Between October 20, 1989, and October 27, 1989, aftershocks of the Loma Prieta earthquake were recorded at five locations on a variety of site conditions in the West Oakland, California region. We show evidence of P-to-S conversions that have comparable characteristics on both mud and alluvium sites, suggesting that these conversions are generated at the base of the alluvium layer. The inferred transfer function for the sedimentary layer and the travel-time observations for direct and converted phases can be used to constrain the depth of the sedimentary layers. Both the amplitude and many characteristics of the observed transfer functions on mud and alluvium are well predicted with one-dimensional modelling. The results suggest that, in some cases, separate resonance peaks result from multiple sediment layers. ¿ American Geophysical Union 1990