TOMOGRAPHY OF THE TRANSITION ZONE FROM THE INVERSION OF HIGHER-MODE SURFACE-WAVES

Higher-mode surface waves have a uniform depth resolution to about 1500 km depth in the period range 60-250 s. Therefore, these modes can be used for retrieving the S'-wave velocity structure in the mantle and, more specifically, in the transition zone. The procedure used for computing a tomographic model of the mantle is divided into two steps. First, the iterative inverse technique developed by Stutzmann and Montagner (Geophys. J. Int., 113: 669-683, 1993) is used for determining higher-mode phase velocities and a spherically symmetric velocity model associated with each path. A 'path' is defined to be the minor or major are (along a great circle) connecting a small epicentral area, in which several earthquakes are selected, and a station. The use of several earthquakes improves the depth resolution. Simultaneously for each datum, the difference between the spectrum of the observed seismogram and the sum of the synthetic spectra corresponding to the different modes is inverted to retrieve (1) the phase velocity dispersion curves of the fundamental mode and the three first higher modes and (2) the spherically symmetric S-wave velocity model corresponding to this path. In a second step, the higher-mode phase velocity dispersion curves and the S-wave velocity model corresponding to each path are regionalized without a priori constraint to obtain their lateral variations. This method has been applied to long-period GEOSCOPE data. We present preliminary results obtained with about 300 seismograms corresponding to 86 paths for the R1 or R2 trains. The three-dimensional S velocity model has been then expanded in spherical harmonics, to compare it with other models and to determine characteristics of the transition zone.