Popular Science Monthly/Volume 68/June 1906/The Times and Places of Earthquakes
|THE TIMES AND PLACES OF EARTHQUAKES|
By Professor H. H. TURNER, F.R.S.,
THE occurrence of several disastrous earthquakes and eruptions during the last few months inevitably suggests the question whether all these events may not have a common and determinable origin. To avert any of these disasters, even to modify them in the slightest degree, may be entirely hopeless; but the vaguest foreknowledge of their probable occurrence might be of untold value in saving life and property. Has modern research obtained any clues which enable predictions to be made, or promise that prediction may be possible in the near future? It must be frankly admitted that as yet our knowledge is so slight as to have no commercial value; but still, there are one or two clues in the hands of those working at the subject which may ultimately lead them to more directly useful knowledge. We have learnt something of the regions where earthquakes occur, and something of the times when we may specially expect them; and, though the something is in each case a very little, the magnitude of the issues involved lends it interest.
Systematic observation of earthquakes is only about a quarter of a century old, and for fairly complete records of all the shocks occurring in different parts of the globe we can date only from 1892. Before that date information could only be collected on the spot, and was thus frequently lost; but it was realized about 1890 that a series of earthquake observatories, with delicate instruments, could obtain records of shocks in any quarter of the globe, and identify the spot with certainty, even if there were no witnesses of the actual occurrence. From the records of these observatories it appears that there are every year some 30,000 minor shocks of earthquake in different localities, but of these only 60 are 'world-shaking' and observable from a great distance. Such numbers indicate immediately that, from one point of view, the San Francisco earthquake can not be regarded as exceptional; it was only one event out of 60 per annum. What rendered it disastrous was the existence of a great town in the shaken locality. But was the neighborhood known to be a dangerous one? Was it at any rate, suspected, so that the building of a great city there was an error of judgment? and is it advisable to the city in the same place? These are questions of the gravest importance; and it is well worth while to review the little knowledge already accumulated with the utmost care to see whether it will give us even provisional answers to them.
Professor Milne, in the tenth report of the British Association committee, refers the 'world-shaking' earthquakes observed in the six years 1899-1905 to thirteen great earthquake regions, designated by the first thirteen letters of the alphabet. Three of these, I, J and L, are responsible for only five, three and two shocks respectively, and are thus of small importance compared with the others, which average about forty shocks each. Excluding them for the present, the remaining ten regions lie approximately in two rings on the earth's surface, a configuration which is most strikingly apparent when the regions are marked on a globe. The more important ring includes the following seven regions: A (Alaskan coast), B (Californian coast), C (West Indies), D (Chilian coast), M (South of New Zealand), F (Krakatoa region), E (Japan). Its center is among the conspicuous group of islands which includes Tahiti, and the radius of the ring is about 65 degrees. The other ring has its center at the opposite point of the earth, which is in the Sahara desert; and at a radius of 50 degrees from this center lie regions G (between India and Madagascar), H (the Azores) and K (Tashkend). Now, this is not merely a convenient geographical summary, but a physical fact of vital importance, according to recent researches by Professor Jeans. In a remarkable paper read before the Royal Society in 1903 he gave reasons for believing that the earth is by no means a sphere or a spheroid, as we have been accustomed to think, but is of a pear-shape. Under gravitational stress it is continually approaching the spheroidal form—the pear is being crushed into a sphere by its own attraction; and the result is a series of earthquakes. These naturally occur in the weakest places, and if any one will experiment in crushing a pear towards a spherical shape, or even draw a diagram and consider where the weakest points would be, the reasons for the existence of two rings of greatest weakness will readily suggest themselves. The ends of the pear are the centers of these rings, one in Africa, one in the Pacific; and when once this is pointed out, the pear-shape of the earth is, according to Professor Sollas, 'obvious to mere inspection; it is a geographical fact and not a speculation.' Professor Sollas is indeed responsible for the particular suggestion above sketched; for Professor Jeans had originally proposed a different axis, which he withdrew in favor of the obvious improvement. The confirmation of Professor Sollas's view from the distribution of earthquake centers is remarkable. It does not seem, however, quite certain which is the blunt end of the pear; it has been hitherto placed in Africa, but there seem to be several reasons for regarding Africa as the stalk end. This point can not, however, be dealt with here. The important thing is that there seems to be a real reason for the occurrence of earthquakes in these particular regions, and that they will probably continue to occur there. Professor Jeans's conclusions have recently been examined by Lord Rayleigh, who announced at the Royal Society only a few weeks ago that he found them generally confirmed, and that we must regard our earth as at present in a state far from stable.
