Earthquakes and other earth movements

by John Milne

2. The position of the moon with regard to the earth, which in every twenty-seven days is once near and once distant.

3. The phases of the moon—whether full or new moon (syzygies), or whether first or last quarter (quadratures).

4. The equinoxes, the position of the sun in the equator, and the relative position of the earth.

5. The position of the moon relative to the equator.

6. The concurrence of the ‘centrifugal force’ of the earth with the last quarter of the moon.

7. The entrance of the moon on the ecliptic—the so-called nodes.

Assuming that earthquakes are wholly consequent on these attractions, it at once becomes possible to predict their occurrence. This Falb does, and when his predictions have been fulfilled he has certainly gained notoriety.

He commenced by the predictions of great storms. In 1873 he predicted the destructive earthquake of Belluno, which earned for himself a eulogistic poem, which he has republished in his ‘Gedanken und Studien über Vulkanismus.’ After this, in 1874, he predicted the eruption of Etna. He also explained why, in b.c. 4000, there should have been a great flood, and for a.d. 6400 he predicts a repetition of such an occurrence.

When we approach the question of the extent to which the attraction of the sun and moon may influence the production of earthquakes, a question which we have to answer is, whether it is likely that the attractive power of the moon is so great that it could draw up the crust the earth beyond its elastic limits. We know what it can do with water. It can lift up a hemispherical shell 8,000 miles in diameter about two or three feet higher at its crown than it lifts the earth. Even supposing the solid crust to be lifted 100 times the apparent rise of the tide, is it likely that a hemispherical arch 8,000 miles in diameter when it is raised 200 feet at its crown could by any possibility suffer fracture? If an arch is 12,000 miles in length, all that we here ask is, whether the materials which compose the arch are sufficiently elastic to allow themselves to be so far stretched that the crown may be raised 200 feet. The result which we should arrive at is apparently so obvious that actual calculation seems hardly necessary. If we regard the earth as being solid, the question resolves itself into the inquiry as to whether a column of rock, which is equal in length to the diameter of the earth, or about 8,000 miles, can be elongated 200 feet without a fracture. This is equivalent to asking whether a piece of rock one yard in length can be stretched one seventy thousandth of a foot. Considering that this is a quantity which is scarcely appreciable under the most powerful of our microscopes, we must also regard this as a question which it is hardly necessary to enter into calculations about before giving it an answer. To vary the method of treating such a question, may we not ask what is the utmost limit to which it would be possible to raise up or stretch the crust of the earth without danger of a fracture? Thus, for instance, to what extent might a column of rock be elongated without danger of its being broken? From what we know of the tenacity of materials like brick and their moduli of elasticity, it would seem possible to stretch a bar of rock 8,000 miles in length for approximately half a mile before expecting it to break. As to whether there is a wave, the height of which is equal to half this quantity, running round our earth as successive portions of its surface pass beneath the attracting influences of the sun and moon, is a phenomenon which, if it exists, would probably long ago have met with a practical demonstration.

The deformation which a solid globe or spherical shell would experience under the attractive influences of the sun and moon has been investigated by Lamé, Thomson, Darwin, and other physicists and mathematicians.