all this was merely coincidental or whether a reason for it can be found. 



At once the question arises: Have other great earthquakes shown any 

 relationship to the phases of the moon and sun? The answer appears to 

 be in the affirmative. Figure 29, which is a pictorial representation 

 of the plane of the ecliptic and the orbit of the earth around the sun, 

 shows the relative positions of earth, moon and sun on the occasion of 

 seven of the largest earthquakes the earth has known. In four of these 

 cases (Lisbon, November 1, 1755; Mino-Owari , Japan, October 28, I89I; 

 San Francisco, April I8, I906; Alaska, March 27, I96U), the moon was in 

 opposition, at syzygy (opposite side of the earth from the sun). Of 

 the remaining three (Assam, India, June 12, l897; Chile, May 22, I96O; 

 Yakutat, Alaska, September 10, l899), the first two were on occasions of 

 new moon with the moon in conjunction, at syzygy. Of the limited niomber 

 of cases thus examined the single exception to the syzygy position of the 

 moon at the time of earthquake was the occasion of Yakutat, Alaska, 

 September 10, l899, but even here the moon is only about three days past 

 its syzygy position of conjunction. Moreover, this M - 8.6 earthquake 

 was preceded by one of magnitude M = 8.3 in almost the same area, just 

 six days earlier on September k, l899 , when the moon would have been 

 about three days ahead of its syzygy position. One might suppose that 

 the approach to syzygy of the moon triggered the first quake and relieved 

 enough of the strain to delay the second and prevent what might otherwise 

 have been a monstrous earthquake when the moon was exactly in conjunction. 



This sampling of cases implies that earth tides are indeed a factor 

 in the causation of earthquakes.* Zetler (1966) has briefly discussed this 

 question, and referred to the work of Allen (1936) and Knopoff (196U). 

 Knopoff failed to find any significant correlation between times of earth- 

 quake occurrence in California and earth tidal potential. Allen, however, 

 had warned of the inefficacy of correlations of global events that paid 

 no attention to the local nature of faults and earth stresses. Zetler 

 appears to concede that insufficient attention has really been given to 

 the horizontal component of the tide-producing force in work of this kind. 



We ourselves (Wilson, et al, I962), had drawn attention to an obser- 

 vation of the Japanese, as reported by Davison (1936), that the highest 

 frequencies of occurrence of aftershocks following the great earthquake 

 of Mino-Owari, in 189I, were timed at new and full moon, resulting in 

 notable recurrence periods of 2k hours, li+.8 days and 29.6 -days. Davison 

 (1936) also remarked that a similar effect on aftershock activity had 

 followed the Calabrian earthquake of 1783, in Italy. Clearly in these 

 cases, lunisolar tide-generating forces must have been instrumental in 

 promoting strain-release in the aftershock adjustments. 



Recently, Ryall , Van Wormer and Jones (1967) have established a 

 definitive correlation between aftershock activity (following the 

 Truckee, California, earthquake of September 12, 1966), and earth tide 

 cycles of about 2k and 12 1/2 hours, so that the question of lunisolar 

 influence upon earthquakes no longer appears in doubt. 



* See note on Figure 29, page k2. 



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