Page:Great Neapolitan Earthquake of 1857 Vol 2.djvu/370

Rh their divergence. Again, on the north of the 2 mile circle, the extreme of divergence northward of the 2 mile circle of Laviano, Valva, and Altamura, and the large spread to the northward, of the isoseismals eastward of the great interrupting north and south ridge, between Avigliano and Lauria, e.g., the swelling of the curve, from Buccino to Monte Peloso; point to an impulse-producing surface, extending northward, within about a mile of Balvano. We therefore come to the conclusion that the whole horizontal dimension of the focal cavity was about nine geographical miles.

But to answer the observed conditions, direction of the wave-paths, normal or nearly normal, to the longest, of which within any isoseismal, the surfaces of the walls of the cavity must be; and to answer the observed emergences near the centre, as at Vietri de Potenza, Auletta, Polla, &c.; the horizontal line, joining the north and south ends of the focal cavity, cannot be a right line: the horizontal section, of the fissure or lamelliform cavity, must therefore be a curve, and the only curve that will answer the conditions, is that drawn by a broad red line or band on the Maps A and B, between the points marked $$f$$ and $$f'$$. It passes from near Balvano, on the north, close to Vietri di Potenza, Caggiano, and Pertosa, and to the north and west of Polla.

The curve so traced, is one of contrary flexure, and the effects necessarily following, from the concavity and convexity of the surfaces (of impulse,) which it generates with the vertical dimension, will, on careful consideration and comparison with the maps, be found singularly in good accord with the irregularity or distortion of the isoseismals; and with the mutual relations between both; and the physical conditions of surface, &c., operative in their co-determination.

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