Popular Science Monthly/Volume 27/October 1885/A Study of Recent Earthquakes
SINCE communication between the extremities of the earth has become both easy and rapid, our ideas on many subjects have been modified and have become more precise. Facts that formerly appeared singular and extraordinary are recognized as frequent and habitual. This is the case with reference to earthquakes. Numerous telegrams, a few months ago, told in every part of the civilized world of the shocks from which Andalusia, in Spain, was suffering. To the astonishment and lively curiosity which these phenomena excited was added a deep emotion which disasters of so dramatic and painful a character would cause. The interest in the study of these things is also enhanced by the additional knowledge it gives us respecting the constitution of the crust of the earth—knowledge which, constantly increasing, enables us the better to comprehend the different parts of the mechanism of these subterranean perturbations.
Among the more recent earthquakes was the one that destroyed most of the Island of Scio. On the 3d of April, 1881, about an hour and forty minutes after noon, the city of Scio and thirty or forty villages in the southern part of the island were disturbed with a violent trepidation. The shaken and cracked houses were still standing, when, a few minutes afterward, a second shock, equally violent, came on, and finished the work of the first. With it five thousand persons were buried under the rubbish. A little while afterward four thousand other persons were killed. Hardly had the people recovered from the terror of one shock, than others came on, causing general panic and stupor. Hardly a quarter of an hour would pass without a new shock, and the wounded who had succeeded in extricating themselves from the rubbish were buried in it again. "Death," said an eye-witness, "seemed to pursue its victims with fury. In less than an hour Scio was an utter ruin." The agitations of the ground continued, with only short interruptions, for a year. During 1879 and 1880 Scio had suffered from frequent tremors, sometimes repeated as many as ten times in a day. Mitylene and Smyrna were also similarly affected, but none of the shocks were strong enough to cause great anxiety. They were, as it were, the subterranean preparation for the catastrophe that was to burst out a few months afterward.
The disaster that desolated the Island of Ischia two years afterward excited no less of emotion. The main shock, on the 28th of July, 1883, was accompanied by a fearful rumbling, which was estimated to last about twenty seconds. There was an extremely violent upward movement that broke up the houses, followed by an undulatory pulsation. The points most disturbed were aligned along the two deep fractures of strata that traverse the island at right angles to one another, crossing nearly under Casamicciola.
Less than a month after the shocks at Ischia followed the terrific explosion of the volcanic Island of Krakatoa, near Java, with all its unparalleled accompaniments: the planting of a deep sea where had been a mountain; the prodigious masses of pumice and stones from the volcanic throat causing intense darkness for hours at long distances; the finer particles scattering in the atmosphere and disturbing its transparency and causing the red lights for months; the marine waves propagated to the ends of the ocean with the speed of the tides; the aerial waves making the circuit of the globe, according to barometrical registrations, in two opposite directions; the thirty thousand human beings that perished; and the villages and cultivated lands which it blotted out—all caused a most vivid impression in all civilized lands.
Now it is Andalusia, one of the finest parts of Europe, that is struck with disasters. The shock that was felt on the 22d of December, 1884, on the western coasts of Spain and Portugal, and as far as the Azores and Madeira, seemed to be a forerunner of the one, of incomparably greater intensity, that took place three days afterward in another part of the Iberian Peninsula. At about nine o'clock in the evening of the 25th of December, the southern part of Andalusia was so roughly shaken that fifty-six towns and villages in the provinces of Malaga and Granada were devastated in less than ten seconds; and twenty of these places were nearly entirely destroyed. Among them were Albania, Arenas el Rey, Albunuelas, Periana, Zaffaraya, and Venta de Zaffaraya. These places, situated above the center of the agitation, are scattered over a surface of which the principal dimension does not reach forty miles; but the movements of the ground extended far beyond this region, to Seville on the west, Cape Gadez on the east, and Molena de Aragon. The movements provoked phenomena of different kinds. Crevasses, several miles long and several yards wide, were opened at various places. From one of them, near Santa Cruz, exhaled fetid gases, having the odor of sulphureted hydrogen, and there burst from the same fissure a copious spring of sulphurous water with a temperature of about 90º; while at a short distance from this point the thermal springs of Alhama, that have been in use from antiquity, were heated to a higher temperature and acquired a sulphurous character.