The lessons to be learnt from the distribution of earthquakes in space are accordingly tolerably plain in theory, though in practise we may not be able to take advantage of them. If we would be particularly safe from earthquakes, we must take up our abode near one of the ends of the pear—either in Africa or in the Pacific. There is also a region of safety between the two dangerous rings—in America generally, for instance, excluding the west, or in Siberia. But the dangerous regions include so vast and so valuable a part of the earth's surface that it is impracticable to leave them unoccupied. Moreover, our knowledge is as yet not specific enough. In the dangerous regions themselves, some parts are much more dangerous than others; for instance, Japan, which is reckoned above as a single region, can be divided into at least fifteen distinct seismic districts. As observations are accumulated we may be able to make similar partitions of the other regions. For the present the general attitude towards earthquakes will probably be similar to that towards other dangers, such as those of travels and voyages for instance; the risks must be incurred. We know that there are at times fatal tornadoes; but other interests are at stake, and we put to sea in the hope that none will occur during our voyage.
We come to the second point, the distribution of earthquakes in time. Are there seasons of special activity such as the recent occurrence of several disasters seems to suggest? Here our knowledge is slighter still, and the observed facts have not yet been coordinated by a mathematical investigation. Still there seems to be some evidence in support of the view that exceptional irregularities in the rotation of our earth may be responsible for an increased number of earthquakes at particular times. That the evidence is slight must be attributed to the shortness of the time during which it has been possible to obtain it, and not necessarily to inherent weakness in the evidence itself. The brevity of the earthquake record has been mentioned above; that of irregularities in the earth's rotation is longer; but the discovery that such irregularities existed was made only twenty years ago, though the phenomenon was then traced back through the old observations. The irregularities are systematic in character, and the law governing them is approximately known already; so that, if the presumed connection between them and earthquakes is confirmed, we may be able to predict periods of great earthquake frequency. Such periods would be in some respects analogous to the times of spring-tides. It is a familiar fact that at new and full moon the tides are much greater than when the moon is at the quarters. The reason is that we have two tide-raising bodies, the moon and the sun, which sometimes act in concert, and then we get large tides; sometimes in opposition, and then we get small tides. If the influence of these two bodies were more nearly equal, instead of the moon being so predominant a partner, we can imagine times when the tides would be barely perceptible. Similarly there are apparently two contributors to the variation in our earth's rotation, which sometimes act in unison and sometimes in opposition. They are more nearly equal in influence than are our moon and sun; and consequently there are times when these two contributors nearly balance one another and the axis of rotation remains almost steady. But in due time the contributors reinforce one another and the axis acquires a considerable 'wobble.' Each end of the axis then describes a curve composed of wide sweeps and sharp bends; and the evidence seems to be that at the sharp bends we are particularly liable to earthquakes. The exact statement of the case as given by Professor Milne in his Bakerian lecture, 'Recent Advances in Seismology,' delivered before the Royal Society, on March 22, last, is as follows:
In a period of nearly thirteen years (1892 to 1904) I find records for at least 750 world-shaking earthquakes, which may be referred to three periods continuous with each other, and each two-tenths of a year or 73 days' duration. The first period occurred when the pole movement followed an approximately straight line or curve of large radius, the second equal period when it was undergoing deflection or following a path of short radius, and the third when the movement was similar to that of the first period. The numbers of earthquakes in each of these periods taken in the order named were 211, 307 and 232—that is to say, during the period when the change in direction of motion has been comparatively rapid the relief of seismic strain has not only been marked, but it has been localized along the junctions of land blocks and land plains where we should expect to find that the stress due to change in direction of motion was at a maximum. Until the magnitude of these induced stresses has been estimated it would be premature to assume that the frequency under consideration is directly due to change in direction of pole movement, it being quite as likely that both phenomena may result from a general cause.
