Great Neapolitan Earthquake of 1857/Part II. Ch. VI
CHAPTER VI.
ENTRANCE WITHIN THE MEIZOSEISMAL AREA—AULETTA—GREAT EARTH FISSURES.
I had no sooner passed into this hollow, than it became evident, that all at once, I had got within the radius of formidable earthquake violence: on every side ruined and prostrate buildings presented themselves. Descending towards Auletta, I can see Buccino greatly elevated, and some six miles to the north: many of the people here call it Bugille, pronounced like French, without the final vowel. This corrupt pronunciation of names is frequent, and renders recognition by maps often difficult; the same name is often pronounced half a dozen different ways by as many persons—an existing example of that jargon of living tongues, betwixt closely adjacent places, of which Sismondi gives so vivid a picture in those very regions, in his 'History of the Literature of the South of Europe in the Middle Ages.'
At half a mile in a right line from Auletta, I pass a ruined house, of one story, of about twelve feet in height, and about twenty feet square in clear of walls, which are two feet thick, pretty well built, and not above ten or fifteen years of age. This and a much larger building, a sort of farmstead, at the opposite side of the road, with a great spread of gable, and front parallel to this little house, present evidences of great violence here, and extremely steep emergence of the wave. The house (Fig. 132) is fissured from wall plate and ground, in the north
and south walls, near the quoins, and over the entrance door, the lintel of which, a single slab of limestone of about 12 in. by 22 in. wide, is fractured right through by the rocking of the wall at either side of it. The roof is all fallen in, except one purlin, which is still in situ, and proves, on examining its ends in the gable walls, that the east gable, and the roof along with it, all came to the eastward, at the first movement of shock, drawing the purlins from their sockets in the west gable; some were drawn quite out, and the roof at once fell in; others returned by the back stroke of the wave, and drove the
ends of the remaining purlins, back again into their sockets like battering rams, throwing out portions and dislocating the west gable. The north-west quoin has had a long wedge-shaped mass projected right outwards, showing a very steep angle of emergence. The dislocations generally, here and at the opposite side of the road, give evidence of a wave-path from E. to W., and an angle of emergence of upwards of 45° with the horizon. I did not measure any Draft Pl. 102
AULETTA, SHOWING THE DIRECTIONS OF THE LANDSLIP,
AND LONG FISSURES IN THE SOIL.
EYE SKETCH
angles, however. In this house a whole family were crushed beneath their humble roof: they had been asleep, and, awakened by the first movement or by the noise, had tried to escape, but the broken lintel, had jammed the obdurate oaken door, just within which their bodies were found collected, beneath the mass of tiles and timber.
Auletta stood upon an elevated knoll, jutting with a S. E. direction from the E. slope of the mountain, of its own N. and S. valley, with the Tanagro sweeping past its base to the southward, and joined by a small torrent on the right bank from the Auletta valley. The bottom of the proper valley of the Tanagro here presents a broad level plateau of a mile or so across, upon which, at the river and beneath the base of the knoll, are some other houses, &c., with a poor and now half-ruined locanda, that also go by the name of Auletta. The town itself on the top, had about three thousand inhabitants, some fine large houses inhabited by official persons, and a mass of poorly built ones, all of the nobbly limestone rubble. It is mediæval, and was fortified in the middle of the sixteenth century, the castello alone now being the only visible fragment of this.
Referring to the eye-sketch (Fig. 132), the spur or elongated knoll upon which it stands has a general direction of N. 60° W. The N. E. side is extremely steep. From the highest point of the remains of the castello I found by the theodolite the general angle of the scarp to be 41° from the vertical. The opposite, or S. W. side, slopes much more gently. The N. W. junction with the mountain range, is a little depressed below the summit at the town, and undulatory. The spur consists of coarse calcareous breccia, in heavy irregular beds, with a nearly N. and S. strike, and dipping 35° to the S. and S. W. These are visible here and there all over the steep scarp, and at several points in ascending from the locanda, along the road at the S. W. flank. The character of the rock, which is the average of most of the breccia hereabouts, is seen in Photog. No. 133 (Coll. Roy. Soc.), taken from within forty or fifty feet of the rock; the pebbles are from three to ten inches diameter, and extremely round. The steep scarp is almost bare rock, but that to the S.W. is covered to within one-third of the height, or less, from the top, with diluvial clay, and olives grow over the whole slope. The clay is perhaps thirty to fifty feet in thickness near the base, between the road up towards the town and the Tanagro, thinning off to nothing as we ascend.
