Seismic properties of the mohorovicicˇdiscontinuity

425 THE MOHO DISCONTINUITY N.I. Davydova (Editor), 1975. Seismic Prop. erties o f the MohoroviciE Discontinuity. (I.P.S.T. translation) Wiley, Chichester, 188 pp., £8.40. This book is a collection of papers with the purpose of drawing the attention of seismologists to a more comprehensive analysis of the characteristics of seismic waves from the Moho discontinuity which are predominant components of the seismic wave field. It consists of 9 selected articles on studies carried out in recent years by the Institute of Physics of the Earth, Academy of Sciences of the USSR, on the frequency and amplitude characteristics of seismic waves from deep-seated boundaries. Special attention is given to the nature of velocity distributions near the Moho discontinuity obtained from kinematic and dynamic wave characteristics, and theoretical wave propagation models are used as a basis of interpretation. In the foreword it is pointed out that, up to the fifties, the crust was modelled by homogeneous layers. In order to reach a better correlation between the observed ratios of reflected and refracted wave intensities in oceanic as well as continental areas, theorists introduced velocity-depth functions with both gradients and first-order discontinuities. Furthermore a detailed study of the kinematic features of seismic waves resulted in the introduction of inversion zones (low-velocity channels). However, even these complex velocity models with a first-order Moho discontinuity do not explain the observed pattern of arrivals as a whole and particularly not the properties of the arrivals from the Moho discontinuity. They do not explain either the poor correlation between observed Moho reflections and their absence in cases of near-normal incidence or the formation of compact and complicated wave groups with a rather bad correlation between phases within these groups. In order to find an explanation of these phenomena, transition zones consisting of irregular thin layers had to be introduced into the model computation and, in addition, deep faults had to be taken into account. Despite the considerable advance in seis-

mic interpretation by modelling techniques, both theoretical and experimental, there is still a large gap between the actual structure and the model approximation of today. This book can be regarded as a contribution to filling this gap. The first paper shows more generally the possibilities of DSS techniques ir~ studying the properties of deepseated seismic interfaces. Three papers deal with the frequency and amplitude characteristics of waves in relation to properties of the Moho discontinuity. In these and the following three papers material from parts of the USSR and the Pacific Ocean is analyzed. In two further articles emphasis is laid on the identification of deep faults cutting through the Moho discontinuity. Another paper concerns the interpretation of shear and converted waves regarding the properties of deep-seated boundaries. The two last papers emphasize the importance of amplitude considerations in the analysis of the seismic wave field. Taking into account absolute wave amplitudes, further parameters such as the coefficients of reflections and refraction, absorption and dispersion can be introduced into the theoretical wave propagation models and, in such a way, the ambiguity inherent in seismic interpretation can be reduced and additional information on physical properties of crust and upper mantle obtained. This book presents papers of individual authors and, besides a foreword defining the aim of this special collection of papers, there is no attempt to summarize the results. The articles are well provided with references, mainly to the Russian literature. As a collection'of articles on an important question of the earth's internal structure and on modern interpretation techniques, it can be highly recommended to geophysicists who are concerned with studies of crustal and upper mantle structure and, more generally, the interpretation of DSS data. A. Vogel, Berlin

GEOLOGY OF THE ENGLISH CHANNEL Sir Kingsley Durham and A.J. Smith (Editors), 1974. A Discussion on the Geology o f