The districts near the Sierra Nevada and its ramifying spurs have frequently been the center of subterranean commotions; and it is an important fact, not to be neglected, that the shocks have many times, as in the present case, been repeated for several weeks in succession.
Among the movements which the ground undergoes during earthquakes, vertical shocks of great energy may sometimes be felt. During the earthquake in Calabria in 1783, houses were thrown up into the air, as if by the explosion of a mine; and at Riobamba, in Colombia, in 1812, several persons were cast bodily upon a hill more than three hundred feet high. These motions are called succussions, or subsultory tremors. The most frequent movements, and generally the most extended, are the undulatory ones, which are propagated horizontally, like the waves which we can observe at any time on a liquid surface. Like those waves, they may, when continued for some minutes, cause a kind of sea-sickness. Sometimes the terrestrial undulations are so strong as to bend over trees till their limbs touch the ground. These two forms of tremor may be associated together, or they may succeed one another at very short intervals. Various instruments—seismographs or seismometers, analyzers, and pendulums—are used to determine their intensity, direction, and duration, and register their characters.
The intensity of the shocks is extremely variable. Sometimes they are hardly perceptible, or marked only by low rumblings; often they are so strong that works of masonry are overthrown by them. For this reason special modes of construction are employed in countries subject to earthquakes, as adapted to oppose the least resistance.
In the most usual undulatory movement, the agitation is naturally stronger at the top of buildings than at their base. Thus, in the theatre at Madrid, on the 25th of December last, the upper gallery was visibly shaken, while the parquette was unmoved. For the same reason, the motions are incomparably less sensible in the interior of mines than on top of the ground. M. Domeyko relates that he was once at the bottom of the silver-mines of Charnacillo, more than six hundred feet underground, during an earthquake that destroyed his own house and several others right above him, while he did not feel the least agitation.
The duration of the shocks is generally very brief, sometimes not more than a second or two. The undulatory movements are more prolonged. A few instants are sufficient to produce the most disastrous effects. Three shocks, each of which was estimated to be not more than four seconds long, destroyed more than 20,000 persons in and around Carácas in March, 1812; and the convulsion at Pdobamba, in 1797, killed as suddenly 30,000 victims.
But, however overwhelming and disastrous in reference to our persons and buildings earthquakes may be, it must be borne in mind that the amplitude of their movements is wholly insignificant in proportion to the dimensions of the globe whose epidermis they shake. The phenomenon is rarely limited to a single shock. Generally, several shocks follow one upon another at short intervals. In many cases, the movements are repeated for months and years, with pauses of a variable duration, so as to form as a whole, till they are totally extinguished, what might be called a seismic period. After the shock which overthrew Thebes, on the 18th of August, 1851, the commotions continued in Bœotia for eleven months, occurring sometimes as often as three times in twenty-four hours. Long series of shocks disturbed a part of Scotland during the two years from the 20th of October, 1839, to the 7th of December, 1841. Hundreds of similar examples might be cited.
The chain of the Alps has furnished examples of seismic periods in many of its parts. Series of shocks were felt at Pignerol, in Piedmont, from the 2d of April, 1808, till the 17th of May following, during which time not a day passed but some movement was felt. Sometimes the tremors were noiseless, sometimes they were accompanied with commotions preceding the destruction of buildings. The commotions were renewed on the 26th of September, the 28th of October, and the 22d of November; and in the next year on the 13th of March and 26th of June. Similar periods of seismic action were observed in le Valais in 1755, on Lake Gardo in 1866, on Monte Baldo in 1868, and at Belluno in 1873. The present period in Andalusia is of the same kind. The prelude of the 23d of December, which disturbed a part of the Spanish Peninsula, the great earthquake of the 25th of December, and that long succession of shocks which still continued on the 9th of March with sufficient violence to cause new ruins, belong to the same series. As Humboldt has remarked, it is noteworthy that series of this kind are produced more especially in countries distant from volcanoes.
