CHAPTER XVII.
OF THE DECAY OF THE WAVE OF SHOCK AND ITS GRADUAL OR PER SALTUM EXTINCTION.
This will be most conveniently treated of under distinct subsections. In Chap. IV. the great and wide-spread conditions of surface configuration, as affecting the extent of diffusion of the wave of shock, and the forms of the isoseismals, have been already discussed. The present Chapter has reference to some of those conditions of like class, which modify seismic action at particular spots within those great areas, and determine, either reduced or increased earthquake energy, at those localities, or change the directions of its action, and the extent of its propagation in different directions.
1st. Local reduction in the intensity of the shock wave, amounting almost to sudden extinction, may be produced by deep and wide fissures extending transverse to the wave-path, and filled in with loose or discontinuous or soft materials.
The nature of this action has been explained, as developed upon its largest scale in Chap. IV., and in Part II. in loc., its effects, as remarkably exemplified at Muro and Bella, have been detailed.
This retarding action is capable, of extremely local or
