THE RIVIERA EARTHQUAKE OF FEBRUARY 23RD, 1887.
Few earthquakes have aroused a more widespread interest than those
which struck the thronged cities of the Riviera on February 23rd,
1887. The first and greatest of the shocks occurred at about 6.20
A.M., the second nine minutes later, and the third, intermediate in
strength, at about 8.51 A.M.[47] All three shocks were of destructive
violence, the damage wrought by them extending along the coast and for
a short distance inland from Nice to beyond Savona. Most of the injury
to property and nearly all the loss of life were, however,
concentrated on the eastern side of the frontier; and it therefore
fell to the lot of the Italian Government to provide for the
scientific investigation of the earthquakes, as well as to meet the
wants of those deprived of home and support. Professors Taramelli and
Mercalli, who two years before had studied the earthquakes in
Andalusia, were again nominated, the former to examine the geology of
the central regions, and the latter to report on the seismic
phenomena. Their joint memoir forms one of the most complete accounts
that we possess of any earthquake, and is the chief authority for the
description given in this chapter. Another valuable monograph is that
prepared by Professor A. Issel, of Genoa, who received an independent
appointment from the same Ministry. A third official commission was
also sent to estimate the amount of damage caused by the earthquakes
in the Italian towns and villages. In France, the destruction of
property was much less serious, and attention was confined chiefly to
the records of the shock provided by magnetographs and other
instruments in distant observatories. In Switzerland, the effects
remarked were merely those due to the evanescent vibrations of a
remote earthquake; but many interesting records were collected by the
permanent seismological commission established in that country.
DAMAGE CAUSED BY THE EARTHQUAKES.
Owing to variations in the nature, foundation, and site of buildings,
there is always great diversity in the destructive effects of an
earthquake. In one and the same town, most of the houses may be razed
to the ground, while in their midst may be found some that are
shattered but still standing, and others perhaps that are practically
unharmed. The stronger after-shocks often complete the ruin of the
partially damaged houses; though in such cases the real loss is as a
rule comparatively small.
The close succession of the two strong after-shocks of February 23rd
made it impossible as a rule to separate their effects from those due
to the first shock; but it has been roughly estimated that about
one-quarter of the total damage was caused by the two after-shocks
together. To them also must be referred in part the comparatively
small number of wounded, many persons buried beneath the ruins having
no doubt perished from subsequent falls before they could be
extricated.
Taking all three shocks together, the total loss to property,
according to Professor Mercalli, must be valued at about 22 million
francs in Italy alone. For the province of the Alpes Maritimes in
France, full details are wanting, but the loss there cannot fall far
short of three million francs. The total amount of damage must
therefore be placed at about a million pounds. From the figures given
by the official commissions, it appears that the earthquakes were most
disastrous at Diano Marina and Diano Castello; while other places,
such as Oneglia, Bussana, Baiardo, Pompeiana, and Vallecrosia,
suffered only a little less severely. At Mentone about 155 houses, and
at Nice about 61 houses, were rendered uninhabitable, and many others
were badly injured.
In Italy, 633 persons were killed, 432 seriously wounded, and 104
slightly wounded; in France, 7 persons were killed and 30 seriously
wounded, the number of persons slightly wounded being unknown. The
majority of the deaths occurred in two or three places. Thus, at Diano
Marina, 190 persons were killed and 102 wounded; at Baiardo, 220 were
killed and 60 wounded; at Bussana, there were 53 killed and 27
wounded. The death-rates were, however, comparatively small, amounting
for the above places to not more than 8-1/2, 14, and 6-1/2 per cent.,
respectively; figures which only slightly exceed those obtained for
places in the meizoseismal area of the Andalusian earthquake.
Though the damage can hardly be regarded as excessive, it was
nevertheless largely due to the peculiar architecture prevalent in the
Riviera. Arches in the walls are common even in the upper storeys,
and, in Oneglia and Diano Marina, if not also in other places, the
floors are nearly always brick arches abutting against the walls and
without other lateral support. Professor Mercalli believes that, in
private houses, more than 90 per cent. of the dead bodies were crushed
beneath these fallen arches. The height of the buildings is also great
in proportion to the foundation and to the thickness of the walls; and
the main walls are interrupted by numerous apertures, from the corners
of which nearly all the fissures sprang. In some of the coast towns,
the houses are built of rounded stones gathered from the beach, or of
rubble with stones of all shapes and sizes, bound by cement of the
poorest quality. Lastly, as much of the damage due to previous
earthquakes had been badly repaired, it is evident that the
destructiveness of the Riviera earthquakes must to a great extent be
referred to preventable causes.
The occurrence of the principal shock shortly after six on the morning
of Ash Wednesday must also have increased the death-rate; for many
persons, after a night of amusement, had lain down for a short time
and were sleeping heavily; while others had already risen and were
collected in the churches; the circumstances in either case rendering
escape more difficult.
