Relying, however, on knowledge obtained from the study of more recent shocks, it seems to me probable that the two foci formed parts of one fault with a general north-west and south-east direction. The slip causing the first part of the double shock apparently took place within the south-east focus, and was followed after a few seconds by one within the north-west focus, greater in amount as well as more deeply seated. In consequence of these displacements there were local increases of stress, causing numerous small slips within or near both princ.i.p.al foci; and, if we may judge from some slight shocks felt at La Sala, accompanied also by other minor slips in the intermediate region of the fault.

REFERENCE.

MALLET, R.--_The Great Neapolitan Earthquake of 1857: The First Principles of Observational Seismology_, etc. 2 vols 1862.

FOOTNOTES:

[3] _Irish Acad. Trans._, vol. xxi., 1848, pp. 51-105 (read Feb. 9, 1846).

[4] _Brit. a.s.soc. Reports_, 1850, pp. 1-87; 1851, pp. 272-330; 1852, pp. 1-176; 1853, pp. 117-212; 1854, pp. 1-326; 1858, pp. 1-136.

[5] _A Manual of Scientific Enquiry_, edited by Sir J.F.W. Herschel, 1849, pp. 196-223.

[6] _Irish Acad. Trans._, vol. xxii., 1855, pp. 397-410.

[7] The linear dimensions of the isoseismal lines are obtained by measurements from Mallet's maps. The areas are given by him in geographical square miles.

[8] Mallet, by some accident, omitted the losses at Polla and neighbouring towns from this estimate. Mercalli (_Geologia d'Italia_, pte. 3, p. 324) gives the number of killed as more than 12,300.

[9] Mallet does not make use of the term _epicentre_; he speaks of the line FE as the _seismic vertical_. The modern and accepted terms are used above

[10] _j.a.pan Seismol. Soc. Trans._, vol. xi., 1887, pp. 175-177.

[11] _Ital. Seismol. Soc. Boll._, vol. ii., 1896, pp. 180-188.

[12] Professor Omori gives the mean direction as S. 71 W., but this was obtained from observation on lamps with square, as well as with circular bases.

[13] Twelve measurements chosen at random from Professor Omori's list gave a mean direction of S. 78 W.

[14] When the accuracy of all the observations seemed equally probable, he adopted the mean of the two extremes as the true direction.

[15] If _a_ be the amplitude of a simple harmonic vibration, _T_ its complete period, _v_ its maximum velocity, and _f_ its maximum acceleration, we have v=2*pi*a/T and f=4*pi^2*a/T^2

[16] _Earthquakes and other Earth Movements_, pp. 81-82.

[17] Obtained from the formula: T=2*pi*a/v=2*pi*x*(1/3)/12

[18] If we take the maximum velocity to be 12 feet per second, and the period to be one second, the amplitude would be about 11-1/2 inches.

[19] Vol. ii., p. 299. The punctuation of the original is not followed in the above extract.

[20] _British a.s.sociation Report_, 1851, pp. 272-320.

CHAPTER III.

THE ISCHIAN EARTHQUAKES OF MARCH 4TH, 1881, AND JULY 28TH, 1883.

Separated from Italy by a distance of not more than six miles, Ischia and the intermediate island of Procida strictly form part of the Phlegraean Fields, the well-known volcanic district to the north of Naples. Ischia, the larger of the two islands, is six miles long from east to west, and five miles from north to south, and contains an area of twenty-six square miles. In 1881, the total population was 22,170, that of Casamicciola, the largest town, being 3,963.

VOLCANIC HISTORY OF ISCHIA.

The central feature of Ischia is the great crater of Epomeo (_a_, Fig.

14). On the south side, and partly also on the east, the crater-wall has been broken down and removed; the portion remaining is about 1-1/2 mile in diameter from east to west, and reaches a height of 2,600 feet above the sea-level. All the upper part of the mountain is composed of a pumiceous tufa, rich in sanidine and of a characteristic greenish colour. At two points, to the west near Forio and to the north between Lacco and Casamicciola, this tufa is seen reaching down to the sea; but, in all other parts, it is covered by streams of trachitic lava, by more recent tufas, or by a deposit of marly appearance, which is regarded by Fuchs as resulting from the decomposition of the Epomean tufa.

