CHAPTER III.ToC

Mallet was one of the first seismologists to realise the significance of the earthquake-sound; and he attended closely to the subject, though finding thesound even more elusive of precise observation than the shock.

The chief result obtained by him was the comparative smallness of the area over which the sound was heard. He estimates it at little more than 3,300 square miles, or about one-twelfth of that over which the shock was felt. It extends north and south from Melfi to Lagonegro, and east and west from Monte Peloso to Duchessa and Senerchia. The sound was thus confined to the region in which the shock attained its most destructive character.

Towards the north and south ends of the sound-area all observers described the sound as a low, grating, heavy, sighing rush, lasting from twenty to sixty seconds, some adding that it was also of a rumbling nature. Near the centre and the east and west boundaries, the sound was distinctly more rumbling; it was shorter in duration, and began and ended more abruptly.

The earthquake, Mallet remarks, "began everywhere with tremors; the sounds generally arrived at the same time; the apparent direction of movement of the tremulous oscillations appeared rapidly to change, and still more rapidly to increase in amplitude; then the greatshoveof the destructive shock arrived, in some places rather before, in some a little after, the moment of loudest sound, and it died away suddenly (i.e., with extreme rapidity) into tremors again, but differing in direction from that of the great shock itself."[19]

The earthquake-sound will be described more fully in the chapter dealing with the Hereford earthquakeof 1896, in which it will be found that the phenomena recorded by Mallet are equally characteristic of the slighter shocks felt in this country.

In 1857 little was known about the velocity of earthquake-waves. Experiments had been made by Mallet himself in 1849 in the neighbourhood of Dublin. These gave 825 feet per second for the velocity in dense wet sand, 1,306 feet per second in discontinuous granite, and 1,665 feet per second in more solid granite.[20]The only earthquake for which the velocity had been calculated was the Rhenish earthquake of 1846, the value ascertained by Schmidt being 1,376 French feet, or 1,466 English feet, per second.

The accurate public measurement of time, which, as Mallet remarks, is one of the surest indications of advancing civilisation, was, however, unknown in the kingdom of Naples; and his attempt was therefore fettered by the rarity of precise estimates of the time of occurrence. Throughout the whole disturbed area only six good records could be obtained, and three of these (at Vietri di Potenza, Atella, and Naples) were derived from stopped clocks, witnesses of rather doubtful value. At Montefermo and Barielle the time was at once read from a watch, and at Melfi from an accurate pocket chronometer. The times given vary from 9h. 59m. 16s.P.M.(Naples mean time) at Vietri di Potenza to 10h. 7m. 44s. at Naples. Allowing for the supposed change of direction by refraction atthe Monte St. Angelo range on the way to Naples, Mallet finds the mean surface velocity to be 787 feet per second. Omitting the Naples record, and taking account of the calculated depth of the focus, the mean velocity becomes 804 feet per second.

A great earthquake rarely, if ever, occurs without some preparation in the form of a marked increase of seismic activity. Perrey records several shocks during the two years 1856-57 that were felt at places as far apart as Naples, Melfi, and Cosenza. On December 7th, 1857, a slight shock, with a report from beneath like the explosion of a mine, was felt at Potenza. Then came the great earthquake on December 16th, at about 10P.M.

This was followed by numerous after-shocks—how numerous it is impossible to say, for the records are of the scantiest description. For some hours the ground within the meizoseismal area is said to have trembled almost incessantly. At Potenza many slight shocks, both vertical and horizontal, were felt during the night, and for a month or more they were so frequent as to render enumeration difficult. Mallet's last record is dated March 23rd, 1858, when four slight shocks were felt at La Sala and Potenza, but occasional tremors were reported to him until May 1859.

The most important of all these after-shocks was one felt about an hour after the principal earthquake. Everywhere far less powerful, it was yet strong enough to shake down many buildings at Polla that had been shattered by the great shock.Towards the south at Moliterno, and towards the north at Oliveto and Barielle, it evidently attracted very little attention. So far as can be judged from the evidence given by Mallet, the disturbed area seems to have been approximately of the same form and dimensions as the meizoseismal area, and elongated in the same direction, but concentric with the north-west focus.

On the other hand, if we may rely on too brief evidence, several after-shocks recorded only at Montemurro, Saponara, Viggiano, or Lagonegro, were probably connected with the south-east or Montemurro focus.

Mallet's theories have suffered perhaps more than any other part of his work from the recent growth of our knowledge. From a historical point of view, some reference to his explanation of the origin of the Neapolitan earthquake seems desirable, and his own conscientious work demands it. On the other hand, his conclusions are, for the present at any rate, superseded, and it will therefore be sufficient to describe them briefly.

