present be out of place.

An isoseismal line is a curve which passes 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.

Fig. 1.—Diagram to illustrate simple harmonic motion.ToList

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 principal 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 principal 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.

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 glass 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 classes. The first consists of slight local shocks, caused by the fall of rock in underground passages; 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 class 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 class, 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 mass 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 Japanese 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:

CHAPTER II.ToC

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.