Taking account, however, of this accidental increase in the number of
victims, Professor Mercalli considers that the earthquake of 1887 was
the most disastrous of all those which have visited either the
Riviera or northern Italy in the last three centuries; though, during
the nineteenth century, there were three Italian earthquakes of far
greater destructive power, but all confined to the southern part of
the peninsula--namely, the Neapolitan earthquakes of 1805 and 1857,
and the Ischian earthquake of 1883.
PREPARATION FOR THE EARTHQUAKES.
It is difficult, as usual, to specify the exact moment when the first
earthquake of the 1887 series took place; but it is evident that the
preparation for the great shock was very brief. At Oneglia, it is
alleged that faint shocks and sounds were observed many times, chiefly
at night, during the month preceding February 23rd; though they were
not at the time supposed to be of seismic origin. A slight shock is
also reported from Diano at about midnight on February 21-22.
The first undoubted shock occurred on February 22nd, at about 8.30
P.M., or ten hours before the principal earthquake. Though very
slight, it was felt throughout the Riviera and in part of Piedmont.
Another shock, also weak, took place at about 11 P.M.; and a third,
sensible only in the eastern part of the Ligurian Apennines, on
February 23rd, at about 1 A.M.; at which time the tide-gauge at Genoa
recorded some abnormal oscillations. An hour later, a more important,
though by no means a strong, shock occurred; this was perceptible all
over the Riviera, in Piedmont, and in Corsica; in other words, it
disturbed a region agreeing closely with the central area of the
disastrous shock. At about 5 A.M., a fifth shock, somewhat weaker
than the preceding, was felt over the same area, concurrently, or
nearly so, with another abnormal oscillation of the tide-gauge at
Genoa; while a sixth shock was noticed at several places a few minutes
before the great shock.
During the night of February 22-23, nervous persons in many towns and
villages were agitated without apparent reason. Birds and animals,
more sensitive than human beings to faint tremors, were more
distinctly affected, especially for some minutes before the
earthquake. Horses refused food, were restless or tried to escape from
their stables, dogs howled, birds flew about and uttered cries of
alarm. As these symptoms were noticed at more than one hundred and
thirty places within the Italian part of the central area, there can
be little doubt that they were caused by microseismic movements for
the most part insensible to man.
ISOSEISMAL LINES AND DISTURBED AREA.
The only complete map of the isoseismal lines is that drawn by
Professor Mercalli.[48] In this map, reproduced in Fig. 33, the
continuous curves represent the principal isoseismal lines; the dotted
curves define the disturbed areas of two of the stronger after-shocks.
The meizoseismal area, bounded by the curve marked 1 in Fig. 33, is
also shown on a larger scale in Fig. 34. At the places denoted by
small circles in the latter figure, the principal shock was
"disastrous," some of the houses in each being either totally or
partially ruined. At those marked by a small cross, the shock was
"almost ruinous"; in other words, numerous houses were damaged, but in
no case was the injury of a serious character. The meizoseismal area
is thus a narrow band, skirting the Riviera coast from Mentone to
Albissola, a distance of 106 miles, and extending inland for not more
than from nine to twelve miles. The greatest intensity, corresponding
to the ruin of many houses with considerable loss of life, was
reached at only a few places between Bussano and Diano Marina, all
lying within a littoral band about twenty miles in length and three to
three and a half miles in width. If, however, the epicentre had lain
on land, the area would have been much greater, Professor Mercalli
estimates about four times greater, than its actual amount.
[Illustration: FIG. 33.--Isoseismal lines of the Riviera
earthquake. (_Mercalli._)]
The curve marked 2 (Fig. 33) bounds the "almost ruinous" zone; its
expansion towards the north and contraction towards the west,
north-west, and east, being its most noteworthy features. The next
zone, that of slight damage, is contained between the isoseismals 2
and 3, the latter curve probably grazing the north end of Corsica.
Beyond this lies the "strong" zone, in which the shock was generally
felt without causing any damage to buildings. Its boundary (marked 4)
passes near Marseilles, Como, and Parma, and includes nearly the whole
of Corsica; towards the north-west, in the valley of Aosta, it curves
in towards the isoseismal 3.
In the outermost zone of all the shock was "slight," and towards the
margin was only just perceptible. The boundary, which of course
defines that of the disturbed area, reaches as far north as Basle and
Dijon, to Perpignan on the west, Trento, Venice, and Pordenone on the
east, and to the south as far as Tivoli (near Rome) and the northern
end of Sardinia. In eastern Switzerland, it shows a marked curve
inwards; possibly, as Professor Mercalli suggests, from the vibrations
having to cross the northern Apennines in a direction nearly at right
angles to their axis. Except for this bay, however, the curve differs
little from a circle, the centre of which lies in the sea, a little to
the south of Oneglia, close to the position assigned by other evidence
to the epicentre. The radius of this circle being about 264 miles, it
follows that the disturbed area must have contained about 219,000
square miles--by no means a large amount for so strong an earthquake.
