Part 1 (1/2)

A Study of Recent Earthquakes.

by Charles Davison.

PREFACE.

The present volume differs from a text-book of seismology in giving brief, though detailed, accounts of individual earthquakes rather than a discussion of the phenomena and distribution of earthquakes in general. At the close of his _Les Tremblements de Terre_, Professor Fouque has devoted a few chapters to some of the princ.i.p.al earthquakes between 1854 and 1887; and there are also the well-known chapters in Lyell's _Principles of Geology_ dealing with earthquakes of a still earlier date. With these exceptions, there is no other work covering the same ground; and he who wishes to study any particular earthquake can only do so by reading long reports or series of papers written perhaps in several different languages. The object of this volume is to save him this trouble, and to present to him the facts that seem most worthy of his attention.

The chapter on the j.a.panese earthquake is reprinted, with a few slight additions, from a paper published in the _Geographical Journal_, and I am indebted to the editor, not only for the necessary permission, but also for his courtesy in furnis.h.i.+ng me with _cliches_ of the blocks which ill.u.s.trated the original paper. The editor of _Knowledge_ has also allowed me to use a paper which appeared four years ago as the foundation of the ninth chapter in this book.

CHARLES DAVISON.

BIRMINGHAM, _January, 1905._

A STUDY OF RECENT EARTHQUAKES.

CHAPTER I.

INTRODUCTION.

I propose in this book to describe a few of the more important earthquakes that have occurred during the last half century. In judging of importance, the standard which I have adopted is not that of intensity only, but rather of the scientific value of the results that have been achieved by the study of the shocks. Even with this reservation, the number of earthquakes that might be included is considerable; and I have therefore selected those which seem to ill.u.s.trate best the different methods of investigation employed by seismologists, or which are of special interest owing to the unusual character of their phenomena or to the light cast by them on the nature and origin of earthquakes in general.

Thus, the Neapolitan earthquake possesses interest from a historical point of view; it is the first earthquake in the study of which modern scientific methods were employed. The Ischian earthquakes are described as examples of those connected with volcanic action; the Andalusian earthquake is chiefly remarkable for the recognition of the unfelt earth-waves; that of Charleston for the detection of the double epicentre and the calculation of the velocity with which the vibrations travelled. In the Riviera earthquake are combined the princ.i.p.al features of the last two shocks with several phenomena of miscellaneous interest, especially those connected with its submarine foci. The j.a.panese earthquake is distinguished from others by its extraordinary fault-scarp and the very numerous shocks that followed it. The Hereford earthquake is a typical example of a twin earthquake, and provided many observations on the sound phenomena; while the Inverness earthquakes are important on account of their connection with the growth of a well-known fault. The great Indian earthquake owns few, if any, rivals within historical times, whether we consider the intensity of the disturbance or the diversity and interest of the phenomena displayed by it--the widespread changes in the earth's crust, both superficial and deep-seated, and the tracking of the unfelt pulsations completely round the globe.

TERMS AND DEFINITIONS.

Some terms are of such frequent use in describing earthquakes that it will be convenient to group them here for reference, others more rarely employed being introduced as they are required.

An earthquake is caused by a sudden displacement of the material which composes the earth's interior. The displacement gives rise to series of waves, which are propagated outwards in all directions, and which, when they reach the surface, produce the sensations known to us as those of an earthquake.

The region within which the displacement occurs is sometimes called the _hypocentre_, but more frequently the _seismic focus_ or simply the _focus_. The portion of the earth's surface which is vertically above the seismic focus is called the _epicentre_. The focus and epicentre are often spoken of for convenience as if they were points, and they may then be regarded as the centres of the region and area in which the intensity was greatest. This is not quite accurate, but to attempt a more exact definition would at present be out of place.

An _isoseismal line_ is a curve which pa.s.ses through all points at which the intensity of the shock was the same. It is but rarely that the absolute intensity at any point of an isoseismal line can be ascertained, and only one example is given in this volume. As a rule, the intensity of a shock is determined by reference to the degrees of different arbitrary scales. These will be quoted when required.

In every strong earthquake there is a central district which differs in a marked manner from that outside in the far greater strength and complexity of the phenomena. As this district includes the epicentre, it is sometimes referred to as the _epicentral area_, but the term _meizoseismal area_ is more appropriate, and will be employed accordingly.

The district over which an earthquake is perceptible to human beings without instrumental aid is its _disturbed area_. In like manner, that over which the earthquake-sound is heard is the _sound-area_.

A great earthquake never occurs alone. It is merely the most prominent member of a group of shocks of greater or less intensity, and is known as the _princ.i.p.al shock_ or _earthquake_, while the others are called _minor_ or _accessory shocks_, and _fore-shocks_ or _after-shocks_ according as they occur before or after the princ.i.p.al earthquake. When the sound only is heard, without an accompanying tremor being anywhere perceptible, it is more accurately called an _earth-sound_, but is frequently for convenience numbered among the minor shocks.

[Ill.u.s.tration: FIG. 1.--Diagram to ill.u.s.trate simple harmonic motion.]

The movement of the ground during a vibration of the simplest character (known as simple harmonic motion) is represented in Fig. 1.

The pointer of the recording seismograph is here supposed to oscillate along a line at right angles to AB, and the smoked paper or gla.s.s on which the record is made to travel to the left. The distance MP of the crest P of any wave from the line AB represents the _amplitude_ of the vibration, the sum of the distances MP and NQ its _range_, and the length AB the _period_ of the vibration. From the amplitude and period we can calculate, in the case of simple harmonic motion, both the _maximum velocity_ and _maximum acceleration_ of the vibrating particles of the ground.[1]