Most of the wave-paths, as we have seen, pass within three miles of a point almost coincident with the village of Caggiano. Of the remainder, six traverse a spot about two miles farther to the south-west, and three cross another about two miles farther to the north-east. Neglecting other points of intersection, but taking account of the observed emergences at Vietri di Potenza, Auletta, Polla, etc., Mallet infers that the horizontal section of the focuswas a curve (indicated by the dotted line in Fig. 9) not less than ten miles in length, and passing from near Balvano on the north, close to Vietri di Potenza, Caggiano, and Pertosa, to a point about two and a half miles west of Polla. Again, he remarks, the observed emergences at places near the epicentre indicate that the vertical section of the seismic focus was either more or less curved, or more probably a surface inclined towards the south-east. He concludes, therefore, that the seismic focus was a curved fissure, 10 miles long and 3½ miles in height, and with its centre at a depth of 6½ miles below the level of the sea.

The production of this great fissure, accompanied, perhaps by the injection into it of steam at high pressure, was regarded by Mallet as the cause of the principal earthquake. He imagines that the rent would start at or near the central point of the focus and then extend rapidly outwards in all directions. In the initial stage, vibrations of very small amplitude would alone be transmitted, and these would give rise to the early sounds and tremors. As the rending proceeded, the vibrations would increase in strength up to a certain point when they produced the shock itself. After this, they would decrease; and, in the final stage, would give place to the small vibrations corresponding to the sounds and tremors that marked the close of the earthquake.

The rush of steam at high pressure into the focus Mallet does not seem to have considered essential, though he evidently regarded it as possible, indeed probable; and he suggests that it may have been in part the cause of the earthquake which occurred an hour later. Though feeling sceptical as to theexistence of any general law of increase of underground temperature, he assumes it, for the sake of illustration, to be 1° F. for every 60 feet of descent. This would give a temperature of 339° F. at the upper limit of the focus, 643° F. at its central point, and 884° F. at its lower margin. If the focus were filled with steam at each of these temperatures, the corresponding pressures on its walls would be 8, 149, and 684 atmospheres, respectively. As the steam may be supposed to be admitted suddenly and to be unlimited in supply, Mallet infers that it might exist at the tension due to the highest of these temperatures, in which case it would be capable of lifting a column of limestone 8,550 feet in height (or about one-half the depth of the upper margin of the focus), and would exert a pressure on the walls of the focus of 4.58 tons per square inch, or of more than 640,528 millions of tons upon its whole surface.

So many pages have already been given to this interesting earthquake that I must sketch still more briefly my own view as to its origin. There were, I believe, two distinct foci with their centres about twenty-four miles apart along a north-west and south-east line, and it was to this arrangement that the elongation of the meizoseismal area was chiefly, though not entirely, due. The evidence is insufficient to determine whether the earthquake was caused by fault-slipping; it is in no way opposed to this view, but if the Neapolitan earthquake stood alone, we should hardly be justified in drawing any further inference. Relying, however, on knowledge obtained from the study of more recent shocks, it seems to me probable that the two foci formed parts of onefault 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 principal 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.

Mallet, R.—The Great Neapolitan Earthquake of 1857: The First Principles of Observational Seismology, etc. 2 vols 1862.

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

[4]Brit. Assoc. 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 termepicentre; he speaks of the line FE as theseismic vertical. The modern and accepted terms are used above.

[10]Japan 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]Ifabe the amplitude of a simple harmonic vibration,Tits complete period,vits maximum velocity, andfits maximum acceleration, we havev= 2πa ÷Tandf= 4π²a÷T²

[16]Earthquakes and other Earth Movements, pp. 81-82.

[17]Obtained from the formula:T= 2πa÷v= 2πx⅓÷ 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 11½ inches.

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

[20]British Association Report, 1851, pp. 272-320.

Separated from Italy by a distance of not more than six miles, Ischia and the intermediate island of Procida strictly form part of the Phlegræan 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.

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 1½ 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 Phlegræan 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 assigned the formation of the trachitic masses 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 altitude, 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 (kandl) 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.

Geological sketch-map of Ischia.Fig.14.—Geological sketch-map of Ischia. (Mercalli.)[21]ToList

Fig.14.—Geological sketch-map of Ischia. (Mercalli.)[21]ToList

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 centuryB.C.The eruptions of Marecocco and Zale are referred to aboutB.C.470; and those of Rotaro and Tabor (q) to between the years 400 and 352B.C.Another eruption is said to have occurred inB.C.89, but the site of it is unknown; and three others are recorded on doubtful authority about the yearsA.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 passing 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 passages 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 theformer, 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.

