Robert Mallet

“ If the body be not free, if the line of wave transit passing through its centre of gravity pass within the base or through any other support, it does not move in the first semiphase of the wave; but if it be free in the opposite direction, it will be displaced in the second semiphase of the wave; but as the wave movement is now reverse to that of its transit, the inertia of the body acting still contrary to the applied velocity, now impels the body in the same direction as the wave transit. ”

“ There was every reason for my believing that the vase remained where I found it, half embedded in a flower-border, in the locked-up garden of the prior, and untouched since the earthquake; and the splintered and disjointed state of the limestone blocks of the pier, itself indicated the extent to which it had vibrated. The corresponding vase was not thrown though loosened, nor was the pier as much shattered, though I could not discover any very certain cause for the difference in effect upon both. Each vase had been steadied when in place by a slender iron dowal. ”

“ The angle of emergence for any given depth of focus diminishes as a function of the distance of any given point of the surface from the seismic vertical; and I have shown elsewhere that the power of the shock to overthrow objects is, cæteris paribus, a maximum at a determinate distance all round the seismic vertical; and this distance would be equal all round, were the earth homogeneous, and the focus or centre of impulse confined to a mathematical point. The centre of impulse in nature, however, occupies determinate, and often large dimensions. ”

“ By the first, we seek to obtain information as to the depth beneath the surface of our earth at which those forces (whether volcanic or otherwise) are in action, whose throbbings are made known to us by the earthquake, and thus to make one great and reliable step towards a knowledge of the nature of these forces themselves; and this is the great and hopeful aspect in which seismology must be viewed and chiefly valued. ”

“ It is only by first gaining some commanding point, whence a general view over the whole field of ruin can be had, and observing its places of greatest and least destruction, and then by patient examination, compass in hand, of many details of overthrow, house by house and street by street, analyzing each detail and comparing the results, as to the direction of force, that must have produced each particular fall, with those previously observed and compared, that we at length perceive, once for all, that this apparent confusion is but superficial. ”

“ A shock in whatever general direction acting here upon the houses and towers, perched on declivities, now rocky, now diluvial, and scattered here and there and facing every point of the compass, must produce effects in the highest degree complicated, or even unaccountable. ”

“ Now, we occasionally find walls that are extremely massive in proportion to their altitude, and built of small stones or brick laid in bad masonry, and with almost bondless mortar; such walls have little or no resilience, and, when thrown more or less out of plumb, or fissured, with suitable conditions, afford a rude approximate measure of the horizontal amplitude of the earth wave, by the range of movement impressed at the level of the centre of oscillation. ”

“ A little further on, after passing a torrent that falls into the Tanagro from the south, I look up in extremely steep lateral valley—Il Vallone Petroso—in which many loose surface blocks of limestone, show themselves to have been shaken from their positions and rolled over: from the road at this point there cannot be less than 3,500 feet vertical, of calcareous beds above me. The geological evidences of violent dislocation and elevation at all sides in the mountain formation are strikingly grand. ”

“ Were we at liberty to consider the vibration of the wave with vertical emergence performed in like manner, the movement of any point of matter upon the surface due to it would be limited to two motions—one directly up and down, due to the amplitude, and the other forward and backward horizontally, due to its altitude and to the senses. ”

“ 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. ”

“ The fissure is a flowing curve similar in horizontal plan to the contour of the hill side, and its line of direction is just that most favourable, to a throw off and slip of the clay masses, upon the sublying rock, by the jog of the earthquake, which is unquestionably the nature of its formation. ”

“ When the wave is nearly normal, and transverse to the ridge of one of those large heavy timbered roofs, with the ends of the principals resting directly upon the masonry, of the top of the side walls, or occasionally perhaps upon a heavy wall plate of ill-squared timber, the inertia of the whole is so enormous, that in almost every instance, the wall first reached by the wave was thrown outwards, by the shove from the roof and its own inertia together, and the whole roof then dropped nearly plumb down upon, the area of the building, crushing everything before it. ”

“ In this vitalizing climate, but a few years will elapse, before the frosts of winter and its torrential rains, will have pulverized and reduced to interstitial soil, much of the dry rubbish: plants and seedlings will root in it; forest-trees will send their searching roots downwards through it, and beneath their fostering shade, a jungle of shrubs, weeds, mosses, and coarse grass, will spring up and conceal with verdure the harsh colouring and form, of the heaps that once were a city ”

“ The relation of the buttress to the wall, as a support against transverse forces of a statical sort, is no longer the same, when the overthrow is produced by a force applied with the rapidity of the wave of shock; there may not be time, to transmit its own stability to the remainder of the wall.
Where the buttress is at the same time a tower rising much beyond the height of the remainder of the building, these generally tend to mutual destruction ”

“ As the water of the Calore thus divided between the subterraneous and the open channels at Polla is very muddy, and contains much calcareous sediment, it is obvious that that portion which passes through the subterraneous channel is filtered, or at least deposits much of its solid material on the passage, and is no doubt now forming new and strangely situated beds of clayey limestone, within the cavernous heart of the mountain range through which it passes, and which here separates, as by a huge wall, the valley of the Tanagro, from the Piano di Diano. ”

