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.
[Illustration: 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 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.
[Illustration: 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.
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.
[Illustration: FIG. 16.--Isoseismal lines of the Ischian
earthquake of 1883.]
Outside Ischia, the shock was felt distinctly in all the island of
Procida and in Vivara; on the mainland, by some as far as Pozzuoli
and by several persons in Naples, which is twenty miles from
Casamicciola. The seismograph at the university of this city
registered two small shocks, the first at 9.10 P.M., and the second
and stronger at 9.25 P.M.; and De Rossi states that at about 9.30 P.M.
the seismographs at Ceccano, Velletri, and Rome recorded a shock
consisting of very slow undulations. There are again no materials for
estimating the size of the disturbed area, but there can be no doubt
that it was much less than that of a moderately strong British
earthquake.
POSITION OF THE EPICENTRE.
Owing to the limited size of the disturbed area, time-observations,
even had they been available, would not have sufficed to determine the
position of the epicentre, and both Dr. Johnston-Lavis and Professor
Mercalli therefore had recourse to Mallet's method, the former relying
chiefly, as before, on fissures in damaged buildings, and the latter
on the overthrow or displacement of columns and other objects.
Dr. Johnston-Lavis measured the azimuth of the wave-paths at
sixty-five places, and at about one-third of these was able to make
two or more observations. The azimuths converge towards the same
region as in 1881, but the area covered by their intersections is
larger. The meizoseismal band of maximum vertical destruction
indicated by shading in Fig. 16 is also of the same form and slightly
greater extent, reaching from the upper part of Lacco to a little
south of 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.
[Illustration: FIG. 17.--Diagram of wave-paths at seismic
vertical of Ischian earthquake of 1883. (_Johnston-Lavis._)]
The six angles of emergence that would give the greatest depth below
the epicentre were all measured at places in the south of the island
close to the line joining Panza and Barano, and it will be noticed
that five of these apparent depths are much greater than those
obtained from the other wave-paths. Excluding these observations, the
remaining eighteen give depths ranging from about 450 to about 3,350
feet, and a mean depth of 1,730 feet,[24] or nearly one-third of a
mile, that is, almost exactly the same as the mean depth found from
the earthquake of 1881.
The six exceptional angles of emergence come from the district of
divergent azimuths to the south of Epomeo. Three of the corresponding
azimuths 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.
[Illustration: 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 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.