Earthquakes

Originally published in Saint Pauls (Virtue & Co.) vol.3 #16 (Jan 1869).

Within the last few years a change has passed over the views entertained by men of science respecting those vibrations of the earth's crust which,—insignificant as they. must be considered in relation to the dimensions of our globe,—constitute yet the most stupendous and alarming phenomena known to the human race. The skill with which physical causes have been analysed, and the critical acumen with which historical records have been discussed, have sufficed to dispel much of that mystery and misconception with which the subject of earthquakes had for a long time been surrounded. We now know not only what really takes place when a portion of the earth's crust is affected by an earthquake, but also what are the purposes subserved in the earth's economy by a phenomenon which seems, at first sight, simply destructive. We proceed to discuss briefly some of the more prominent results of modern researches into the history, character, and work of the earth's subterranean forces.
        In reasoning respecting the action of the earth's internal forces in long-past periods, geologists formerly fell into the mistake of denying to nature the fulness of time which she requires to work out her ends. As Sir Charles Lyell remarks, they "represented nature as having been prodigal of violence and parsimonious of time." They busied themselves in inquiring what might be the nature of forces which could produce, by a single effort, those various formations which are the study of the geologist. They naturally came to the conclusion that the subterranean action in remote ages surpassed enormously the corresponding effects of internal heat in our own times. The instantaneous upheaval of mountain-chains such as the Andes and the Himalayas could only be effected by the action of paroxysmal explosions, in comparison with which the most violent throes of modern earthquakes sink into absolute insignificance. And even without leaving our own country, we can find traces of subterranean action which,—if we could suppose it to have operated instantaneously, or for but a brief period,—must have incomparably exceeded even such eruptive explosions as those which caused the great earthquakes of Riobamba, Lisbon, and Calabria. "We have 'faults' in England," says Sir Charles Lyell, "in which the vertical displacement is between 600 and 3,000 feet, and the horizontal extent thirty miles or more, the width of the fissures, since filled up with rubbish, varying from 10 to 50 feet." The upthrow or downthrow of such enormous masses of rock,—if produced instantaneously,—"would imply tremendous catastrophes," he remarks, "wholly foreign to the ordinary course of nature."
        It is important that the bearing of the error into which geologists had been led by a want of care in the examination of the phenomena presented to their notice should be thoroughly understood. So long as it was held that the old manifestations of the powers of nature exceeded those presented in our own day, or within historical times, in the enormous proportion inferred by ancient geologists, we were prevented from applying to the examination of the earth's strata any of the experience which we derive from observing the effects of recent processes of change. The geologist could hardly form more exact notions of the former state of our earth than the astronomer can form of the condition of her companion planets. It was in vain that the effects of long-past processes were brought under the very hammer of the geologist when he was free to ascribe these effects to causes wholly dissimilar from those now in operation.
        Therefore, the first and most important step towards our present acquaintance with the laws regulating the action of the earth's subterranean forces, was the discovery that in ancient geologic eras precisely the same forms of disturbance were in action as are going on at the present day. It will hardly be believed that the old views respecting geologic evidence,—the seemingly irrational theories of catastrophes and cataclysms indulged in by the rival factions of the Vulcanists and the Neptunists,—held their ground up to quite recent times. In fact, few circumstances seem more promising to those who take interest in the progress of geology than the fact that the science has made these enormous strides during the last sixty years, after so many centuries of inaction or of misdirected efforts. "The cultivation of geology," says one of the most eminent modern geologists, "began at a later period than that of astronomy; and in every step which it has hitherto made towards sound theoretical principles it has had to contend against many violent prepossessions. But when we compare the result of observations in the last half century with those of the three preceding centuries, we cannot but look forward with the most sanguine expectations to the degree of excellence to which geology may be carried even by the labours of the present generation."
