Geological control on the three-component spectra of Rayleigh-wave microseisms

Microseisms in the band 1 to 15 Hz propagate principally as multi-mode Rayleigh waves. Comparison of some previously published spectra from sites over unconsolidated sediments, with theoretical Rayleigh-wave dispersion curves shows a high correlation between observed broad spectral maxima and theoretical group-velocity minima. This gives practical support to a recent prediction of this relationship. Comparison of three-component spectra with theoretical Rayleigh-wave dispersion curves and particle motion figures shows that fine structure in the observed spectra can be correlated with changes in the particle motion figures for different Rayleigh modes. Thus both broad and fine spectral features are affected by local geology and can give useful control when inverting microseism data to obtain a seismic model. The Rayleigh wave nature of microseisms implies that direct interpretation of spectra in terms of body-wave seismic resonances of the earth is incorrect. However, since an approximate correspondence exists between theoretical group velocity minima and body-wave resonant frequencies, some spectral maxima do occur near such frequencies.