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 furnishing 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]
A few terms describing the nature of the shock are also in common use among Italians and Spaniards. An _undulatory_ shock consists of one or several waves, the movement to and fro being along a nearly horizontal line; a _subsultory_ shock of movements in a nearly vertical direction; while a _vorticose_ shock consists of undulatory or subsultory movements crossing one another in different directions.
ORIGIN OF EARTHQUAKES.
Earthquakes are grouped, according to their origin, into three cla.s.ses. The first consists of slight local shocks, caused by the fall of rock in underground pa.s.sages; the second of _volcanic_ earthquakes, also local in character, but often of considerable intensity near the centre of the disturbed area; while in the third cla.s.s we have _tectonic_ earthquakes, or those directly connected with the shaping of the earth"s crust, which vary in strength from the weakest perceptible tremor to the most destructive and widely felt shock. Of the earthquakes described in this volume, the Ischian earthquakes belong to the second cla.s.s, and all the others to the third.
That tectonic earthquakes are closely connected with the formation of faults seems now established beyond doubt. They occur far from all traces of recent volcanic action. Their isoseismal lines are elongated in directions parallel to known faults, and this is sometimes the case in one and the same district with faults that occur at right angles to one another. Indeed, when several isoseismals are carefully drawn, it is possible from their form and relative position to predict the position of the originating fault.[2] The initial formation and further spreading of the rent may be the cause of a few earthquakes, but by far the larger number are due to the subsequent growth of the fault. The relative displacement of the rocks adjoining the fault, which may amount to thousands of feet, occasionally even to miles, is the result, not of one great movement, but of innumerable slips taking place in different parts of the fault and spread over vast ages of time. With every fault-slip, intense friction is suddenly brought into action by the rubbing of one ma.s.s of rock against the other; and, according to the modern view, it is this friction that gives rise to the earthquake waves.
In most earthquakes, the slip takes place at a considerable depth, perhaps not less than one or several miles, and the vertical slip is so small that it dies out before reaching the surface. But, in a few violent earthquakes, such as the j.a.panese and Indian earthquakes described in this volume, the slip is continued up to the surface and is left visible there as a small cliff or fault-scarp. In these cases, the sudden spring of the crust may increase and complicate the effects of the vibratory shock.
FOOTNOTES:
[1] If _a_ is the amplitude of the vibration and T its period, the maximum velocity is 2*pi*a/T and the maximum acceleration 4*pi^2a/T^2
[2] See Chapter VIII., on the Hereford and Inverness earthquakes.
CHAPTER II.
THE NEAPOLITAN EARTHQUAKE OF DECEMBER 16TH, 1857.
Half a century ago, seismology was in its infancy. On the Continent, Alexis Perrey of Dijon was compiling his earthquake catalogues with unfailing enthusiasm and industry. In 1846, Robert Mallet applied the laws of wave-motion in solids, as they were then known, to the phenomena of earthquakes; and his memoir on the Dynamics of Earthquakes[3] may be regarded as the foundation-stone of the new science. During the next twelve years he contributed his well-known Reports to the British a.s.sociation,[4] and prepared a series of instructions for the observation and study of earthquake-shocks.[5]
The latter, it is worth noting, contains an outline, but hardly more than an outline, of the methods of investigation which he developed and employed eight years afterwards in studying the Neapolitan earthquake.
The history of Mallet"s preparation for his great work is somewhat strange. No one else at that time possessed so full a knowledge of earthquake phenomena. It was, however, a knowledge that had little, if any, foundation in actual experience; for, when he was awakened by the British earthquake of November 9th, 1852, he failed to recognise its seismic character. Although this shock disturbed an area of about 75,000 square miles and was felt in all four parts of the kingdom, the paucity of observations and the absence of durable records combined in preventing the successful application of his new modes of study.[6]
Nevertheless, with confidence unshaken in their power, he awaited the occurrence of a more violent shock, but five years had to pa.s.s before his opportunity came towards the close of 1857.
So destructive was the Neapolitan earthquake of this year (Mallet ranks it third among European earthquakes in extent and severity), that nearly a week elapsed before any news of it reached the outer world. Without further loss of time, he applied for and obtained a grant of money from the Council of the Royal Society, and proceeded early in the following February to what was then the kingdom of Naples. Armed with letters of authority to different officials, he visited the chief towns and villages in the meizoseismal area; and, in spite of unfavourable weather and the difficulties of travelling in a country so recently devastated, he completed his examination in little more than two months. It was a task, surely, that would have baffled any but the most enthusiastic investigator or one unspurred by the feeling that he possessed the key to one of the most obscure of Nature"s problems.
Mallet"s confidence in the accuracy of his methods was almost unbounded. His great report was published four years later; but he seems to have regarded it almost as a text-book of "observational seismology" and the results of his Neapolitan work as mere ill.u.s.trations. His successors, however, have transposed the order of importance, and rank his two large volumes as the model, if not the inspirer, of many of our more recent earthquake monographs.
[Ill.u.s.tration: FIG. 2.--Isoseismal Lines of the Neapolitan Earthquake of 1857. (_Mallet._)]
ISOSEISMAL LINES AND DISTURBED AREA.
The position of the meizoseismal area, to which Mallet devoted most of his time, is indicated by the small oval area marked 1 in Fig. 2, represented on a larger scale in Fig. 9. It is 40 miles long and 23 miles wide,[7] and contains 950 square miles. Within this area, the loss of life was great and most of the towns were absolutely prostrated.
The next isoseismal, No. 2, which is also shown more clearly in Fig.
9, bounds the area in which the loss of life was still great and many persons were wounded, while large portions of the towns within it were thrown down. Its length is 65 miles, width 47 miles, and area 2,240 square miles. The third isoseismal includes a district in which buildings were only occasionally thrown down, though none escaped some slight damage, and in which practically no loss of life occurred. This curve is 103 miles long, 82 miles wide, and includes 6,615 square miles. Lastly, the fourth isoseismal marks the boundary of the disturbed area, which is 250 miles long, 210 miles wide, and contains not more than 39,200 square miles; an amount that must be regarded as strangely small, and hardly justifying Mallet"s estimate of the Neapolitan earthquake as the third among European earthquakes in extent as well as in seventy.
DAMAGE CAUSED BY THE EARTHQUAKE.