42 



Symposium on Microseisms 



Figure 3. Palisades resonant seismograph traces for May 5, 1952. Time shown is GCT. 



western North Atlantic Ocean two and occa- 

 sionally three hurricanes were present si- 

 multaneously on 25 days and were in a posi- 

 tion to cause ambiguity at most of the hurri- 

 cane tracking stations. This does not include 

 cases when strong low-pressure areas could 

 also have produced ambiguous results. 



In this connection it may be of interest 

 to consider one of the cases given by Gilmore, 

 namely the hurricane of September 3 to 9, 

 1950. It should be noted that for a portion of 

 this interval and preceding the time of Gil- 

 more's study, two hurricanes existed in oppo- 

 sition to Bermuda. One was to the south and 

 approaching, and the other to the north, and 

 receding, as shown in Figure 1. Figure 2 

 shows a first amplitude maximum (24mm) cor- 

 responding exactly to the time of velocity maxi- 

 mum (for the waves across the net) and 

 showed the same trend. It should be noted 

 that the value of 30,000 ft/sec was the aver- 

 age of the measured velocities across the Ber- 

 muda tripartite station for the minority of 

 the waves present. Most of the waves re- 

 corded at each element of the net appeared to 

 arrive at exactly the same time indicating in- 

 finite velocities or a standing wave pattern 

 from oppositely moving wave trains. By se- 

 lection of waves with measurable time differ- 

 ences, azimuths pointing toward both storms 

 were obtained. The amplitudes during this 

 interval (September 4-6) seem to be clearly 

 the result of two hurricanes. The later, larg- 

 er amplitude build-up was produced by the 

 close approach of the storm from the south. 

 Certainly estimates of intensity as well as po- 

 sition would be affected by the presence of two 



or more storms. If simultaneous hurricanes 

 are well-separated, some stations in the detec- 

 tion net would respond uniquely to one storm, 

 but a prior knowledge of their existence and 

 position would be necessary before the data 

 would be useful. Another possible solution is 

 given in the second part of this paper. 



2. Relatively local high wind areas can 

 introduce considerable ambiguity in the ap- 

 plication of amplitude distribution studies. As- 

 suming that the microseisms of local wind 

 and hurricane origin can be successfully dis- 

 tinguished (which is not always possible), the 

 effect of microseisms of local origin must be 

 removed before amplitude ratios are deter- 

 mined. This has frequently proved a matter 

 of some difficulty and much ambiguity in the 

 writer's study of hurricane microseisms. In- 

 terpretation of the hurricane intensity would 

 be similarly subject to error. 



3. The size and shape of the generating 

 storm can also effect estimates of their posi- 

 tion and intensity. Microseism amplitudes 

 have been described as being a function of both 

 area and intensity of an atmospheric disturb- 

 ance [Donn, 1952a]. The strongest micro- 

 seisms observed by the writer were produced 

 by a large area cyclone with 35 to 40 knot winds 

 while closer but smaller hurricanes with 100 

 knot winds generated much smaller micro- 

 seisms at the same station. It is believed that 

 the intense wind areas of hurricanes vary suffi- 

 ciently in size so that estimates of velocity 

 based only on microseism amplitude can be un- 

 reliable. Elongated storms should result in 