There are two distinct periods in the geological history of Ischia.

The first, a submarine period, probably began with the dawn of the quaternary epoch, for all the marine fossils of the island belong to existing species. About this time, Epomeo seems to have originated in eruptions occurring in a sea at least 1,700 feet in depth--eruptions that preceded the formation of Monte Somma and were either contemporaneous or alternating with those that gave rise to the oldest trachitic tufas of the Phlegraean Fields. The destruction of the south wall may have occurred much later through some great eruptive paroxysm, but more probably, as Professor Mercalli suggests, through early marine erosion and subsequent subaerial denudation. To the submarine period must also be a.s.signed the formation of the trachitic ma.s.ses which compose Monti Trippiti, Vetta, and Garofoli (_b_, _c_, _d_, Fig. 14), on the east side of Epomeo; and, in part only, those of Monte Campagnano and Monte Vezza (_f_, _g_).

At or near the close of the elevation, many violent eruptions occurred on the south-west of Epomeo, during which was formed the south-west corner of the island, including Monte Imperatore and Capo Sant' Angelo (_h_, _i_).

In the second or terrestrial period, when the island had practically attained its present alt.i.tude, the eruptive activity was almost confined to the eastern and northern flanks of Epomeo. At the beginning Monte Lo Toppo (_j_) was formed by a lateral eruption. In the north-west corner of the island, Monte Marecocco and Monte Zale (_k_ and _l_) owe their origin to a gigantic flow of sanidinic trachite, issuing probably from the depression which now exists between them. Lastly, towards the north-east, are the recent lateral craters of Rotaro, Montagnone, Bagno, and Cremate (_m_, _n_, _p_, _s_), the first two being the most regular and best preserved in the island.

[Ill.u.s.tration: FIG. 14.--Geological sketch-map of Ischia.

(_Mercalli._)[21]]

The earliest eruption of the historic, or rather human, period appears to have taken place from Montagnone, and probably also at about the same time from the secondary crater of Porto d'Ischia (_u_), about the beginning of the eleventh century B.C. The eruptions of Marecocco and Zale are referred to about B.C. 470; and those of Rotaro and Tabor (_q_) to between the years 400 and 352 B.C. Another eruption is said to have occurred in B.C. 89, but the site of it is unknown; and three others are recorded on doubtful authority about the years A.D. 79-81, 138-161, and 284-305. The last outburst of all took place after the series of earthquakes in 1302 from a new crater, that of Cremate (_s_), which opened on the north-east flank of Epomeo, and from which a stream of lava, called the Arso (_t_), flowed down rapidly and, after a course of two miles, reached the sea.

After the first eruptions to which it owed its origin, the central crater of Epomeo apparently remained inactive. All the later eruptions occurred either on the external flanks of the mountain or on radial fractures of the cone.[22] Trippiti, Lo Toppo, Montagnone and the Lago del Bagno (_b_, _j_, _n_, _p_) lie in one line, Vetta and Cremate (_c_, _s_) on another, and Garofoli and Vatoliere (_d_, _e_) on a third, all pa.s.sing through a point near the town of Fontana, which occupies the centre of the old crater of Epomeo.

Professor Mercalli points out that the lateral eruptions of Epomeo differ in one respect from those of Etna and Vesuvius. In these volcanoes the lava ascends to a considerable height in the central chimney, and by its own weight rends open the flanks of the cone. In Epomeo, it appears to traverse lateral pa.s.sages at some depth, perhaps far below the level of the sea, and to rend the mountain by means of the elastic force of the aqueous vapour, etc., which it contains. It will be seen how important is the bearing of this difference on the occurrence of the Ischian earthquakes.

The eruptions that have taken place during the last three thousand years agree in several particulars. They either occurred suddenly, or, at any rate, were not preceded by a stage of moderate Strombolian activity; they were always accompanied by violent earthquakes; and all succeeded intervals of long repose. As the eruption of 1302 happened after at least a thousand years of rest, the lapse of six more centuries does not justify us in concluding that Epomeo is at last extinct.

