Precursors to the S or shear wave can cause serious error in interpretation of seismograms, particularly if only the vertical component is available. A series of synthetic seismograms are constructed using either asymptotic ray theory or the Fourier transform of a crustal transfer function obtained from a Haskell-Thomson matrix formulation to show the contribution of S- to P-wave conversions as precursors to the shear wave. Errors of 1 to 6 sec may occur if only the vertical component of seismic displacement is available. Errors may occasionally occur if horizontal seismograms are used, particularly when seismic stations are situated on thick sedimentary basins. In this case, particle-motion studies and arrays of recording instruments are required to avoid errors which may be as large as 2 sec. The problem arises at all distances and is of particular importance when ts−tp times are used to determine epicentral distances, first-motion studies, and in the study of premonitory phenomena using the ratio ts/tp for earthquake prediction.