POSITION OF THE EPICENTRES.
It is evident, from the form of the meizoseismal area shown in Fig.
33, that a mere fringe of it lies upon land, and that the epicentre
must be situated some distance out at sea. Other facts may be
mentioned which point to the same conclusion. There were, for
instance, no purely vertical movements observed, even in the districts
where the damage done by the shock was greatest. Nor were any large
landslips to be seen in those areas; there were no lasting changes in
the underground water-system; and in general, as Professor Mercalli
remarks, all the superficial distortions of the ground which are so
characteristic of the epicentral area of a great earthquake were
conspicuous by their absence. There is evidence, again, of some
disturbance of the sea-bed in the death and flight of fishes from
great depths and in the seismic sea-waves recorded by the tide-gauges
at Genoa and Nice. These phenomena will be described in a later
section, but reference should be made here to an interesting
observation at Oneglia on the occurrence of some of the stronger
after-shocks. Persons on the coast, it is said, saw the sea curling
and moving, and immediately afterwards the shock was felt.
In determining the position of the epicentre, Professor Mercalli had
recourse as usual to observations on the direction of the shock,
especially those derived from the oscillation of lamps or other
suspended objects, the projection or fall of bodies free to move,
fractures, etc., in damaged houses, and the stopping of pendulum
clocks. Such observations were made at 120 places--72 in the western
Riviera and the Alpes Maritimes, and 48 at Piedmont, Lombardy, and
Tuscany.
At many of these places the movement was extremely complicated. In
nearly all parts of the area most strongly shaken, for instance, the
direction of the shock changed more than once; and it was therefore
necessary to select whenever possible the principal direction of the
shock at each place. In some towns, such as Oneglia, Mentone, Antibes,
Cuneo, etc., the shock had two dominant directions, and these appeared
to be sensibly at right angles to one another; an inclination which,
as Professor Mercalli suggests, may be due in part to the
approximation of the real directions to those of the principal walls
of the houses in which the observations were made.
Most of the lines of direction, when plotted on the map, converge
towards an area lying between the meridians of Oneglia and San Remo,
and between nine and fifteen miles from the coast. For places near the
epicentre, the most trustworthy, in Mercalli's opinion, are those made
at Oneglia, Mentone, Taggia, Bordighera, Castel Vittorio, Nice, and
Genoa; and the points in which these lines Intersect one another are
Indicated by small crosses on the map of the meizoseismal area (Fig.
34). All of them lie at sea at distances between six and fifteen miles
to the south of Oneglia. The most probable position of the principal
epicentre is that marked by the small circle A, which is situated
about fifteen miles south of Oneglia.
[Illustration: FIG. 34.--Meizoseismal area of the Riviera
earthquake. (_Taramelli and Mercalli._)]
There are, however, several lines of direction which can have no
connection with this epicentre. Besides the east and west lines at
Nice, Mentone, and Antibes, there are others at the same places which
run north and south or nearly so. Professor Mercalli believes that
they were due to vibrations coming from a second focus lying to the
south of Nice, and there are also several lines of direction at more
distant places which converge towards the neighbourhood of the
corresponding epicentre.
This conclusion receives unexpected support from some of the best
time-records. At the railway-stations of Loano and Pietra Ligure, the
times of occurrence were given as 6h. 20m. 5s. and 6h. 20m.
respectively--estimates which are probably accurate to within a few
seconds; for, at the moment of the shock, the officer who brought the
exact time along the railway-line from Genoa was at Loana, and had
just passed through Pietra Ligure. On the other hand, the estimates
for Mentone and Nice--namely, 6h. 18m. 35s. and 6h. 19m. 43s., if not
equally exact, cannot err by many seconds, certainly not by so much as
one minute. Since the distances of Loana and Pietra Ligure from the
principal epicentre are 31 and 32 miles, and those of Mentone and Nice
28 and 37 miles, it is therefore clear that the vibrations which
arrived first at Nice and Mentone must have come from a local focus,
where the impulse preceded that at the principal focus by several
seconds.
DEPTH OF THE PRINCIPAL FOCUS.
Inaccurate as are all the methods of determining the depth of focus,
it seems probable, as Professor Issel argues, that the principal
Riviera focus was situated at a considerable distance from the
surface. In no part of the meizoseismal area was the shock a really
violent one; yet its intensity must have faded very slowly outwards,
for it was strong enough to stop clocks at places in Switzerland and
elsewhere not less than 250 miles from the origin.
