Symposium on Microseisms: Held at Arden House, Harriman, N.Y. 4-6 September 1952, Sponsored by the Office of Naval Research, and the Geophysical Research Directorate of the U.S. Air Force. (1953)

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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.

It is imperative that pressures on the ocean bottom at large depths be measured in some detail. We need to know whether or not standing waves under a storm are as common as microseisms. Currently without such knowledge the theory that microseisms originate under the deep sea stands hedged and inviolate. If there is no increase of microseisms with a storm, then there were perforce no standing waves—if they increase at some stations and not at others, the latter were protected by a barrier. If they appear to approach from the wrong direction at a station, then they were refracted or reflected at a barrier on their way from storm to station. When two ideas persist for as long as the above (1) correlation of microseisms with deep sea phenomenon (2) correlation of microseisms with coastal phenomenon, one not violently interested may suspect there is something to both of them. Miss van Straten's paper in this symposium is admirable in bringing this out. The purpose of a symposium such as this is primarily to broaden the minds of the members—not to offer each an opportunity to convert the others. As one member remarked, after many years observing micro- seisms at one station one may learn pretty well with what to correlate them. But this does not mean that he would be equally successful in an- other geographic locality. It would even appear that microseisms in Eu- rope and America are not so comparable as we might expect. It is to be hoped that each member of the conference will go home to review his own data with new possibilities in mind. University of California at Berkeley Berkeley, California