Use Of Gaussian Beams To Study The Effects Of Interface Irregularities On Love Waves

Abstract

Gaussian beam summation is applied to study the effects of dipping and irregular interfaces on Love waves. Gaussian beams are approximate solutions of wave equation under the parabolic approximation. Closely related to ray theory, this approach is free of some limitations of ray solutions. For instance, it is possible to compute the wave field near to caustics because of the intrinsic smoothing effects of Gaussian beams. Given a source expansion in terms of beams and a simple ray tracing scheme that considers appropriate reflexion coefficients, the solution is computed in the frequency domain for a series of receivers along the surface of a simple layered structure. A frequency-wavenumber analysis allows to identify dispersion curves of Love waves. The procedure is tested for a flat layer case and excellent agreement with exact solution is found. The effects of upand down-dip propagation on dispersion characteristics of Love waves are presented. Some cases of smooth and sharp transitions between different structures are also described. INTRODUCTION Locally generated surface waves play a significant role in the local site response of sediment filled geological structures during earthquakes. Several approaches that explicitly take this fact into account have been recently proposed (see e g Calderon et al [1], Hisada et al. [2], Fujiwara and Takenaka [3] and Sanchez-Sesma et al. [4]). In these studies, however, one basic assumption is that the layered structure of the sediment is flat. Earlier studies of lateral effects on the propagation of surface waves include the work of Boore [5] Transactions on the Built Environment vol 3, © 1993 WIT Press, www.witpress.com, ISSN 1743-3509