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 eight heads.
(a) There is evidence that the earthquake frequency in Japan is subject to a periodicity associated with the lunar day.
(b) 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.
(f) 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 declination, for the simple reason that fully as prominent 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 particularperiod no tidal stresses can be directly associated.
