On Lunar Periodicities in Earthquake Frequency. 465 



The most important of these is the fact that the nodical month, 

 which has no direct connexion with tidal stress periodicity, is 

 characterised by harmonic amplitudes greater, on the average, than 

 those corresponding to the other months. This is particularly 

 evident in the graphs. 



There are, however, other features which favour the hypothesis of 

 seismic tidal stress, such as the occurrence in the vicinity of perigee 

 of the Anomalistic 1st Harmonic amplitude ; the lagging, by one 

 day, behind full and new moon of the Synodic 2nd Harmonic maxima ; 

 the distinctly greater amplitude of the Synodic 2nd Harmonic as 

 compared with those of the other harmonics a fact which is in 

 accord with the fortnightly succession of spring tides. 



It is, certainly, a striking fact that the same statistics which, when 

 grouped according to an approximately twenty-eight days' period, give 

 a prominent 1st harmonic should, when grouped according to an 

 approximately thirty days' period, give a comparatively small 1st 

 harmonic but a prominent 2nd harmonic. 



4. General Conclusions. The conclusions are summarised under 

 ight heads. 



(a) There is evidence that the earthquake frequency in Japan is 

 subject to a periodicity associated with the lunar day. 



(6) The lunar half-daily period is particularly in evidence, both by 

 reason of its relative prominence and the regularity with 

 which, in each of two groups of the several seismic districts, 

 its phase falls in relation to the time of meridian passage of 

 the moon. 



(c) There is no certain evidence that the loading and unloading 



due to the flow and ebb of ocean tides have any effect on 

 seismic frequency. 



(d) Hence we must look to the direct tidal stress of the moon, in 



its daily change, as the most probable cause of a range in 

 frequency which does not exceed 6 per cent, of the average 

 frequency. 



(e) There is distinct evidence, both as regards amplitude and 



phase, of a fortnightly periodicity associated with the times 

 of conjunction and opposition of the sun and moon. 

 {/) No definite conclusion can be drawn from the apparent 

 monthly and fortnightly periodicities which seem to be 

 associated with the periodic changes in the moon's distance 

 and decimation, for the simple reason that fully as pro- 

 minent harmonic components exist when the statistics are 

 analysed according to the periodic change in the moon's 

 position relative to the ecliptic, and with this particular 

 period no tidal stresses can be directly associated. 



VOL. LX. 2 N 



