The Subterranean World

by G. Hartwig

But beyond these limits slighter earthquakes are of far more common occurrence than is generally supposed, and probably they leave no part of the world entirely undisturbed. From the year 1821 to 1830 no less than 115 earthquakes have been felt to the north of the Alps, and since the year 1089, 225 are cited in the annals of England. Some of these earthquakes seem to have but just stopped at the point when a slight increase of their force would have covered the land with ruins. In 1574, on the 26th of February, between five and six in the evening, an earthquake was felt at York, Worcester, Hereford, Gloucester, and Bristol. Norton Chapel was filled with worshippers; they were nearly all overthrown, and fled in terror, thinking that the dead were unearthed or that the chapel was falling. Six years later, on the 6th of April, at 6 P.M., all England was thrown into consternation. The great bell at Westminster began to toll; the students at the Temple started up from table and rushed into the street, knives in hand; a part of the Temple Church fell, and stones dropped from St. Paul’s. Two stones fell in Christ’s Church, and crushed two persons. In rushing out of the church many were lamed, and there was a shower of chimneys in the streets. At Sandwich, the occurrence was marked by the violence of the sea, which made ships run foul of each other; and at Dover a part of the fortifications fell with the rock which supported it.

On the 6th October 1863, a movement, though gentle when compared to the preceding instances, was felt from the English Channel to the Mersey, and from Hereford to Leamington and Oxford. The Malvern range was about the centre of the area, as it has often been before. Even in alluvial Holland, six or eight slight earthquakes have been felt during the last century. The industrious researches of Kluge show that, during the eight years from 1850 to 1857, no less than 4,620 earthquakes—a great proportion of which (509) fell to the share of Southern Europe—have been noticed in both hemispheres; and when we remember that a very considerable part of the globe is still either totally unknown or removed by the barbarous condition of its inhabitants from all intercourse with the scientific world, and that, consequently, the above list must necessarily be incomplete, it is very probable that not a day passes without some agitation of the surface of the earth in some place or other.

A violent earthquake almost always consists of several shocks following each other in rapid succession. Sometimes they are preceded by slighter vibrations; at other times they suddenly convulse the land without any previous notice. In most instances, each shock lasts but a few seconds; but this is enough to ruin the work of ages. Three violent commotions within five minutes destroyed the town of Caraccas on March 26, 1812; and the earthquake which, in 1692, desolated Jamaica lasted but three minutes. On January 11, 1839, two shocks within thirty seconds covered Martinique, and the whole range of the Lesser Antilles, with ruins. But a violent earthquake, though itself but of short duration, is generally followed by a series of secondary shocks, which are repeated at gradually widening intervals and with decreasing energy, so that if these subsequent tremors be taken into account, it may often be said to last for weeks, or even months. Thus, to cite but one instance, the earthquake of October 21, 1766, destroyed the whole town of Cumana in a few minutes, but during the following fourteen months the earth was in a constant vibratory motion, and scarce an hour passed without a shock being felt.

In countries where earthquakes are comparatively rare (for instance in the south of Europe) the belief is very general that oppressive heat, stillness of the air, and a misty horizon are always forerunners of the phenomenon. But this popular opinion is not confirmed by the experience of trustworthy observers, who have lived for years in countries such as Cumana, Quito, Peru, and Chili, where the ground trembles frequently and violently. Humboldt experienced earthquakes during every state of the weather, serene and dry, rainy and stormy.

Brute animals, being more sensitive than men of the slightest movement of the earth, are said to evince extraordinary alarm, and it has been often observed that even the dull hog shows symptoms of uneasiness previous to the shock. During the great Neapolitan earthquake of 1857 an unusual halo-like light was seen in the sky before, and not long before, the shock. Mr. Mallet was at first inclined to look upon this notion as a superstitious tale; but, finding it widely diffused in a country where communication is bad and news travels slowly, no longer doubted that it was founded on fact.[^[10]] Conjectures would be useless as to its nature, but future observation directed to the point may determine whether some sort of auroral light may emanate from the vast depths of rock formation under the enormous tensions and compressions that must precede the final crash and rupture; or whether volcanic action, going on in the unseen depths below, may give rise to powerful disturbances of electric equilibrium, and hence to the development of light; just as from volcanic mountains in eruption lightnings continually flash from the huge volumes of steam and floating ashes above the crater. Humboldt is also of opinion that, though in general the revolutions which take place below the surface of the earth are not announced beforehand by any meteorological process, or a peculiar appearance of the sky, it is not improbable that during violent shocks some change may occur in the condition of the atmosphere. Thus, during the earthquake in the Piedmontese valleys of Pelis and Clusson, great alterations were observed in the electrical tension of the atmosphere without any appearance of a thunder-storm.

