Page:Great Neapolitan Earthquake of 1857 Vol 2.djvu/333

268 homogeneous medium. The outward passage of the wave, at like points of the phase, will not be in spherical but in ellipsoidal shells, whose greatest axis, will be perpendicular to the plane of the fissure.

1st. If the plane of the fissure be horizontal, and at a considerable depth beneath the surface, this greatest axis will be vertical, and the main power of the shock will be expended within a very limited area of the earth's surface, and the wave within it will emerge steeply; while its decay in passing outwards from the seismic vertical, will be extremely rapid, and the form of the isoseismals, in a homogeneous medium will depend upon the horizontal contour of the fissure. The phenomena will be very much those, of the Melfi shock of 1851, modified by the actual heterogeneity of the earth.

2nd. If the plane of the fissure be vertical, (suppose, to fix our ideas running north and south,) then in a homogeneous medium, the greatest axis of the ellipsoid, will be east and west, and the horizontal distances, upon the surface of the earth, for equal effort, greatest in the east and west azimuths, and least in those north and south, so that the form of the isoseismals will be elliptic.

3rd. If the plane of the fissure be inclined, more or less, to the horizon, as $$f\ f$$, (Fig. 347,) $$e\ e$$ being the earth's surface, then the greatest axis of the ellipsoid will be inclined also, and in the direction $$s\ c\ d$$, which will be the direction of greatest effort, and the overthrowing power of the wave will be unequal, both from amplitude, and direction of emergence, for objects situated at equal distances, as at $$s$$ and $$p$$, from the seismic vertical $$\mathrm {V\ O}$$, in the azimuth of thie greatest axis, i. e. in the azimuth of a plane perpendicular to the walls of the fissure and near it. Whence it results, that the isoseismals must, in this case, take the form of ovals or