledge of the magnitude of the discrepancy a heterogeneity larger than 1 per cent with correlation length 200 km is unlikely. Copyright © 1993, Wiley Blackwell. All rights reserved