Great Neapolitan Earthquake of 1857/Part I. Ch. XI

by Robert Mallet

CHAPTER XI.

SECOND CLASS OF CONDITIONS MODIFYING EFFECTS
SHOCK—FIRST ASPECT OF RUINED TOWNS.

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This disposes, by anticipation, of the third class of modifying conditions. As respects the second, nothing is more remarkable and puzzling to an unpractised observer, who enters a town situated upon tolerably level ground, than the apparent caprice by which the fall of the buildings is characterized. He finds one whole side of a street cleared down; turning into the next, within a few hundred feet, he needs be on the watch, to discover any signs of injury. Further on, whole districts of streets have disappeared, and one heap of rubbish, stones, and beams occupies their place; yet, not far off, long lines of houses are but fissured. In some large streets the houses are down here and there, in the most irregular order, some of the very loftiest stand pretty safely—some of the humblest are in dust.

These abrupt changes from safety to destruction are still more remarkable, if the town, in place of resting upon the plain, or on pretty level ground, occupy the summit and flanks of some "colline," or conoidal hill. Here, perhaps, at one side of a line passing over the crest of the hill, nearly everything is demolished; at the other, little damage has been done. Until by the help of such observations as have been described, upon some one or more of the injured but not overthrown large and cardinal buildings, the observer has got some clue to the direction of transit of shock, he is completely bewildered; and if he has no clear notion formed on this point, he will be much disposed to coincide at first, with the opinions chattered around him by the principal townsmen, the Syndic, the Judice, the Sotto Intendente, &c., who accompany him, and assure him that the shock was in every direction "tutti, tutti direzioni,"—that it was "orizontale e vorticoso;" and point out, in proof of this, that the buildings have actually fallen in all possible directions, which is undeniable.

A town formed of streets of adjoining and abutting houses, may be viewed, as respects each block of buildings, as a vast cancellated single structure, in which, in certain directions at least, every portion adds stability to all the rest.

The larger the single block of houses, or the longer the line at the sides of a street, if the shock be in the direction

of its length, the less, cœteris paribus, is the injury done; but there are other causes also in play. Those of some of the principal phenomena may be best illustrated by reference to Fig. 65, an imaginary block plan, of part of an earthquake-shaken town. Whole districts at $qq$ are prostrated. At both ends of the great church, and about $m$, and along the street from $h$ to $e$, houses have fallen, and others are grievously injured; yet these are perhaps amongst the best built and least ancient houses in the place. Passing along the main central street, after debouching over heaps of stone at $e$, it is found, that along the greater part of its length, fissures here and there, a few chimneys, or a side wall down, as we look up some "vico," are all the signs of earthquake visible.

The great church, when examined, tells us that the horizontal direction of shock was from $a$ to $b$. Returning now over the ground, and examining the place with the compass in hand, we find that the main street presented its length to the direction of movement; its stiff front and rere walls, its multiplied floors, have saved it. The street $e$ to $h$, on the contrary, although of houses of the same date, height, and character, &c., is nearly destroyed; it was nearly transverse to the line of movement. The street at $m$ has escaped much better, though much injured, yet it is nearly parallel with $eh$, and the houses are as high if not higher; but we find they are some of the newest and best-built houses in the town. Some houses at the right of the Chiesa Madre are thrown down, but no others in the Piazza, have suffered. The rubbish has probably all been cleared from the Piazza in the market-place, and we cannot tell which way the houses fell, but we look at the fallen apse or campanile or transept, and find that it fell against one of these, and upon these houses. We now get out of the highways of

Photo Pl.66		Photo Pl. 81
	⁠
Vincent Brooks, lith. London
(illegible text).	⁠	Interior of the Cathedral at (illegible text). looking North West

the place, and find a large district whose length is almost parallel with the well-preserved main street and yet is a mass of rubbish. No house stands to tell us of what they were; but, on examining the character of the stones, and of the timbers, the shattered doors and window-shutters, and so forth, we find this was one of the poorest quarters of the town, and, as always happens (except in rapidly growing cities), one of the oldest. The buildings here are in heaps, because before the shock they were tottering and ready to fall. The whole matter has become clear, and the fact has been learned, that entire towns give little information to the seismic observer, and that but of a general and often uncertain character—that single buildings are his proper objects.

Had the direction of shock been $a'$ to $b'$, in place of what was assumed, a little examination of the figure will make it evident that the whole train of phenomena would have been changed. Hence the prodigious complexity of phenomena presented by towns that have been subjected to two or more shocks in different or in orthogonal directions in quick succession. It is needful to state these circumstances, trivially simple as they appear, because hitherto they have been unnoticed and unregarded by previous earthquake describers. Even the scientific reporters of Calabrian and other earthquakes, who multiply such facts, seem to have had no clear conception of their cause and connection.

The Photogs. Nos. 66, (67 and 68, Coll. Roy. Soc.,) illustrate also some examples. In No. 66, a street in Polla, the direction of shock, was nearly perpendicular to the plane of the picture, and from the observer. The face of the houses, built well and with a batter, here receiving the shock nearly end on, remained in great part standing, and so saved all beyond them, while all lower down that part of the city was destroyed.

In part of a street in the humbler part of Pertosa, Photog. No. 67, (Coll. Roy. Soc.,) the shock was nearly in the perpendicular, to the plane of the picture, and from the observer. One side of the street, that on the right hand, is almost quite down; the opposite one was comparatively safe. The houses were much of the same character at both sides; but on one side the shock pulled out the fronts, that have fallen away from the roofs and joists, which had no hold upon them owing to the method of construction, while it pressed inwards, against the firmly resisting edges of the floors and roofs the other side, which the wave, in its return vibration, had not sufficient velocity in its horizontal component to bring down.

In Photog. No. 68, (Coll. Roy. Soc.,) some of the highest houses in Polla were left standing tolerably safe, though full of apertures, and arched galleries in front; while the same street, lower down and near the front of the picture, was choked with the ruins of houses, of only half the height or less. The former were new and well built in comparison with the latter.

If in place of the town standing upon pretty level ground, as previously assumed, it stand upon a hill summit, and on its flanks, the difference of devastation, at different sides, is usually great. If, as in Fig. 69, the shock emerge in the direction $a$—$b$, the buildings at the right and left, are differently affected, because their grasp upon the ground at their foundations is different, the relative direction of emergence different (as will be more fully examined when treating of the relations of geological formation to shock), and often (as at Saponara) chiefly because the houses that fall at the steep side to the left hand, fall against and upon,

those below them, and the accumulated ruin falls in an avalanche of rubbish down the hill-side.

So also if, in place of a steep angle of emergence, the line of shock were nearly horizontal, as from left to right, in the figure, the whole of the steep side of the town, during the first half vibration, tends to fall from the hill-side, and therefore away from all the natural abutments that their foundations excavated from the hill-side afford to the sides nearest the hill-top, but all of which generally are operative in favour of the houses at the other side of the hill.

This difference may be much compensated by the nature of the rock formation beneath the town, as well as by the form of the hill, &c., but this belongs to a future part of the subject.