202 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1934 



may conceivably be brought about by secondary causes outside the 

 solid earth. It is not surprising, therefore, that many investigators 

 have endeavored to trace connections between the occurrence of 

 earthquakes and other natural phenomena. There have been nu- 

 merous attempts to find periodicities and regular variations in the 

 frequency of earthquakes, but so far the only two that appear to be 

 well established are the solar diurnal and annual variations found 

 by Davison. It is found that on the whole earthquakes are more 

 frequent during the night than during the day, and more frequent 

 in winter than in summer. A possible explanation, suggested by 

 Davison and by Omori, is that these variations are due to the annual 

 and diurnal changes in atmospheric pressure. The 11-year period 

 has been examined, and it appears that in all parts of the world 

 earthquakes are more frequent in the years of many sun spots. Davi- 

 son also investigated a 19-year period, and in this case the maximum 

 frequencies of the Northern Hemisphere correspond with the mini- 

 mum frequencies of the Southern Hemisphere; the period is the 

 same as the nutation period of the earth, and it appears probable 

 that the strains associated with the movements of the earth's axis 

 are factors in determining when earthquakes shall occur. 



A large earthquake is usually followed by a series of after-shocks, 

 the daily numbers of which at first decline very rapidly. In a few 

 days, however, the frequency curve flattens out but, as was demon- 

 strated by Omori, it shows fluctuations which occur at intervals of 

 a few days. Davison has examined these periodicities recently and 

 finds that they are related to the phases of the moon. The occurrence 

 of after-shocks was also investigated by Turner, who neticed that 

 they appeared to recur at intervals of multiples of 21 minutes; the 

 reality of this period, however, cannot be said to be definitely 

 established. 



MICROSEISMS 



Although seismology is usually considered to be concerned only 

 with earthquakes in the ordinary sense of the word, it also includes 

 other shakings of the earth which are not due to fractures or sudden 

 disturbances. At nearly every seismological station the records 

 frequently show continuous oscillatory movements known as 

 microseisms. The oscillations are much greater in winter than in 

 summer, and they appear to be associated in some way with atmo- 

 spheric storms; in fact, in some parts of the world attempts have 

 been made to use them as a means of forecasting the approach of 

 stormy weather. The exact nature of the connection is still in 

 doubt. According to one theory microseisms are due to sea waves 

 breaking on exposed coasts, but from a recent survey of microseismic 

 disturbance during 1 month all over the world A. W. Lee has come 