        The careful examination of the effects of ancient subterranean action having at length forced geologists to the conclusion that the condition of the globe in long-past eras corresponded much more closely to its present state than they had been disposed to imagine, it was seen, for the first time, that in the various phenomena presented by the earth's crust, we have exhibited to us, as in a vast museum, specimens of the effects we may attribute to modern subterranean action. Hence the modern seismologist, instead of confining his attention to the effects of earthquake shocks occurring jn our own day, is able to analyse all the different stages of disturbance by examining the effects of processes which have been in action for thousands upon thousands of years. The aid thus afforded him it is almost impossible to over-estimate. In fact, without this aid our views respecting the action of the earth's subterranean forces must necessarily have been altogether imperfect; since, in any single age, these forces produce no effects sufficiently imposing to afford a distinct conception of the extent and importance of the subterranean action. "We must consider geognostic phenomena," says Humboldt, "in their true correlation, and observe them in their transition from a merely dynamic concussion, and from the upheaving of continents and mountain masses, to the production and effusion of gases and liquids, of boiling mud, and of red-hot and molten earths, which harden into crystalline rocks." It is only by duly comparing the geologic records of past action with what takes place in our own day, that seismologists have been enabled to arrange side by side groups of phenomena which, at first sight, seem so utterly heterogeneous,—to form, as Humboldt expresses it, "a grand picture of nature, in which all these phenomena become fused into the single conception of the reaction of the interior of a planet upon its crust and surface." "To the inquiring spirit," he adds, "it is given to mount from link to link in the chain of phenomena, till the point is gained at which, in the ancient consolidation of our planet, in the first transition of the conglobated matter from the vaporous form, the internal heat of the earth,—that heat which is not due to the sun's action,—was originally developed."
        Equally important towards the formation of the modern theory of earthquakes has been the intelligent examination of their dynamical effects. The labours of Mallet and Hopkins were the first by which light was thrown on the obscure and seemingly complex relations presented by earthquake phenomena.
        In examining the effects of an earthquake shock, we must remember that usually three constituents of the earth's globe are affected,—viz., first, the solid mass lying above the seat of the shock; secondly, the ocean lying above this solid mass; and thirdly, the superincumbent atmosphere. Let us consider these three constituents separately.
        In the first place, then, a shock is communicated to a portion more or less extensive of the earth's solid crust. In considering its effects we must dismiss from our minds the ordinary conception of the earth's rigidity. The earth is only rigid—in appearance even—when considered in comparatively small masses. Large portions of the earth's crust are "elastic," and partake of that property which belongs to all elastic bodies—the power of transmitting waves of disturbance. Such waves, when originated by an earthquake shock, travel vertically as well as horizontally, but much farther horizontally than vertically. We can tell little of the rate of propagation of the vertical wave, because the distance it has to travel before reaching the surface is so short. Respecting the velocity of the horizontal wave, we are better able to judge. It has been estimated that on an average the land-wave transmitted horizontally travels at the rate of about 2,000 feet per second, or nearly twenty-three miles per minute. If we could watch this wave, we should see it rushing on with this enormous velocity, and presenting precisely the appearance of low, broad waves at sea. Any part of the wave, considered separately, may be looked upon as travelling forward in a straight line. But the complete wavefront forms a circle, ellipse, or else some irregular oval figure, constantly expanding, around the centre of concussion. The ordinary conceptions about vorticose motions, despite the evidence which Humboldt collected in their favour, must be abandoned as untenable. Every well-authenticated fact in the history of earthquakes may be referred to the action either of the vertical wave or of the horizontal one. Strictly speaking, even these two waves must be looked upon as belonging to a single globular wave of compression, of which the centre lies at the origin of the concussion.
        In connection with the wave-like motion of the earth's solid crust we must remark that there is also a wave of sound which travels four or five times as fast. It is to the passage of this wave that we must attribute the rumbling sounds commonly observed to precede the shocks of earthquakes.
        The second effect which has to be considered, is the transmission of a great sea-wave when the centre of concussion lies beneath the bed of the ocean. This wave will, of course, be transmitted according to the usual laws which regulate the movement of oceanic waves—in other words, its rate of motion will depend on the depth of the seas over which it passes. It will, therefore, travel independently of the solid wave, and may arrive at the shores of the various countries which experience the real earthquake shock at times which show no correspondence with the latter occurrence. We see at once, then, that this oceanic wave must be looked upon as a phenomenon wholly distinct from that recognised in many earthquakes in which the land-shock is accompanied by a recession of the sea, followed almost instantaneously by the inrush of a mountain-wave. The explanation of this phenomenon is one of the most interesting points of Mr. Mallet's inquiry.
