Earthquakes and other earth movements

by John Milne

A conclusion that we are led to as one result of these valuable investigations is, that if the interior of the earth be fluid, and covered with a thin shell, then enormous elastic tides must be produced. A consequent phenomenon, dependent on the existence of these tides, would be a marked regularity in the occurrence of earthquakes. As this marked regularity does not exist, we must conclude that earthquakes are not due to the attractive influences of the sun and moon acting upon the thin crust of the earth covering a fluid interior. The periodicity of earthquakes corroborates the conclusions of Sir William Thomson, who remarks that if the earth were not extremely rigid the enormous elastic tides which must result would be sufficient to lift the waters of the ocean up and down so that the oceanic tide would be obliterated.

Assuming that the earth has the rigidity assigned to it by mathematical and physical investigators, we nevertheless have travelling round our earth, following the attractions of the moon and sun, a tidal stress. This stress, imposed upon an area in a critical state, may cause it to give way, and thus be the origin of an earthquake. Earthquakes ought therefore to be more numerous when these stresses are the greatest.

The periods of maximum stress or greatest pull exerted by the moon and sun will occur when these bodies are nearest to our planet—that is, in perigee and perihelion, and again when they are acting in conjunction or at the syzygies. That earthquakes are slightly more numerous at these particular periods than at others is a strong reason for believing that the attractions of the moon and sun enter into the list of causes producing these phenomena.

Had there been a strongly marked distinction in the number of earthquakes occurring at these particular seasons as compared with others, we might have attributed earthquakes to the existence of elastic tides of a sensible magnitude. As the facts stand, it appears that the maximum pulls exerted by the moon and sun are only sufficient to cause a slight preponderance in the number of earthquakes felt at particular seasons, and therefore that these pulls only result in earthquakes when the distorting effort has been exerted on an area which, by volcanic evisceration, the pressure of included gases, and other causes, is on the verge of yielding.

Earthquakes and the tides.—If we assume that earthquakes are in many cases due to the overloading of an area and its consequent fracture, such loading may occur by the rising of the tide. A belief that the earthquakes of Japan were attributable to the tides may be found in the diary of Richard Cocks under the date November 7, 1618, who remarks:—

‘And, as we retorned, about ten aclock, hapned a greate earthquake, which caused many people to run out of their howses. And about the lyke hower the night following hapned an other, this countrey being much subject to them. And that which is comunely markd, they allwais hapen at a hie water (or full sea); so it is thought it chauseth per reason is much wind blowen into hollow caves under ground at a loe water, and the sea flowing in after, and stoping the passage out, causeth these earthquakes, to fynd passage or vent for the wind shut up.’[131]

Although we may not acquiesce in Cock’s views respecting the imprisoned wind, it would seem that a comparison of the occurrence of earthquakes and the state of the tide would be a legitimate research. Inasmuch as the stresses which are brought to bear upon an area by the rising of the tide are so very much greater than those due to barometrical changes, it is not unlikely that a marked connection would be found. But it must be remembered that because researches, so far as they have gone, tend to show that earth movements are more frequent when an area is relieved of a load, it is not unlikely that the greatest number of earthquakes may be found to occur at low water. Prof. W. S. Chaplin attempted to make this investigation in Japan, but not being able to obtain the necessary information respecting the tides, was compelled to relinquish this interesting work.

Every foot of rise in a tide is equivalent to a load being placed on the area over which the tide takes place of sixty-two pounds to the square foot. This load is not evenly distributed, but stops abruptly at a coast line. Lastly, it may be observed that many coast lines are not simultaneously subjected to stresses consequent upon this load. Japan, for instance, may be regarded as an arch placed horizontally. The area near the crown of this arch is loaded by the tidal wave crossing the Pacific before the areas near the abutment, and farther there is a horizontal pressure at the crown which, if Japan were like a raft, would tend, as the tide advanced, to straighten its bow-like form, but as the wave passed its abutments to increase its curvature.

Prof. G. Darwin has calculated the amount of rise and fall of a shore line due to tidal loads (see [p. 336], ‘Earth Pulsations’). The result of these calculations apparently indicates that these loads may have a considerable influence upon the stability of an area in a more or less critical condition.

Mr. J. Carruthers suggests that tidal action may hold a general but indirect relationship to volcanic and seismic action by the retardation it causes on the earth’s rotation. By this retardation the polar axis tends to lengthen, and tensile stresses are induced, resulting in fracture. The fluid interior of the earth, being no longer restrained, would move polewards, and, leaving equatorial portions unsupported, this would gradually collapse. The primary fractures would be north and south, while the secondary fractures would be east and west.[132]