In the disturbed regions we may generally remark a tract of limited extent in which the movement is particularly energetic. It corresponds, without doubt, with the deep-seated center of impulsion, and has been called the epicenter. Here is where the vertical shocks or succussions are most usually felt. From these radiate the undulatory movements, the speed of propagation of which has been estimated at from 1,100 to 1,500 feet per second, or about that of sound in the air. Sometimes the area of disturbance is very limited, even when the convulsion is most violent; at other times it is very extensive, as was the case in the Lisbon earthquake of 1755, which affected a surface equivalent to one-thirteenth of that of the globe. Most frequently the linear dimensions of the agitated surfaces are included within from 65 to 600 miles, or from one four-hundredth to one fortieth of the circumference of the earth.
The area of disturbance is usually irregular in form, and not circular, as was once supposed. Sometimes it is much lengthened in one direction, following the lines of neighboring mountains or other important accidents of structure. During the earthquakes of 1783 and October, 1876, the chain of the Apennines served as a protecting wall to the eastern provinces of the Italian Peninsula. While one side of the chain was assailed by thousands of shocks, which caused great disasters, nothing whatever was felt on the opposite slope. In the Alps, most of the earthquakes take place in the lateral, north and south spurs, which are formed of sedimentary beds, while the central chain, composed of crystalline rocks, is not disturbed. The Andes of South America form a natural bulwark which the strongest convulsions of the Pacific littoral, while they extend a long distance parallel to the chain, hardly ever cross; and, if occasionally a few shocks are propagated beyond it, they become extremely weak.
The movements are very unequally perceptible within the area of disturbances; and between two points shaken by the same impulsion there may be intermediate points that continue quiet. These are sometimes called bridges or arches. The shocks are frequently accompanied by noises resembling heavily loaded wagons rolling over the pavement, or subterranean thunders or roarings; but their intensity bears no kind of proportion to that of the agitation. The great earthquake of Riobamba, in 1797, was silent. But the sounds have relation to the rocks that transmit them. Then there are subterranean rumblings that are associated with shocks like the bramidos of Guanajuato, in Mexico, in 1784, continuing for a month, under terror of which the inhabitants left the city. There were flashes as of lightning, alternating with long rollings, like that of distant thunder. This phenomenon gradually passed away. The noises associated with earthquakes seem to be of the same nature as those that accompany eruptions. The latter are propagated through the ground, not the air, for hundreds of miles. But nothing else that is known of this kind reaches the proportions of what took place on the 26th of August, 1883, at the eruption of Krakatoa, the sounds of which were heard within the whole area of a circle of 30º, or nearly 2,000 miles radius, or a diameter of one sixth the circumference of the globe.
The ocean basins are also disturbed, as is proved by the shocks suffered by ships on the deep, without any apparent external cause, and which give an impression as though the vessel were running upon a shoal. The movements of the littoral, also, however slight may be their intensity, are transmitted to the liquid mass. The sea retires from the shore, leaving the bottom dry, sometimes for several miles. Then it returns swiftly upon itself, and, overleaping its normal limit, precipitates itself with fury, and as if in assault, toward the interior of the country, as an enormous wave, which has been frequently known, as in Chili, to reach a height of 100 or 125 feet. Then it retires, carrying out upon the deep whatever it has gathered up in its passage. This terrible oscillation is repeated three or four times with decreasing energy, unless the movements of the ground persist. These invasions of the sea, or tidal-waves, are often more dreaded by the people, who have had experience of them, than the shocks on land. The huge wave* are also propagated in the ocean to a very great distance from the center of disturbance. Twelve hours after an earthquake that destroyed the city of Simoda, Japan, in December, 1854, a formidable wave was precipitated upon the Californian coast, 5,600 miles away. In 1868 a wave of similar origin destroyed Arequipa and Arica, Peru, and ingulfed 30,000 persons. It seemed to have come from Honolulu, in twelve homes, or with a speed of 450 miles an hour. The most striking example is that of the wave that followed the Krakatoa explosion, which traveled over a distance of 11,890 miles, or half-way round the globe, in twenty hours and fifty minutes, or at the rate, according to M. Bouquet de la Grye's estimate, of about 900 feet a second.