Professor Mercalli regards Mallet's method with greater favour than
most seismologists. He points to the gradual increase in the angle of
emergence from the outer zones disturbed by the Riviera earthquake
towards the meizoseismal area, where several good observations were
made from fissures in walls parallel to the dominant direction of the
shock. The angles of emergence which he considers as most trustworthy
are those of 35° at Taggia, 40° at Oneglia, and about 30° at
Bordighera. The corresponding depths for the focus are 10.4, 10.4, and
11.6 miles, giving an average of about 10-3/4 miles.
There are no similar observations forthcoming for the depth of the
secondary focus near Nice and Mentone; but Professor Mercalli observes
that it must have been shallower than the other, for the vertical
component of the vibrations from this focus was much less sensible
than that of the motion coming from the principal focus.
NATURE OF THE SHOCK.
_The Double Shock._--In the valuable collection of records made by
Professors Taramelli and Mercalli there appears at first sight to be
the utmost diversity in the evidence with regard to the nature of the
shock. Thus, in the province of P. Maurizio alone, the shock was
described as subsultory first and then undulatory or vorticose at 25
places, undulatory and then subsultory at 22, undulatory and then
subsultory and again undulatory or vorticose at 13, and subsultory
first, then undulatory, and finally subsultory and vorticose at two
places. It is clear that the shock was of considerable duration, not
less than half-a-minute as a rule, and that there were several phases
in the movement; and it would seem that one or more of these phases
may have passed unnoticed owing to the alarm occasioned by the shock,
and to the fact that most of the observers were asleep when the
earthquake began. Defects of memory must also have an influence not to
be neglected, for, even with the simple shocks felt in the British
Isles, persons in the same or neighbouring places differ greatly in
their testimony.
But, if we confine ourselves to the accounts of careful persons alone,
the discrepancies to a large extent disappear. Indeed, all over the
ruinous area (Fig. 33) the shock maintained a nearly uniform
character. At Oneglia, for instance, there were two well-marked
phases, the first of which began with a brief subsultory movement,
followed by more horizontal undulations of longer period; a pause,
lasting but for an instant, was succeeded by vibrations which, though
not vertical, were highly inclined to the horizon; they continued
throughout the second phase, but, towards the end, new undulations
were superposed, and these, coming from different directions, resulted
in an apparently vorticose movement. Professor Mercalli represents the
motion diagrammatically by the curve _a_ in Fig. 35. At Diano Marina,
as will be seen from the curve _b_, the shock again consisted of two
phases, each beginning with a few subsultory vibrations and ending
with horizontal undulations of much longer period. In the first phase,
the undulations were marked by a dominant direction, but, towards the
close of the second phase, there was no determinate direction, and the
impression was again that of a vorticose shock. At Savona, the
movement, which is represented by the curve _c_, must have lasted from
twenty-five to thirty seconds. It also consisted of two phases, with
subsultory vibrations and undulations in the same order; and it was
noticed that the second part of the shock was much stronger than the
first. According to some observers, the concluding movements were
vorticose.
[Illustration: FIG. 35.--Nature of shock of Riviera earthquake.
(_Taramelli and Mercalli._)]
In the zone surrounding the ruinous area, the vertical component of
the motion was observed to diminish with the intensity; but, in other
respects as well as in duration, the shock retained the same general
form. At Genoa, Turin, Acqui, Alessandria, Antibes, and other places,
two distinct phases were perceived, occasionally separated by a brief
pause, the first being invariably the weaker. At some places, the
observers speak of two shocks at about 6.20 A.M., separated by an
interval of a few seconds; and this division was noticeable as far as
Salò on the shore of Lake Garda and Vicenza in Venetia. Only in
Switzerland and other districts near the boundary of the disturbed
area did the weaker part of the shock become insensible, the other
consisting of horizontal oscillations, remarkable for their slowness
and regularity, and lasting for as much as twenty or thirty seconds.
We may thus conclude, with Professor Mercalli, that the earthquake
resulted from the almost immediate succession of two distinct shocks,
in each of which the nearly vertical vibrations were more marked at
the beginning, while the slower undulations predominated towards the
close, those of the second phase generally becoming vorticose through
the superposition of movements coming from different directions. The
second part of the shock in all of the more carefully written accounts
is described as the stronger, especially as regards the subsultory
vibrations in the meizoseismal area; except in the immediate
neighbourhood of Nice, where the second phase was generally regarded
as the weaker, or at any rate as not stronger than the first.
_Origin of the Double Shock._--These observations show, not only that
the principal earthquake consisted of two distinct shocks, but also
that the shocks originated in different foci. For, if the vibrations
of both had started from one focus, the second shock would have been
everywhere the stronger; instead of which there was a small area near
Nice where the intensity of the first was the greater. This points
clearly to the existence of another focus situated not far from Nice;
and it is evident that the greater intensity of the first part in that
district was due solely to the proximity of this focus, for, still
farther to the west, at Antibes, the second part was again the
stronger.
