70 



Symposium on Microseisms 



mentary strata prevailing throughout the 

 central United States. 



Considering the great amplitude differ- 

 ences found for the different frequencies of 

 motion, it is apparent that a "flat" seismograph 

 response curve is useless for studying the mi- 

 croseismic spectrum over a wide frequency 

 range. In such a case, the limiting factor is 

 the dynamic range of the recorder, which will 

 seldom exceed 40 db (100:1), whereas the phe- 

 nomenon to be observed has an amplitude range 

 of at least 60 db (1000:1). Under these con- 

 ditions, analysis of the seismogram by any 

 method will not reveal useful information on 

 the short period components — such informa- 

 tion being irretrievably lost in recording. 

 Two approaches are possible in obtaining broad 

 band microseismic coverage. One is a method 

 used here, employing the selectivity character- 

 istics of low frequency filters; or equivalently, 

 using tuned detectors. The second approach is 

 to use a seismograph response characteristic 

 that is nearly the inverse of the microseismic 

 spectrum. In this case, the product of ground 

 motion and seismograph magnification will be 

 nearly constant, and all ground frequency com- 

 ponents will be registered equally well on the 



recorder. Fourier analysis or autocorrelation 

 methods may then be used to obtain true 

 ground amplitudes over a wide frequency 

 range. _ It may be observed that the short peri- 

 od Benioff seismograph when critically damped; 

 has nearly the correct characteristics for the 

 spectra discussed here, since the Benioff re- 

 sponse decreases nearly as the cube of the 

 period for periods longer than one half second. 

 Mechanical seismographs, or seismographs 

 with capacitance — bridge transducers will not 

 have as great a microseismic band coverage, be- 

 cause their response decreases as only the 

 square of the period on the long period side of 

 the peak magnification. 



Conclusions from the foregoing are that 

 the microseismic spectrum at Pinewoods is es- 

 sentially continuous in the range from Vfc to 5 

 seconds, increasing at a rate about proportional 

 to the third or fourth power of the period, de- 

 pending to some extent on meteorological con- 

 ditions over the adjacent regions on the North 

 Atlantic Ocean ; that the ocean is the source of 

 at least part of the short period microseismic 

 activity; and that the shorter period micro- 

 seisms suffer a more rapid attenuation due to 

 distance traveled than the longer periods. It 



Figure 4. Microseismic Spectra for Pinewoods, 21 August 1952. Hurricane at nearest approach 

 to station. 



