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Spatial coherence structure and attenuation of the Lg phase, site effects, and the interpretation of the Lg coda

TELEDYNE GEOTECH ALEXANDRIA LABORATORIES, P.O. BOX 334, ALEXANDRIA, VIRGINIA 23313

Abstract

Analyses at seismic arrays showed that the intersensor coherence in the early, most energetic part of the Lg wave train decreases faster perpendicularly to the direction of propagation than parallel to it. This indicates a significant amount of forward scattered energy with a spread in direction of arrival. Following this part of the Lg wave train, the intersensor coherence rapidly decreases, indicating that most of the energy arriving is omnidirectional. The amplitude levels of this part of the wave train are too high to be explainable by the single scattering mechanism used in the interpretation of coda Q. The energy in the late coda is also often too high by this standard. The strong dependence of the Lg envelope shapes on local geology also indicates that the Lg coda is generated by processes much more complex than the scattering of primary Lg into waves of the same modal composition, and mode conversions play a significant role in the scattering process.

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