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 1.5P.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 massive, 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.

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, 1¼ 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, asdrawn, it is about 3 miles long, 2 miles wide, and 5 square miles in area.

Isoseismal lines of the Ischian earthquake of 1881.Fig.15.—Isoseismal lines of the Ischian earthquake of 1881. (Johnston-Lavis.)ToList

Fig.15.—Isoseismal lines of the Ischian earthquake of 1881. (Johnston-Lavis.)ToList

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.

In determining the position of the epicentre, Mallet's method was closely followed. Fissures in buildings were used for the most part, in two out ofevery three cases; and occasional measurements were made from objects overthrown, projected, or shifted, and also from the personal experiences of observers. The attempt to apply the method was, however, fraught with difficulties. The heterogeneous structure of the island was no doubt responsible for many divergent azimuths; the irregularity of the buildings both in form and material and their variety of site furnished other sources of error; even the smallness of the area was a disadvantage in lessening the number of trustworthy records.

Measurements were made at 55 places altogether, but in most cases they were the results of isolated observations, not the means of several at each place. On this account, I have not reproduced in Fig. 15 the azimuths shown in Dr. Johnston-Lavis's map of the earthquake. A large number of them clearly converge towards an area lying to the west of Casamicciola; and, from their arrangement, Dr. Johnston-Lavis concludes, though the evidence does not seem to me quite strong enough for the purpose, that they emanated from a fracture running from a little west of north to a little east of south.

This conclusion is, however, justified by other evidence. In the centre of the injured district, Dr. Johnston-Lavis has traced a meizoseismal band, in which the shock must have been nearly or quite vertical. "The damage inflicted on buildings included within this band was," he says, "very characteristic of the nature of the shock; the walls having received but slight injury, whilst almost every floor and ceiling had been totally destroyed. In fact," he adds, "many houses would have required no other repairs than the replacing of the divisions betweenthe different storeys." The shaded central area in Fig. 15 represents this band, passing in a nearly north and south direction from a point midway between Campo and the upper part of Lacco on the north, through the west part of Casamenella and Campo, to a point near Frasso on the south; the length of the band being thus about two-thirds of a mile.

If the central line of this band is produced towards the south, as indicated by the dotted line, it grazes the west side of Fontana, where, as we have seen, there was a second meizoseismal area, much smaller than the other and surrounded by a district in which houses were almost uninjured. That the shock in this town was vertical or nearly so, is shown by the nature of the damage (p. 52) and also by the testimony of the inhabitants. I will give Dr. Johnston-Lavis's explanation of this detached meizoseismal area when discussing the origin of the Ischian earthquakes; but the evidence seems to me to favour either the existence of two distinct foci or, more probably perhaps, the extension of the fissure to the south with an increased impulse beneath the centre of Epomeo.

At nine places, Dr. Johnston-Lavis was able to make measurements of the angle of emergence, in every case from fissures in buildings, and therefore liable to sources of error already referred to. On the other hand, owing to the small depth of the focus, there would probably be less general refraction of the wave-paths than in the Neapolitan earthquake. The depths indicated by these observations vary betweenabout 615 and 2,885 feet, a difference that is no greater than might be expected, as the size of the focus was no doubt comparable with that of the district in which observations were made. The mean depth Dr. Johnston-Lavis finds to be about 1,700 feet, or a little less than one-third of a mile.

The limited depth of the focus is also evident from the nature of the shock. It was only within the actual meizoseismal band that the shock was subsultory or vertical throughout; at a short distance from the epicentre, the movement was both subsultory and undulatory; while near the third isoseismal, and in most of the region outside, the movement was entirely undulatory or lateral. An observer at Perrone (which lies 1-2/3 miles east of the epicentre) gives the following account of the shock:—"I was standing on my balcony (this faces Casamicciola) admiring the scene ... when I felt the house rock, feeling at the same time as if something was rolling along beneath the ground. This movement was accompanied by a sound like this, Boob, boob—boob— — boob— — — boob— — — — boob. Both noise and movement seemed to come from Casamicciola.... In a few seconds, in the distance over the town arose a terrific cloud of white dust, so that I imagined the town on fire.... I felt hardly any, if any, subsultory movement, but as I leant upon the balcony rails, I was alternately pressed against them and then drawn away."

