Higher mode interference and observed anomalous apparent Love wave phase velocities

Abstract

For a representative selection of spherical earth models compatible with seismic observations, there is a significant frequency range over which the fundamental and first higher Love mode group velocity curves approach each other closely or actually overlap. Higher Love modes can be excited comparably to the fundamental mode for both shallow and deep sources under a variety of circumstances, and thus higher mode interference is an important factor to be taken into account in the proper interpretation and analysis of Love waves. Simple theoretical computations reveal the nature of the effect of mode contamination on measured phase velocities, and biases in the selection and analysis of surface wave data make it appear likely that fundamental mode Love waves experiencing higher mode interference will exhibit anomalously high apparent phase velocities, as observed in the United States midcontinent and in Japan. Thus it is suggested that it may not be necessary to resort to complex or anisotropic models to explain these observations. Consideration of the effects of mode interference, as well as knowledge of source parameters (type, depth, orientation), are important in determining dependable fundamental and higher Love mode phase velocity dispersion over a broad frequency band. Phase velocity filtering across a large array could effectively separate the modes. Love waves traversing continental regions from the source will suffer little or no contamination from higher modes for wave periods of less than about 60 sec and thus are preferred to oceanic paths in obtaining reliable fundamental mode data in this restricted frequency range.