Popular Science Monthly/Volume 24/February 1884/The Causes of Earthquakes
|THE CAUSES OF EARTHQUAKES.|
THE causes of earthquakes have long been the subject of many conjectures. The numerous investigations of later years have contributed much to define their characters; and several data recently acquired tend further to make their mechanism clear. It is known that the shocks are by no means distributed at haphazard over the surface of the globe. The countries where the strata have preserved their original horizontal position, like the north of France, a part of Belgium, and the most of Russia, are privileged with tranquillity. Violent commotions are manifested particularly in regions that have suffered considerable mechanical accidents, and have acquired their last relief at a recent epoch, like the region of the Alps, Italy, and Sicily.
The tracts that are simultaneously disturbed by the same shock most frequently comprise arcs of from 5° to 15°, or from 300 to 1,500 kilometres. They rarely include a much more considerable fraction of the globe; although the celebrated catastrophe at Lisbon on the 1st of November, 1755, extended over some 17° or 18°, into Africa and the two Americas, or over a surface equal to about four times that of Europe.
The detailed examination of many earthquakes has enabled us to determine the center of the shocks as well as the contours of the disturbed areas. From the manner in which the latter surfaces agree with the lines of pre-existing dislocations, several of the most distinguished geologists, including Mr. Dana, M. Suess, and Albert Heim, have considered the shocks in question as connected with the formation of chains of mountains, of which they may be a kind of continuation.
In fact, the crust of the earth everywhere shows the enormous effects exercised by the lateral pressures that have been in operation at all epochs. The strata, bent and bent over again many times through thousands of metres of thickness, as well as the great fractures that traverse them, are the eloquent witnesses of these mechanical actions. Notwithstanding the apparent tranquillity now reigning on the surface of the globe, equilibrium does not exist in the earth, and commotions have not been arrested in its depths. The proof of this is found, not only in earthquakes, but also in the slow movements of the soil, of elevation and depression—a kind of warping, which has continued to manifest itself within historical times in all parts of the globe. It is conceivable that slow actions of this kind, after more or less prolonged strains, may end in abrupt movements, as Élie de Beaumont supposed. We can see, also, in experiments intended to imitate the bending of strata, how gradual inflections lead all at once to fractures and outbursts. Simple cavings-in, in deep cavities, have also been regarded as possibly giving rise to earthquakes; and this opinion has been adopted by M. Boussingault after the well-known observations he made in the Andes. There is, in fact, nothing to prove that disturbances of these different kinds do not take place in the interior of the globe; but we may certainly consider them as the general cause of earthquakes. These shocks are, however, most commonly in evident connection with volcanoes; and it is in the neighborhood of the latter that they are especially frequent. As is well known, every volcanic eruption is announced by precursory earthquakes, the violence of which is stilled when an outlet is opened for the vapor of water which is successively the cause of the subterranean agitations and the projecting agent of all the eruptions. The tension of the vapor in the volcanic reservoirs must be very high. Thus, that pressure which forces the lava up to more than 3,000 metres above the sea, to the top of Etna, can not be less than a thousand atmospheres.
An attentive study of the phenomena confirms the attribution of the cause of the shocks, however violent they may be, to the vapor of water. It is sufficient for this to be the case for vaporization to take place at a temperature of 1,000° C. (1,800° Fahr.), approximately that of lava, and under a volume equivalent to that of the water in the liquid state whence the vapor is derived. Under these conditions, we must suppose the vaporization to be total, for the critical temperature, above which the liquefaction of vapor can not be realized, is, according to M. Clausius, 332° C. (629° F.). The pressure, of which it is also possible to make an approximate estimate, then becomes comparable to that of the most powerfully explosive gases, and is, consequently, capable of producing very considerable dynamic effects. These effects may also be produced at a much lower temperature than that of lavas at 500° C. (900° F.); for example, if we suppose that the volume imposed upon the vapor is so limited as to correspond to a density of 0·8 or 0·9. No doubt such conditions are realized in the lower regions of the globe, where water is confined within limited spaces, and as hot as the melted rocks which we see gushing out from the surface at a temperature of 1,000° C. (1,800° F.) or more. We shall see, however, that such depths and such a temperature are not necessary.The vapor of water when superheated acquires a power of which the most terrible boiler-explosions could give no idea if we had not the result before our eyes. The tubes of the best quality of iron that I used in observing the action of superheated water in the formation of silicates had an inside diameter of twenty-one millimetres and were eleven millimetres thick. They sometimes exploded, and were projected into the air with a noise like that of the firing of a cannon. Before bursting, the tubes swelled out into bulbous forms, and rents were opened in the middle of the bulbs. If the iron had no flaws and according to the estimate that it would preserve up to 450° C. (810° F.), the temperature to which it was raised, the same tenacity it had when cold, such rents must have indicated a pressure of several thousand atmospheres. A few cubic centimetres of water were sufficient to produce an effect like that; and, considering the small dimensions of the inside of the tubes as compared with the volume of the water, the vapor must have reached a density of about 0·9. If we apply the data we possess to the depths of the globe, it is not difficult to conceive very simple dispositions in which the vapor of water, under the conditions we have just determined, will suddenly provoke shocks or series of shocks that will too often make themselves felt on the surface. Whatever conception we may form of the volcanic reservoirs, we must admit it to be very probable that solutions of continuity exist between the soft or fluid masses in fusion and the solid masses superposed over them. Moreover, cavities may also exist in the solid rocks themselves that lie over the soft masses. On the other hand, the incessant losses, which these internal reservoirs suffer in consequence of the enormous volumes of water in the condition of vapor which they disengage every day, are probably repaired by supplies from the surface.