        When the land-wave travels underneath the ocean, it carries on its back, to use Mr. Mallet's expressive verbiage, a great oceanic wave. Now a little consideration will show that the effect of this will be to cause a recession of the ocean just before—so nearly preceding as to appear coincident with it—the arrival of the land-wave upon any line of coast. For the mass of water upheaved above the solid wave must be drawn from the water in front of and behind that wave. The effect of this will clearly be, that just before the land-wave reaches a coast-line, the water which would have lain between the wave-front and the coast-line will all be drawn up—or "propelled" up, it matters not which view we take of the mode of its motion—to the top of the land-wave. Thus, for a moment, the shore will be left dry, but as the land-wave, with its superincumbent load of water, travels onwards, there will be seen the appearance of a mountain-wave of water pouring in upon the shore. We shall presently have to mention several remarkable instances of this phenomenon.
        The third wave is produced by the agitation of the atmosphere. Its effects are merely exhibited—where the wave consists of adequately swift vibrations—in the form of "sound," which "must be low in note, and must reach the ear considerably after the shock has been experienced,"
        Let us now consider a few instances of the particular forms of disturbance we have mentioned above.
        The great catastrophe which occurred in Ecuador and Pern on February 4, 1797, supplied evidence on several of the most interesting questions connected with the phenomena of earthquakes. The perpendicular, or explosive motion, was exhibited with remarkable distinctness at Riobamba. The bodies of many of those who perished were flung so forcibly into the air that they were afterwards found upon the summit of the hill La Culla, which rises to the height of several hundred feet on the farther side of the small river Lican. Humboldt saw among the skeletons of men on this height large quantities of rubbish from the stone buildings of Riobamba. Another circumstance which points to the action of a very energetic explosive force was the fact. that in several instances articles belonging to one house were found beneath the ruins of others at a considerable distance. We can only account for this remarkable circumstance by supposing that the latter buildings were flung high into the air, and that in the interval which elapsed before they reached the ground, the ruins of other houses were brought beneath them by a rapid horizontal movement.
The same great earthquake afforded very remarkable evidence of the nature of the great horizontal land-wave which travels outwards in all directions from the centre of concussion. It will be understood that this centre must have lain almost directly under the town of Riobamba, so that all other places affected by the earthquake must have suffered through the action of the horizontal or surface-wave. We can form some notion of the intensity of this wave of disturbance from the fact that every town and village within a region 120 miles long and 60 broad was levelled to the ground. Many places were actually buried beneath enormous masses flung down from the surrounding mountains. It is related that the form of the surface throughout this region was entirely altered. So remarkable were some of the effects as to suggest to an experienced observer like Humboldt the notion of vorticose or rotatory concussions,—a notion not justified, as we have seen, by the results of a more caroful consideration of all the phenomena presented during the progress of earthquakes. The appearances which deceived Humboldt may be accounted for very readily when we remember that variations in the thickness of the earth's crust would suffice to render the linear advance of any part of the surface-wave more or less irregular. Those appearances are very well worth noticing, however, as affording evidence of the singularly complicated effects which may be looked for from the simple progression of a great surface-wave along an irregular portion of the earth's crust. Walls were twisted round without being cast down; rows of trees which had been parallel, were found to have become deflected in a very singular manner; and, lastly, the ridges of cultivated fields were found to have assumed a new direction through the effects of the earthquake. About twenty minutes after the overthrow of Riobamba, a fearful grinding noise was heard under the cities of Quito and Ibarra, which lie upwards of a hundred miles from the centre of disturbance.
        We have seen how this remarkable earthquake illustrates the effect of the vertical and horizontal land-waves. But as the centre of concussion did not, in this instance, lie beneath the ocean, the great sea-wave considered by Mallet receives no illustration from the earthquake of Riobamba. We now proceed to consider some of the more remarkable earthquakes which have been accompanied by oceanic disturbances.
