A Study of Recent EarthquakesThe after-shocks in 1883 were much more numerous than in 1881. Between 9.25 P.M. on July 28th and noon on August 3rd, twenty-one slight shocks were recorded at Casamicciola. At 2.15 P.M. on August 3rd, a violent shock occurred that caused further damage at Forio, and even at places so far from the epicentre as Fiaiano, Barano, and Fontana, and increased the displacements of the landslips on Epomeo. This shock was also registered at the observatory on Vesuvius.
After this the shocks became less frequent and slighter, twelve being felt at Casamicciola during the remainder of the year, and six in the first half of 1884. Several shocks and rumbling noises were also observed in other parts of the island. Among them may be mentioned noises heard at Fontana on August 12th and 15th, and a slight shock at the same place on August 17th; also on September 4th, at 10.30 and 10.40 A.M., slight shocks at Barano, Serrara, and Forio. On March 27th, 1884, at 2.7 P.M., another strong shock occurred; strongest at Serrara, where the shock was subsultory and accompanied by noise; and less strong, though still subsultory, at Ciglio, Panza, Forio, Fiaiano, and Casamicciola, and very slight at Ischia. The series seems to have ended during the following summer, with a slight shock at Casamicciola on July 21st, and a stronger one on July 23rd, felt from Casamicciola on the north to Serrara on the south.
Most of the after-shocks must have originated in the neighbourhood of Casamicciola, but it is worthy of notice that more than one centre was in action. Several were recorded at Ischia only. Others, as mentioned above, affected chiefly the south part of the island, and especially the small towns of Serrara and Fontana.
CHARACTERISTICS OF ISCHIAN EARTHQUAKES.
After the eruption of 1302, there succeeded a period of comparative repose in Ischia. The revival of activity dates from 1762, and, since that year, there have been four great earthquakes, namely, those of 1796, 1828, 1881, and 1883. In every respect but that of increasing intensity, these earthquakes were apparently identical; each, as Professor Mercalli says, was merely a replica on a different scale of those that preceded it. The princ.i.p.al features in which they resemble one another, and differ from the average tectonic earthquake, are the coincidence of the epicentres, the small depth of the foci, and the sudden onset of the princ.i.p.al shock.
1. _Coincidence of Epicentres._--In Fig. 14, which is copied from Professor Mercalli's map, are shown the areas in which buildings were seriously damaged by these four earthquakes. The curves for 1796, 1828, and 1881 are approximately concentric. In 1796, the shock was disastrous only to the west of Casamicciola; in 1828, according to Covelli, "the ground most injured was not precisely the region of Casamicciola, but that which lies between the district called Fango and that known as Casamenella, situated to the west of Casamicciola, and a short distance from it."[26] The epicentres may have varied slightly in size, but, in position, it is clear that all four were nearly or quite coincident. The meizoseismal bands in 1881 and 1883 were also similar in form and elongated in the same direction.
In the last two earthquakes there was, as we have seen, very distinct evidence of a secondary meizoseismal area surrounding Fontana, and it is remarkable that this was also noticeable in the earthquake of 1828.
"Besides the centre of vibration in the district of Fango," says Covelli, "another less powerful centre showed itself in the locality of Fontana; this made itself felt more heavily than in surrounding localities; as if another centre of movement had taken place from that part, independent of the former."
2. _Small Depth of the Foci._--Mallet's method, as noted above, cannot be trusted to yield accurate estimates of the focal depth, or to indicate more than its order of magnitude. But it is remarkable that the depths calculated by Dr. Johnston-Lavis for the last two earthquakes are both only a little less than a third of a mile, and it is probable that the actual depth did not differ very greatly from this amount. The nature of the shock, vertical or nearly so close to the epicentre and horizontal at a short distance from it, is merely personal testimony of the same character as fissures in masonry, and of course points to the same result.
[Ill.u.s.tration: FIG. 18.--Diagram showing connection between depth of focus and rate of decline in intensity.]
