of post-cards, one of which was to be returned to him at the end of each week with a record of the shocks which had been felt. "The barricade of post-cards was then extended farther northward, with the result of surrounding the origin of certain shocks among the mountains, while others were traced to the sea-shore. By systematically pursuing earthquakes, it was seen that many shocks had their origin beneath the sea, ... but it was seldom that they crossed through the mountains forming the backbone of the island."[1] When the country had been thus mapped out, it was possible to choose the most advantageous sites for the observatories.
It would carry us too far into technical matters to describe the method of searching into the bowels of the earth for the actual point of disturbance. It must here suffice to say that if a shock be accurately-timed at various places, and if the approximately circular ring where it was most severe be determined, it is possible to find with fair accuracy the spot or spots under which it originated and the depth of the earthquake-center. Even without accurate time-observations, Mallet was able to show that the Neapolitan shock originated between three and seven miles below the surface. The Yokohama earthquake of 1880 appears to have had its center at a depth of from one and a half to five miles. Notwithstanding that one earthquake has been estimated as originating at a depth of fifty miles, it is probable that in all cases the center of shock is only a few miles below the surface.
The vagueness as to the position which has been assigned for the center of disturbance in the case of particular earthquakes probably depends less on the difficulty of tracing back the vibrations to their origin, than on the fact that the shocks do not usually originate in a single point, but rather along a line of a mile or two in length.
As to the way in which seismic activity is distributed geographically over the earth's surface, certain broad conclusions have been fairly well ascertained. If a map be shaded, so as to represent the frequency of earthquakes, we see that the shading has a tendency to fall into bands or ribbons, which generally follow the steeply sloping shores of continents and islands; and it is probable that the actual origins of the shocks are generally situated under the sea not far from the coast.
It is a further interesting peculiarity that the most important bands fall end to end, so that they may be regarded as a single ribbon embracing nearly half the earth. It may be suspected that this ribbon really meets itself and forms a closed curve, but this can not be proved as yet. We may begin to trace its course at Cape Horn, whence it follows the Andes along the whole western coast of South America. At the north of that continent it becomes somewhat broader, but its course is clearly marked along the line of the West Indies from Trinidad to Cuba. Hence it passes to the mainland in Mexico, and then runs along the whole western coast of North America. We then trace the line
- ↑ Milne, "Earthquakes," p. 189.