At the bed of the Tanagro, below the bridge that passes over the great military road, barom. reads 29.76 in., thermo. 51° (13th February). At the locanda, which is on the level of the valley piano or bottom, (14th February,) barom. 29.50 in., thermo. 52°; and at the summit of the town on top (13th February), barom. 29·41 in., thermo. 48°. These reduced (see table, Appendix), give for the respective levels above the sea—
Bed of the Tanagro . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
576.0 feet. |
Valley piano, or bottom . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
647.6 feet.„ |
Summit of Auletta . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
889.5 feet.„ |
The spur is therefore 242 feet nearly above the valley bottom. It can be scarcely half a mile in a right line across the base in a direction A to B of section, and perhaps two miles the long way, in its projection from the main slopes.
A knowledge of the magnetic declination, as affecting all my determinations, rendered frequent observations to ascertain its amount important; unfortunately, from the season of the year, and inclemency of the weather in these mountain regions, the observation of the sun or pole star was practicable but seldom. I therefore, in addition to such solar or stellar observations as were possible, took magnetic bearings, from many elevated known points, of others visible from them, and recognizable again upon the two great maps of the country, Zannoni's and Bachler D'Albe's, so that, by comparing the observed bearings with those of the maps, the declination might be checked. I took the following bearings at Auletta:—
| Villa Carusso bears from Auletta 7° | W. of N. | |||
|
30' | W. of N. | ||
|
3' | W. of N. | ||
|
0' | W. of N. | ||
|
0' | E. of N. | ||
|
E. of N. |
These give from 14° to 14° 50' declination W.
On the 13th February I was able to see the sun's disc, though not perfectly clear, and take observations. At 9h. 0m. Greenwich time by chronometer, the centre of the sun's disc bears 24° E. by compass. Taking Auletta to be in lat. 40° 30' N., long. 15° 23' E.
| The hour angle at time of observation | = 32° 25' | 34".50 | |||
|
= 36° 20' | East | |||
|
= 24° 0' | East | |||
|
= 12° 20' | West |
Auletta has suffered much from the shock, most so, at its highest portions, and upon the N. E. side of the summit, wher the most substantial buildings stood, and upon the S. W., where were some of the poorest and worst.
The general appearance of the locality is seen in Photog. No. 134 (Coll. Roy. Soc.) from the road sloping up to the town, and Photogs. Nos. 135 and 136 give two views nearly at right angles to each other, of some of the most instructive buildings at the upper part of the town (No. 135), looking, about N. W.
The propped wall, in both Photogs. is the same, and is square to the four parallel walls, which in No. 136 are observed all shorn off and thrown, from the western quoins. This is a good example of the parallelism of angle at which such fractures form in large masses.
At the lower part of the town, and upon the slope towards the N. W. leading to it, the fractures all indicate a very steep emergence from the eastward—upwards of 45°; but upon the summit, the buildings show a lower apparent angle of emergence, and greater dislocation. This obviously arises from the fact that, as the wave-path hereabouts was in some direction from E. to W., and therefore diagonally transverse, to the narrow or thin direction of the spur, upon which the town stands; the mass of the spur itself vibrated
Photo Pl. 136 |
Photo Pl. 135 | |
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Vincent Brooks, lith. London. |
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At Auletta. |
Auletta. |
with the blow as an elastic pendulum, so that its proper motion at top, which was quam prox. horizontal, was added to that of the emergent wave, thus reducing the direction to one of less emergence, and increasing the range and velocity of movement upon top. The great majority of the fractures at Auletta, indicate a wave-path E. and W. by compass; those of the castello, which is prostrated to the level of the first story, and stands upon the very brink of the precipitous side, when reduced give one of 115° W. of N.; but the buildings are not isolated, and the direction has been probably perturbed by this, and by longitudinal vibration in the mass of the spur on which it stands. On the whole, the wave-path here is E. and W.
The effects, of the form, elevation above its base, substance, and direction with respect to wave-path, of the collines or spurs, upon which so many of these towns are perched, in modifying the results of the shock upon them, are strikingly seen here, as well as at Castelluccio, which we have passed, and at the little village of Petina, which, from a point between this and Pertosa, I can descry with the telescope, perched high up, upon the south scarp of Monte Alburno, at least a thousand feet above me. It stands upon a level sort of short, stumpy, buttressed spur, jutting out from the steep mountain slope, which in form is like a piece of artificial earthwork, the little town standing upon the level platform on top, with a steep scarp in front of it, the mountain rising abruptly behind, and the scarp sloping in and getting lost in the mountain sides to the E. and W. of the town. The terrace upon which it stands, however, is not earth, but solid limestone—a projection, of the great horizontal strike of the beds, of the great scarp of Alburno, as in Fig. 137, which dip pretty sharply to the south. I find upon inquiry here that Petina, which is just five
Italian miles S. W. of Auletta, and about six from Pertosa, has nevertheless suffered absolutely nothing, although these latter towns are in great part prostrated.
