GLOBAL PHASE-VELOCITY MAPS OF LOVE AND RAYLEIGH-WAVES BETWEEN 40 AND 150 SECONDS

Although much is known of the 3-D structure of the Earth, existing mod els do not make use of much that is known about the large structural p erturbations near the surface. It has long been known, for example, th at continental and oceanic crustal structures are quite different, and that these differences are evident in the dispersion of Love and Rayl eigh waves sampling continental and oceanic paths. Such differences ar e largest at periods of less than about 100 s. Existing global models do not adequately account for such data, and make allowances for crust al structure in a very approximate way, owing to the incompleteness of information on the global distribution of crustal parameters. As a re sult, variations in, for example, crustal thickness translate themselv es into model artefacts extending to great depth. This can be seen as one aspect of the imperfect resolution of the existing global models. In order to construct higher resolution models of the Earth's outer sh ell (0-200 km depth), it is necessary to gain more precise knowledge o f near-surface structure by incorporating data that have sensitivity t o the details of the depth distribution of heterogeneity near the surf ace. As a first step we analyse a large data set of fundamental-mode R ayleigh and Love waveforms to obtain global phase-velocity maps in the period range 40-150 s. Minor and major are phase velocities have been determined from about 24000 digital GDSN and GEOSCOPE seismograms rec orded between 1980 and 1990. In order to make such measurements in an automatic way, we have developed a method, using non-linear waveform i nversion, in which velocity and amplitude, as a function of frequency, are expanded in B-splines. The waveform data are inverted for the B-s pline coefficients, with the application of an explicit smoothness con straint that protects against unwanted effects, such as those due to n otches in the amplitude spectra, and avoids some of the problems assoc iated with the phase ambiguity. The cost function (which is minimized in a least-squares sense) presents many local minima, and a good initi al model is needed; this is derived by integration of group velocities . The measurements made using this new technique are then used in a gl obal inversion for phase-velocity distributions of Love and Rayleigh w aves, expressed in terms of a spherical harmonic expansion. We show re sulting phase-velocity maps up to degree and order 40. These maps are corrected for possible artefacts due to the truncation of the spherica l harmonic expansion. We present a detailed resolution analysis which shows that global lateral resolution for surface-wave tomography is of the order of 2000km. Love-wave phase velocities show a high correlati on with known upper mantle structure at long periods and with crustal structure at shorter periods. Similarly, Rayleigh-wave phase velocitie s correlate well with known tectonic features, but show no clear crust al signature owing to their different sampling of the structure with d epth.