As might be expected from the movable nature of water, the wave-motion of earthquakes is frequently propagated to surprising distances over the sea. The Chilian earthquake of 1835 produced oscillations of the ocean that made themselves felt on the Sandwich Islands at a distance of 5,000 nautical miles. On Mauai, the sea retreated 120 feet, and then suddenly returned with a tremendous wave that swept away the trees and houses on the beach. In Hawaii, a large congregation had assembled for divine service near Byron’s Bay. Suddenly the water began to sink, so that soon a great part of the harbour was laid dry. The spectators hurried to the shore to admire the astonishing spectacle, when a wave, rising twenty feet above the usual tide-mark, inundated the land, destroyed sixty-six huts, and drowned eleven of the islanders, though the best swimmers in the world. So far from its starting-point did the South American earthquake seek its victims. Fifteen hours and a half after the great earthquake of Arica (1868) the water-wave undulating over the vast Pacific was felt at Chatham Islands, a distance of 6,300 miles, and an hour later at New Zealand.
The enormous powers which come into action during a great earthquake show themselves not only in the destruction of edifices and the wide-spread ruin so produced, but in the changes which they effect in the configuration of the soil. Wherever masses of earth rest loosely upon a sloped surface of subjacent rock, or where steep mountain crests overlie wet and unctuous beds of shale, or where the rock itself is composed of coherent material, or where river-banks are formed of precipitous masses of clays, or where the corroding waters have undermined the ground, the violent commotion caused by an earthquake cannot fail to produce landslips, fissures, and falls of rock. In 1571, on the 17th of February, the ground opened all at once at the ‘Wonder,’ near Putley, not far from Marcle in Herefordshire; and a large part of the sloping surface of the hill—twenty-six acres, it is said—descended with the trees and sheep-folds, and continued in motion from Saturday to Monday, masses of ground being turned round through half a circle in their descent. This was a great landslip, said to have been occasioned by an earthquake.
Earth-fissures were formerly supposed to be occasioned by a stretching of the ground, occasioned by the wavy nature of the shocks; but Mr. Mallet has shown that no earthquake wave can possibly produce any such stretching, and considers them as cases of small and incipient landslips caused by the shaking downwards of a loose mass. His own observations left no doubt in his mind that the descriptions, given by the Neapolitan Academy in their Historical Account of the Earthquake of 1783, of the earth-fissures therein produced, and designated constantly by the pompous term ‘voragines,’ are gross exaggerations, and that the well-known Jamaica earth-fissures, that were said to have opened and closed with the wave, and to have bitten people in two, must be regarded as audacious fables.
‘The vulgar mind, filled from infancy with superstitious terrors as to “the things under the earth,” is seized at once by the notion of these fissures of profound and fabulous depth with fire and vapour of smoke issuing from within their murky abysses; but they should cease to belong to science.’
Enormous landslips are sometimes occasioned by earthquakes, but their extent depends less upon the power and energy of the shock than upon the conditions of unstable equilibrium presented by great masses of loose material, through the configuration of the country. In consequence of landslips or dislodgements of large masses of rock, alterations in the flow or distribution of the waters frequently take place. Thus, brooks or rivers are not seldom dammed, and temporary ponds or lakes created.
Permanent elevations of the land have been observed after some earthquakes. Thus, after the violent shocks of November 19, 1822, a great part of the coast of Chili was found to be raised several feet above its previous level, and after the great earthquake which occurred in New Zealand in the night of January 23, 1855, a large tract of land was found to be permanently upraised from one to nine feet. Before the shock there had been no room to pass between the sea and the base of a perpendicular cliff called Muka-Muka, except for a short time at low water, and the herdsmen were obliged to wait for low tide in order to drive their cattle past the cliff. But immediately after the upheaval, a gently sloping raised beach, more than 100 feet wide, was laid dry, affording ample space at all states of the tide for the passage of man and beast.
These permanent elevations have often been attributed to the immediate agency of earthquakes; but Mr. Mallet proves this assumption to be a fallacy, as the impulse of the earthquake wave even right above the focus is utterly incapable of raising the level of the land by a height much more than instrumentally appreciable, and there is not the least evidence that any part of even this elevation is permanent. That earthquakes occur along with, and as part of, a train of other circumstances which do produce permanent elevation occasionally, and that earthquakes are probably always the signals that the forces producing elevation are operative, is another matter, with which that erroneous or loosely expressed view should not be confounded.
The causes of earthquakes are still hidden in obscurity, and probably will ever remain so, as these violent convulsions originate at depths far below the reach of human observation. Mr. Mallet came to the conclusion that the depth of the original Calabrian shock in 1857 did not exceed seven or eight miles, and deduces from all the facts known as to the movements of earthquakes, that the subterranean points where the shocks originate perhaps never exceed thirty geographical miles, so that, even supposing the central nucleus of the earth to be fluid, they cannot possibly be due to the reaction of the internal ocean of molten stone upon the solid shell with which it is enveloped, but must have their seat within the latter. The existence of reservoirs of fused matter at various depths in the solid earth-rind is quite sufficient to account for all seismic and volcanic phenomena; for it is evident that whenever rain-water, or the waters of the sea percolating through rocks, gain access to these subterranean lakes of molten stone, steam must be generated, the pressure of which will in many cases rend and dislocate the incumbent masses.
‘During such movements,’ says Sir Charles Lyell, ‘fissures may be formed and injected with gaseous or fluid matter, which may sometimes fail to reach the surface, while at other times it may be expelled through volcanic vents, stufas, and hot springs. When the strain on the rocks has caused them to split, or the roofs of pre-existing fissures or caverns have been made to fall in, vibratory jars will be produced and propagated in all directions, like waves of sound through the crust of the earth, with varying velocity, according to the violence of the original shock, and the density or elasticity of the substance through which they pass. They will travel, for example, faster through granite than through limestone, and more rapidly through the latter than through wet clay, but the rate will be uniform through the same homogeneous medium.’
According to Mr. Poulett Scrope, the originating cause of the earthquake must be sought in the expansion of some deeply seated mass of mineral matter, owing to augmentation of temperature or diminution of pressure. By this expansive force, the solid rocks above are suddenly rent asunder, and whether below the sea or not, their violent disruption produces a jarring vibration, which is propagated on either side through their continuous masses in undulatory pulsations.