There is thus a striking agreement in the inferences drawn from
observations on the direction, time of occurrence, and nature of the
shock. In the face of such concurring testimony, little doubt can
remain as to the existence of two foci, one to the south of Oneglia
and the other to the south of Nice, the initial impulse at the latter
being decidedly the weaker, and preceding that at the eastern focus by
an interval of some seconds, long enough at any rate for the resulting
vibrations to reach the Oneglia focus and to spread beyond it before
the vibrations from that focus started on their outward journey.
_Seismographic Records._--In 1887, the Riviera and the districts
adjoining it were unprovided with accurately constructed seismographs.
The observatories at Alessandria, Milan, Monza, Parma, Florence, and
other places in Italy contained seismoscopes and other pendulums, and
these all registered the fact that an earthquake had occurred, and in
many cases traced a series of elliptical or elongated curves. A record
of the shock was also given by a Cecchi seismograph at Perpignan in
France, but the distance from the epicentre was too great to allow
details to be shown. The most valuable record was that obtained from a
Cecchi seismograph at the observatory of Moncalieri, near Turin, about
ninety miles north of the principal epicentre.
In this seismograph, the pendulums are provided with pointers, the
tips of which touch vertical sheets of paper attached to the sides of
an upright rectangular box. When an earthquake occurs, this box is
made to descend slowly with a uniform velocity, while the moving
pointers trace curves upon the smoked paper. The north-and-south
component of the horizontal motion is inscribed on the sheet of paper
facing west, and the east-and-west component on the paper facing
south.
[Illustration: FIG. 36.--Seismographic record of the Riviera
earthquake at Moncalieri. (_Denza._)]
During the principal Riviera earthquake, the former pendulum furnished
an indistinct record, while the other traced the diagram reproduced in
Fig. 36. The movement, as here represented, began at about 6h. 21m.
50s. A.M. (mean time of Rome) with a series of small tremors, which
lasted for about twelve seconds. Then followed some large
oscillations, always in a nearly east-and-west direction, which at 6h.
22m. 21s. gave place to a second series of tremors similar to those at
the beginning of the shock, but of greater amplitude. These continued
for at least twelve seconds, at the end of which time the motion of
the smoked paper ceased. The total duration of the movement at
Moncalieri cannot therefore have been less than forty-three seconds.
Interesting as this record is, it is doubtful how far it represents
accurately the movement of the ground. The Moncalieri instrument was
erected before the modern type of seismograph was designed, in which
some part remains steady, or very nearly steady, during the
complicated movements of the ground that take place in an earthquake.
It will be noticed that the curve in Fig. 36 shows no sign of the
division of the shock into two distinct parts, and this may perhaps be
due to the swinging of the pendulum itself; in which case, the curve
described by the pointer would be the resultant of the oscillations of
the ground and the proper motion of the pendulum.
SOUND-PHENOMENA.
The sounds that preceded and accompanied the Riviera earthquake have
attracted but little study, although they seem to have been widely
observed. No attempt was made to define the limits of the area over
which they were audible; but Professor Mercalli states that in the two
outer zones (Fig. 33) the sound generally passed unobserved. It was,
however, heard near Piacenza in Lombardy and Reggio in Emilia, places
which are about 115 and 140 miles from the principal epicentre.
In the area in which the shock was most violent, the sound resembled
that of trains and vehicles in motion; while, outside this area it
generally appeared to be like the hissing of a violent wind. In only a
few places was it compared to detonations, the crashes of artillery or
distant thunder. Some observers describe the sound as appearing at
first as if a strong wind were rising, and then as the roaring of a
heavy railway-train passing.
Nearly all the observers, who were awake at the beginning of the
earthquake, agree in asserting that the sound distinctly preceded any
movement of the ground. From this, as in the case of the Andalusian
earthquake, Professor Mercalli infers that the sound-vibrations
travelled with the greater velocity; but, as will be shown in Chapter
VIII., the general precedence of the sound admits of another and more
probable explanation.
THE UNFELT EARTHQUAKE.
If the Andalusian earthquake first drew general attention to the
distant spread of unfelt earth-waves, the Riviera earthquake showed
that this was no isolated phenomenon. We know now that the propagation
of such waves is only limited by the surface of the earth, but in 1887
some doubt was felt at first as to the nature of the disturbance,
whether it was magnetic or mechanical in its origin.
In 1884, the only observatories at which magnetographs were disturbed
were those of Lisbon, Parc Saint-Maur (near Paris), Greenwich, and
Wilhelmshaven. In 1887, the magnetographs registered the Riviera
earthquake at these and several other observatories, the distribution
of which is shown in Fig. 37. In this sketch-map, the position of the
principal epicentre is represented by the small cross, while the
nearly circular line shows the boundary of the disturbed area.
[Illustration: FIG. 37.--Distribution of observatories at which
magnetographs were disturbed by the Riviera earthquake.]
