706 DABBYSHIBE [CHAP. 20 



and setting up oscillations in the atmosphere which are transmitted to the 

 ground. But M.Bath in Sweden has reported (1949, 1950, 1951, 1953) that micro- 

 seism activity in Scandinavia is increased when the cold front crosses the coast 

 rather than the shelf. These views require that some form of coupling exists 

 between the atmosphere and the sea-bed. Some papers on this effect have been 

 published by Haskell (1951) and Press and Ewing (1951), but it is difficult to 

 obtain a quantitative agreement between these ideas and actual observations. 

 It is unfortunate that very few examples have been published of simultaneous 

 spectra of waves and microseisms on this coast. Recently, however, Carder and 

 Eppley (1959) have published plots of wave period and microseism period, 

 wave amplitude and microseism amplitude for several points along the American 

 coast, and there is often a close correspondence. It is possible the discrepancies 

 on other occasions could be explained if detailed spectrum analysis of the two 

 sets of waves were carried out. The wave interference theory is getting in- 

 creasing support from workers in this area as a result of more detailed analysis. 

 It is probably more difficult to check the theory in this region than many 

 other places because there may be crustal discontinuities which prevent the 

 passage of microseisms from the deep ocean. The effect of refraction (discussed 

 in section 3, page 707) is also very complicated here. Refraction may also 

 provide an explanation for the dependence of microseism period on the depth 

 at the storm ; microseisms of a certain period generated at one area tend to 

 be focused more strongly in particular directions than those of other periods 

 so that for one particular recording station certain storm locations would 

 favour certain periods. Another explanation for this dependence would be the 

 resonance frequency of the overlying water column, which follows from 

 Longuet-Higgins's theory. 



2. The Nature of Microseisms 



It was shown by Lord Rayleigh that in a semi-infinite solid with a free sur- 

 face, a certain type of wave motion was possible. With it the solid particles 

 moved in ellipses in a retrograde fashion (opposite to that in water waves) and 

 the vertical axis of the ellipse was longer than the horizontal. 



The motion was expressed by 



X = 0.42a sin 17, z = 0.62a cos rj, 



where -q varies with the time. These waves are called Rayleigh waves and are 

 not dispersive. The ratio a;/z = 0.68 is called the Rayleigh constant. 



If there is a semi-infinite solid with a thin layer of another medium on top, 

 it was shown by Love that another kind of surface wave is possible. These 

 waves are called Love waves and the motion is entirely horizontal and per- 

 pendicular to the direction of wave propagation. Under these conditions, 

 Rayleigh waves are still possible but their character becomes now more compli- 

 cated as they are dispersive and the Rayleigh constant depends on the densities 

 of the layers. 