At Fontana, however, the undulatory shock was replaced by a vertical one. This was the universal experience, though one or two persons felt a slightlateral movement immediately after. At Valle (near Barano) and Piejo, both places about a mile from Fontana, the vertical component was also perceptible.

The after-shocks were few and of slight intensity. Dr. Johnston-Lavis gives the following dates: March 7th, 12.5A.M.and midday; March 11-12, 15-16, 17-18, 27 (?), April 5th and 6th, and July 18th, 8.30P.M.The only shock of the series marked as strong occurred at midnight on March 15-16 at Casamicciola. The last of all, that of July 18th, consisted of a rumble and slight shock, and was most perceptible at Fango.

Undeterred by the experience of 1881 or by the warnings of seismologists, Casamicciola was rebuilt, only to suffer more complete disaster. On July 28th, 1883, at 9.25P.M., occurred the most destructive earthquake of which we have any record in Ischia. The shock lasted about fifteen seconds, and before it was over clouds of dust were rising above the ruins of Casamicciola, Lacco, and Forio; 1,200 houses were destroyed, 2,313 persons were killed, nearly 1,800 in Casamicciola alone, and more than 800 seriously wounded. "No better idea," says Dr. Johnston-Lavis, "of the absolute destruction of buildings could be conceived than what was actually realised at Casamicciola and Campo. Looking, on the following Monday, over the field of destruction, I could discover (with few exceptions) the wall-stumps only remaining."

Dr. Johnston-Lavis again spent about three weeks in the island, examining the effects of the new shock with equal zeal and wider experience. His monograph is now our chief work of reference on Ischian earthquakes. Inquiries were also made by several Italian seismologists, among others by Professor M.S. de Rossi, the organiser of earthquake-studies in the peninsula; by Professor L. Palmieri, the founder of the Vesuvian observatory; and especially by Professor G. Mercalli, whose valuable memoir supplements the report of Dr. Johnston-Lavis in some important particulars.

The interval between July 18th, 1881, when the last shock of that year was felt, and July 28th, 1883, was one of almost complete quiescence. Early in March 1882, a few slight shocks were noticed at Casamicciola. On July 24th, 1883, a watch hanging from a nail in a wall was seen to swing at 6A.M.and 9A.M., and, on the same morning, at about 8.30, a slight shock, accompanied by a rumbling sound, was felt at Casamicciola. Again, on the 28th, about a quarter of an hour before the great shock, one observer at Casamicciola states that an underground noise was heard, and that some persons in consequence left their houses.

Many assertions have been made with regard to variations witnessed a day or two before the shock in the hot springs, such as an increase of flow or temperature and changes in their volume and purity. Fumaroles are alleged to have burst out with violence, and even flames to have been seen. The statements,though widely quoted, can hardly be said to rest on satisfactory evidence. On the other hand, Dr. Johnston-Lavis arrived in the island within twenty-four hours after the shock, and, before another day had elapsed, he had examined most of the places where the phenomena were said to have occurred, but could find no remarkable change nor any signs of such having taken place. It is also known, as he remarks, that the temperature of the Ischian springs and fumaroles sometimes varies considerably without any earthquake following, that of the water of Gurgitello occasionally changing by as much as 30° or 40°. We may therefore, I think, conclude that, except for one or two shocks and underground noises too slight to cause general alarm, there were no decisive heralds of the great earthquake.

The curves in Fig. 16 represent the isoseismal lines as drawn by Dr. Johnston-Lavis. As in the earthquake of 1881, they bound respectively the areas of complete destruction, partial destruction and slight damage to buildings, the course of the outer line being to a great extent conjectural owing to the small extent of land traversed by it. The first isoseismal is about 2½ miles long, 1½ miles broad, and 3 square miles in area; the second about 4 miles long, 3½ miles broad, and 11 square miles in area; and the third about 6½ 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 thesurrounding country, though the difference was of course less marked than on the previous occasion.

Isoseismal lines of the Ischian earthquake of 1883.Fig.16.—Isoseismal lines of the Ischian earthquake of 1883.ToList

Fig.16.—Isoseismal lines of the Ischian earthquake of 1883.ToList

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 9.10P.M., and the second and stronger at 9.25P.M.; and De Rossi states that at about 9.30P.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.

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 Frasso, 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 passed within a few hundred yards of Fontana.

Measurements of the angle of emergence were made by Dr. Johnston-Lavis at twenty-four places, and inevery 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 passing through the epicentre, and the shorter another through Fontana. The short lines onthe 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.

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