MODERN STUDIES OF EARTHQUAKES.
By GEORG GERALAND.
The investigation of earthquakes, seismology, has become in the present day an independent subject of scientific interest. In lands where earthquakes are frequent, as in Italy and Japan, seismic observations have been officially systematized over the whole country, with central and branch stations at which the work is never still. A net of seismic observations of all nations is being more and more closely woven over the whole earth, and there are yearly and monthly collations of observations of even the slightest shocks. Seismic literature is, therefore, nearly inexhaustible, and theory and praxis are in constant vogue; in short, seismics has grown to be a separate branch of science, and to demand independent treatment, calling for the energy and labor of many students. What gives it so great importance? What is the condition of our present knowledge and its history? What will be reached in the future through the competition of the nations? These questions possess a high scientific as well as culture-historical interest. We here attempt to answer them.
The first really scientific description of an earthquake—that of Lisbon—with its far-reaching accompanying phenomena, was the work of the greatest contemporary thinker, Kant, and it is not too much to say that his paper opened a new epoch in the knowledge of earthquakes. That terrible event and the extreme terror which it caused everywhere were followed in 1783 by the likewise extremely destructive earthquake of Calabria. The attention of the people was thus directed to this mysterious mighty activity of the earth, and was kept especially lively in Italy, the country of Europe most subject to earthquakes. The newly rising science of geology therefore found in the last third of the last century in these phenomena a problem of prominent importance. Geologists were the first to apply themselves to seismic studies, as the most widely current explanation of the phenomena is still a geological one. The scientific interest of the question prevailed over the practical. More attentive observation was given to earthquakes, the accounts of them scattered through the ancient chronicles were collated, and the already very numerous seismic notes of great earthquake manifestations—such as those by Hoff, Perry, Mallet, Volger, Fuchs, etc.—constituted a very important factor in the study. One of the earliest results of the inquiry was to show that directly perceptible earthquakes are not perceptible everywhere; that they are most common on the great upfoldings of the earth's crust on the mountain chains, such as the Andes, Alps, and Himalayas; and that, further, they are connected with the shores of the Pacific, the Antilles, and the Mediterranean, and with places also where great breaches and various disturbances are evident; that they are at home likewise in volcanoes; and that they are most frequent in the northern hemisphere, and when the earth is nearest to the sun. The descriptions of powerful shocks furnish us evidence of a double movement of the earth's crust—an alternate up-and-down vibration and an often very marked wave motion. The destruction which earthquake shocks and waves inflict on buildings, and the remarkably rapid and wide spread of the tremblings over the surface of the earth, have been very diligently inquired into; and when, in 1856, Naples and Calabria were visited by a great earthquake, an English investigator, Robert Mallet, made a full study of it, and believed that by comparing the direction of the rents in walls and buildings, which were assumed to correspond with that of the tremblings, he could identify the focus of the shocks in the earth's interior, and the course of the wave movement over its surface—a view which has long prevailed in seismology. Still more important was the work of the geologist Karl von Seebach, of Göttingen, on the great earthquake in central Germany, which kept the northern part of the plains of the upper Rhine, around Mayence, Grossgerau, and Darmstadt, disturbed for several years after 1869. Von Seebach's chief effort was to obtain the most exact data possible as to the time of the beginning of the shocks from as many places as possible, from which he might deduce the spot where the shocks began and were strongest, the epicenter which lay directly over the point in the earth's interior where the movement originated. From them he also deduced a series of localities where the shocks were simultaneous and of equal intensity, which could be connected by certain nearly circular lines called homoseists. As the distance of these from the epicenter increases, the undulations take place later and are weaker, and facts may be thus furnished from the velocity of propagation of the shocks can be computed. The observations are also important because von Seebach undertook through a simple mathematical calculation to determine from them the situation of the forces of the subterranean point where the undulations originated.
With these investigations, the process of annihilating time and space by steam and the applications of electricity was also going on. By the effect of this great event, the conditions of earthquake investigation were revolutionized. A comparative study of the phenomena, fundamental and essential to a science of seismology, on the basis of material furnished from all the regions of the earth, was rendered possible. An earthquake service was organized in Japan, by J. Milne, of England; one had already been organized for a considerable time in Italy, and the results obtained at the two places of observation so widely separated corresponded. Japanese, Indian, and American earthquakes could be simultaneously studied in Italy, Russia, Germany, and England; and thus a new, hitherto undeveloped field was gained, the scope of which has already extended far beyond its merely geological aspect.
