Part 4 (2/2)
The first isoseismal is about 2-1/2 miles long, 1-1/2 miles broad, and 3 square miles in area; the second about 4 miles long, 3-1/2 miles broad, and 11 square miles in area; and the third about 6-1/2 miles long, 6 miles broad, and 30 square miles in area. The curve drawn by Professor Mercalli (Fig. 14) coincides nearly with the second of these lines.
At Fontana, the damage exceeded that in the surrounding country, though the difference was of course less marked than on the previous occasion.
[Ill.u.s.tration: FIG. 16.--Isoseismal lines of the Ischian earthquake of 1883.]
Outside Ischia, the shock was felt distinctly in all the island of Procida and in Vivara; on the mainland, by some as far as Pozzuoli and by several persons in Naples, which is twenty miles from Casamicciola. The seismograph at the university of this city registered two small shocks, the first at 9.10 P.M., and the second and stronger at 9.25 P.M.; and De Rossi states that at about 9.30 P.M.
the seismographs at Ceccano, Velletri, and Rome recorded a shock consisting of very slow undulations. There are again no materials for estimating the size of the disturbed area, but there can be no doubt that it was much less than that of a moderately strong British earthquake.
POSITION OF THE EPICENTRE.
Owing to the limited size of the disturbed area, time-observations, even had they been available, would not have sufficed to determine the position of the epicentre, and both Dr. Johnston-Lavis and Professor Mercalli therefore had recourse to Mallet's method, the former relying chiefly, as before, on fissures in damaged buildings, and the latter on the overthrow or displacement of columns and other objects.
Dr. Johnston-Lavis measured the azimuth of the wave-paths at sixty-five places, and at about one-third of these was able to make two or more observations. The azimuths converge towards the same region as in 1881, but the area covered by their intersections is larger. The meizoseismal band of maximum vertical destruction indicated by shading in Fig. 16 is also of the same form and slightly greater extent, reaching from the upper part of Lacco to a little south of Fra.s.so, and being therefore nearly a mile in length. The centre of maximum impulse was in the same position as in 1881, or possibly a little more to the south.
Professor Mercalli's observations were made at forty-eight places, and in only six cases were they the same as those used by his predecessor.
He also notices that most of the azimuths converge towards Casamenella, and intersect within an elongated area. This area runs in the same direction as Dr. Johnston-Lavis's meizoseismal band, but is less elongated, and situated a short distance farther to the south, though on the whole the agreement between the two areas is remarkably close.
There was again apparently a second epicentre at Fontana. In this town, according to Dr. Johnston-Lavis, there were two distinct types of damage. As in 1881, there was evidence of a vertical blow, the only one that absolutely ruined houses; but, in addition, there was another independent set of fissures, quite as widely distributed as the others, though evidently caused by a less violent movement. These indicated a wave-path with a low angle of emergence coming from between north and north-north-west, or almost exactly in the line of meizoseismal band. To the south of Fontana, however, there is a group of places, including Panza, Serrara, Barano, etc., where the azimuths diverged rather widely from the epicentre at Casamenella. These azimuths are twelve in number, and it is worthy of notice that they all intersected the crater of Epomeo, while half of them pa.s.sed within a few hundred yards of Fontana.
DEPTH OF THE SEISMIC FOCUS.
Measurements of the angle of emergence were made by Dr. Johnston-Lavis at twenty-four places, and in every case from fissured walls. The greater part of the diagram on which his results are depicted is reproduced in Fig. 17. The horizontal line, as in Fig. 13, represents the level of the sea, the longer vertical line one pa.s.sing through the epicentre, and the shorter another through Fontana. The short lines on the left of the former show the incipient wave-paths to places lying east of the epicentre; those on the right, with one exception, represent the wave-paths to places west of the same meridian. Small horizontal marks are inserted on the vertical lines to show the depth in tenths of a mile below the level of the sea.
[Ill.u.s.tration: FIG. 17.--Diagram of wave-paths at seismic vertical of Ischian earthquake of 1883. (_Johnston-Lavis._)]
The six angles of emergence that would give the greatest depth below the epicentre were all measured at places in the south of the island close to the line joining Panza and Barano, and it will be noticed that five of these apparent depths are much greater than those obtained from the other wave-paths. Excluding these observations, the remaining eighteen give depths ranging from about 450 to about 3,350 feet, and a mean depth of 1,730 feet,[24] or nearly one-third of a mile, that is, almost exactly the same as the mean depth found from the earthquake of 1881.
The six exceptional angles of emergence come from the district of divergent azimuths to the south of Epomeo. Three of the corresponding azimuths pa.s.s within one-quarter of a mile from the centre of Fontana, and none of the other three more than three-quarters of a mile from the same point. Though disbelieving in a subsidiary focus below this town, Dr. Johnston-Lavis has calculated its mean depth, supposing it to exist, and found it to be about 1,560 feet below the sea level, a result which is remarkably close to the calculated mean depth of the focus near Casamenella.
NATURE OF THE SHOCK.
In the meizoseismal band, preliminary tremor and rumbling sound were alike absent. So sudden, indeed, was the onset of the earthquake, that the survivors generally found themselves beneath the ruins of their houses before they were conscious of any shock. The destruction, practically instantaneous, was wrought by four or five vertical blows, so powerful that, according to some observers, Casamicciola seemed to jump into the air. Then followed undulations, not noticed by all, that appeared to come from every direction. The shock lasted altogether fifteen seconds or more,[25] and was accompanied by a rumbling noise, in the midst of which were detonations as of thunder or of great blows given upon an empty barrel.
In the immediate neighbourhood of the meizoseismal area, at Perrone, Pennella, and Lower Lacco, the subsultory movement was still the more prominent; but, farther away, as at Panza, Testacchio, Barano, Ischia, and Bagno, the subsultory motion was followed by distinctly horizontal undulations, while outside the island of Ischia only slow undulatory movements were perceptible.
LANDSLIPS.
The dotted areas in Fig. 16 indicate the sites of the only landslips of importance that were precipitated by the earthquake of 1883. Two of these occurred on the north slope of Epomeo, and the third on the west flank of Monte Rotaro. The materials of the Epomean landslips had evidently been separated for some time by shallow fissures from the adjoining rock, for the surfaces of the fissures were discoloured by fumarolic action. Immediately after the earthquake a cloud of dust was seen to rise from the spots; the ma.s.ses, already detached laterally, were merely set in motion by the shock; and they continued to slide down during the following days either through the action of the after-shocks or of the heavy rains that followed.
All over the island, however, fissures and minor landslips occurred.
At two places on the north coast the steep cliffs of incoherent tufa were so much damaged that, according to Dr. Johnston-Lavis, ”large quant.i.ties of their materials were thrown into the sea. The water then sorted out the pieces of pumice, which in many cases were of very large size, and were seen floating about in the neighbourhood for some days,” giving rise to the supposition that a submarine eruption had taken place to the north of the island.
AFTER-SHOCKS.
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