Trains moving through the Rio Grande Valley at Socorro are a known isolated source of short-period microseisms. Power spectra at five different locations indicate that the train noise is confined to a narrow band (< 3 cps) of frequencies. In general, the frequency limits and peaks of the power spectra shift to lower values with an increase in distance from the source. Peaks in the power spectra, which range from 1.80 to 4.75 cps, do not correlate with mechanical interactions of the train with the track.
Particle motions for randomly selected samples of train noise are extremely complex and no dominant component of motion—Rayleigh or Love—can be identified. However, for samples of noise showing the same regular frequency on all components of ground motion, a strong fundamental mode Rayleigh wave can generally be found.
Phase-velocity measurements at a distance of about 17000 feet from the train indicate that a major amount of the train noise at Socorro is being propagated as a fundamental mode Rayleigh wave. The average of over two hundred phase velocity determinations was 1400 ft/sec, a value that corresponds closely to 0.92 of the shear velocity of the surface layer at the recording location. Other observations at the same location indicate normal dispersion of fundamental mode Rayleigh waves, but the change in phase velocity with frequency (in the range of 2 to 5 cps) appears to be quite small.