The effect of 3‐D structure on Lg propagation patterns

Abstract

The Lg phase is one of the most prominent features of many regional seismograms and its amplitude is known to be sensitive to major changes in crustal structure. This sensitivity can be exploited to investigate the nature of crustal heterogeneity, but also reduces the utility of the analysis of the Lg phase for characterizing seismic sources. The method of ray diagrams, based on the constructive interference of multiple S reflections, gives a visual measure of the interaction of guided wavetrains with variations in crustal structure. The technique is extended to 3‐D heterogeneity and by representing the S ray systems in plan view through their basement reflection points, a semi‐quantitative measure of the disruption of Lg at different phase velocities can be made. In addition, the tilt of the ray systems can be used as an indication of the degree of interconversion between Rayleigh and Love wave energy. Two simple crustal models are examined to investigate the effects of oblique incidence and transverse topographic gradients on multiple S reflections using ray diagrams. The method is then applied to several real earth models of the crust in central Asia using topographic data and assuming isostasy. Interpretations of the ray diagrams for paths across the major tectonic features compare well with many of the observations from previous studies (Ruzaikan et al. 1977; Ni & Barazangi 1983), and suggest that the gross characteristics of Lg propagation can be attributed to the shape of the crustal waveguide. Copyright © 1990, Wiley Blackwell. All rights reserved