A review of the effects of anisotropic layering on the propagation of seismic waves

Abstract

Summary. This paper reviews recent work, much of it unpub~shed, on the effects of anisotropy on seismic waves, and lays the theoretical background for some of the other papers in this number of the Geophysical Journal. The propagation of both body and surface waves in anisotropic media is fundamentally different from their propagation in isotropic media, although the differences in behaviour may be comparatively subtle and difficult to observe. One of the most diagnostic of these anomalies, which has been observed on. some surface-wave trains, and should be evident in body-wave arrivals, is generalized, three-dimensional polarization, where the Rayleigh motion is coupled to the Love, and the P and SV motion is coupled to the SH. This coupling introduces polarization anomalies which may be used to investigate anisotropy within the Earth. 1 Introduction A material displaying velocity anisotropy must have its effective elastic constants arranged in some form of crystalline symmetry. The behaviour of both body and surface waves in such anisotropic structures differs from that in isotropic structures, and the variation of velocity with direction is only one of the anomalies which may occur, where we use anomaly to mean differences in behaviour from that expected in isotropic material. Within an anisotropic material three body waves propagate in any direction, having different and varying velocity, and different and varying polarization. Away from directions of crystal symmetry there may be anomalous phases, body and surface waves will have anomalous polarizations, and energy propagation of body and surface waves will not be parallel to the propagation vector. It appears intuitively that many of the anomalies can be attributed to the subtle interplay of the three varying body waves, making the variations of these anomalies difficult to predict. Similarly, smd differences in the structure, such as the thickness of the layer, can make radical changes in the anomalous behaviour. In this paper, we shall describe the type of phenomena to be expected in seismic waves from the presence of a layer of anisotropy within the Earth. A more complete treatment of the mathematics for the general problem of a plane layered structure containing a layer of anisotropy can be found in Keith (1975) for body waves, and Crampin (1970) and Taylor & Crampin (1977) for surface waves.