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

Higher-mode surface waves have a uniform depth resolution to about 150 0 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 com puting a tomographic model of the mantle is divided into two steps. Fi rst, the iterative inverse technique developed by Stutzmann and Montag ner (Geophys. J. Int., 113: 669-683, 1993) is used for determining hig her-mode phase velocities and a spherically symmetric velocity model a ssociated with each path. A 'path' is defined to be the minor or major are (along a great circle) connecting a small epicentral area, in whi ch several earthquakes are selected, and a station. The use of several earthquakes improves the depth resolution. Simultaneously for each da tum, the difference between the spectrum of the observed seismogram an d the sum of the synthetic spectra corresponding to the different mode s is inverted to retrieve (1) the phase velocity dispersion curves of the fundamental mode and the three first higher modes and (2) the sphe rically symmetric S-wave velocity model corresponding to this path. In a second step, the higher-mode phase velocity dispersion curves and t he S-wave velocity model corresponding to each path are regionalized w ithout a priori constraint to obtain their lateral variations. This me thod has been applied to long-period GEOSCOPE data. We present prelimi nary results obtained with about 300 seismograms corresponding to 86 p aths for the R1 or R2 trains. The three-dimensional S velocity model h as been then expanded in spherical harmonics, to compare it with other models and to determine characteristics of the transition zone.