Three of the observatories, those of Nice, Lyons, and Perpignan, lie
inside this area. At Nice (which is thirty-seven miles from the
principal epicentre), M. Perrotin states that the magnetograph curves
show nothing of any interest, except a notable magnetic perturbation
on the vertical force curve, the time of which, however, is not
stated.[49] At Lyons (211 miles), the declination, horizontal force
and vertical force, magnets were all disturbed at 6h. 25m. 47s. A.M.,
and Perpignan (264 miles), all three magnets, but especially those for
the declination and horizontal force, were set abruptly oscillating
at 6h. 25m. 20s.
Elsewhere in France, the disturbances were noticed at the
observatories of Parc Saint-Maur and Montsouris, near Paris (about 447
miles), and at Nantes (538 miles). At Parc Saint-Maur, all three
curves show a very clear trace of the earthquake at 6h. 25m. 35s., the
oscillations lasting several minutes, and at Montsouris they also
began at the same time. At Nantes, the perturbations were so slight
that they escaped notice on a first examination.
In Austria, disturbances were observed at Pola (295 miles) and Vienna
(506 miles), beginning at 6h. 28m. 35s. and 6h. 30m. 35s.,
respectively. They reached Brussels (522 miles) at 6h. 29m. 27s., and
Utrecht (600 miles) at 6h. 28m. 38s.[50] At Wilhelmshaven (690 miles),
only the vertical force magnet was affected, the oscillations
beginning at 6h. 30m. 35s., and lasting for fourteen minutes. At 6h.
27m. 55s., the declination and horizontal force magnets of Greenwich
observatory (642 miles) were set vibrating, but no similar
disturbances were revealed by the vertical force curve or by the two
earth-current registers. At Kew (652 miles), the horizontal force
magnetograph was moved by the earthquake at about 6h. 29m. 55s. The
curves at Stonyhurst and Falmouth show no sign of any disturbance, nor
do those at Pawlovsk in Russia, or Seville. At Lisbon (951 miles),
however, the three curves indicate disturbances at 6h. 32m. 35s., but
so feeble are they that they would have escaped discovery if the
occurrence of the earthquake had been unknown.
The effects registered on the magnetograms are quite different from
those which correspond to ordinary magnetic perturbations; but they
are not unlike those produced by the action of the momentary currents
which are used for making the hour-marks, except that the
earthquake-oscillations lasted several minutes (see Fig. 21). In each
case, then, the magnetic bars must have received a succession of
several or many impulses.
Now, the effect of these impulses on each magnet must depend on the
relations which exist between the period of oscillation of the magnet,
the rate of damping of such oscillations, and the interval between the
successive impulses. Also, the apparent commencement of the phenomena
may be delayed if two impulses of contrary sense should follow one
another before the bar is perceptibly displaced. It is therefore to be
expected, as M. Mascart points out, that the disturbances of the three
instruments need not be of the same order of magnitude, that with
different forms of apparatus the effects may be very variable, and
that the deflection of one instrument may precede that of another at
one and the same place.
In all the magnetographs, the record is made on photographic paper,
which travels so slowly that the time of a movement can only be
ascertained to the nearest minute. As the disturbances on the French
curves were apparently almost simultaneous, and as no two of the
others differed in time of occurrence by more than five minutes, there
is thus some colour for M. Mascart's contention that the magnetic
apparatus registered, not the movements of the ground, but the passage
of electric currents produced in the ground at a certain epoch of the
earthquake.[51]
On the other hand, it is important to notice that, in the central part
of the disturbed area, at Nice, two, if not all three, of the
magnetographs were unaffected at the time of the earthquake.
At first sight, this fact seems equally opposed to a mechanical
explanation of the disturbance. But, when the vibrations are very
rapid, as they are in the neighbourhood of the epicentre, the magnetic
bars, owing to their mode of suspension, have not sufficient time to
be sensibly deflected in the brief interval between successive phases
of the impulse. The magnetograms of the Montsouris observatory show,
for instance, hardly any perceptible trace of disturbance during the
passage of railway trains along two adjacent lines. The farther,
however, the earth-waves travel from the origin, the longer becomes
the period of their vibrations. In Switzerland, they were remarkable
for their slowness, even to the unaided senses. Thus, at places more
or less remote from the Riviera, the magnets would receive impulses at
intervals approximating to their own periods of vibration, and they
would then oscillate freely for some time.
Again, notwithstanding some variations, it will be remarked that on
the whole the retardation of the initial epoch of the disturbances
increases with the distance from the epicentre. It thus seems clear, I
think, that the cause of the disturbances must be sought in the shock
itself; although their initial epochs at different places are too
roughly defined for ascertaining the velocity with which the
earth-waves travelled.
EFFECTS OF THE EARTHQUAKE AT SEA.
