The effect of intrinsic oceanic upper‐mantle heterogeneity on regionalization of long‐period Rayleigh‐wave phase velocities

Summary. Rayleigh‐wave phase velocities at very long periods (185–290 s) are investigated and regionalized, taking into account the lateral heterogeneities within ocean plates revealed by earlier studies at shorter periods. The two‐station method is applied to a few ‘pure‐age’ oceanic paths, and is shown to be compatible with the average Earth model C2 (Anderson & Hart 1976) below depths of 180 km. Under this assumed oceanic model, regionalized for age above 180 km, continental velocities are then derived from a set of experimental great‐circle values, both new or taken from previously published studies. The results basically agree with earlier studies (Dziewonski 1970; Kanamori 1970), although they exhibit less scatter than Kanamori's model. Results are successfully checked against a set of values derived by the two‐station method from a pure continental path. Although the shield velocities are substantially different from the mean oceanic ones, they still fall within the range of variation of oceanic velocities with the age of the plate. This makes velocities derived theoretically from Jordan's (1975a, b) models of deep continent—ocean lateral heterogeneities, inconsistent with the present set of experimental data. Finally, we show that Dziewonski's (1971) model S2 reconciles all experimental seismic data relative to shields, without being significantly different from oceanic models below 240 km.