68 



Symposium on Microseisms 



station was found on August 21, when the hur- 

 ricane was at a minimum distance from the 

 stations. As seen in Figures 3 and 4, not only 

 did the general activity increase, but the shape 

 of the spectra changed so that amplitudes were 

 found to increase nearly as the fourth power 

 of the period. A comparison of all of this data 

 is shown in Figure 5 relative to the Harvard 

 spectrum on 23 August 1950, here used as a 

 standard spectrum. If all amplitudes are re- 

 duced to ratios of those at Harvard on the 

 normal day, the following emerges. First, on 

 a normal day the long period noise, presumed 

 to originate in the Atlantic, is reduced by only 

 a small amount in traveling the distance be- 

 tween Harvard and Pinewoods, while the short 

 period noise (also presumed to originate in the 

 ocean) is reduced by a factor of about 3:1. 

 This is interpreted as being in agreement with 

 Dr. Bath's conclusions, that there is a greater 

 extinction rate for short period waves than for 

 long period waves. Second, on the day of the 

 hurricane's near approach, both stations 

 showed a marked increase in amplitude, with 

 The long period waves increasing in size much 

 more than the shorter periods. It may be ob- 

 served that Pinewoods, at roughly 50% great- 

 er distance from the storm than Harvard, 



showed an amplitude increase of 500% at 5 

 seconds, while Harvard showed an increase of 

 1,000%' at 5 seconds; short period amplitudes 

 about doubled at each station. This, incidental- 

 ly, tends to verify the assumption that the 

 short period (1.5 second) microseisms also 

 originate in the ocean, as evidenced by the in- 

 crease in amplitude associated with the hurri- 

 cane. 



Further knowledge of the microseismic 

 spectrum in the vicinity of Troy, New York, 

 was obtained on November 12, 1950. On that 

 date, between the hours of 1100 to 1200 E.S.T., 

 means of a vertical shaking table, so that the 

 total response characteristic at any filter set- 

 ting was determinable by simply adding the 

 known filter response to that of the seismome- 

 ter. Results are shown in Figure 6. The first 

 line shows electrical noise in the system, which 

 may be seen to be rather unimportant corn- 

 instrumental data were obtained showing mi- 

 croseismic amplitudes over a considerably wid- 

 er frequency range. The instrumental setup 

 consisted of a conventional vertical component 

 electromagnetic seismometer whose output was 

 amplified, played through a Krohn-Hite Ultra- 

 Low Frequency Band-Pass Filter (model 330- 



SOTS 



s a 7 a 



Figure 2. Microseismic Spectrum for Pinewoods, 23 August 195 2. Normal day. 