It is eminently to be desired that a mathematical investigation of the point should be undertaken; but the difficulties are very great, and as yet no one has had the time and courage to attack them. It will be seen, then, that the seismologist is as yet not able to give forecasts of any commercial value, though he is by no means without hope of doing so.
There are, however, some lessons of immediate practical importance which have been learnt by seismological study; we may again quote from Professor Milne's Bakerian lecture:
At the Imperial University of Tokio a platform was constructed which by means of powerful machinery could be made to reproduce earthquake motion of varying intensity. On this table large models of masonry, wood, and metal designed to resist expected seismic accelerations were tested. This table has been to the builders in Japan what a testing tank in a dockyard has been to constructors of large vessels. The ultimate result of these and other investigations has been to modify and extend the rules and formulæ of ordinary construction, and now in Japan, as opportunity presents itself, new types of structure are springing up. These have withstood violent shakings which have materially damaged ordinary types in the neighborhood. While much has thus been done to reduce the loss of life and property, the Japanese government, stimulated by the results of this experience, has been encouraged to extend its support to seismological investigations in general.
In 1886 the chair of seismology was established at the Imperial University, and since 1892 there has been in existence a seismological investigation committee, which has already issued 70 quarto volumes. At the Central Meteorological Observatory in Tokio records are received from nearly 1,500 observing centers.
From these paragraphs it will be seen that there are questions which merit the close study of engineers and architects whose work lies in the dangerous regions, though but little attention has been paid to them except by that wonderful little people who have already taught us more than they learnt from us. It is some consolation, doubtless, to reflect that modern seismology owed its origin to Englishmen. It was the little community of Englishmen who were invited in 1880 to 'pitch their tents on the trembling soil of Japan,' in order to teach the Japanese something of western civilization, who began to study these earthquakes, and enlisted the sympathy of the Japanese government in the matter. The sequel in this case as in others suggests comparison, not perhaps between the disciple and his master, but between the treatments which they have received at the hands of the world in general and governments in particular. While seismological research has been stimulated and rewarded in Japan in the manner above indicated, Professor Milne's heroic exertions in England have met with very little recognition. Practically single-handed he has organized forty stations all over the world, where records are obtained, and has carried on the correspondence and clerical work necessary to keep them in communication and coordinate the records. Until recently the only assistance accorded him of any kind was a small grant of about £20 a year made by the British Association—all they could afford in view of the numerous claims on their small funds—which barely sufficed to buy the paper and chemicals for his own recording station at Shide. Appeals for government aid have so far been fruitless; though recently one or two welcome private donations have been forthcoming.
It will, no doubt, be objected to this comparison that an important consideration has been omitted. Seismological questions are of urgent practical importance in Japan, but not in England. That is true, and we all hope that it may remain true; but our guarantee is not absolute. Whether the regions of danger are permanent or shifting is just one of the questions which the whole world is interested in answering, and which can be answered only by patient and laborious research. The British Isles are far from being in a specially safe region; in fact, they lie almost exactly on the smaller dangerous circle above-mentioned, through Tashkend, the Azores and the Indian region; and though earthquake activity seems to be at present limited to these three regions, and so far as it strays in our direction seems to find an outlet rather beyond us (in the region labeled J by Professor Milne, between Iceland and the North Cape, where three earthquakes were recorded in six years), we have no right to assume that this state of things is more than temporary.
During the last year or two, however, more has been done in Europe generally to follow the lead of Japan; international cooperation in seismological work has been organized in Germany; and though the adhesion of some important countries is not yet certain, owing to various difficulties which need not be noticed here, it is hoped that these may be smoothed away in time. If so we may look forward to a welcome strengthening of the corps of workers in seismology, though there is still more than enough work for them all to do.
- From the London Times.