Earthquakes may also effect permanent changes in the relief of the land, not only in the shape of crevasses and the overturning of rocks; slight though appreciable elevations have also been observed, as in Chili, in 1822, 1835, and 1837. In the last case, marine shells, still alive and adhering to the rocks on which they had grown, appeared above the level of the sea, and served as indisputable witnesses of the change of level which had been suddenly produced.
Movements of another class are extremely weak, and can not be perceived without the aid of special and delicate instruments. In 1869 M. d'Abbadie, examining the surface of a mercurial bath in his observatory at Abbadia, discovered very slight but frequent variations in the situation of the vertical, from which he inferred that the ground is not always motionless, even when it has all the appearances of being so. The same fact has since been confirmed in many places. Abrupt oscillations that have been frequently perceived in the astronomical glasses at the observatory of Pulkowa, and were observed at Nice on the 27th of November, 1884, are also revelations of disturbances in the terrestrial crust. The French Academicians Bouguer and La Condamine had already come to a similar conclusion in 1741, when they were measuring zenith distances of stars. It could hardly have been anticipated that the observation of the stars would reveal processes that were going on down in the interior of our planet.
These movements, not directly perceptible by our senses, are subjected to an attentive daily study in Italy, at twenty-eight stations, scattered from one end of the peninsula to the other; and the results of the observations are centralized at the geodynamic observatory in Rome under the direction of M. Rossi. The movements are distinguished as very rapid and prolonged tremors (tremiti), and microseismic undulations, characterized by their extreme slowness. The observations of each day are depicted on a map of Italy by means of conventional signs, so that they may be followed in all their details as well as in the aggregate, at a single glance. Weak as these phenomena may be, they are well worthy of attention by reason of their continuous and general character; for they reveal an internal labor under the foundations of the ground, that never stops.
The crust of the earth also suffers displacements of a secular slowness, unaccompanied by any sudden movement; a class of phenomena which would never have been made known, if the mean level of the sea did not offer, at the shore, an invariable beach-mark by which to measure them. Tracts, which have manifestly been submerged within historical times, are now above the level of the sea, and constitute what are called raised beaches; while, on the other hand, forests, described in history as partly submarine, are now, in consequence of the depression of the soil, wholly under water. Such changes of level, very numerous and well established in all parts of the globe, are sometimes repeated in an oscillatory fashion of alternate elevations and depressions. They were formerly attributed to changes in the level of the sea, but the movement is in the land. They are in continuance of analogous changes which took place on vast scales during all the ancient geological periods. They are not to be confounded with superficial erosions and delta formations, which are quite different in character as well as in cause.
In view of these facts we are justified in saying that the crust of the earth is very far from being still. At every instant and in many of its parts it is undergoing very pronounced and often violent shocks. More frequently the movements are simply thrills, which can be discovered and studied only by a kind of auscultation. They are really continuous and of different kinds. It remains to inquire to what subterranean causes they should be attributed.
Numerous as the observations on earthquakes maybe, they concern merely the external manifestations of a phenomenon the source of which is completely hidden from us, separated by a considerable thickness of rocks. Hence we have no clear, certain data by which to support an explanatory hypothesis. The phenomena have been variously ascribed to subterranean electric storms, to the influence of the sun, supposed to be potent over the interior regimen of the planets as well as upon their course in their orbits; to thrusts of the liquid or semi-liquid masses of the interior against parts of the solid crusts, which may be caused by the same forces as produce the tides; to sudden reductions of atmospheric pressure; or to the fall of immense masses of rocks in vast interior cavities.
