Amplitude Distribution of Storm Microseisms 



47 



low pressure area developing over near ocean 

 waters with relatively high winds in the cool 

 air over local shallow waters near the station. 

 These winds appear responsible for the initial 

 2-second microseism increase. The rapid in- 

 tensification of the "low" from 0030 to 1230, 

 May 17, shown on charts "B" and "C", respec- 

 tively occurring over deeper and more distant 

 waters appears to be responsible for the gener- 

 ation of the long-period microseisms. This is 

 consistent with all previous studies of similar 

 situations. Wind velocities decreased over 

 shallow waters as the "low" intensified and 

 with this decrease occurred the decrease in 2- 

 second activity. The conventional records 

 show irregular long-period microseisms dur- 

 ing this interval. 



From the three cases given here, the reso- 

 nant instruments seem capable of distinguish- 

 ing between microseisms generated in different 

 environments. The amplitude variations of 

 several minutes duration, referred to earlier, 

 are even more prominent on the records of case 

 3. Since these amplitude groups cannot be 

 traced as simultaneous events among the differ- 

 ent traces they suggest an origin in different 

 parts of the generating area, and on the basis of 

 this and data given in earlier studies, water 



depth is considered to be the dominating factor. 

 If these groups were recorded with the same 

 pattern simultaneously at different stations 

 having equivalent instruments, it would tend to 

 prove the relation of these groups to conditions 

 in the generating areas. It may be of interest 

 to compare these records with a wind-velocity 

 recording, Figure 7, made during the passage 

 of a storm. The wind record has the same time 

 scale and shows similar amplitude variations. 

 No conclusions are drawn from the obvious 

 similarities, for doubtless, ocean wave records 

 would have a similar appearance on this scale. 

 The groups in all of these phenomena are re- 

 lated to conditions of origin and propagation, 

 and their complete explanations are equally 

 difficult. 



In view of the response of the resonant 

 seismometers as given in the study so far, their 

 use in hurricane tracking seems to have definite 

 advantages. 



Part III — Amplitude Studies Applied To Prob- 

 lem Of Microseism Origin. Gilmore refers to 

 the commencement of microseism storms with 

 the primary generating hurricanes at a dis- 

 tance so great as to preclude the simultaneous 

 arrival of swell in local waters. It is believed 



5/12-10- 



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12 — W*V*s* »W^ 



%***"rt^ft*A'V»^'* , ' * —''i^.V—* V\i^V* * «B 



5/13-02 



04- 





Figure 8B. Weston microseisms of May 11-13, 1946, recorded by long-period vertical. 



