Chapter 6
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 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.
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 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 (_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.
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 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 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 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.
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.
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.
POSITION OF THE EPICENTRE.
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 of every 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 between the 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.
DEPTH OF THE SEISMIC FOCUS.
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 between about 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.
NATURE OF THE SHOCK.
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 slight lateral movement immediately after. At Valle (near Barano) and Piejo, both places about a mile from Fontana, the vertical component was also perceptible.
AFTER-SHOCKS.
The after-shocks were few and of slight intensity. Dr. Johnston-Lavis gives the following dates: March 7th, 12.5 A.M. and midday; March 11-12, 15-16, 17-18, 27 (?), April 5th and 6th, and July 18th, 8.30 P.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.
EARTHQUAKE OF JULY 28TH, 1883.
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 9.25 P.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.
PREPARATORY SIGNS.
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 6 A.M. and 9 A.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.
ISOSEISMAL LINES AND DISTURBED AREA.
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 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.
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 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.
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 passing 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.
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 pass 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 masses, 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 quantities 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.
The 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 principal 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 principal 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.
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 passing 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 pass through the centre of the old crater of Epomeo. Along the line of this band, occur the fumaroles of Monte Cito and Ignazio 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 repetition 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 principal 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 passages 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. DAUBRÉE, A.--"Rapport sur le tremblement de terre ressenti à 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." _Japan 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 principal trachytic masses, 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 masses and craters are denoted by the following tables:--
_a._ Epomeo. _b._ Trippiti. _c._ Vetta. _d._ Garofoli. _e._ Vatoliere. _f._ Campagnano. _g._ Vezza. _h._ Imperatore. _i._ C. St. Angelo. _j._ Lo Toppo. _k._ Marecocco. _l._ Zale. _m._ Rotaro. _n._ Montagnone. _p._ Bagno. _q._ Tabor. _r._ P. Castiglione. _s._ Cremate. _t._ Arso. _u._ Porto d'Ischia.
[22] It is possible that Monte Campagnano may form an exception to this statement.
[23] Shocks were felt in the island in 1559 and 1659, but one at least was of external origin.
[24] Prof. Mercalli, from the five estimates of the angle of emergence which he considered most reliable, found the mean depth to be about 3,280 feet.
[25] Professor de Rossi estimated the mean duration as not much exceeding ten seconds. Dr. Johnston-Lavis, on the other hand, considers the general estimate of fifteen seconds as far too low. In one case, at Casamicciola, he ranks it as high as thirty-one seconds.
[26] Quoted from the useful translation of Covelli's memoir given by Dr. Johnston-Lavis.
[27] Baldacci supposes that the thermal springs and fumaroles of Forio, Stennecchia, Montecito, Casamicciola, and Castiglione lie along a tangential fracture starting from Forio and passing by Casamicciola to near Punta di Castiglione. Mercalli, however, argues forcibly against this inference.
[28] Professor Mercalli adds, as a fourth point of contact between Ischian earthquakes and volcanic phenomena, the changes in the fumaroles and hot springs which preceded or accompanied or followed the earthquakes of 1828, 1881, and 1883.
[29] "Fontana," he says, "occupies the centre of the great crater of Epomeo..., and therefore lies immediately over the ancient chimney, which in all probability is filled by an old plug of consolidated trachyte, which must descend to the igneous reservoir. Any mass of igneous matter, that might determine the further rupture of a collateral fissure, would result in the conduction of any changes of pressure or vibrations, along the column of highly elastic trachyte; whilst the same earth-waves would be annulled or absorbed by the inelastic tufas surrounding it, so that the blow would be struck perpendicularly to the surface, and in a small area with well defined limits. The undulatory sensations, after the principal local shock, were those that arrived from the great centre of impulse beneath Casamenella."
[30] The above paragraph is a summary of the reasoning stated with admirable clearness by Dr. Johnston-Lavis. It should be mentioned that the late Professor Palmieri, relying on the extremely limited disturbed area, dissented from this view; but his difficulty is met by supposing the focus to be small as well as shallow, a supposition that is supported by the shortness of the meizoseismal band, as well as by the elongation of the isoseismal lines in the direction perpendicular to this band.