Microseismic Period Spectra and Related Problems Scandinavian Area 



69 



A) , and recorded by means of a Brush recorder. 

 The Krohn-Hite is a selective filter which will 

 pass frequencies of an input signal, with no 

 loss in gain, within any desired band between 

 2,000 cps and 0.02 cps (or 50 seconds period). 

 High and low cut-off frequencies are independ- 

 ently adjustable. Outside of the pass band, the 

 response falls off at 24 db per octave. Using 

 this detector system, it was possible to examine 

 the microseisms within a one octave region of 

 the spectrum, and to vary the pass band so 

 that it was possible to observe activity in sev- 

 eral regions of the spectrum without interfer- 

 ing "noise" from other frequencies of ground 

 noise. The seismometer was calibrated by 

 pared to the recorded microseisms. Although 

 the total instrumental magnification at suc- 

 cessively larger periods decreases roughly as 

 the cube of the period due to the response of 

 the seismometer (T = 1.2 seconds, damping 

 critical) it was found necessary to further re- 

 duce the gain for longer periods ; from this we 

 infer that the ground amplitudes increase fast- 

 er than the cube of the period. Calculated true 

 ground amplitudes are shown on Figure 7, 

 which shows an amplitude increase about pro- 

 portional to the fourth power of the period in 

 the range 0.5 < T <. 5 seconds. Indications 



are that the ground amplitude was maximum 

 at 5.0 seconds, decreasing slowly for longer 

 periods. Measurements on wave periods be- 

 yond 5 seconds are not shown, since there were 

 evidences of instrumental instability at ex- 

 tremely long periods. It was estimated, how- 

 ever, that the waves with periods exceeding 

 ten seconds had about 20% of the amplitude 

 of the five second microseisms. Synoptic me- 

 teorological conditions on this date have not 

 been studied. 



No evidence for the existence of discrete 

 microseismic "bands" at Harvard or Pinewoods 

 was found during the investigation described 

 here. In all cases, the sprectrum appeared to 

 be continuous, or nearly so, with no wave peri- 

 od exhibiting amplitudes departing to any 

 marked extent from the very regular rate of 

 increase of amplitude with period. On the 

 other hand, Macelwane and his colleaques have 

 published examples showing narrow and sharp- 

 ly defined short period microseismic bands. It 

 is tentatively suggested that the difference in 

 these findings is attributable to regional geolog- 

 ical differences; both Harvard and Pinewoods 

 are situated on relatively homogeneous meta- 

 morphics, in contrast to the horizontal sedi- 



Figure 3. Microseismic Spectrum for Harvard, 21 August 1952. Hurricane at nearest approach 

 to station. 