This could have happened only through another advance that has been made in our century, which has first rendered a real seismology, a scientific knowledge of the seismic conditions of the earth, possible through the immense development of technics, by which a system of instrumental observation of earthquakes was established. Only through this could the acquisitions of recent times be utilized. While formerly observations were macroscopic and touched only earthquakes that could be directly felt, they now cover essentially microscopic tremors of the earth's crust, of less than a thousandth of a millimetre, that are wholly imperceptible to human senses; and we can read them, enlarged at our pleasure, on our photographically registering seismometers. We already had instruments which correctly indicated the time of the beginning and possibly the direction of a shock; but we needed and have invented new instruments—various sorts of horizontal and vertical pendulums—for the observation and representation of the whole course of the movement. The vertical indicating instruments are much used in Italy, and the horizontal ones almost exclusively in England, Japan, and Germany. The horizontal pendulum was invented in Germany in 1832 by Hengler, adapted to scientific use by Professor Zöllner, of Leipsic, and afterward applied in that form by English, German, and other observers. The most complete shape and the one best adapted to extremely delicate seismic observations was given to it by the late German astronomer and geographer Dr. Ernst von Rebeur Paschnitz, of Merseburg. Having undergone a few small changes, fixed in a threefold combination it serves as our most sensitive and accurate seismometer. Its movements and its very exact time markings are photographically represented. The pendulum box is only forty centimetres in diameter. In consequence of its convenience and cheapness, its self-action and its serviceability, it is becoming adopted more and more generally as an international instrument.
Microseismic investigation and its wide extension over the earth have raised seismology another step during the last twenty years, so that it may be said that really exact seismic research began with it. Modern seismology has confirmed many of the older results, such as the localization of earthquakes on the shores of the Pacific, the Mediterranean and in the mountain chains of the earth, and also the importance of homoseists and the epicenter. It has, on the other hand, greatly modified the former estimates of the velocity of propagation of the shocks. It has cast much doubt on speculations as to the seasons in which earthquakes are more or less frequent; and it has demonstrated the inadequacy of former methods of determining the central focus. It has furthermore brought us much that is new. First is the momentous fact that the earth's crust is never at rest; that it undergoes a multitude of very diversified movements besides those of the earthquake. Thus a periodical swelling, a flood wave, is produced by the attraction of the moon; and other heavings are induced by the daily and annual course of the sun's heat. But such movements and other similar ones do not come within the scope of this article.
Real earthquakes, or movements that originate in the depths of the earth, also appear in very different forms. First are the directly perceptible shocks, from the powerful ones that create great disturbances to the merely local ones often hardly remarked. Of the immediate workings of these shocks, microscopic instruments have taught us nothing essentially new. But very many macroscopic movements, often continuing for several hours, but which are not felt, have been revealed, that have been shown in many instances to be distant effects of other strong earthquakes; effects which are sometimes propagated over the whole surface of the earth. There is, furthermore, another series of movements, only partly explained as yet, of a peculiar sort: first, small, quickly passing disturbances, which appear in the photographic reproductions of the curves as larger or smaller knots, and which are regarded with great probability as distant effects of minor seismic movements most likely imperceptible anywhere. They can not be local earthquakes, for they give entirely different curves. There also appear, with considerable regularity, at certain seasons of the year, very slow movements of the ground, called pulsations; and finally the multitude of vibrations called tremors, which assume various forms. Sometimes they come as forerunners, accompaniments, or followers in close association with those great disturbances that originate in distant earthquakes; sometimes as shocks of minute intensity in separate groups, which it has not yet been possible to account for; and in other cases they are traced to the shaking of the ground by the wind. It is hardly necessary to observe that the seismic apparatus should be most carefully guarded against disturbance by the movements of trade, wagons, etc., so that the problem shall not be complicated by them.
The theory of the nature of earthquake shocks, their transmission and their velocity, has been set in a new light by the labors of Augustus Smith, of Stuttgart. From some calculations of their velocity made by G. von Nebeur, it is found that the earthquake of April 17, 1889, in Tokio, Japan, was perceived in Potsdam, Prussia, nine thousand kilometres distant, in thirteen minutes; that of October 27, 1894, in Santiago, Chili, in Rome, eleven thousand five hundred kilometres distant, in seventeen minutes, and in Charkow, Russia, two thousand kilometres from Rome, between one and two minutes later. It reached Tokio at the same time, after a transit of seventeen thousand four hundred kilometres.
Still another task of modern seismology is the investigation of earthquakes at sea, or seismic movements of the bottom of the ocean, and the manner in which they are propagated through the water, of which a very fine cartographic representation has been published by Dr. C. Rudolph, of Strasburg.