I have shown by experiment that these supplies may be transmitted through the pores of some kinds of rocks. Simple capillary action, in conjunction with gravity, may force water to penetrate against very strong counter-pressure, from the superficial and cooler regions of the globe, to deep and hot regions, where, by reason of the temperature and pressure it acquires there, it becomes capable of producing very great mechanical and chemical effects. If we suppose that water penetrates, either directly or after a halt in a reservoir where it has remained liquid, to masses in fusion, so as to acquire there an enormous tension and an explosive force, we shall have the cause of the anterior real explosions and of the instantaneous shocks due to gases at high pressure. If the cavities, instead of forming a single reservoir, are divided into several parts or distinct compartments, there is no reason why the tension of the vapor should be the same in the different receivers, provided they are separated by walls of rock. The pressure may even be very different in two or more of them. This admitted, if a separating wall is broken by excess of pressure or melted by the heat, vapor at high pressure will be set in motion, and in the presence of the solid masses upon which it will strike it will behave just as if there had been an instantaneous formation of vapor, as we supposed in the former case.
It is very hard to establish, as has been attempted, a clear line of demarkation between the character of the earthquakes of volcanic regions proper and of regions without volcanoes, such as Portugal, Asia Minor (Chios, April 3, 1881, five thousand victims), Syria, Algeria, and the rim of the Mediterranean generally. In both classes, the characteristic manifestations which we perceive are the same. If, as some assume, the internal movements of the rocks were a cause of real earthquakes, it could only be because those internal movements mechanically developed heat, and in that way provoked the formation of vapor. But, in the recently disturbed regions we have especially in view, which are the seat of so frequent shocks, another cause is much more probable. There doubtless remain in them interstices and interior cavities that permit the access of water to the hot regions. The depth of the centers of disturbance of earthquakes has been estimated, in different cases, by calculations only grossly approximate, at eleven kilometres, twenty-seven kilometres, and thirty-eight kilometres. In any case, such depth, though very slight in comparison with the length of the radius of the earth, is great enough for the temperature at the normal rate of increase to be very high; and the same will also be the case with the water that may be present there. Now, as we have already seen, a temperature of 500° C. (900° Fahr.) is sufficient to cause water to explode with violence.
It is certainly in the largest number of cases very difficult to admit collisions of solid bodies in the interior as the moving causes of earthquakes. How, for instance, can we conceive that so violent and extensive an earthquake as that of Lisbon on the 1st of November, 1755, was produced in this way? John Mitchell (Royal Society, 1760, vol. x, p. 751) drew from this memorable example the conclusion that the vapor of water intervenes in these shocks as well as in the eruptions of volcanoes. Manifest effects of a class of internal explosions, undoubtedly due to the production or sudden moving of a great quantity of superheated vapor, are exhibited at the present epoch, and are not rare. Such explosions, for instance, are exceptionally formidable in the region of Java, and the mind is naturally led to the one which has just convulsed the zone between that island and Sumatra, which has caused the disappearance of the island of Krakatoa and its mountains, has raised other mountains, and has claimed more than forty thousand victims.
At a period more remote from us, the explosive force of interior gases gave rise to very remarkable circular cavities, which have been called "craters of explosion," and are well known. Examples of them are found in Auvergne (Lake Pavin) and in the district of the Eifel, where the stratified beds have been sharply cut as if with a punch. What gases thus put in motion are capable of, as a mechanical power, could hardly have been suspected till since the explosive effects of gun-cotton, nitroglycerine, and dynamite, have been known. The effects of compressed air in the air-gun and of the powder-gases in fire-arms have been wonderfully surpassed, for we now measure explosive pressures of six thousand atmospheres and more. In the experiments in which I have had occasion to observe gases at high pressure in order to explain the action that a meteor coming with planetary speed is subjected to on the part of the atmosphere into which it plunges, I have been surprised at witnessing the great energy of gaseous masses. They engrave themselves deeply, as if with a burner, into the pieces of steel that are opposed to them, and of themselves reduce a part of it to an impalpable dust shot into the atmosphere as if it were volcanic ashes. It is no less surprising—and this observation is of much importance in explaining the problem that occupies us—to remark the tenuity of the gaseous mass that produces such results. Yet its force causes ruptures which the pressure of a weight six hundred thousand times heavier than the gas could not effect!In short, gaseous movements under high pressure, put in operation from time to time by a simple mechanism like what Nature can and does present, will account for all the essential features of earthquakes. Much better than the hypothesis of interior collisions of solid bodies, they explain the effect of the shock, resembling the blows of a ram, their violence, their frequent succession, and their recurrence in the same regions after many centuries; they explain also the production of earthquakes in regions of dislocation, especially in those in which the disturbance is recent, and their subordination to deep fractures of the crust of the earth.
Earthquakes seem to be volcanic eruptions that are suppressed because they can not find any outlet, nearly as Dolomieu thought. The motive power of gases, the immense effects of which we can see in the protuberances or jets shot out from the sun with prodigious speed and of enormous dimensions, appears to be sufficiently considerable in the depths of the globe also to explain all the effects of earthquakes.—Translated for the Popular Science Monthly from the Revue Scientifique.