Periodicity of Earthquakes. 



473 





Am pi. 



Epoch. 



Barometric Gradient. 



mm. 







per 112 miles. 



h m 







P.M. 



1st component (24 hours) ... 



•106 



50 



2nd „ (12 hours) ... 



•057 



10 5 



3rd „ ( 8 hours) ... 



•010 



5 3. 



4th „ ( 6 hours) .. 



•006 



4 



With regard to the after-shocks of Japanese earthquakes, 

 the records are of such short duration that satisfactory results 

 can only be obtained by a detailed comparison during the 

 intervals over which they extend. It is worthy of notice, 

 however, that the epoch of the first seismic component occurs 

 with some persistence early in the day, and does not differ very 

 greatly from that of the first component for barometric pressure. 



It seems evident, therefore, that we cannot attribute the 

 diurnal variation of seismic frequency exclusively to that of 

 barometric pressure or of wind-velocity. But it is not im- 

 probable that it may result from a combination of both 

 phenomena ; that the diurnal periodicity of ordinary earth- 

 quakes may be due chiefly to that of wind-velocity, and the 

 diurnal periodicity of after-shocks chiefly to that of baro- 

 metric pressure. In support of this suggestion, it may be 

 mentioned that the amplitudes of the first component of wind- 

 velocity and earthquake-frequency at Tokio are both greater 

 in summer than in winter, and that the same amplitudes for 

 the whole year are both greater at Manila than at Tokio. 



In the case of after-shocks, a reason maybe given for the 

 efficacy of barometric pressure if, as seems not improbable, 

 the principal earthquakes were due to the sudden elevation of 

 one rock-mass adjoining a fault. In a movement so abrupt 

 as this must be, the disturbed mass would almost certainly 

 overshoot its position of equilibrium, and immediately after- 

 wards would begin to settle back to it by a succession of 

 minor slips, at first numerous but gradually becoming less 

 frequent along the whole line of displacement. In such a 

 condition the elevated rock-mass would, in some part or other, 

 be constantly on the point of giving way, and a slight increase 

 of barometric pressure would probably be sufficient to pre- 

 cipitate a slip resulting in an after-shock. 



Imperial Meteorological Office at Tokio. The hourly means for the 

 Italian stations are given in Buchan's 'Atmospheric Circulation,' 

 pp. 17, 18. The hourly means of wind-velocity for Tokio are obtained 

 from the ' Report of the Meteorological Observations for the ten years 

 1876-1885 made at the Imperial Meteorological Observatory of Tokio ' ; 

 those for Manila from the ' Osservatorio Meteorologico de Manila, 

 Observaciones venjicadas ' (1890-9-S). 



