Lateral variations of S velocity in the upper mantle from higher Rayleigh modes

Summary. The generalized inverse theory has been applied to interpret several sets of higher mode data, previously obtained for the United States and the Pacific Ocean. The depth-resolving power of these data allows us to find the distribution of S velocity down to about 600 km. The main lateral variations of S velocity are found in the uppermost 250 km, the south-western United States showing the lowest velocities and the central-north-eastern United States the highest velocities. Between 250 and 500 km an opposite situation seems to occur, western velocities being the greatest ones, but these lateral variations are 3 to 5 times less than above and they cannot be surely established under the variance estimated for the data. Finally no lateral variations are resolved between 500 and 700 km. Some remarks may be made about the corresponding absolute models: (1) the agreement is good with published models, built with the fundamental mode alone; (2) the slight lowvelocity zone which is not required when inverting the fundamental mode alone in the central and north-eastern United States, is required when highermode data are added; (3) a rather strong increase of the S -velocity gradient is found near 360 km depth, both for the average data across the United States and the Pacific Ocean.