But the most conclusive evidence on which we have to rely is the extraordinary intensity of the shock at the centre of a very small distributed area. In Great Britain, an earthquake felt over a district of equal size would hardly at the centre exceed the trembling produced in a station platform by a pa.s.sing train. The curves in Fig. 18 show how the rate of decline in intensity depends on the depth of the focus. They are drawn on the supposition that the intensity at any point on the surface varies inversely as the square of its distance from the focus; the curves _a_, _b_, _c_ corresponding to foci situated at depths of one-third of a mile, one mile, and two miles respectively, and the figures below the horizontal line denoting the distance in miles from the epicentre. Thus, the rapid decline of intensity from the epicentre outwards shows that, in each of the four great Ischian earthquakes, the depths of the focus must have been very small.
3. _Suddenness of the Shocks._--In 1796, we have no record of preparatory shocks, but the evidence is scanty; in 1828 and 1881, none are mentioned; in 1883, one or two tremors and underground noises, possibly of seismic origin, gave warning to a few. Fore-shocks, for all practical purposes, were conspicuous by their absence.
Still more remarkable is the sudden advent of the great shocks. There were no preliminary tremors or rumbling sound, no animals showed signs of uneasiness and no birds fluttered screaming from trees or ground.
The shock of 1828, says Covelli, "was announced by three powerful blows coming almost vertically, from below upwards;" and the same words apply equally well to the earthquakes of 1881 and 1883. The destruction of houses in every case was practically instantaneous, and coincident with the first vibration.
In all respects, tectonic earthquakes differ widely from the Ischian shocks. The epicentres of successive earthquakes are rarely coincident, but show a distinct tendency to migration along certain lines; the decline in intensity outwards from the epicentre is nearly always very gradual, and therefore indicative of a comparatively deep-seated focus; they are almost invariably preceded either by a series of slight shocks and rumbling sounds, or, in an unstable district, by a marked increase in their frequency. Distinctions, so great as these are, evidently remove the Ischian shocks from the category of tectonic earthquakes.
ORIGIN OF THE ISCHIAN EARTHQUAKES.
On the other hand, the Ischian earthquakes possess several features which connect them closely with true volcanic earthquakes.
1. They originate beneath the northern slope of Epomeo--a volcano that we have no reason to consider absolutely extinct, but rather as one subject to eruptions at long intervals of time--in a region as yet unoccupied by parasitic craters, but having the same relation to the central cone of Epomeo as those in which the recent craters of Monte Rotaro, Montagnone and Cremate are situated.
2. In both the earthquakes of 1881 and 1883, the epicentre is an elongated band, the axis of which, if produced, would pa.s.s through the centre of the old crater of Epomeo. Along the line of this band, occur the fumaroles of Monte Cito and Ign.a.z.io Verde and the thermal springs of the Rita and Capitello. These facts, as Professor Mercalli suggests, lead us to believe that the foci of the earthquakes coincide with a radial fracture of the volcano, the course of which, as traced by him, is represented by the continuous line in Fig. 14.[27]
3. Except in their relations with actual eruptions, the Ischian earthquakes resemble closely the true volcanic earthquakes which from time to time shake the flanks of Etna. These are marked by great intensity of the shock at the centre of a comparatively small disturbed area, epicentres often elongated radially to the cone, frequent repet.i.tion with similar characters in the same districts; and as a rule they precede by a short interval, but sometimes accompany or follow, volcanic eruptions.[28]
Two other phenomena may be referred to as probably indicating some connection between Ischian earthquakes and the structure and history of Epomeo.
We have seen that, in the three earthquakes of 1828, 1881, and 1883, there is distinct evidence of a second meizoseismal area at Fontana, within which the shock was mainly subsultory. Dr. Johnston-Lavis, though recognising the possibility of the existence of two epicentres, prefers another explanation.[29] But the wide extension of the southern boundary of the area of destruction in 1883, and the limitation of several of the after-shocks to the south of the island, seem to me to favour the existence of a second focus beneath the crater of Epomeo, though, it may be, not entirely detached from the chief focus beneath Casamenella.
Again, as Professor Mercalli remarks, all historic eruptions on the flanks of Epomeo were accompanied by very violent earthquakes; while, previously to 1302, only one disastrous earthquake, so far as known, occurred in the island without being attended by an eruption. It should be noticed also that the princ.i.p.al shocks during the recent revival of activity (_i.e._, since 1762) show a continual increase in intensity, whether this be measured by the damage to buildings, the loss of life, or the extent of the area of destruction (Fig. 14).