Numerous and exact studies, bringing into clear view the relations of earthquakes with the geological structure of the countries subject to them, have given us a better comprehension of their organic causes. An important fact, developed by patient statistical research, is the great inequality in the geographical distribution of the phenomena. There are vast regions in which they are very rare and feeble, and others where the agitations are frequent and often very violent. But it is a significant fact in this connection that the frequency of the disturbances is not so much associated with geographical position as with peculiar characters in the constitution of the crust of the earth. Thus, many earthquake regions are characterized by the presence of active volcanoes. A striking example of such association is presented in the narrow tract between the Andes and the Pacific Ocean, in South America, particularly in Colombia, Ecuador, and Chili. "On the coasts of Peru," says A. von Humboldt, "the sky is always clear; neither hail nor stones nor fierce lightnings are known; the subterranean thunder attending the earthquake-shocks takes the place of the thunder of the clouds. By long habit and the general opinion that only two or three destructive shocks are likely to occur in a hundred years, the people of Lima are but little more afraid of earthquakes than those of the temperate zones are of hail-storms." In this region, between the sixteenth and twenty-fourth degrees of latitude, there are eighteen volcanoes; Chili, eminently subject to earthquakes, has thirty-three active volcanoes, between 33º and 43º south. Very different conditions prevail east of the Cordilleras, where vast countries like Brazil have no earthquakes. Farther north, on the isthmus, there are regions where the shocks are so frequent that one of them has been called "Cuscuttan," or the hammock. The single state of Nicaragua has twenty-four volcanoes. Along the coast of Asia is a zone of volcanoes and earthquakes about 9,000 miles long. It begins at Barren Island in the Bay of Bengal, crosses Sumatra, Java, the Moluccas, and the Philippines, bends around by Formosa and the neighboring archipelagoes to Japan, and then to the Kurile Islands and Kamchatka, and ends at last in the Aleutian Islands. Through all this zone the volcanoes are numerous and active, and in some parts of it at least, as in Japan and the Philippine Islands, the earth is never at rest. The seismograph at Manila is always in motion, even when the ground seems still, and a year never passes without a severe shock. The connection between earthquake crises and volcanic crises is also shown by alternations in their activity. Every volcanic eruption is heralded by precursory tremors, whose violence is calmed down as soon as a volcanic outlet is opened for the escape of the vapors. Now, the vapor of water is the recognized cause of volcanic eruptions, and constitutes, in all parts of the earth, the most abundant and most constant emanation from them. It is the agent that throws out from the depths to the surface the lavas, which, despite their high temperature, hold it incorporated in their paste; in the same way that carbonic acid, dissolved in water, forces the liquid impetuously out of a mineral-water or a champagne bottle; and it also shoots quantities of solid matter, stones, lapilli, and cinders, violently into the atmosphere. It is logical to believe that this vapor is likewise the cause of the agitations that accompany volcanic crises. Agreeably to this idea, Kircher and Humboldt regarded volcanoes as safety-valves against earthquakes.
Other countries, again, where there are no volcanoes, are disturbed with no less energy and frequency, and that over great areas. Of such is the southern part of the basin of the Mediterranean. Syria with Palestine, Asia Minor, Turkey in Europe, Greece and the archipelagoes, Italy, Sicily, the southern part of the Iberian Peninsula, and a part of its western coast around Lisbon, have shown evidences of this predisposition within historical times. In each of these countries are districts or places that have been associated with most disastrous convulsions. In them we may discern a common and essential characteristic in the shape of a dislocation of the constituent strata, which is revealed for the most part in a mountainous relief.
