A formulation is presented to elucidate the layered structure down to a depth of over 200 km from long-period body-wave records of teleseismic deep-focus earthquakes. This formulation utilizes the ratios of vertical to horizontal motions of P and SV waves, and the ratio of SH motion to horizontal motion of SV waves. A strong advantage of the basic assumption of this method is that a postulation of horizontal parallel layering is limited to a few hundred kilometers. A two-step procedure is proposed and justified which makes a separate determination of the structure of crust and upper mantle. In each step, the truncated transfer ratio is used in which factors naturally incorporated in observed spectra, such as finiteness of the record, data window, instrumental response, source function, and later phases are all taken into account. Methods are developed to evaluate effects of the noise and the deviation of the direction of wave approach on the transfer ratio. This formulation makes it possible to elucidate the fine, regionalized structure of the crust and upper mantle such as transitional layering and details of the upper-mantle low-velocity zone.
A routine procedure for determining fine structure of the crust and upper mantle around recording stations is proposed. This cyclic procedure is a combined study of all available seismological methods such as body-wave transfer ratios, surface-wave dispersion, travel times, and synthetic seismograms, and a study of body-wave transfer ratios is in the heart of it. This approach leads to an unambiguous estimation of the layered structure in the upper part of the Earth.