INVERSION FOR LATERALLY HETEROGENEOUS UPPER-MANTLE S-WAVE VELOCITY STRUCTURE USING ITERATIVE WAVE-FORM INVERSION

In contrast to previous work, which treats the Earth's lateral heterog eneity as an infinitesimal perturbation to a spherically symmetrical s tarting model, we conduct iterative linearized waveform inversion for the Earth's laterally heterogeneous structure. We use the Direct Solut ion Method (DSM) (Geller et al. 1990a) to calculate synthetic seismogr ams and their partial derivatives for a laterally heterogeneous earth model. We invert surface-wave data from the IDA and GEOSCOPE networks. We expand the lateral heterogeneity of rigidity in spherical harmonic s up to angular order number 8 and use three parameters to specify the depth dependence of each harmonic, giving us a total of 240 unknowns. The short-wavelength lateral heterogeneity (s = 4, 6 and 8) in the de eper part of the upper mantle obtained by this study differs significa ntly from M84A. The relative improvement in the variance reduction as compared with model M84A is about 20 per cent for the IDA data and mor e than 100 per cent for the GEOSCOPE data.