Amplitude Distribution of Storm Microseisms 



53 



microbarographic activity." 



Microseism storms of the short-period, ir- 

 regular type shown here have been shown 

 (Donn 1952 a) to grade continuously into regu- 

 lar long-period microseisms as the generating 

 atmospheric disturbance moves to distant and 

 deeper waters, with the microseism period ap- 

 pearing to be more a function of water depth 

 than distance. This is shown on simultaneous 

 recordings with Palisades short- and long-peri- 

 od seismographs in Figure 13, with the generat- 

 ing cyclone shown in Figure 14. The difference 

 in the type of microseisms seems to be one of 

 the position and environment of the generating 

 disturbance. Microseism period is shown to in- 

 crease continuously until the generating cy- 

 clone reached deepest ocean water. The reverse 



effect is expected when fronts or storms ap- 

 proach coastal stations from shoaling seas. 



Conclusions — 1. The amplitude distribution 

 and micro-ratio technique for locating hurri- 

 canes and estimating their intensities is a 

 strictly empirical procedure. Only further ap- 

 plication will determine its operational value. 

 Although the serious effects of refraction and 

 possibly of short-crested microseism waves are 

 eliminated by this new procedure there are still 

 a number of serious adverse factors which 

 would at present prevent application to all 

 hurricanes and to early positions of many if not 

 most hurricanes. 



2. The use of resonant seismographs 

 seems to permit the study of a narrow micro- 



Figure 13. Comparison of microseism data 'from long- and short-period Palisades instruments for 

 October 26-29, 1949 showing shift of microseism energy to longer period with the retreat of the 

 cyclone shown in Figure 14. 