In some whole countries the sedimentary strata, which form a notable part of the thickness of the earth's crust, have remained horizontal or nearly so, as they were deposited. In other countries, and over considerable areas, the corresponding strata are raised up, bent, and contorted in different ways, having been subjected to dislocations through enormous thicknesses. Such lifts and foldings can not have taken place in solid masses without being accompanied by many and important fractures. The principal classes of such fractures, which are nearly vertical, are called faults. They crop out and cut the surface of the ground, sometimes for tens and hundreds of miles, and are of indefinite depth, or descend to below where it is possible for man to penetrate; whenever a fault is produced, the two sides are displaced, and must rub hard upon one another; and vast rocky surfaces are thus engraved, striated, and polished: thence they are called mirrors in the language of the miners.
The occurrence of these facts is not confined to mountain-chains, but may be observed in countries that are marked by only slight prominences, but which have undergone similar actions through all their constituent strata. It is evident that the solid envelope of the globe has undergone dislocations at many epochs in its history. The manifestations of gigantic force thus plainly registered are the effects of ancient back-foldings and of lateral or horizontal pressures. It is as if the crust of the earth had become too large for the supporting nucleus, and had, to keep in contact with it, to shrink up and bend upon itself. These foldings and fractures have given origin to chains of mountains.
Now, the geological study of earthquakes has shown that their centers of impulsion are in relation with the ground-lines of fracture and dislocation. The disturbed bands are usually longitudinally parallel to the chains. A recent example of this linear disposition has been added in the latest earthquake in Andalusia, the major axis of which, according to M. Fouqué, is parallel to the mountain-crests of the province, as well as to the numerous faults that cut it up. Another important point to be noticed is that the countries in which the mountains have most recently acquired their latest relief are the ones in which these subterranean agitations are particularly frequent.
The Andalusian masses which have been so rudely disturbed within a few months partake of all the structural conditions that have just been noticed. The Sierra Nevada is among the youngest chains of mountains on the globe. The tertiary strata around it have been powerfully lifted up, sometimes to more than three thousand feet above the sea, without having their horizontality destroyed. According to M. de Botella there are also, in different places at the foot of the chain, strata regarded as quaternary that have been tilted into an inclination of 65º. Furthermore, numerous faults furrow the country, while the parts that have been most disturbed, according to Macpherson, are upon the faults that terminate the crystalline mass of the Sierra Tejea and Almijara. The numerous thermal springs of the region are further evidences of the deep fractures that traverse it.
Similar conditions, dislocations, and recent age, are found in other regions subject to subterranean perturbations. They appear notably in that part of the Mediterranean basin which we have spoken of as especially agitated, although it is distant from volcanoes; in the Apennines, the Lebanon, and the mountainous masses of Dalmatia and Croatia bordering on the Adriatic. The configuration of the northern coasts of that sea, so exceptionally slashed and cut by deep indentations, results from the complexity of the fractures that have determined the outlines of their principal features. Even the chain of the Alps, where shocks are felt nearly every year, acquired its final relief only at a comparatively recent epoch. It is conceivable under such conditions that the interior masses are not yet at equilibrium nor wholly subsided, and that they contain vacant spaces affording room for further sinkings.
According to what seems to be the dominant opinion of the day, there are two kinds of earthquakes: those which are due to volcanic actions, and of which the vapor of water is the prime mover; and those which are the effect of such ruptures of the equilibrium of the solid masses as we have just been considering. It is hard for the mind to admit two causes so different for phenomena which in most of their features so resemble one another. The demarkation between them is hard to define. On the western coast of South America and in Venezuela earthquakes facing volcanic ranges and those distant from such phenomena present the same manifestations. The supposition that the phenomena are due to the rubbing together of the solid parts of the crust also encounters a serious objection in the remarkable repetition of shocks during the same crises. In fact, this reiteration of shocks by the hundred and the thousand, for weeks and months, is one of the most characteristic accompaniments of earthquakes.
In view of these periods of disturbances, the cause, instead of being exhausted by a few shocks at short intervals, as would happen under the supposition that the first cause is the action of solid masses upon one another, should be one that could re-enforce itself after having been temporarily weakened. This is an essential fact, which any proposed solution must explain. We first remark that water, confined in a close space which it fills, will come, when it reaches a high enough temperature, to have a power which it is hard to represent in figures.
