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Symposium on Microseisms 



cular or a wave phenomenon? The physicist 

 now finds it is both ! Not enough data is avail- 

 able at present for settling this microseism 

 problem. I hope that both Gilmore and Dinger 

 will continue their hurricane tracking inde- 

 pendently so that with increased data an ef- 

 fort can be made to solve the baffling problem. 



Dr. van Straten's suggestion that we take 

 as a starting point that microseisms are gen- 

 erated by interfering swells is an excellent one 

 and should be the object of intense experi- 

 mental work. Our work with cold front micro- 

 seisms has convinced us that these frontal 

 microseisms originate in the Great Lakes, pos- 

 sibly, as Dr. van Straten suggests as the result 

 of interfering swells. Further and more pro- 

 nounced activity is then noticed when the cold 

 front enters the Atlantic. This may well be 

 the masking action that Kammer and Dinger 

 observed in hurricane micros. Our reason for 

 the conviction that the Great Lakes are a source 

 of micros is the following : When we moved our 

 tripartite station to Poughkeepsie to get away 

 from New York traffic, we noticed a persistent 

 source of two second micros to the West. We 

 recorded from many directions, but there was 

 a persistent source to the West. Since the 

 Hudson River is a sizable body of water to the 

 West we felt it necessary to test this as a pos- 

 sible source of local micros. We set up a sta- 

 tion at West Park on the West side of the 

 Hudson, opposite Poughkeepsie. The source 

 of the micros was still to the West, eliminating 

 the Hudson River as a possible origin. More- 

 over, there were two persistent time intervals 

 indicating two persistent directions — one due 

 West and one North West. Lake Erie was due 

 West of our Station and Lake Ontario, North 

 West. This threw suspicion on the Lakes as 

 the source of our micros. As a first test we set 

 up a third station in the Western part of North 

 Carolina, at Hot Springs. This is due South of 

 Lake Erie and as far South of the Lake as 

 Poughkeepsie is to the East of the Lake. At 

 Hot Springs the source of our micros was due 

 North. Since Lake Erie is due North of Hot 

 Springs and due West of Poughkeepsie we are 

 now convinced that Lake Erie is the source of 

 our persistent two second cold front micro- 

 seisms. We propose in the near future to 

 record both water and ground activity on the 

 shores of one of the Great Lakes. In this con- 

 nection it might be worth mentioning in pas- 



sing that while working on frontal microseisms 

 in Fort Schuyler, one instrument set up on a 

 concrete pier in the Sound recorded the propel- 

 ler pattern of each passing tug. 



We agree with Dr. van Straten that most 

 micros are complex waves. Manly has given 

 a simple method of analyzing such wave trains 

 by inspection. He uses the fact that when two 

 waves of different periods combine, the result- 

 ing wave assumes the period of the wave of 

 greater amplitude, the amplitude oscillating, as 

 in ordinary beats between the sum and differ- 

 ence of the amplitudes of the combining waves. 

 In the resultant wave, if the separation of 

 peaks at the maxima is greater than the separa- 

 tion of peaks at the minima, the frequency of 

 the component of greater amplitude is greater 

 than the frequency of the component of less 

 amplitude, otherwise the reverse is true. Sup- 

 pose we have a microseismic wave train in 

 which the time interval between two group 

 maxima is one minute. Suppose there are 12 

 peaks between the maxima. Then the fre- 

 quency of the component of greater amplitude 

 is 12 per minute and the period of this compon- 

 ent is therefore 5 seconds. If the separation 

 of peaks at the maxima is less than the separa- 

 tion of peaks at the minima, the frequency of 

 the lesser component is greater than that of the 

 major — namely 13 per minute. Hence the 

 period of the lesser component is 4.6 seconds. 

 Suppose the amplitude of the maxima is 

 11.4 mm and that of the minima is 2.2 mm. 

 Since these are respectively the sum and differ- 

 ence of the constituent amplitudes, the con- 

 stituent amplitudes are 6.8 and 4.6 mm respec- 

 tively. Hence our microseismic wave can be 

 analyzed into two waves of periods 5 and 4.6 

 seconds and of amplitudes, 6.8 and 3.6 mm. 



When three waves combine the treatment 

 becomes more complicated, but it is given by 

 Manley (1945). 



In conclusion I wish to thank Dr. van 

 Straten for the privilege of having been able to 

 pre-digest her very masterful resume of storm 

 and surf microseisms. 



REFERENCES 



Manly, R. G., Wave Form Analysis, New York, John 

 Wiley and Sons, 1945. 



