Gabriel Auguste Daubrée

“ 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. ”

“ 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 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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 ”

“ 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. ”

“ 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. ”

“ 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 ”

“ 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. ”

“ 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. ”

“ 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 33° and 43° south. Very different conditions prevail east of the Cordilleras, where vast countries like Brazil have no earthquakes. ”

“ 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. ”

“ 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. ”

“ 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. ”