Page images
PDF
EPUB

CHAPTER II

CAUSES OF EARTHQUAKES

The Three Views-The Dislocation Theory-The Volcanic Theory-Views of Humboldt and Boussingault-The Tidal Theory-Perrey's Inquiries and Laws-The Statistical Method of Investigation-Milne's Catalogue in Japan-De Montessus de Ballore-Sources of Earth Stresses-The Contractional Hypothesis-The Ideas of Babbage and Herschel-Isostatic Equilibrium-Prof. George Darwin's Discussion of Stress-differences and their Distribution-The Relative Magnitudes

'HE originating or ultimate causes of earthquakes have

THE

been the subjects of controversy for more than a century. It would be curious and amusing, rather than useful, to go back into the eighteenth century to exhume buried and forgotten hypotheses put forth to explain these phenomena. They seem to indicate the fanciful childhood of the science which endeavours to deal with problems of earth physics. It will be more profitable to limit ourselves to those explanations which, though they may be more or less erroneous, are neither childish nor absurd.

During the first half of the nineteenth century these might be divided into three groups: Ist, those which attributed earthquakes to sudden downthrows or collapses of the ground; 2nd, those which attributed them to volcanic action; 3rd, those which attributed them to the

action of a liquid interior of the earth upon an external rocky crust under the disturbing influence of tidal forces. As none of these theories have been fully disposed of, and as all of them still have more or less vitality at the present time, we may in a preliminary way briefly advert to them.

The downthrow, or downfall, or Einsturztheorie has had for its advocates J. J. Scheuchzer, in his day an eminent Swiss geologist'; Boussingault,' a far more illustrious naturalist; Albert Necker, and G. H. Otto Volger. These authors were disposed to give the largest possible extension to the downfall theory. Such downthrows, instantly followed by great and far-reaching earthquakes, had been witnessed and verified in the calamities of Port Royal in Jamaica, 1692, in the terrible Calabrian quake of 1783, in the New Madrid disaster, 1811-12, in the Rann of Cutch near the mouth of the Indus, 1819, and in Murcia in 1829. The studies of Boussingault, Humboldt, and Darwin in the Chilian, Peruvian, and still more northerly Andes had established the occurrence of many others under similar conditions.

That a considerable number of the greatest and most

'Scheuchzer unfortunately is remembered at the present time chiefly through a blunder he made in describing the fossil bones of a large lizard as human remains, and as homo diluvii testis. But this does him a great injustice, for he

was an acute observer.

2" Sur les tremblements de terre des Andes." Annales de Chimie et Physique, vol. lviii., 1835.

3" On a Probable Cause of Certain Earthquakes." Phil. Magazine, vol. iv., 1839.

4" Untersuchungen über das Phänomen der Erdbeben in der Schweitz." Gotha, 1857-58, drei Bande.

The above references are taken from Dr. Rudolf Hoernes's Erdbebenkunde.

forcible of modern earthquakes were attribtuable to sudden displacements in the rocks seems to be well established. The chief error of the Einsturztheorie seems to have been the extreme lengths to which its advocates carried it. Not content with claiming its validity in those cases where the sudden downthrow was well attested and still open to the inquest of the observer, with the instantaneous sequence of the earthquake proven by unquestionable evidence, it was sought to apply the same action to quakes in which no evidences of downthrow could be pointed to. It was by many thought necessary to postulate large subterranean cavities into which great masses of overlying rock might drop without the least sign upon the surface that any such cavity had existed, or that any such drop had occurred.

The more recent views of downthrows in the outer shell of the earth do not require the pre-existence of large subterranean cavities into which great blocks or masses of the earth may suddenly fall. They are regarded rather as the results of unequal stress in the outer shell which have become so great that the rocks must yield to them along lines of least resistance as if they were plastic instead of rigid. The masses involved seek a readjustment or equilibrium of stress, and the motion may be upward in one locality and downward in an adjacent one, and probably also compounded with horizontal shifting in either case. At no time and at no place is any cavity supposed to exist any more than a cavity under the water surface is required to explain the sinking of the wave-crest into the wave-trough.

In those instances where the downthrow had surely occurred the inquiring mind was left in the utmost perplexity

to imagine how a vacuity large enough to receive the fallen mass could have existed. Caves near the surface, usually subterranean watercourses dissolved out of limestones, are about the only vacuities in the earth of which we have positive knowledge. But they are insignificant in size compared with vacuities which would be necessary to accommodate the masses involved in such downthrows as those of New Madrid, Murcia, or the Rann of Cutch. The deeper the imagination penetrates into the earth the greater becomes the pressure of the overlying rocks, and the greater the improbability of any cavity. The disputant who postulates them must be required to bear a heavy burden of proof. The downthrow theory cannot claim full acceptance beyond those instances where the evidence of the downthrow is patent to every eye upon the surface of the ground and when the instantaneous sequence of the earthquake is attested by satisfactory evidence.

One other error was made by the advocates of this theory, and that was the endeavour to discredit all other theories unduly, especially the volcanic theory. At that time (the first half of the nineteenth century) controversies among geologists and zoologists were very bitter. It seemed as though the odium theologicum had taken possession of them and generated an asperity surpassing even that of clashing creeds and political prejudice. The volcanic theory was a rival to their own, and with a bias that at times was almost passionate they were apparently disposed to deny it any but the most insignificant part in the category of causation.

The belief that earthquakes were associated with volcanic action, that they were caused by it, or were in some way dependent upon it, is as old as Aristotle. It appears in the works of Pliny, Strabo, and Pausanias, and was universal throughout the Middle Ages. These opinions were, however, tinctured with more or less superstition and not a little absurdity until near the beginning of the nineteenth century. The volcanic theory first took on its scientific form under the treatment of two of the most illustrious naturalists of that period, Leopold von Buch, and Alexander von Humboldt. Curiously enough, both of them were pupils of the Neptunist Werner, and if early education could have biased minds of such calibre it must have tended to array them against a Plutonic source of causation. Both of them, however, held to the view that the earth's interior was composed of molten matter, and that volcanic action was the

chief geotectonic agency. Humboldt, however, regarded volcanism as being the reaction between the hot interior and the cold exterior, and regarded volcanos as safetyvalves. So long as they remained open the interior forces. could not, he thought, accumulate to any dangerous extent in their immediate neighbourhood. But at a distance from the vent they might become more menacing. He seemed to be much impressed with an opinion which he found almost universal in the Andes among dwellers in the vicinity of volcanos, that so long as the crater continued to steam freely and to discharge regularly puffs of dust and lapilli there was no danger of serious developments. But if activity ceased at the upper cone there was reason for grave forebodings.

« PreviousContinue »