MODERATOR'S COMMENTS 



Perry Byerly 



University of California at Berkeley 



The idea that microseisms in the range of period from about 4 to 8 

 seconds are best correlated with marine phenomena has been widely agreed 

 upon by seismologists for a long time. It is also generally agreed that they 

 are surface waves — not body waves. 



The theory that they are caused by surf breaking on rocky coasts 

 was advocated by Weichert's school. Good correlations were established 

 between high surf on the Norwegian coast and microseisms in northern 

 Europe (and even Asia). However, the correlation was not so good with 

 southern European stations at equal distance. This weakness was met 

 by the assumption of geologic barriers between northern and southern 

 Europe — called sometimes "deep seated faults." That heavy surf must im- 

 part some energy to the earth is unquestioned ; that such energy would pro- 

 duce earth waves as nearly regular as microseisms seems unlikely consid- 

 ering the irregularities of coasts and of surf. 



A parallel theory, advocated strongly by Banerji and Cherzi two 

 decades ago, gave as the source of these microseisms some phenomenon ac- 

 companying storms far at sea. A number of particular cases were cited 

 where the correlation seemed clear enough. The great objection then to 

 this theory was physico-mathematical. Internal pressures due to water 

 waves in deep water die off too rapidly. It was physically impossible for 

 energy in the air to be transmitted through the ocean to its bottom. 



The idea that microseisms must be Rayleigh waves is an old one. 

 Many efforts have been made to get the direction of approach by analyzing 

 components on this assumption. Then the tripartite method of getting 

 direction of approach, free of the Rayleigh wave assumption, was applied 

 simultaneously in America and Europe. This method as first applied sug- 

 gested strongly that the center of storms at sea was the source of micro- 

 seismic waves. However, an exhaustive pursuit of the tripartite method 

 showed: 1) not always was the direction of approach that of the deep sea 

 storm center, and 2) not at all tripartite set-ups did microseisms rise equal- 

 ly for storms at a given distance. Refuge was again taken in the assump- 

 tion of geologic barriers. Their duty is to shield when microseisms are 

 not observed accompanying a storm, and to reflect or refract when the 

 computed direction of approach does not point to the storm center. 



Although the surf theory has gone out of date there have remained 

 those who connect their microseisms with cold fronts passing over the 

 coasts and with storms only when they reach shallows near the coast. The 

 surf is, however, disavowed as an intermediary. 



For all theories to date one seems to have little difficulty in pointing 

 out exceptions. 



Only recently has the transfer of energy from the atmosphere over 

 the deep ocean, through the water, to the earth become theoretically pos- 

 sible. If the ocean waves are standing waves then second order terms 

 become effective. It appears that for such ocean waves of reasonable am- 

 plitudes the amplitudes in microseisms may be explained if the area cov- 

 ered by the water waves is reasonably large. The period of the micro- 

 seisms should be half that of the ocean swell and has been so observed in 

 England. 



