Synthesis of seismic surface waves

Summary. The reflection method is used to produce complete sets of Rayleigh wave dispersion curves in a given phase velocity-frequency window for horizontally stratified media, including modes of very high numerical order, and theoretical surface wave seismograms are then synthesized. The difficulties encountered when attempting to complete large numbers of dispersion curves are discussed. Particular problems arise from models with a low-velocity zone, when curves in a certain portion of the dispersion dia-gram split into two families, the crustal and the channel modes. The reflection method provides a convenient framework in which to examine this pheno-menon heuristically and so devise a method to overcome the difficulties. Seismograms are produced by mode summation and it is found that body-wave behaviour, as well as surface-wave features can be synthesized. The effect of truncating the mode series at a number comparable with the number of modes used in previous studies is examined. It is found that although the S-coda at longer ranges is not adversely affected, the arrivals attributable to body-waves are severely distorted. T h s must call into question the validity of synthetic seismograms generated by summation of only a few modes.