Can Sea Waves Cause Microseisms 



87 



If the previous literature is re-examined, 

 bearing the wave-interference theory, and 

 what we already know about waves, in mind, 

 some of the apparent contradictions to which 

 emphasis has been given appear explainable. 

 The example given by Whipple and Lee 

 (1935) of almost identical isobaric charts of 

 two depressions south-east of Greenland, one 

 associated with intense microseismic activity 

 and the other with practically none, is not such 

 an obstacle when the previous histories of the 

 two depressions are studied. One had moved 

 rapidly northwards over the ocean, with plenty 

 of opportunity for wave interference, whereas 

 the other had developed over the land. Similar 

 attempts to estimate wave interference might 

 explain why less microseismic activity was 

 found with a depression over the mouth of the 

 St. Lawrence river and an anticyclone over the 

 Great Lakes than when the positions of the 

 depression and anticyclone were reversed; or 

 why, with a shallow depression off the east 

 coast of Japan, the microseisms were larger on 

 the coast of China while the wind was stronger 

 off the coast of Japan. 



There is, however, not much to be gained 

 by studying cases which are not fully docu- 

 mented. We must, as Dr. Longuet-Higgins 

 emphasizes, learn more about the conditions 

 which give rise to wave interference ; we must 

 select examples in which the metorological con- 

 ditions are sufficiently simple for us to be cer- 

 tain of the connection between the storm and 

 the microseisms, and we must measure the 

 waves and the microseisms as precisely as 

 modern techniques will allow. It is possible 

 that some of the present misunderstanding is 

 due to faulty interpretation of records from 

 seismometers that are highly tuned to the 

 short-period end of the microseism range, and 

 faulty estimation of the sea surface or wave 

 and microseism recordings, in which the size of 

 a long period oscillation can be underestimated 

 owing to the interruption of its swing by minor, 

 shorter, waves. 



The wave-interference theory is, to say the 

 least, an excellent working hypothesis, and if it 

 is subjected to further question and experi- 

 ment, of the standard set by Dr. Longuet-Hig- 

 gins and his co-workers, we must move rapidly 

 towards a full solution. 



It seems to me that the subject has now 

 been put on a systematic basis, and that its 

 progress must be more rapid. In spite of some 

 setbacks we shall soon be in a better position to 

 take full advantage of the practical possibili- 

 ties. 



I think that Dr. Longuet-Higgins's histori- 

 cal note gives a proper account of the develop- 

 ment of the new theory. 



REFERENCES 



Bernard, P., Etude sur I'agitation microseismique et 

 ses variations. Ann.' Inst. Phys. Globe, v. 19, pp. 

 1-77, 1941. 



Darbyshire, J., Identification of microseism activity 

 with sea ivaves. Proc. Roy Soc. A., v. 160, pp. 439- 

 448, 1950. 



Whipple, F. J. W., and Lee, A. W., Notes on the theory 

 of microseisms, Mon. Not. Roy. Ast. Soc. Geophys. 

 Suppl., v. 3 pp. 287-297, 1935. 



Discussion 



Jacob E. Dinger 



Naval Research Laboratory 



As a discussion of the theoretical paper 

 "Can Sea Waves Cause Microseisms," I should 

 like to present some of the data and interpreta- 

 tions obtained by the Naval Research Labora- 

 tory on various field trips during the hurricane 

 seasons of the past several years. 



The data considered here is concerned with 

 hurricanes which have followed paths in the 

 Western Atlantic and Caribbean. It has been 

 a primary objective of this work to obtain evi- 

 dence which might help to determine where the 

 area of microseism generation is with respect 

 to the hurricane center and to determine under 

 what condition a hurricane can generate micro- 

 seisms. In furthering this objective it has be- 

 come of interest to study the data in the light 

 of various theories to see if the data lends sup- 

 port to any of these theories. 



During the hurricane seasons of 1948-1951 

 records of microseisms have been obtained at 

 points in the Bahamas, Florida, North Carolina 

 and Washington D. C. as various hurricanes 

 have followed varying paths in the Western 

 Atlantic. The following observations have in 

 general been true for all these hurricanes: 



(1) Storms which generate in the Middle 

 Atlantic and approach the seismo- 

 graph locations do not produce ap- 

 preciable microseismic activity until 

 the storm moves over the continental 

 shelf or, over the shallower waters 

 surrounding the Islands of the Carib- 

 bean Sea. This same observation is 

 pointed out by Donn (1952). 



(2) As the storm recedes, the microseisms 

 continue at a much higher level of 

 amplitude as compared to the same 

 distance from the seismograph loca- 

 tion during the approach of the storm. 



(3) The point of nearest approach is not 

 necessarily the time of maximum 

 amplitude. 



The above observations can be interpreted 

 as giving evidence that the storm must move 