        We must premise that the great earthquake which has recently taken place in Ecuador and Peru would appear to have been followed by a remarkable oscillation of the sea. The waves of disturbance travelled even as far north as California; and the inhabitants of Callao were alarmed for the safety of their city by the continual inrush of gigantic waves. Yet, it must be remarked, that the seat of disturbance was not, in all probability, beneath the ocean in this instance. It is far more probable that the ocean-waves, large as they were, had their origin in the horizontal land-wave. As soon as this wave reached the shore, it must necessarily have communicated a portion of its motion to the ocean; and it serves to afford some conception of the vastness of the land-wave to notice that it had sufficient power to communicate a disturbance of such magnitude to the waters of the Pacific Ocean.
        It is possible that the same explanation may even be extended to the remarkable instance of oceanic disturbance observed when Callao was destroyed in 1747. Of twenty-three ships and vessels which were in the harbour of Callao, nineteen were sunk; and the other four were carried by the waves to a great distance inland, and left on dry ground at a considerable height above the sea. The same great wave which carried these ships so far from their element, destroyed every vestige of the town save a small fragment of the fort of Vera Cruz. Of 4,000 inhabitants, only 200 escaped. Indeed, one account of the catastrophe ascribes a yet more terrible destructiveness to the great wave. According to this account, "all the inhabitants of Callao were destroyed except one. The man who escaped, standing on a fort which overlooked the harbour, saw the sea retire to a great distance, and then return like a vast mountain. He heard a cry of 'Miserere!' rise from all parts of the city, and then in a moment all was silent,—where the town had once flourished, there was a wide sea. But the same wave which overwhelmed the town drove past him a small boat, in which he flung himself and thus escaped."
        Our reason for assuming that, in this instance, the centre of concussion was under the dry land, and not under the ocean, is the circumstance that the whole of Peru was shaken by the disturbance in a manner which seemed to indicate the close proximity of the centre of concussion. It is also related that a volcano burst forth in Lucanas, which poured forth such enormous quantities of water that the whole country was overflowed; and in the mountain near Pataz, called Conversiones de Casamarquilla, three other volcanoes broke out, and poured enormous torrents of water from their cones.
        In fact, it would seem as if Peru were specially liable to the occurrence of that particular form of disturbance of which we have had an instance in the recent earthquake. It is related by Ulloa that in 1687 a great convulsion took place, during which the sea first retired and then returned in a mountainous wave which destroyed Callao and its inhabitants. Wafer relates that the same wave drowned men and beasts for fifty leagues along the shore, and carried ships a league into the country. Ulloa, Wafer, d'Acosta, and other historians, describe other earthquakes which produced similar inundations along one part or another of the coast of Ecuador and Peru.
        But in all these instances the seat of disturbance lay in all probability beneath some part of the great range of the Andes. In order to obtain an example of sub-oceanic disturbance, we must consider the case of earthquakes whose chief effeets on land have taken place far from the neighbourhood of volcanoes or volcanic ranges.
        The great earthquake which overthrew Lisbon in 1755 affords a remarkable instance of the class of disturbance we are considering.
        The first shock of this great earthquake was the shock of the great land-wave. We shall presently see that this wave had run in from a point considerably to the west of Lisbon. By this shock the city was shaken to its foundations. The houses were swung to and fro with such violence that the upper stories were flung bodily to the ground, crushing thousands of the wretched inhabitants to death beneath the ruins of their own dwellings. Those who had escaped rushed to the great square in front of St. Paul's Church, hoping to find in this open space a refuge from the falling ruins of the city. But when they drew near, they were still more terrified by the awful scene of desolation which presented itself than they had been by the ruin of their homes. It was the festival of All Saints, and St, Paul's, like all the other churches, had been crowded with worshippers. But the great church had been flung bodily to the ground, and the immense multitude which had filled it had perished beneath its ruins. And now those who had escaped from the different churches were to be seen flying in crowds to the banks of the Tagus for safety. Others sought the heights of St. Catherine's Hill, but these were quickly destroyed; for a second land-wave followed soon after the first, and the church, which crowned the hill, was rocked violently to and fro until it fell, crushing beneath its ruins the great multitude which had been crowded around it.
