. Compendium of meteorology. Meteorology. 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 structu
. Compendium of meteorology. Meteorology. 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 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 shght motion between the groups. T
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