SECT. 1] SUB-OCEANIC STRUCTURAL EXPLORATION BY SEISMIC SURFACE WAVES 111 



2. The General Nature of Seismic Surface Waves 



The data for the study of the ocean floor through the apphcation of surface- 

 wave techniques consist of seisniograms from a large number of stations for a 

 large number of earthquakes. 



Each seismogram is a record relating to some particular component of ground 

 motion as a function of time. Three factors affect the character of this record. 

 They are (1) the nature of the source; (2) the nature of the propagation of 

 the waves ; and (3) the response of the instrument. Generally speaking, the 

 third factor, instrument response, is understood well enough to be compensated 

 for, and thus removed. 



Of the other two factors, excitation and propagation, the greatest effect 

 upon the character of the seismogram clearly results from propagation. The 

 seismic-wave train from a natural earthquake with a duration measured in 

 seconds, or fractions of a second, at the source commonly lasts many tens of 

 minutes at a distant station. It is to the toj)ic of propagation effects that most 

 of this chapter will be devoted. Excitation is of considerable importance, 

 however, and should become increasingly important in the future as our 

 methods for studying this XDhenomenon improve. Some observations relating to 

 various kinds of excitation will be discussed later. 



Providing instrumentation is adequate and the source is of the proper size, 

 the seisniograms from a seismograph station at some distance from the source 

 will include waves of a number of identifiable types. Body waves of the con- 

 ventional P (compressional) and S (shear) types as well as various reflected 

 and/or refracted combinations of these types will be registered largely during 

 the early portion of the seismic recording. In general, these waves will have 

 penetrated deep within the earth, and it is from such waves that most of our 

 knowledge of the earth's deep interior, the core and the mantle, has been 

 derived. When the source is not too deep, the early body waves will be followed 

 by trains of dispersed long-period waves of relatively large amplitudes and long 

 durations (Fig. 1). These are the surface waves. Their amplitudes are large be- 

 cause the waves are confined to the surface and near-surface media, and the 

 frequency content, the duration and the dispersion result largely from wave- 

 guide propagation within the earth's upper layers. Measurement of such 

 dispersion provides us with much of our information on the earth's crust and 

 upper mantle. 



The seismic surface-wave trains may consist of waves of two general types. 

 These are named after their discoverers, A. E. H. Love and Lord Rayleigh. 



Love waves have velocities in general slightly greater than Rayleigh waves 

 of the same period, and are characterized by surface -f)article motion which is 

 transverse to the direction of propagation and in the horizontal plane. This 

 particle motion is that of the component of shear waves usually labelled SH, 

 and, in fact, Love waves may be thought of as SH waves trapped within the 

 surface wave guide. 



Rayleigh waves have particle motions which are retrograde elliptical and in 



