Page:Great Neapolitan Earthquake of 1857 Vol 2.djvu/339

274 The meizoseismal curve, is almost a regular oval, whose longer axis is to the shorter, in the ratio of 30 : 12, and the seismic vertical, close to Caggiano, is very nearly in the focus of the minor or north end of the oval or about one geographical mile to the south and west of it.

What has produced this great distortion of figure, from a circle, with the seismic vertical in the centre? The maps show, that besides the great Apennine chain, passing through the area, at a small angle to the major axis, from between Tito and Potenza, on the north, to Lauria on the south, another great continuous chain, namely, that of the east flank of the Val di Diano, rises almost precisely at Caggiano (the seismic centre), and stretches down, nearly in a right line, and almost parallel to the major axis, as far as Tramutola, where it joins in with the former chain.

The impulse, from its very starting-point, was delivered "end on," along the latter chain, and so obliquely upon the first mentioned, as to produce much of its effect, longitudinally. These indicate, in brief, the main circumstances of surface configuration, that produced the very lengthened propagation of the wave, from the focus towards the south-east.

But other conditions conspired with these, to limit and shorten its propagation, in the orthogonal direction, or in that of the minor axis of the curve. To the north of the seismic vertical, rounding from Castelluccio and Buccino on to Avigliano, we have one of the extensions of the great east and west chain of the Apennines, opposed transversely to its progress, and at an average distance from the focus, of not more than six geographical miles. Again, from the same point (Castelluccio) we have, stretching thence along the whole length of the S.W. boundary of the curve,