We seem, on the contrary, to be drawing near another epoch of activity. During the four and a half centuries that followed the eruption of 1302, we have no record of Ischian earthquakes.[23] Then, suddenly, on the night of July 28-29, 1762, Casamicciola was visited by sixty-two shocks, some of which were very strong and damaged buildings. On March 18th, 1796, another severe shock took place, but destructive only in the neighbourhood of Casamicciola, where seven persons were killed. On February 2nd, 1828, the area of damage, though concentric with the former, enlarged its boundaries; 30 persons were killed and 50 wounded. On March 6th, 1841, and during the night of August 15-16, 1867, further shocks injured houses at Casamicciola, but without causing any loss of life. Slight tremors occurred at various dates in 1874, 1875, 1879, and 1880, leading up to the disastrous earthquakes here described, those of March 4th, 1881, when 127 persons were killed, and July 28th, 1883, which resulted in the death of 2,313 persons and the wounding of many others.

EARTHQUAKE OF MARCH 4TH, 1881.

The Ischian earthquakes have been fortunate in their investigators. In the spring of 1881, Dr. H.J. Johnston-Lavis, the chronicler for many years of Vesuvian phenomena, was residing in Naples. Impressed by a recent perusal of Mallet's report on the Neapolitan earthquake, and wishing to test the value of the methods explained in the last chapter, he crossed over to Ischia on March 5th; and to his unwearied inquiries extending over more than three weeks and lasting from thirteen to sixteen hours a day, we are indebted for most of what we know about the earthquake of 1881.

On March 4th, at 1.5 P.M., the great shock occurred abruptly, without any warning tremors. Its effects were aggravated by the faulty construction of the houses. The walls are of great thickness, loosely put together, and connected by mortar of the poorest quality. The chimneys and roofs also are ma.s.sive, and the rafters are so slightly inserted in the walls that they were drawn out with the rocking of the houses. In such cases, the destruction was often so complete that no fissures were left available for measurement.

ISOSEISMAL LINES AND DISTURBED AREA.

The isoseismal lines as drawn by Dr. Johnston-Lavis are represented by the curves in Fig. 15. The isoseismal marked 1 bounds the area of complete destruction; it is about 1 mile long from east to west, 2/3 of a mile broad, and contains an area of not more than half a square mile. The next isoseismal (2) marks the area of partial, but still serious, destruction; this is nearly 2 miles long from east to west, 1-1/4 miles broad, and 2 square miles in area. Within the isoseismal 3, buildings were more or less slightly damaged. The course of this curve is somewhat doubtful, but, as drawn, it is about 3 miles long, 2 miles wide, and 5 square miles in area.

[Ill.u.s.tration: FIG. 15.--Isoseismal lines of the Ischian earthquake of 1881. (_Johnston-Lavis._)]

Outside the last curve, the shock diminished rapidly in intensity. At Monte Tabor and Bagno, it was very slight; in the town of Ischia, only about half the people were conscious of any movement; and at Capella, a small village to the south, it was not felt at all. Again, the shock was perceptible, though only faintly, in the neighbourhood of Campagnano, at Serrara to the south of Epomeo, and at Panza near the south-west corner of the island. On the other hand, at Fontana, which occupies approximately the centre of the crater of Epomeo, there were evidences of a distinctly stronger shock. No house actually fell, and side walls were but little injured; but the roofs, which are of great weight, suffered considerable injury.

In the adjacent island of Procida, the shock was felt distinctly by many people, and by some, though slightly, at Monte di Procida, Misenum, and Bacoli, on the coast of Italy. No record whatever was given by the seismographs in the university of Naples and the observatory on Vesuvius. We have of course no means of estimating the exact size of the disturbed area, but in this respect, disastrous as the earthquake was in the neighbourhood of Casamicciola, it was clearly inferior to all but the very weakest earthquakes felt in the British Islands.