Microseismic Period Spectra and Related Problems Scandinavian Area 



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through the cooperation of Dr. L. Don Leet of 

 Harvard and Dr. Roland F. Beers of R. P. I. 

 Seismometers were of the capacitance-bridge 

 type, and ground motion was recorded on pa- 

 per by means of a Brush Instrument Company 

 penmotor after suitable amplification. Prior 

 to installation, both seismographs were cali- 

 brated by means of a shaking-table at the 

 Pinewoods Observatory, and a field calibra- 

 tion technique was developed to insure that 

 changes in the instrumental constants would 

 be known. The seismometers were operated 

 with a free period of 1.5 seconds and with crit- 

 ical damping. 



Both stations were in operation during the 

 period from August 20 through August 23, at 

 which time an intense hurricane was moving 

 parallel to the Atlantic coast line between Cape 

 Hatteras and a point south of Greenland. This 

 storm produced a rapid rise in microseismic 

 amplitudes at both stations, reaching a maxi- 

 mum on the early morning of the 21st, and 

 decreasing to essentially the normal level by 

 the 23rd. Measurements were made of the 

 microseisms on August 21, near the time of 

 the storm's least distance of about 200-350 

 miles from the stations, and again on August 



23, when the storm was about 1700 miles away. 

 The measurement technique consisted of find- 

 ing amplitudes (peak to trough and associated 

 periods for the largest nearly sinusoidal groups 

 appearing on the records ; special attention was 

 directed toward finding groups with as widely 

 different periods as possible. Seismogram am- 

 plitudes were reduced through the experiment- 

 ally known steady-state response curves to 

 ground motion in microns, and plotted as a 

 function of period on logarithmic paper. Dis- 

 played in this manner, the points show a rather 

 surprising regularity. Figures 1 and 2, for 

 the "normal" day, when the storm had moved 

 off to a great distance, show the observed spec- 

 tra at Harvard and at Pinewoods. Within the 

 range from 1.4 to 5.0 seconds, the peak micro- 

 seismic ground motions were found to increase 

 very nearly as the cube of the period. Impor- 

 tant differences were observed, however. Note 

 that while both stations showed ground motion 

 for the long period (5.0 seconds at about 0.7- 

 1.0 microns, the shorter (1.5 second) periods 

 were noticeably smaller at Pinewoods, approxi- 

 mately 120 miles inland from Harvard. 



A marked change in the spectrum at each 



Figure 1. Microseismic Spectrum for Harvard, 23 August 1952. Normal day. 