“ The fissures evidently had in no case run down plumb into the soil, but sloped in the same direction, but with a less angle to the vertical than the hill side on which it was found.
There is hence nothing very surprising in their occurrence. They are all in deep soft diluvial clays, of great specific gravity, very fine grained, and almost free from gravel and stones—a tenacious clay loam, that when wetted gets at once into a sticky paste, and soon runs into a greasy cream, so that a very small declivity and moisture alone, produce frequent land slips. ”

“ Fracture seldom or never occurs through the solid stone, in masonry, but always at the mortar joints, and generally by their loss of adhesion, to the stone at the faces of the joint. It rarely occurs through the brick, in brickwork, and only when the cohesion of the brick itself, is less than that of the cement. ”

“ The average slope of the rock, beneath the soil of the great fissure here, cannot be less than 25° the horizon, and may be much more. That of the fissure is still steeper. The "work done" by the shock, in originating the actual transport of material, due to these fissures is not very large; it amounts merely to reducing the friction and coherence of the mass, so as to permit the descent, through probably not more than 3 inches vertically, of several millions of tons of earth, the movement of descent being produced by gravity only. ”

“ There are some fissures, in all the walls, but the great mass are in those running north and south. These make angles with the vertical of 30° to 40°. They cross in both directions, but the larger and greater number, are inclined at top towards the north.
The fissures in the walls running east and west are all more nearly vertical, than those in north and south walls. They prove that the building rocked in all directions, but that the main force was one emergent from the north towards the south, at an angle with the horizon of between 50° and 60°. ”

“ The shock through the limestone reached the Certosa from the Colline of Padula, the nearest point of rock in the path, through an intervening stratum of clays and gravels, by which there must have been some loss of vis vivâ. If we throw off the decimals for this correction, which we can only estimate, we have finally, for the surface velocity of translation, 240 yards per second in the limestone, and 239 yards per second in the clays and gravels, measures approaching so nearly to equality, as to warrant one or both of the following conclusions. ”

“ Polla was an important place; originally, as its name imports, one of the ancient foundations of Magna Græcia. Nothing older than middle-age architecture remained, however, before the earthquake, and of this the Castello, near the summit of the town, was the most prominent. Its position in the rich country around, had produced its rapid modern growth to nearly seven thousand inhabitants, and most of its buildings were comparatively modern and pretty well built. ”

“ When there are two or three floors, over each other, that thus carry away the walls, they all (walls and floors) go towards , the walls wholly falling outwardly. It will readily be conceived, what an inextricable mass of confused ruin, houses thus thrown down present, for the side walls that run parallel with the joists (or beams) , fissured before transversely to the line of wave transit, are always more or less shaken down also, by the tremendous descent of the floors and walls, upon them falling. ”

“ From the extent to which destruction had proceeded, and the vile class of the masonry, measurements of the widths of fissures, were uncertain and the angles of fracture ill defined. From measurements and general judgment together, however, it appeared to me that the angle of emergence here must have been extremely steep, steeper than the indications at the Villa Carusso, and certainly not less than 70° with the horizon, possibly as much as 75°. ”

“ No roads whatever, suited to wheel traffic, exist throughout the kingdom, except the five great military ways, and these are perfectly unconnected by branches, with any but a few great towns: hence all produce has to be carried by mules, or by hand; and journeying off the military road can only be accomplished in the same way, or on foot.
It results from the perched positions, of almost all these towns that they are exposed to the severest effects of every earthquake shock. They are rocked as on the tops of masts. ”

“ Petina has owed its immunity, first to the peculiarly strong form of the terrace upon which it is perched to resist any vibration in the mass itself, secondly, to the fact that from where I stand at Auletta Bridge, there is about 1,000 feet of piled-up limestone beds between me and Petina, so that any shock emergent at a steep angle, here or further eastward, must have passed up transversely through all these successive plates of variable hardness, and none in absolute contact with each other, and so the vis vivâ of the shock be enormously reduced before reaching the elevation of the village. ”

“ 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. ”

“ The common construction of provincial Neapolitan floors and roofs, has been already briefly described. Were the floors of an earthquake-shaken house perfectly homogeneous, formed, as if of a single parallel plate of cement or beton, self-supported and free from all timbering, the lines of fracture would be almost perfectly regular, and follow in accordance with those of the side and end walls; so that with a normal or subnormal wave, fractures would extend crossways, parallel to the walls, and generally uniting the fissures wherever these were situated in the latter ”

“ If, as in Fig. 69, the shock emerge in the direction —, 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. ”

“ The torrent separates these beds to the south from the cretaceous limestone which here makes its appearance once more. Sloping up steeply to the north for more than half a mile, with a bare, weathered, and water-furrowed surface, without a blade of vegetation, extend parallel, and nearly vertical beds of green, grey, and purple clays, alternating with beds of yellow, soft, clayey sandstone, many yards in thickness, standing in ridges high above the worn-down clays. ”

“ Were Seismology an older and more mature branch of science than it is, it would be impertinent to enter at any length into the means and methods by which it is to be pursued, in the observation of earthquake phenomena. ”