Page:Great Neapolitan Earthquake of 1857 Vol 2.djvu/322

Rh shore, I believe to be much exaggerated, and hence the areas larger than they ought to be.

Seismal area alone, however, affords no test of comparative seismic energy. If the depth of focus were the same, the area would be, cœteris paribus, a measure of comparative energy.

The focal depth, however, in different earthquakes, in the same region, may, for anything we yet know, differ greatly, and either a very deep focus, with such a form of focal cavity, or such other conditions, as shall produce wave-paths, chiefly emergent at very steep angles; or a very shallow focus, although of equal intensity of original impulse, may equally result in great limitation of area shaken; and yet within it, or within its central portion, the destruction may be great or absolute. The former of these conditions, seems to have been in operation in the Melfi shock of 1851, whose emergence at and around Melfi, was extremely steep, and its destructive energy within the central region, tremendous; but the total area shaken, was very moderate, and the decay of energetic effort in passing out from the centre, extremely rapid.

In fact, the impulse in this case, seems to have been delivered upward, as through a funnel, formed by the volcanic formations about Melfi and Vulture, encircled by the limestone and murgie rocks.

Each isoseismal area as marked, is in fact, the partial integration of the total seismic effort, for the space, and within the conventional limit as to effect, fixed upon for each; and distances proportional to the eccentricity of the seismal curve at any points from the seismic vertical taken along any radii indicate points of equal effort, (apart from

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