Earthquakes are generally attended with sounds, sometimes like the howling of a storm, or the rumbling of subterranean thunder, at others like the clashing of iron chains, or as if a number of heavily laden waggons were rolling rapidly over the pavement, or as if enormous masses of glass were suddenly shivered to pieces. As solid bodies are excellent conductors of sound (burnt clay, for instance, propagating it ten or twelve times more rapidly than the air), the subterranean noise may be heard at a vast distance from the primary seat of the earthquake. In Caraccas, in the grass-plains of Calabozo, and on the banks of the Rio Apure, which falls into the Orinoco, a dreadful thunder-like sound was everywhere heard on April 30, 1812, without any simultaneous trembling of the earth, at the time when, at the distance of 158 geographical miles, the volcano of St. Vincent, in the Lesser Antilles, was pouring out of its crater a mighty lava-stream. This was, according to distance, as if an eruption of Vesuvius were heard in the north of France. In the year 1744, during the great eruption of the volcano Cotopaxi, a subterranean noise like the firing of cannon was heard at Honda on the Magdalena river. But the crater of Cotopaxi is 17,000 feet higher than Honda, and both points, situated at a distance of 109 geographical miles, are moreover separated by the colossal mountain masses of Quito, Pasto, and Popayan, and by numberless ravines and deep valleys. The sound was certainly transmitted, not through the air, but through the earth, and must have proceeded from a very considerable depth.

But noise is not the necessary attendant of an earthquake, for many instances are known in which the most violent shocks have been completely noiseless. No subterranean sounds were heard during the terrific earthquake which destroyed Riobamba on February 4, 1797, and the same circumstance is mentioned in the narratives of many of the Chilian earthquakes.

The phenomenon of sound, when unaccompanied by any perceptible vibration, makes a peculiarly deep impression on the mind, even of those who have long inhabited a country subject to frequent earthquakes. They tremble at the idea of the catastrophe which may follow. A remarkable instance of a long protracted noise without any trembling of the earth occurred in 1784, at the wealthy mining town of Guanaxuato in Mexico, where the rolling of subterranean thunder, with now and then a louder crash, was heard for more than a month, without the slightest shock, either on the surface of the earth, or in the neighbouring silver mines, which are 1,500 feet deep. The noise was confined to a small space, so that a few miles from the town it was no longer audible. Never before had this phenomenon been known to occur in the Mexican highlands, nor has it been repeated since.

Earthquake shocks are either vertical or undulatory. A vertical shock, which is felt immediately above the seat or focus of the subterranean disturbance, causes a movement up and down. Like an exploding mine, it frequently jerks movable bodies high up into the air. Thus, during the great earthquake of Riobamba, the bodies of many of the inhabitants were thrown upon the hill of La Culla, which rises to the height of several hundred feet at the other side of the Lican torrent; and during the earthquake of Chili in 1837, a large mast, planted thirty feet deep in the ground at Fort San Carlos, and propped with iron bars, was thrown upwards, so that a round hole remained behind.

Although to the inhabitants of a shaken district the undulatory wave or vibration of an earthquake appears to radiate horizontally outwards from the spot on the surface where it is first felt, the force does not really operate in a horizontal direction, like a wave caused by a pebble on the surface of a pond; for at every point, except that immediately above the focus of the shock, it comes up obliquely from below, causing the ground to move forwards and then backwards in a, more or less horizontal direction. As a ship, yielding to the oscillatory movements of the waves, alternately inclines to one side or the other, so, during the more violent undulations of the soil, the objects on its surface are momentarily moved from their vertical position, and often considerably inclined towards the horizon. Thus during the great earthquake which convulsed the valley of the Mississippi[Mississippi] in 1811–12, Mr. Bringier, an engineer of New Orleans, who was on horseback near New Madrid, where some of the severest shocks were experienced, saw the trees bend as the wave-motion of the earthquake passed under them, and immediately afterwards recover their position. The transit of the wave through the woods was marked by the crash of countless branches, first heard on one side and then on the other. It must have been awful to see the giants of the forest thus move to and fro like a corn-field agitated by the wind!