        But now the great sea-wave was to work the destruction of thousands who had escaped from the falling ruins of the streets and churches of Lisbon. The banks of the Tagus were crowded with terrified wretches, whose "misereres" sounded like the dirge of the ruined city. Of a sudden, as they prayed, a strange heaving swell was seen to traverse the broad expanse of the river. The waters seemed as though they were being sucked away to meet an enormous wave which was sweeping in with inconceivable rapidity from the sea. Another moment and the whole multitude was engulfed. It is related,—with what amount of truth is not certainly known,—that of the vessels anchored in the river, of an enormous quay recently built at great expense, and of the multitudes which crowded quay and shores and vessels, not a single trace was ever afterwards discovered.
        But the end was not yet. A third land-wave rushed in upon the town, the shock it produced being scarcely less violent than the two preceding ones—and immediately after, the river was again traversed by an enormous wave. Then followed other shocks and other inrushes of the sea, producing effects so disastrous that "it was generally believed," said an eye-witness, "that the city of Lisbon was doomed to be entirely swept from the face of the earth."
        And now that the earthquake was over a new calamity, scarcely less terrible, befel the unfortunate inhabitants of Lisbon. The city was found to be on fire in a hundred places at once, the flames bursting forth with such fury that the whole city presented the appearance of a gigantic conflagration. It is asserted that subterranean fires, issuing from fissures in the earth, were partly the cause of this calamity. But on this point the evidence we have is not very clear. It is certain that flames and smoke issued from many non-volcanic mountains in Portugal during the earthquake; but there are no authentic records of flames having issued from fissures in plains or within the precincts of the city of Lisbon. It is for more probable that the conflagration was caused in another way. Tho day, as we have already mentioned, was one of the great festivals of the Catholic Church. The cathedrals, chapels, and churches were all illuminated with wax tapers and lamps, and as the curtains, waxen work, and timbers were overthrown with the burning lights, each sacred edifice became the centre of an extensive conflagration. In many of the private houses also, the kitchen and other fires had been flung over the woodwork of the floors. Thus on every side there was fire, while few were left to contend with the ravages of the destructive element. Six days elapsed before an end was finally put to the conflagration.
        We have been led by the contemplation of this great catastrophe,—one of the most sudden and fearful ever experienced by the human race,—to considerations somewhat removed from the subject we are dealing with. We must hasten to consider the evidence which the great earthquake affords us respecting the action of the earth's internal forces.
        The area of disturbance was remarkably wide. The shock was felt distinctly in the Alps, on the coast of Sweden, in Northern Germany, Thuringia, and in small inland lakes near the shores of the Baltic. In the Antigua Islands, Barbadoes, and Martinique, even in the great Canadian lakes, the motion was also sensibly felt. In Algeria and Morocco the agitation of the earth was scarcely less violent than in Portugal. Humboldt estimated that an area of the earth's surface four times greater than that of Europe was disturbed by this remarkable earthquake.
        Rivers and lakes in Great Britain were singularly agitated. The water of Lech Lomond, without any apparent cause, was flung violently against its banks and then sank below its usual level. On a comparison of the epochs at which the first shock was felt at Lisbon and the occurrence of shocks elsewhere, it was estimated by Michell that the movement of the ground travelled at the rate of twenty miles per minute. Bat it is necessary to note the possibility that this estimate may be exaggerated. In forming it, Michell looked upon Lisbon as the centre of disturbance,—in reality, the centre of disturbance lay far away from Lisbon, under the bed of the Atlantic.
        Of the extent of the oceanic disturbance directly caused by the passage of the land-wave we have already had striking evidence. We must note some other very remarkable circumstances. It is related that the great wave which traversed the shores of the Peninsula rose at Cadiz to the height of no less than sixty feet. At Tanjiers it rose and fell eighteen several times. "At Funchal in Madeira," writes Lyell, "it rose full fifteen feet perpendicularly above high-water mark, although the tide,—which ebbs and flows there seven feet,—was then at half ebb. Besides entering the city and committing great havoc, it overflowed other seaports in the island. At Kinsale, in Ireland, a body of water rushed into the harbour, whirled round several vessels, and poured into the market-place."
        The shock was felt in a peculiarly striking manner on the open sea. The captain of the ship "Nancy" records that off St. Lucar his vessel was shaken so violently that all on board thought she had struck upon a hidden reef. He ordered a seaman to heave the lead, but found they were in deep water. Captain Clarke also relates that his ship was shaken and strained as if she had struck upon a rock, "insomuch that the seams of the deck opened and the compass was overturned in the binnacle." Another ship, 120 miles west of St. Vincent, was struck so violently that the men "were pitched a foot and a half perpendicularly from the deck."
