Fig. 81.—Map of the island of Ischia to show how the shocks of recent earthquakes have been concentrated at the crossing point of two fractures (after Mercalli and Johnston-Lavis).
By far the most unstable belt upon the earth’s surface is the rim surrounding the Pacific Ocean, within which margin it has been estimated that about 54 per cent of the recorded shocks of earthquake have occurred. Next in importance for seismic instability is the zone which borders both the Mediterranean Sea and the Caribbean—the American Mediterranean—and is extended across central Asia through the Himalayas into Malaysia. Both zones approximate to great circles upon the earth’s surface and intersect each other at an angle of about 67°. It has been estimated that about 95 per cent of the recorded continental earthquakes have emanated from these belts.
Fig. 82.—A line of earth fracture indicated in the plan of the relief, which may at any time become the seat of movement and resultant shock.
The special lines of heavy shock.—Within any earthquake district the shocks are not felt with equal severity at all places, but there are, on the contrary, definite lines which the disturbance seems to search out for special damage. From their relations to the relief of the land these lines would appear to be lines of fracture upon the boundaries of those sections of the crust that play individual rôles in the block adjustment which takes place. More or less masked as these lines are beneath the rounded curves of the landscape, they are given an altogether unenviable prominence with each succeeding earthquake. At such times we may think of the earth’s surface as specially sensitized for laying bare its hidden structure, as is the sensitized plate under the magical influence of the X rays.
When, at the time of an earthquake, blocks are adjusted with reference to their neighbors, the movements of oscillation are greatest in those marginal portions of direct contact. Corners of blocks—the intersecting points of the important faults—should for the same reason be shaken with a double violence, and this assumption appears to be confirmed by observation. Upon the island of Ischia, off the Bay of Naples, the shocks from recent earthquakes have been strangely concentrated near the town of Casamicciola, which was last destroyed in 1883. This unfortunate city lies at the crossing point of important fractures whose course upon the island is marked by numerous springs and suffioni ([Fig. 81]).
Seismotectonic lines.—The lines of important earth fractures, as will be more clearly shown in the sequel ([p. 227]), are often indicated with some clearness by straight lines in the plan of the surface relief ([Fig. 82]). Lines of this nature are easily made out upon the map of the West Indies, and if we represent upon it by circles of different diameters the combined intensities of the recorded earthquakes in the various cities, it appears that the heavily shaken localities are ranged upon lines stamped out in the relief, with the most severely damaged places at their intersections ([Fig. 83]). These lines of exceptional instability are known as seismotectonic lines—earthquake structure lines.
Fig. 83.—Seismotectonic lines of the West Indies.