PRACTICAL APPLICATION OF MICROSEISMS TO FORECASTING 
By JAMES B. MACELWANE, S. J. 
Saint Louis University 
The term microseisms applies to all elastic wave 
systems which are propagated along the surface of the 
earth and which, on the one hand, are not caused by 
earthquakes and, on the other hand, are not purely 
local man-made disturbances due to traffic or industrial 
activity. We may also exclude from present considera- 
tion the more or less irregular local vibrations produced 
by natural causes, such as varying pressure of the wind 
on a particular structure, wind friction on a landscape, 
freezing and thawing of the ground, jerky movements 
due to cooling of structures or of hills or other 
topographic features, although these are often referred 
to in the literature as a class of microseisms. We are 
thus restricting our attention to elastic surface waves 
which are propagated in the earth’s crust over appre- 
ciable distances. These microseisms seem to be of 
different types and to appear in discontinuous 
frequency bands rather than in a continuous spectrum. 
The type of microseisms to which most study has 
been devoted since the time of Bertelli is characterized 
by wave periods which, in the majority of cases, lie 
somewhere between four seconds and seven seconds, 
that is, by frequencies between 140 and 250 milliherz 
(1 herz equals 1 cycle per second). The reason for this 
emphasis is not far to seek. These frequencies fall 
within the optimum response range of most of the 
SRO ROSSI 
earthquake seismographs in common use and hence the 
microseisms of this type appear as a more or less dis- 
turbing background on most of the earthquake records. 
Some connection between this type of microseisms 
and meteorological conditions in general had been noted 
by many workers beginning with Bertelli. These micro- 
seisms are regular in wave form and appear in a 
succession of groups of a few large waves each with 
short intervals of slight motion between the groups. 
They do not appear at all times but in discrete 
sequences which may last for a perod of hours or days, 
building up to a maximum and dying down again. 
Such a sequence has come to be known as a microseismic 
storm (Fig. 1). 
Four principal theories have arisen concerning the 
nature and origin of this type of microseisms. The first 
group of theories related them to the meteorological 
and geological conditions surrounding the recording 
station. The rise and fall of amplitude was attributed 
by some to the simultaneous arrival of vibrations of 
slightly different frequencies so as to produce an effect 
of beats. The second theory related the microseisms 
to steep barometric gradients on land or sea. A third 
theory, championed principally by Wiechert and other 
scientists of the Géttingen school, and particularly by 
Gutenberg through a period of three decades, ascribed 
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WA Caters 
1312 