In nature, the tension of the vapor in the volcanic reservoirs is exhibiting its energy at every instant; for that which forces the lava out of the crater of Etna, about ten thousand feet above the sea, can not be less than one thousand atmospheres. The conditions necessary to give such tensions can not fail to be realized in the crust of the earth at a certain depth, beyond the domains of real volcanoes, and principally under chains of mountains and dislocated tracts. It is an ascertained fact that, whatever may be the constitution of the ground, the temperature increases regularly as we descend to a lower depth. At the same time water tends to descend, under the joint influence of gravity and capillarity, and may continue to descend till it reaches the deep and hot regions where it can acquire a temperature that will render it capable of producing great mechanical and chemical effects. Hence we can hardly doubt that waters from the surface reach the internal regions, and then make us feel in the shape of tremors and rumbling the power and explosive force which they gain there.
The depth at which the agency from which earthquakes originate should be sought has been the subject of careful studies. The results indicate that it is not seated in the central parts of the globe. This is the inevitable conclusion from such cases as the earthquakes in Calabria, in which the disturbed area was very small.
It is probable that the consolidation of the deep parts under the dislocated regions and especially under chains of mountains of a relatively recent age is not yet completed, and that there are still interstices and interior cavities of high temperature which eventually became filled with water by the action of capillarity. Hence we find the three conditions we have just been considering in the lower parts of the dislocated regions: cavities, water, and a high temperature, all constituting an agency capable of producing considerable dynamic effects at any moment. Suppose a barrel of powder exploded in a cavity situated a hundred metres underground. At the surface we would hear a rumbling explosion, and feel a vertical shock within a limited space, and an undulatory thrill over a wider circle. The phenomenon will be much like that of an earthquake, except that the essential element of repetition will be wanting; for all will be over with the first shock. But, in the majority of earthquakes, the shocks come in succession, as if the cause of them were a regenerating one.
Many ways may be conceived in which these enormous tensions may end in reiterated shocks, according to the hypothesis on which we place ourselves. Thus the water in a cavity having in time reached the temperature of explosion, suddenly displaces some of the walls of its prison. Hence, a first shock, followed by an expansion into the cracks and adjoining cavities, which have lower temperature and tension. Then, the pressure in the original focus of explosion having fallen off, the walls which had given way return upon themselves to their former position, to give way again when the primitive reservoir has regained its lost tension. This flow from cavity to cavity, which, instead of being continuous, is made by bursts and starts, may continue to be reproduced time after time till the principal reservoir is exhausted. But the mechanism is not destroyed then. During the period of calm, following the seismic period, it can be charged again. Something analogous to this takes place in volcanic eruptions, which are separated by the lapse of time necessary to recharge their apparatus by a slow alimentation. Furthermore, reservoirs of water may be suddenly displaced under the effect of contractions of the crust, and may thus be brought into contact with masses having a high temperature.
If we suppose a sea of melted matter to exist beneath the crust of the earth, we should have analogous effects whenever hydrated rocks, broken off from the shell, fall into the ignited masses.
The theory of the agency of vapor is also supported by the rumblings and subterranean thunders which sometimes continue for months and even for years without being attended by shocks, and for which it is difficult to imagine any other causes than sudden condensation, or a flow of gaseous matter at very high tension through a narrow orifice. The vapor, having escaped from its prison, must in many cases resume the liquid state very quickly, on account of the enormous expansion it undergoes. It also has to traverse miles of relatively cold rocks, more or less water-charged and full of cracks. It may thus contribute to the production of thermal springs. Examples are on record, too, of earthquakes, remote from any volcanic point, in which both hot water and gaseous matter have been seen to issue from crevasses.
Thus, the motive force of these formidable disturbances is always active under the feet of the inhabitants of many regions. Against the permanent danger that menaces them, men have at least the remedy of forgetfulness.—Translated for the Popular Science Monthly from the Revue des Deux Mondes.