An earthquake is caused by a series of elastic waves due to a sudden shock in the earth's interior; the visible phenomena at the surface are produced by the outcropping of these waves and by the movements of the soft and inelastic soil, which is set in motion by the outcropping waves. While the elastic waves, which in mode of transmission resemble those of sound, are very regular in hard and homogeneous rocks, the actual movements of a given particle at the surface are highly irregular and confused, as is well shown in the wire model, Fig. 2. This model, constructed from the records of seismographs, gives in magnified form the movements of an earth-particle from the 20th to the 40th second of the shock, the numbers indicating the position of the particle at each successive second. The seismographs referred to are recording instruments, commonly horizontal pendulums, which register on paper strips the various components, horizontal and vertical, of the movements. So delicate are these instruments, that they register even those violent shocks which originate at the very antipodes of the observatory where the instrument is installed.

Magnified model, showing the movements of a surface particle of earth from the 20th to the 40th second of a shock. (Omori).

Fig. 2. - Magnified model, showing the movements of a surface particle of earth from the 20th to the 40th second of a shock. (Omori).

The study of seismographic records has brought to light many highly significant facts, among others that minute and insensible tremors of the earth are almost incessant, but some, at least, of( these tremors are due to atmospheric changes and it is not known how large a proportion of them are of subterranean origin. The term " earthquake" is usually restricted to those movements of the ground which can be felt, though the distinction is a somewhat arbitrary one. Another very important result of the seismographic observations is that when a very distant earthquake is registered, three series of waves are indicated, viz., the ist and 2d phases of the preliminary tremors, and the larger waves of the main shock. Those first to arrive, called the preliminary tremors, are believed to be transmitted through the mass of the earth along the chord of the arc included between the point of origin and the point of observation. The preliminary tremors include two of the three series of waves, known as phases.

The heterogeneous mass of rocks which forms the outermost crust of the earth does not permit the transmission of any simple form of wave-motion, and it is only at a distance of about 10° of arc of the earth's surface (about 700 miles) that the three different kinds of waves begin to appear upon the instrumental records. The preliminary tremors, which pass through the subcrustal region of the earth and travel at a higher rate of speed than the waves which follow the surface, appear, as already mentioned, in two phases. The waves of the first phase are believed to be the normal, or compressional waves, and those of the second phase to be the transverse or distortional waves, the two known kinds of wave motion which can be transmitted through a homogeneous solid. The waves of the third series are longer and slower (i.e. of greater amplitude and period) and constitute the "main shock"; they are believed to follow the curvature of the earth's surface.

Fig. 3.   Seismographic record of the San Francisco earthquake of 1906, U. S.

Fig. 3. - Seismographic record of the San Francisco earthquake of 1906, U. S. Coast Survey observatory, Cheltenham, Md. A, Preliminary tremors, ist phase; B, Preliminary tremors, 2d phase; C, Main shock. The upper record shows the north-south component, and the lower gives the east-west component. (Bauer).

Earthquake regions of the Eastern Hemisphere, (de Montessus de Ballore).

Fig. 4. - Earthquake regions of the Eastern Hemisphere, (de Montessus de Ballore).