Effect of higher mode contamination on measured Love wave phase velocities

In some circumstances the first higher mode of the Love surface wave can be significantly excited in the period range 30–90 sec by earthquakes and may travel at a group velocity comparable to that of the fundamental mode. This difficult-to-separate contamination will produce a large scatter, but no uniform bias, in phase velocities measured from an ensemble of events. The phase velocities measured from a single event may show a bias over a limited range of periods, but this perturbation may be positive or negative. These results argue against an explanation of observed inconsistencies of Love and Rayleigh wave phase velocities as being due to higher mode contamination. The results are also applicable to the effect of multipath interference of two similar-mode waves traveling at an angle with respect to one another.