Part 1 (2/2)

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.

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