The Riviera earthquake, owing to its submarine origin, was marked by
certain phenomena that were absent from the other earthquakes
described in this volume.
_Nature of the Earthquake at Sea._--At the time of the earthquake,
several vessels were close to the epicentral area. One, about three
miles off Diano Marina, was shaken twice at about 6.20 A.M., and so
violently that it seemed as if the masts would be broken off. Another,
about ten miles south of P. Maurizio, also experienced two shocks, a
few minutes apart, as if each time it had struck the bottom. These
observations are chiefly interesting in showing that the double shock
was felt at sea as well as on land. As transverse vibrations are not
propagated through water, it follows that the second part of the shock
cannot, as some maintain, have been composed of transverse vibrations.
_Destruction of Fishes._--During the days immediately following the
earthquake, a large number of deep-sea fishes were found dead or
half-dead either in shallow water or stranded on the beach, especially
in the neighbourhood of Nice. Among them were numerous specimens,
mostly dead and floating, of _Alepocephalus rostratus_, a typical
deep-sea form, several of _Pomatomus telescopium_, _Scopelus
elongatus_, and _S. humboldti_, and many of _Dentex macrophthalmus_
and _Spinax niger_. The death and flight of these fishes must have
been due to a sudden shock, almost like that caused by the explosion
of dynamite, and communicated simultaneously to the whole surface of
their bodies.
_Seismic Sea-Waves._--Immediately after the earthquake, the sea
retired a short distance, variously estimated at from ten to thirty
metres, laying bare some rocks that were usually immersed. At P.
Maurizio, the surface was lowered by a little more than a metre; and
after a few minutes it rose to nearly a metre above its original
level, returning to it after a series of continually-decreasing
oscillations. At San Remo, a fall of about the same amount took place,
the sea returning after five minutes, and a ship anchored in the
harbour broke from her moorings. Again, at Antibes, the sea was
suddenly lowered by about a metre, so that ships afloat in the harbour
were aground for some instants, and then returned with some
impetuosity to its original level.
[Illustration: FIG. 38.--Record of tide-gauge at Nice.
(_Issel._)]
The evidence of eye-witnesses is confirmed by that of the tide-gauges
at Nice and Genoa, the curves of which are reproduced in Figs. 38 and
39. At Nice, the first arrest of the curve in its usual course
occurred at 6.30 A.M.;[52] the sea-level sank somewhat abruptly, and
after a few marked oscillations gradually returned to its normal
position at 7.50 A.M. At Genoa, the shock caused the writing-pen of
the tide-gauge to dent the paper on which the record is made, and soon
afterwards the curve shows a series of irregular oscillations, about
eight taking place every hour, and gradually decreasing until they
ceased to be perceptible about two hours after the principal
earthquake.
[Illustration: FIG. 39.--Record of tide-gauge at Genoa.
(_Issel._)]
MISCELLANEOUS PHENOMENA.
_Connection between Geological Structure and the Intensity of the
Shock._--As with the Andalusian earthquake, faulty construction and
defective materials were responsible for much of the damage caused by
the Riviera earthquake. But, if we may judge from the sharp local
variations in its amount, the nature of the surface-rocks must have
exerted a still more potent influence. At Cervo, for example, the
injury to property amounted to less than £3 per head of the
population; at Diano Marina, only two or three miles to the west, it
rose to £22 per head. The death-rate at Cervo was about one-tenth, and
at Diano Marina about 8-1/2 per cent. Again, at Mentone, the damage
must have been considerable, for about 155 houses were rendered
uninhabitable; while Monte Carlo, only a few miles farther west,
escaped almost unharmed. Now, Mentone and Diano Marina are for the
most part built on clay or alluvial deposits, and Monte Carlo on a
foundation of limestone.
Even within the limits of a single town, variations no less striking
were perceptible. In Mentone, the greatest damage occurred to houses
of two storeys built on alluvial soil in the low-lying parts near the
sea and in the valleys. The effect of the foundation in this part was
well shown in the case of two equally well-built houses not more than
300 yards apart. One in the valley, with doubtful foundations, was
very much shattered; the other, built on rock, was uninjured. The
large hotels, especially those on high ground, suffered least, few of
them having their main walls seriously damaged. These buildings rise
to heights of from four to six storeys, and of necessity have a firm
and solid foundation.
Professors Taramelli and Mercalli have made a careful study of the
subject of this section. The general conclusions at which they arrive
are that the intensity of the shock was greatest at places built on
pliocene conglomerates, beds of clay superposed on compact old rocks,
patches of alluvium, miocene formations of some thickness formed of
repeated alternations of strata of incoherent marls and limestones or
compact sandstones, beds of chalk, or somewhat rotten dolomite.
The shock was also more destructive on the summits of isolated hills
and ridges and on the steep slopes of mountains. The influence of the
form of the ground was, however, subordinate to that exerted by the
nature of the subsoil. Thus, at Mentone, as we have seen, and also at
Nice and Genoa, houses built on rock in elevated positions suffered
much less than those situated on the plains below that are composed of
sand and recent alluvium.