The immunity of Castelluccio from injury arose, as I before remarked, from the long dimension of its well-buttressed knoll being opposed to the line of shock, as well as to the barrier interposed between it and any shock coming from the eastward, b the mass of vertical breccia beds to the east of the town.
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. This is probably also the reason why huge masses, of the towering and ruiniform crags, that form the highest summits of Monte Alburno, far above Petina, have not been brought also toppling down into the valley. As it is, however, I find, according to the information of the guard of gendarmes here stationed, that there have been some heavy falls of rock in the Vallone Petroso, and at other places along this end of the scarp of Alburno.
At Auletta it was alleged to me by these gendarmes, who arrived there within a day or two after the shock, and whose statements were confirmed, by those of some dozens of the poor inhabitants, that large and long fissures had opened in the earth around the town, but that they had since all become closed again, and they doubted that they could now be seen. I gave much scrutiny to this, and having got the corporal of the guard to come with me, he pointed out the place where he had observed one of the largest of these fissures, to the north-west of the town, amongst olive grounds, and in deep clays (at F, Fig. 132). After some time I found myself unmistakeable traces of the fissure, in a continuous sort of little narrow trench, about 12 or 15 inches wide, at the widest places on the surface, but generally not more than 8 inches wide, and of a blunt V shape in cross section, with rounded edges, and not above 8 or 9 inches deep, as in Fig. 132. The whole interior surface between the lips was free from vegetation, which grew, in many places, close np to the edges, and corresponded on opposite ones. There was no denying the evidence of a recent fissure, filled up still more recently, by the slow sinking together of the sides, and by the washing in of earth by rain. I was enabled to trace it along the surface, with but occasional breaks of continuity, where the rain had washed illuvium transversely and filled it, or where the ground had been tilled between the olives, for more than a quarter of a mile. I was informed by the same soldier (whose testimony I had thus proved trustworthy) that he had himself traced the fissure F for nearly two Italian miles in a west and south-west direction, which was one generally coinciding with the horizontal contour along the slope of the hill side. Returning back to whence we started, I found two divergent fissures (as figured), and traced one of these in a S. W. and S. direction for some hundreds of feet, down through the olive orchards parallel to the road up to Auletta. The one from h to k led me to try to follow it in a contour line, along the S. W. slope of the town, and without much difficulty I found it again, where the earth got deeper, and traced at several points, but not continuously (much matter having been washed across it on this steep slope), the fissure . It was a similar little trench to the former, but as in Fig. (a, 132), one side of the greatly higher than the other, smaller in size, and harder to trace than the preceding from the want of surface vegetation and the pebbles rolled into it. In all these it was manifest, that the fissure was the evidence of a great earth slip, and had resulted, not from any direct rending asunder of the ground or rocks beneath it, but that the clay masses had when shaken violently upon the inclined beds of rock upon which they were superposed, slid down bodily by gravity, and parted off from each other at these fissures.
The fissures, by their direction, perfectly sustain this view, but are absolutely opposed to the idea of fracture, either by shock or by unequal or local sudden elevation or depression of the subjacent formations. For the great direction in length, of the fissures, is not far from that of the wave-path here, while it is everywhere but little removed from one, transverse to a line up and down the slope of the hills.
The people generally stated, that these fissures were at first, from one to two palms wide, at the widest, and that in steep places, one of the lips was about a palm above the other, and that in one place, the depth had been probed with a rod to nearly thirty palms. 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. Thus on the road leading from Auletta, along the slope of the hill, towards Villa Carusso, (which is also that to Salvitella and Vietri,) on the N. W. side of the valley, I observed a place where a slip had occurred, upon a bed of not above 12° or 15° slope, which a year or two since had carried away the whole road (here on side cutting), and lowered its surface about 4 feet, for some 400 feet in length, carrying with it the telegraph poles, still standing fast in the soil.
But a small effort of vibratory movement, therefore, must be sufficient partially to dislodge masses of such material, on much steeper slopes. 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.
The rain cuts these deep clays, here, (as everywhere in these provinces) into deep ravines or "nullahs." Some which I crossed in walking from the town of Auletta to Villa Carusso, which I next visited, were 30 to 40 feet in depth, with sides so steep and soil so greasy, that it was with great difficulty they could be crossed, and were it not for the hold on the brushwood at the sides, could not be ascended at all.
Photo Pl. 137.
Vincent Brooks, lith. London
Villa Carusso Near Auletta