THE

PREFACE

HE methods of studying earthquakes which have been developed during the last thirty years differ so much from those which preceded them, that they have justified the name of “The New Seismology." Prior to 1870, the studies published upon this subject were, with few exceptions, little more than narratives of disasters. Prof. Alexis Perrey had, it is true, compiled some extensive catalogues of earthquakes and published some comparisons which were interesting and which suggested inferences concerning the relations between earthquakes and the phases of the moon. Dr. Robert Mallet had investigated, with a minuteness never before attempted, the severe Neapolitan earthquake of 1857, and drew from his studies some remarkable conclusions. Neither Perrey's nor Mallet's work, however, added much of permanent value to the science except the truly scientific purpose and spirit which animated them. In general, the seismology of more than thirty years ago was descriptive only, and might be regarded as just within the outermost pale of scientific philosophy. The new seismology is eminently scientific, and in the strictest sense, for it investigates its phenomena by means of instruments which measure force and motions, speeds and acceleration. Its field is within the great department of physics, and is a

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part of the branch which treats of elasticity and wavemotion.

The standpoint of the new seismology is very different from that of the old. The latter often seemed to regard earthquakes as being one of the formative geologic forces of nature, whose origin was mysterious, but which accomplished important structural results. The new view treats them as pure effects of geologic forces, just as thunder is an effect of the electric discharge, and not the cause of it. As sound is merely the elastic vibration of the air, so, in the view of the new seismology, the earthquake is merely the elastic vibration of the earth-mass. Hence the science becomes in a great measure the investigation of elastic wavemotion in a solid medium. This medium is the earth itself. And since the modes and configurations of this motion depend almost wholly upon the nature of the medium which transmits them, they become a means of investigating some important problems relating to the condition of the earth's interior.

The new seismology may be said to have begun its work with the invention of the seismograph. There are many forms of seismograph, and many investigators have contributed to their development, so many, indeed, that it might seem invidious to single out any one of them. Yet it is only just to mention Prof. James A. Ewing, formerly of the University of Tokio, and now of Cambridge, as the first designer of the most important features of the principal seismographs used for analysing earthquake motion and recording it in conjunction with a time scale. By means of these instruments it became possible to inaugurate an en

tirely new series of investigations of the nature of earthquake motion, and ultimately to create a new branch of science.

In this line of research a leader was necessary; one who was well versed in physical science, having a high faculty of initiative, combined with a candid temperament, and one who was willing to make the study a life-work. For the science is a profound one, and could expect little progress from the fitful zeal and capricious attentions of those who might be momentarily attracted to it and quickly repelled by failure to achieve results. Such a leader was found in John Milne. In the earliest stages of the new science, when it was furnishing more questions than answers, he had the field mostly to himself, but when the results began to appear and to multiply, chiefly through his patience and industry, new workers entered the field, and many of them were of a high order of ability. At present they are quite numerous, and Japan, Germany, Austria, Italy, and England have many learned investigators. Their combined efforts have made the science very instructive and interesting.

The following chapters are intended to summarise their more important results: Chapter I sets forth the nature of an earthquake according to the modern concepts. It defines the technical terms used in discussion, and describes the action taking place on the surface of the ground during a quake of great energy. Chapter II is a general discussion of the causes of earthquakes. Two causes are recog

nised, apparently quite distinct, though possibly they may have interrelations not yet recognised. The first cause is

volcanic; the second is that force which is presumed to be always active in disturbing the rocks which form the outer shell of the earth, resulting in the building of mountains, the folding or shearing of the strata, and the elevation and depression of the earth's crust. Thus we have two groups of quakes: the volcanic and the tectonic. They have in many cases distinct characteristics, and these are described in Chapters III and IV.

Two chapters, V and VI, describe in detail the more important instruments used in seismometry: (1) the best forms of seismoscopes; (2) the higher and more elaborate forms of seismographs, which analyse seismic motion and resolve it into its three co-ordinates; (3) the large Italian vertical pendulums; (4) the most approved forms of the horizontal pendulum for recording the disturbances originating at far-distant parts of the world.

Chapter VII discusses the details of seismic vibratory motion and explains the four kinds of waves with which the inquiry deals. Two of these kinds of wave, the normal and transverse, pass through the earth-mass.

The third kind

appears to travel along great circles around the earth. The definite establishment of this latter form of wave-motion is a surprise to seismologists, and is not as yet wholly free of mystery, though its reality no longer seems to admit of any question. The fourth form is seen only in the epifocal regions of great earthquakes, and does not appear to consist of elastic waves at all, but of secondary effects of other vibrations. They, too, have long been difficult to explain, and, though attested by overwhelming evidence, are still far from being fully understood.

The kinetic elements of harmonic or vibratory motion are space and time,-i. e., the distance traversed and the time of traversing it,-in other words, the amplitude and period of vibration. To this subject a chapter is given. The amplitudes and periods are measured by the seismograph, which thus gives the measured data upon which all kinetic estimates are based. The relations of horizontal and vertical vibration, as disclosed by the seismograph, are briefly set forth.

Passing then to the kinetic aspects of seismic vibration, the subject of intensity is treated in two chapters. For the moment we may regard intensity as the degree of vigour with which the earth, and objects upon its surface, are shaken. Since seismographs are few and far between, there is ordinarily no other way to reckon intensity than by its effects upon structures or loose objects, or upon human sensations. These effects are classified into a conventional scale of intensity ordinarily having ten degrees, ranging from the feeblest sensation of which men may be directly conscious to the energetic shaking which lays a city in ruins. The two most common scales in use, the Rossi-Forel and the Mercalli, are given. It might seem as if this way of reckoning intensity were exceedingly rough guesswork, of little value as a basis of kinetic estimates; yet nothing could illustrate better the highly scientific character of the new seismology than the attempts of Professor Holden and Professor Omori to correlate these purely conventional scales with an absolute scale of intensity, or accelerations.

The chapter on variations of intensity points out the method of computing the depth of origin of an earthquake