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On the Diurnal Periodicity of Earthquakes. [Mar. 5. 



out that a marked diurnal periodicity characterises the after- shocks 

 of great earthquakes in Japan. 



The results of twenty-six registers obtained by means of con- 

 tinuously recording instruments in Japan, the Philippine Islands, 

 and Italy are subjected to harmonic analysis with the following 

 conclusions : — 



(1) The reality of the diurnal variation of earthquake -frequency 

 seems to be proved by the approximate agreement in epoch (mean 

 local time) of the first four components (24, 12, 8, and 6 hours) for 

 the whole year at Tokio and Manila, and for the winter and summer 

 halves of the year at Tokio. 



(2) In ordinary earthquakes, there is in nearly every case a 

 marked diurnal period, the maximum generally occurring between 

 10 a.m. and noon. The semi-diurnal period, though less prominent, 

 is also clearly marked, the maximum occurring as a rule between 

 9 A.M. and noon and between 9 p.m. and midnight. Other minor 

 harmonic components are also occasionally important, the first maxi- 

 mum of the eight-hour component probably occurring about 6.30 a.m., 

 and that of the six-hour component about 3 or 4 A.M. ; but for these 

 two epochs the results are not always concordant. 



(3) Though the materials are insufficient for any general conclu- 

 sion, the weaker shocks seem to be subject to a more marked diurnai 

 periodicity. 



(4) In the case of after-shocks of great earthquakes, the diurnal 

 periodicity is as a rule strongly pronounced. The maximum of the 

 diurnal period occurs within a few hours after midnight, but the 

 epochs of the other components are subject to wide variation, possibly 

 on account of the short intervals over which the records extend. A 

 special feature of after- shocks is the prominence of the eight-hour 

 and four-hour components. 



The epochs of the first four components representing the diurnal 

 variation of seismic frequency are compared in several cases with 

 those for barometric pressure and wind velocity. While the variation 

 of the former cannot be attributed exclusively to either of the latter 

 phenomena, it seems not improbable that the diurnal periodicity of 

 ordinary earthquakes may be due chiefly to that of wind velocity, and 

 the diurnal periodicity of after-shocks chiefly to that of barometric 

 pressure. 



