Earthquakes and other earth movements

by John Milne

Modern views respecting the cause of earthquakes—Earthquakes due to faulting—To explosions of steam—To volcanic evisceration—To chemical degradation—Attractive influence of the heavenly bodies—The effect of oceanic tides—Variation in atmospheric pressure—Fluctuation in temperature—Winds and earthquakes—Rain and earthquakes—Conclusion.

As the results of modern inquiries respecting the cause of earthquakes, we see many investigators chiefly attributing these phenomena to special causes. A few attribute them to several causes. It seems to us that they might be attributed to very many causes which often act in a complex manner. The primary causes are telluric heats, solar heat, and variations in gravitating influences. These may be the principal, and sometimes the immediate, cause of an earthquake. The secondary causes are those dependent upon the primary causes, such as expansions and contractions of the earth’s crust, variations in temperature, barometrical pressure, rain, wind, the attractive influences of the sun and moon in producing tides in the ocean or the earth’s crust, variations in the distribution of stress upon the earth’s surface caused by processes of degradation, the alterations in the position of isogeothermal surfaces, &c.

The part which may be played by these various causes in the production of oscillations, pulsations, and tremors will be referred to.

Earthquakes consequent on faulting.—In the chapter on Earth Oscillations, the causes producing the phenomena of elevation and depression are briefly indicated.

By the variations in stress accompanying elevations and depressions, cracks are produced. Inasmuch as compression would crush the rocks constituting the earth’s crust, we must conclude with Captain Dutton that these cracks are formed by tension. By elevation, the upper rigid crust of the earth is stretched, and fissures are produced. The sudden formation of these fissures or faults gives rise to earthquakes, and perhaps also to volcanic vents. That earthquake and volcanic regions are situated on areas where there is evidence of rapid elevation is strikingly illustrated round the shores of the Pacific.

Lasaulx considered that the earthquake of Herzogenrath was more or less intimately connected with the great mountain fissure—the Feldbiss—which crosses the coal region of the Wurm.[126] The sudden elevation or sinking of large areas at the time of an earthquake may be a consequence of these dislocations.

It has already been pointed out that the earthquake region of Japan is the one where we have evidence of recent and rapid elevation. That certain earthquakes of this region may possibly be the result of faulting we have the evidence of our senses and of our instruments. The sudden blows and jolts which are sometimes felt are indicative of the sliding of one mass of rock across another.

Should the ground be simply torn asunder, this tearing would give rise to a series of waves of distortion, vibrating in directions parallel to the plane of the fissure. Supposing this motion to be propagated to a number of surrounding stations, it would be recorded at each of these as having the same direction. To those situated on a line forming a continuation of the strike of the fissure, the vibrations would advance so to speak end on, whilst to those stations lying in a line perpendicular to the strike of the fissure, the motion would advance broadside on.

Motions like these latter have been recorded in Tokio, where earthquakes which from time observations were known to have come from the faulted and rising region to the south have been registered as a series of east and west motions, or vibrations transverse to this line of propagation.

It must, however, be here mentioned that the registration of only transverse motion may possibly be due to the extinction of normal motion, although this is not generally regarded as probable.