Prograde Rayleigh wave particle motion
Fundamental mode Rayleigh waves generally show retrograde particle motion at the surface of the Earth. If there exists a thick sedimentary layer, however, reversal of the sign of vertical eigenfunction occurs near the surface, resulting in prograde Rayleigh-wave particle motion at the surface. We show that, for structures similar to those found in the Los Angeles basin (with thickness up to 8 km), surface prograde motion may occur within the frequency band 0.05-0.3 Hz. Although it has been suggested that the effect of gravity on waves in unconsolidated surface layer may be important, partitioning of energy between the elatic and gravitational energy shows that the gravitational energy is less than I per cent and thus is not important. The phenomenon is caused by elastic effects, mainly caused by extremely slow shallow seismic velocities. Observation of prograde elliptical particle motion may be difficult, however, because particle motion is largely horizontal and high microseismic noise exists in the same frequency band.