Amplitude Distribution of Storm Microseisms 



43 



poorer fixes than those with circular shapes, 

 especially since the wind velocity may have 

 considerable variation in different sectors of a 

 moving hurricane. 



4. It has been shown [Donn 1951] that 

 amplitudes are a function of the spectrum of 

 the microseism periods and the tuning of the 

 receiving instruments. Much ambiguity could 

 be removed in the study and application of am- 

 plitude distribution by the use of instruments 

 sharply tuned to the periods shown by hurri- 

 cane microseisms for a given region. This 

 would eliminate much local high wind micro- 

 seisms usually of short-period, and might dis- 

 tinguish between microseisms from simultane- 

 ous hurricanes providing they are of different 

 periods. This will be referred to again in 

 Part II. 



5. The use of horizontal-component seis- 

 mographs in making amplitude distribution 

 and ratio studies introduces possible sources 

 of error. The equal-amplitude lines around a 

 station will tend to be elongated in the direc- 

 tion of pendulum movement, as found by Gil- 

 more. This has also been shown in studies of 

 amplitudes from horizontal components ori- 

 ented at right angles, and is consistent with 

 the Rayleigh-wave idea of microseisms. The 

 comparison of amplitudes among several sta- 

 tions would be more uniform if vertical com- 

 ponents were used. It has further been shown 

 [Lee 1935] that amplitudes from vertical com- 



ponents are less affected by local geologic dif- 

 ferences than those from horizontal compo- 

 nents. 



6. It is believed that the amplitude dis- 

 tribution and micro-ratio procedures can op- 

 erate best when microseism amplitudes are 

 much above background since this decreases the 

 error in making comparisons and ratios with 

 other stations. This may explain the difficul- 

 ties in applying the micro-ratio procedure to 

 the first five days of Hurricane No. 1, 1950, 

 referred to earlier. Since the tripartite meth- 

 od requires only the recognition of the first 

 sinusoidal waves of a new microseism storm, 

 it may still be more applicable for early loca- 

 tion of hurricanes. Both procedures can prob- 

 ably be refined beyond their present stages 

 and may be valuable supplementary tools. 



In view of the factors just considered it 

 appears that the converse of the observation 

 that a hurricane of a given intensity will at a 

 given position generate microseisms of a defi- 

 nite amplitude at a particular station, is not 

 always true. However, this is the basis for 

 the application of amplitude distribution data 

 to the determination of the position and in- 

 tensity of a hurricane. 



Part II — Amplitude Studies by Means of Reso- 

 nant Seismometers — It is considered an em- 

 pirical fact at least for east-coast stations, 

 that the microseisms period-spectrum varies 



acgr 



501 00-MAY 5 — 



c 



AAA^y v ^o^^^^^^/^yy! 



Figure 4. Cold front positions (A) and Benioff (B) and Columbia (C) verticals for May 5, 1952. 



