Great Neapolitan Earthquake of 1857/Part II. Ch. XII

by Robert Mallet

Chapter XII.

First Deductions from Facts of the Certosa—Double Shocks.

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I now pass to the deductions to be obtained from the observed facts here.

There is evidence everywhere, of a double if not a triple shock, confirmatory of the statements made at the town of Padula, of oscillation in various directions. The main shock was in the primary wave-path, right along the Vallone 15° W. of north towards the south, and arrived, through the deep clays and loose material of the plain. This was preceded at a very brief interval by a secondary shock, transverse in path to this by a certain angle, and derived from the lateral vibration of the mass of limestone mountain on the range to the north-east. Lastly, the primary shock appears to have been reflected, from the abrupt neighbouring mountain further south, and to have returned again, as an earthquake echo, through the clays, with very diminished force, arriving last upon the scene.

Referring to the Photog. No. 225, of the monument of St. Bruno, it will be seen that many of the obelisks and finials are twisted, and some are overthrown. We have universal evidence of the shock, in the path 15° W. of north to south. Here the finials which are overthrown are thrown directly westward. All those that are twisted are turned from left to right.

Now there are two distinct trains of earthquake causation, by either of which bodies may be twisted on their bases. 1st. By the action of a single shock, when the centre of adherence of the base of the object, lies to one side or other of the vertical plane passing through the centre of gravity, and the line of the wave-path. 2nd. By the conjoiut action of two closely successive shocks. By the first shock, the body is tilted up from its base, but not overthrown, so that for a time greater or less, it rests wholly upon one edge of its base; while thus poised, if another shock bear upon it, in any direction transverse to the first, it acts as usual at the centre of gravity of the body, to displace it by inertia, in the contrary direction to the wave transit; but the body is held more or less, by friction at the edge momentarily in contact with its support, and there only; but this edge must always lie to one side of the vertical plane passing through the centre of gravity, in the direction of the wave-path: hence the tilted body, while relapsing upon its base also rotates, round some point situated in the edge of its base upon which it had been tilted, and thus it comes to rest in a new position, having twisted more or less round a vertical axis.

If the observer look due south at a square pyramid, for example, whose sides stood cardinal, and it be tilted by the first semiphase of a shock from east to west, the pyramid will tilt or rise upon the eastern edge of its base; and if, before it has had time to fall back, it be acted on by another shock from north to south, the pyramid will rotate, upon the bisection or on some other point, of the edge on which it momentarily rested, and will hence come to repose, after having twisted from left to right, or with the hands of a watch.

If the tilting up, had been produced by the second semiphase, of the same shock from east to west, then the pyramid would have risen upon the western edge of its base, and the same direction (north to south) of second shock, would have produced rotation upon that edge, but in a contrary direction to the preceding, or from right to left, or against the hands of a watch.

Again, if, on the first supposition, the first semiphase of the east to west shock, had tilted the pyramid upon its eastern edge of base, but the second shock had been from south to north, in place of the reverse as before, then the rotation would have been from right to left; and if tilted by the second semiphase on the western edge, the second shock, south to north, would produce rotation left to right.

It would therefore appear at first impossible, to determine the direction of motion in transit, of either shock, from such an observation: we can, however, generally discover upon which edge of the base any heavy body of stone or masonry has tilted, by the abrasion or splintering of the arris, and the rotation must have taken place round some point in that edge. If, therefore, we know the direction of either one of the two shocks, we can always discover that of the other, by the rotation observed; and if the time of oscillation of the body be ascertainable, we are enabled to calculate a major limit, for the interval of time that must have elapsed, between the arrival at the twisted body, of the first and of the second shock, when both the wave-paths are known.

With a single instance of such twisting, it may be impossible to decide, whether the twist has been due to one shock, (1st case) or to two shocks in succession, (2nd case); but when several bodies alike or dissimilar, at the same locality, are all found twisted in one direction, it is certain to have been the work of two distinct shocks, for it is beyond the reach of probability, that several bodies, should all happen to have their respective centres of adherence, at the same side of their respective centres of gravity, and unless they have, some will rotate in one, some in the other direction by any single shock; rotation thus produced, being always by the centre of gravity, moving contrary to the first or second semiphase of the wave, and carried round the centre of adherence, by the line joining them as a radius vector; the inertia of motion at the centre of gravity, and the resistance of the point of rotation in the edge of the base, or of the centre of adherence, forming in every case, the extremities of the dynamic couple.

All the effects of the double shock will be understood by examination of the Figures Nos. 235 and 236, in which

Fig. 235 shows the action of any double shock; Fig. 236 the variations of result produced, first, by rotation in the first semiphase A, and second semiphase B, by the same double shock; secondly, the like by rotation, in the first semiphase (C), the first shock being as before, but the second, contrary in direction to that of the previous cases (A and B), and the like for the second semiphase (D), the two shocks being the same (C and D).

Applying this to the facts at the monument of St. Bruno. All the finials, &c., are twisted from left to right; we know that the main shock was from 15° W. of north to the south, it therefore follows, that the shock which first moved them, arrived in a path somewhere between that, and from east to west: by this they were tilted; by the immediately following shock, 15° W. of north to south they were twisted. Neither shock was sufficient, in velocity or range, completely to overthrow any of them, except those which were top-heavy, by having had balls at their summits, which have, except in one instance, been all dislodged.

A great many pyramids and finials on the top of the fountain in the entrance square B, Fig. 1, Diagram No. 240, and Photog. (Coll. Roy. Soc.), presented precisely similar phenomena, as did those on the parapet of the great façade Photog. (Coll. Roy. Soc.), and in divers other places.

The complex forms of these objects, which rendered the ascertainment of the positions of their centres of oscillation on the edges of their bases, difficult and uncertain, unless by experiment, prevents any calculation of a precise character, from their movements, as to the velocity of either shock, nor do we require it.

They give us other valuable information, however. In the case of the parallelopipedal chimney, (F. Fig. 1, Diagram Nos. 238–240, and Fig. 4, same diagram), twisted upon its base, it had rotated upon a point in the western edge of its base at $b$, Fig. 237. We know already that the direction (generally) of the first shock, was from some points east or N. E. towards the west or S. W., the second being from 15° W. of north to south. The chimney stalk had therefore made, one semi-oscillation, and one complete oscillation; that is, it was being acted on by the second semiphase of the wave of the first shock, at the moment when the second shock arrived at it, as in Fig. 237.

The centre of oscillation of the chimney above $b$ thus tilted was, as nearly as could be ascertained, 4.33 ft distant from the edges of the base upon which it tilted $b$ and $b_{2}$. The first shock, east to west, fractured the chimney from its base, and produced in the detached chimney, one semi-oscillation eastward ($\mathrm {A}$, Fig. 237). The chimney then relapsed upon its base ($\mathrm {B}$), Fig. 237, and rising again upon the edge $b_{2}$, leaned over westward ($\mathrm {C}$), Fig. 237, having thus made one complete oscillation in that direction, with the moment of repose ($\mathrm {B}$), when it had fallen back plumb upon its base. Between that moment of repose, and the completion of the oscillation, or almost instantly after it had commenced to fall back ($\mathrm {C}$) from west to east, to reassume its original position of repose, the second shock from the north to south reached it, and twisted it round horizontally, in the manner that has been already explained.