14 



Symposium on Microseisms 



N 



180° 170° 160° 150° 140° 130° 



S 



Figure 3. Typical bearings and velocities observed on 21 November 1950 



measure of the reciprocal of the apparent ve- 

 locity and the orientation of the point with 

 respect to the origin gives the computed direc- 

 tion for the bearing. A wide scatter in direc- 

 tion is apparent when all points are consid- 

 ered, however, the spread in direction is nar- 

 rowed, in this case to about 12 degrees, if only 

 those bearings having an apparent velocity of 

 10,000 ft/sec or less are considered. Figure 4 

 shows the weather map existing at the time 

 the bearings of Figure 3 were taken. The in- 

 terpretation placed on these and similar re- 

 sults for various storms is, that at those in- 

 stances when the computed velocity is a rea- 

 sonable value, the recorded microseisms con- 

 sists of a coherent wave train coming from a 

 single source. 



The question of refraction and reflection 

 was raised in the preceding paper as a limita- 

 tion on the usefulness of the tripartite sta- 

 tion. Donn and Blaik [1952] have also re- 

 ferred to refraction as a possible source of 

 error in pointing to the area of generation of 



microseisms. It may well be that refraction 

 is important, but it is believed that the exis- 

 tence of refraction will be very difficult to 

 identify as long as so much uncertainty exists 

 in knowing where the true area of generation 

 really is. The assumption that refraction is 

 the reason why tripartite bearings do not point 

 to the center of a hurricane or low-pressure 

 area does not seem justified until it is proved 

 that these centers are the area of generation. 

 However, the use of earthquake records to study 

 refraction of seismic waves is a valid approach 

 since in this instance the location of the source 

 is well known. 



To summarize, it can be stated that the 

 tripartite station has definite limitations. To 

 obtain the greatest accuracy from a tripartite 

 station one must (1) use the most advanced 

 technique in instrumentation and (2) some 

 method must be applied which selects the por- 

 tions of the records to be used to compute the 

 bearings so as to insure the use of the most 

 nearly coherent wave trains that exist during 

 a given microseismic storm. 