It therefore seems legitimate to conclude that, in the recent Ischian earthquakes, we have merely so many unsuccessful attempts to force a new volcanic eruption. The pa.s.sages once existing through Epomeo and its parasitic craters having become blocked, the highly heated magma beneath is compelled to find a new outlet. Its tension slowly increasing, the crust above is at last rent, or an incipient rent is enlarged, the fluid rock is injected almost instantaneously with great force into the open fissure, and its sudden arrest by the containing walls is the ultimate cause of an earthquake. With the expansion of the magma, its tension is at once correspondingly reduced, and some time must elapse before it can again reach the critical point at which a further rupture, resulting in a second shock, takes place.[30]
Thus, with each great Ischian earthquake, we are, I believe, advancing a step nearer the time, which may be close at hand or may be very remote, when the fracture will at last reach the surface, and above the site of Casamenella a new parasitic cone will rise, from which, as from Cremate in 1302, a stream of lava may flow down towards the sea.
REFERENCES.
1. BALDACCI, L.--"Alcune osservazioni sul terremoto avvenuto all'
Isola d'Ischia il 28 luglio 1883." _Ital. Com. Geol. Boll._, vol. xiv., 1883, pp. 157-166.
2. DAUBReE, A.--"Rapport sur le tremblement de terre ressenti a Ischia le 28 juillet, 1883; causes probables des tremblements de terre." Paris, _Acad. Sci._, _Compt. Rend._, vol. xcvii., 1883, pp. 768-778.
3. DU BOIS, F.--"The Earthquakes of Ischia." _j.a.pan Seism. Soc.
Trans._, vol. vii., pt i., 1883-84, pp. 16-42.
4. ---- "Farther Notes on the Earthquakes of Ischia." _Ibid._, vol.
viii., 1885, pp. 95-99.
5. JOHNSTON-LAVIS, H.J.--_Monograph of the Earthquakes of Ischia_ (1885).
6. MERCALLI, G.--_Vulcani e fenomeni vulcanici in Italia_ (vol. iii.
of _Geologia d'Italia_, by G. Negri, A. Stoppani, and G.
Mercalli), 1883, pp. 46-50, 331-332.
7. ---- _L'Isola d'Ischia ed il terremoto del 28 luglio 1883_ (Milano, 1884).
8. PALMIERI, L., E A. OGLIALORO.--"Sul terremoto dell' Isola d'Ischia della sera del 28 luglio 1883." Napoli, _R. Accad.
Atti_, vol. i., 1884, pp. 1-28.
9. ROSSI, M.S. DE.--"Il terremoto di Casamicciola del 4 marzo 1881."
_Bull. del Vulc. Ital._, anno viii., 1881, pp. 5-12. (In the same volume are brief notices by different writers on pp. 22, 38-42, 52-53, 67-68, 70-74.)
10. ---- "Raccolta di fatti, relazioni, bibliografie sul terremoto di Casamicciola del 28 luglio 1883, con brevi osservazioni."
_Bull. del Vulc. Ital._, anno xi., 1884, pp. 65-172.
11. ---- "Intorno all' odierna fase dei terremoti in Italia e segnatamente sul terremoto in Casamicciola del 4 Marzo 1881."
_Ital. Soc. Geogr. Boll._, 1881.
12. SERPIERI, A.--"Sul terremoto d'Ischia il 28 luglio 1883."
_Scritti di Sismologia_, Pte. ii., pp. 207-216.
13. ---- "Sul terremoto dell' Isola d'Ischia il 28 luglio 1883."
_Ibid._, pp. 217-232.
FOOTNOTES:
[21] The shaded areas indicate the princ.i.p.al trachytic ma.s.ses, the broken lines represent the boundaries of the craters that are still recognisable, and the dotted lines the boundaries of the areas within which buildings were damaged by the earthquakes of 1796, 1828, 1881, and 1883 (according to Mercalli). The continuous curved line shows the position of the radial fracture with which the earthquakes were probably connected. The trachytic ma.s.ses and craters are denoted by the following tables:--
_a._ Epomeo.
_b._ Trippiti.
_c._ Vetta.
_d._ Garofoli.
_e._ Vatoliere.