S‐P wave travel time residuals and lateral inhomogeneity in the mantle beneath Tibet and the Himalaya

Abstract

The average S wave travel time residuals from 11 earthquakes each in Tibet and the Himalaya imply that there are marked lateral variations in structure beneath Tibet and the Himalaya. Average S‐P wave residuals, measured with respect to the Jeffreys‐Bullen (J‐B) tables, for each of 11 earthquakes in the Himalaya are less than +1 s. Average J‐B S‐P residuals from 10 of 11 earthquakes in Tibet, however, are greater than +1 s, even when corrected for the large crustal thickness of Tibet. The largest values, 2.5, 3.2, 3.8, 4.5, and 4.9 s, are for five events in central and northern Tibet, and they imply that the average velocities in the crust and upper mantle of these portions of Tibet are 4–10% lower than those beneath the Himalaya. Therefore it is unlikely that a shield structure underlies these parts of Tibet, unless these S‐P residuals are due to variations in structure deeper than 250 km. S‐P residuals from two other events each in western and eastern Tibet are 1–2 s late and hence suggest that velocities beneath these areas are also somewhat lower than beneath shields. The mean S wave velocity seems not only to be different beneath Tibet and the Himalaya but also seems to increase from a minimum beneath northern Tibet to values close to those of the average earth toward the east, south, and west.Appendix A is available with entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, DC 20009. Document B84‐010; $2.50. Payment must accompany order.