        It was noticed that not only at Lisbon but at the island of Madeira, and in several other places, the sea retreated from the shore before coming in as a great wave. It is at once seen that these facts are in perfect accordance with Mallet's theory; or may we not rather say, that no other theory is capable of adequately explaining them? As Sir Charles Lyell remarks, if the retreat of the sea had been caused by the simple upheaval of the coast of Portugal, "the motion of the waters when propagated to Madeira would have produced a wave previous to the retreat. Nor could the motion of the waters at Madeira have been caused by a different local earthquake; for the shock travelled from Lisbon to Madeira in two hours, which agrees with the time which it required to reach other places equally distant."
        We have said that the earth's crust is in general sufficiently elastic to transmit waves,—which, indeed, run along the surface almost as regularly as the waves of a disturbed ocean. But there are certain conditions which are opposed to the free transmission of the horizontal land-wave. Amongst these we may include sudden variations or irregularities in the thickness of the earth's crust, and also a want of homogeneity in the arrangement of the strata which compose it. Under these circumstances the wave cannot pass without fracturing the earth's crust. There are many instances of this occurring. In the great earthquake of Calabria in 1783, for example, the whole face of the earth was broken up by the waves of disturbance which repeatedly swept across it. The same sort of thing was witnessed in a yet more striking manner in the earthquake of Jamaica in 1692. We are told that "the ground swelled and heaved like a rolling sea, and was traversed by numerous cracks, two or three hundred of which were often seen at a time, opening and then closing rapidly again. Many people were swallowed up in these rents; some the earth caught by the middle and squeezed to death; the heads only of others appeared above the ground; and some wore first engulfed and then cast up again with great quantities of water."
        Nor must we omit to notice that the work of earthquakes does not terminate with the mere passage of a wave of disturbance across a large portion of the earth's surface. Even where no external difference appears in the face of a country after the occurrence of an earthquake we are yet justified in assuming that internal work has been effected. We have evidence that important changes may take place beneath the earth's crust without in any way affecting the external aspect of things. For example, it is related by Humboldt that since the earthquake of Cumana every shock of the southern coast is felt in the mica-slate strata of the peninsula of Maniguarez, which lies opposite the limestone or chalk hills of the fortress. He adds, that "in the almost incessant undulations of the ground of the valleys of the Mississippi, Arkansas, and Ohio, which occurred from 1811 to 1813, the progress of the motion from south to north was very striking. It was as if subterranean impediments had been gradually overcome, and the wave of commotion then advanced upon each occasion along the way which had been opened up." It appears, therefore, that the circles of concussion enlarge in consequence of the effects of a single powerful shock.
        But besides these internal effects, very marked external effects have been known to follow the occurrence of an earthquake. In the earthquakes which took place in Chili in 1822, 1835, and 1837, the earth was permanently elevated. After the earthquake of 1835, Admiral Fitzroy found beds of shell-fish in a putrid state, but still adhering to the rocks, and raised ten feet above high-water mark. But, remarkable as this fact is, we may note as still more striking the evidence of Mr. Darwin's researches respecting the length of time during which the process of elevation has been in action. He found shells precisely similar to those discovered by Mr. Fitzroy, but at a far higher elevation, near Valparaiso. These shell-beds were a quarter of a mile above high-water mark.
        We see, then, that besides phenomena of motion wholly dynamical, the earthquake is competent to effect important changes in the relations presented by the earth's surface. In fact, we may recognise in the work thus accomplished by earthquakes one of the principal restorative powers of nature. To quote the words of Sir Charles Lyell, "the constant repair of the land, and the subserviency of our planet to the support of terrestrial as well as aquatic species, are secured by the elevating and depressing power of causes acting in the interior of the earth, which, although so often the source of death and terror to the inhabitants of the globe,—visiting in succession every zone, and filling the earth with monuments of ruin and disorder,—are, nevertheless, the agents of a conservative principle above all others essential to the stability of the system."