=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. [Illustration: FIG. 83.—Seismotectonic lines of the West Indies.] =The heavy shocks above loose foundations.=—It is characteristic of faults that they soon bury themselves from sight under loose materials, and are thus made difficult of inspection. The escarpment which is the direct consequence of a vertical displacement upon a fault tends to migrate from the place of its formation, rounding the surface as it does so and burying the fault line beneath its deposits (Fig. 43, p. 60).