Very remarkable displacements of objects are not seldom caused by earthquakes, such as the rotation of the blocks of columns or the turning of statues on their pedestals.
At Lima, which, owing to its repeated destructions by earthquakes, is properly a city of ruins, Professor Dana saw two obelisks with the upper stone on each displaced and turned round on its axis about fifteen degrees in a direction from north to east. These rotations by earthquakes have been attributed by some authors to an actual rotatory movement in the earthquake vibration; but it has lately been shown by Mr. Mallet that this hypothesis is untenable and unnecessary, as a simple vibration back and forth is all that is required to produce a rotatory motion in the stone of a column, provided that stone be attached below more strongly on one side of the centre than on the opposite.
The wave-motion of an earthquake sometimes spreads over enormous spaces. The shocks of the earthquake of New Granada which took place in the night from the 16th to the 17th of June 1826, were noticed over a surface of 750,000 square miles. The earthquake of Valdivia (February 20, 1835) was felt southwards on the distant island of Chiloe to the north as far as Copiapo, in Mendoza to the east of the Andes, and on the Island of Juan Fernandez, 300 miles from the coast. Supposing these effects to have taken place at corresponding distances in Europe, all the land would have trembled from the North Sea to the Mediterranean, and from Ireland to the centre of France.
It is evident that the extent and force of the wave-motion of an earthquake must in a great measure depend upon the nature of the rocks through which it is transmitted. It will vibrate more easily through solid homogeneous masses, while in alluvial deposits, or in a soil composed of sand and loose conglomerate, its undulations will be propagated irregularly and its effects be far more destructive. This is particularly the case where the alluvial deposits repose on a substratum of hard rock. Thus the devastations of the Calabrian earthquake of 1783 were most apparent in the plain of Oppido, in those parts where the newer tertiary strata rest upon granite. The earthquake wave generally follows the direction of mountain-chains, and but rarely crosses them. The great Chilian earthquakes, which often propagate their vibrations to distances of many hundred miles along the western foot of the Andes, remain unfelt on their eastern border; while the earthquakes along the shores of Venezuela, Caraccas, and New Granada rarely transmit their vibrations beyond the high mountain-chains which run parallel with the coast. This is probably due to the numerous dislocations, rents, and caverns which are produced by the elevation of the mountain-chains, and necessarily serve as barriers to the propagation of the earthquake wave.
Severe earthquakes are not seldom accompanied by a violent agitation of the sea. First, at the instant of the shock, the water swells high up on the beach with a gentle motion, and then as quietly recedes; secondly, some time afterwards, the whole body of the sea retires from the coast, and then returns in waves of overwhelming force. The first movement seems to be an immediate consequence of the earthquake affecting differently a fluid and a solid, so that their respective levels are slightly deranged; but the second is a far more important phenomenon. ‘Some authors,’ says Mr. Darwin, ‘have attempted to explain it by supposing that the sea retains its level, while the land oscillates upwards; but surely the water close to the land, even on a rather steep coast, would partake of the motion of the bottom; moreover, similar movements of the sea have occurred at islands far distant from the chief line of disturbance. I suspect (but the subject is a very obscure one) that a wave, however produced, first draws the water from the shore on which it is advancing to break. I have observed that this happens with the little waves from the paddles of a steamboat. From the great wave not immediately following the earthquake, but sometimes after the interval of even half-an-hour, and from distant islands being affected similarly with the coasts near the focus of the disturbance, it appears that the wave first rises in the offing, and, as this is of general occurrence, the cause must be general. I suspect we must look to the line where the less disturbed waters of the deep ocean join the water nearer the coast which has partaken of the movements of the land, as the place where the great wave is first generated; it would also appear that the wave is larger or smaller according to the extent of shoal water which has been agitated together with the bottom on which it rested.’
The following examples sufficiently prove that no storm, however violent, is capable of raising such prodigious waves as an earthquake.
In the year 1692 the town of Kingston in Jamaica was almost totally destroyed by a huge earthquake wave. A frigate which lay in port was carried forwards over the houses and stranded in the middle of the town. In his ‘Principles of Geology,’ Sir Charles Lyell relates that, during the Calabrian earthquake of 1783 the Prince of Scilla had persuaded a great part of his vassals to betake themselves to their fishing boats for safety, and he himself had gone on board. On the night of February 5, when some of the people were sleeping in the boats, and others on a level plain slightly elevated above the sea, the earth rocked and large masses of rock were thrown down with a dreadful crash upon the plain. Immediately afterwards the sea, rising more than twenty feet above the level of this low tract, rolled foaming over it and swept away the multitude. It then retreated, but soon rushed back again with greater violence, bringing back with it some of the bodies it had carried away. At the same time every boat was sunk or dashed against the beach, and some of them were swept far inland. The aged prince was killed, with 1,430 of his people.
After the earthquake which devastated the town of Lima on the 28th of October 1746, the sea rose on the evening of the same day eighty feet above its usual level in the neighbouring Bay of Callao, overwhelmed the town, and destroyed nearly all the inhabitants. Of the twenty-three ships which were lying in the harbour at the time, nineteen immediately sank, while the four others were thrown upon the land at the distance of nearly a league.
Shortly after the shock which desolated Chili on the 20th of February 1835, a great wave was seen from the distance of three or four miles, approaching in the middle of the Bay of Talcahuano with a smooth outline, but tearing up cottages and trees along the shore, as it swept onwards with irresistible force. At the head of the bay it broke in a fearful line of white breakers, which rushed up to a height of twenty-three vertical feet above the highest spring tides. Their force must have been prodigious, for at the Fort a cannon with its carriage, estimated at four tons in weight, was moved fifteen feet inwards. The whole coast was strewed over with timber and furniture, as if a thousand ships had been wrecked. As Mr. Darwin walked along the shore, he observed that numerous fragments of rock, which, from the marine productions adhering to them, must recently have been lying in deep water, had been cast up high on the beach. One of these was six feet long, three broad, and two thick.
During the dreadful earthquake which in 1868 raised the strip of land at the western foot of the Andes from Iburra in Ecuador, to Iquique in Peru, 1,200 miles in length, the receding sea uncovered the bay at Iquique to the depth of four fathoms, and then, returning in an immense wave, a mass of dark blue water, forty feet high, rushed over the already ruined city, and swept away every trace of what had been a town. One spectator, seeing the whole surface of the sea rise like a mountain, ran for his life to the Pampa. The waves overtook him. Fighting with the dark water, amidst wreck and ruin of every kind, carried back into the bay, and again thrown back to the Pampa, wounded and half-naked, he crept for safety into a hole of the sand, and waited sadly for the dawn. At Arica, the British Vice-Consul, alarmed at the first shock, rushed out of the house with his family, and made for the high ground, in just terror of the expected sea-wave. Through the ruined town, amidst dead and dying, half stifled with dust, they reached rising ground, and, looking back, saw a dreadful sequel—the sea rushed in and left not a vestige[vestige] remaining of the lower part of Arica. Six vessels were lost in the bay or tossed over rocks and houses; an American gunboat was whirled away from her moorings, and laid, without a broken spar or tarnished flag, high and dry on the sand-hills, a quarter of a mile from the sea.