_Observations of the Earthquake in Railway-Tunnels._--Observations
made in mines at various times and places have proved that an
earthquake is felt less strongly in deep workings, if felt at all,
than on the surface of the ground. In the railway-tunnels of the
Riviera, as Professor Issel has shown, the same result was established
during the earthquake of 1887.
On the line which runs northward from Genoa to Piedmont, a tunnel more
than five miles in length pierces the hilly ground between Ponterosso
and Ronco, the greatest thickness of rock above being about a thousand
feet. At the time of the earthquake, the tunnel was not everywhere
opened out to its full width, and men were at work in different
sections. Outside, the shock was strong enough to damage buildings.
Inside, at about 200 yards from the south end, only a feeble shock was
felt; at 1,350 and 1,625 yards, some bricks were seen to fall from the
facing, but the shock was not otherwise perceived, and only a few
yards farther nothing unusual was noticed by the men at work.
Again, in an unfinished tunnel, about three-quarters of a mile long,
between the harbour of Genoa and the eastern railway-station, the
vibrations were very slightly felt. Even in the tunnels traversed by
the coast railway from Genoa to Nice--that is, in those situated
within the meizoseismal area--the shock was either very weak or not
felt at all, and not one of the tunnels suffered the slightest injury.
To men at work inside a long tunnel, the conditions for observing
earthquakes are somewhat imperfect, but these facts, nevertheless,
bring out very clearly the inferior intensity of the shock at some
depth below the surface.
AFTER-SHOCKS.
While the unfelt earth-waves of the great earthquake were still
wending their way over the zone that surrounds the disturbed area, the
central regions were again shaken, at 6.29 A.M., by a shock strong
enough to produce fresh ruins in the stricken towns along the coast.
Nearly two and a half hours of quiet followed, broken only by a few
subterranean rumblings in the central part of the meizoseismal area.
Then, at 8.51 A.M., occurred another shock, short and sharp, and
inferior in strength only to the principal earthquake. Both of these
after-shocks were felt in Western Switzerland; indeed, they were
perceptible nearly as far as the great shock; the second, however, a
little farther than the first, for it alone was noticed at such places
as Vicenza, Forlì, and Florence. The shock at 6.29 was usually
described as long and its vibrations as undulatory only; that at 8.51
as rather subsultory than undulatory and of very brief duration. The
latter, however, was followed after an interval of a few seconds by
another shock so weak that it generally passed unobserved. Both shocks
were preceded by a rumbling sound.
During the next two days, tremors and earth-sounds were frequent in
the Riviera; once an hour, on an average, the greater part of the
meizoseismal area was shaken by vibrations more or less slight. But,
between one shock and another, at Diano Marina and Alassio, and even
as far as Nice, it only required attention from a careful observer to
perceive an almost continual throbbing of the ground.
Only one of these shocks, that of February 24th, at 2.10 A.M., was
strong enough to cause slight damage to buildings. It disturbed an
area, not exceeded by any of the later shocks, the boundary of which,
shown by the dotted line A in Fig. 33, extends to the north and east
as far as Piacenza and Spezia, while to the west it includes Cannes.
The centre of the curve so drawn lies on land, but, as the shock was
not felt in Corsica, there is no evidence as to the southerly
extension of the disturbed area; and it is probable, as Professor
Mercalli suggests, that the shock originated in the eastern or Oneglia
focus of the great earthquake.
After February 25th, slight shocks were felt during the next
fortnight, at the rate of three or four a day, until March 11th, when
the last after-shock resulting in slight damage occurred at about 3.12
P.M. The boundary of its disturbed area, represented in Fig. 33 by the
dotted line B, passes a little to the east of Savona, and then through
Alessandria, Moncalieri, and Marseilles. The shock, however, was not
observed in Corsica, so that the exact position of the epicentre is
unknown; but Professor Mercalli believes it to coincide with the
western or Nice epicentre of the principal earthquake. At the moment
of the shock, the sea was observed from Alassio to curl and to rise
slightly, while the tide-gauge at Nice, which had traced a continuous
curve earlier in the day, showed a characteristic notch about 3.7 P.M.
Of the remaining after-shocks, only two attained any notable degree of
strength. One, on May 20th at about 8.15 A.M., disturbed an area
nearly concentric with that of the great earthquake, and with a
boundary coinciding nearly with the isoseismal 2 in Fig. 33. Again, on
July 17th at 11.30 P.M., occurred a shock felt over an area nearly as
large as that disturbed on February 24th at 2.10 A.M., and situated in
the same part of the country.
Altogether, during the year following the Riviera earthquake,
Professor Mercalli records 190 after-shocks, most of them slight or
only just felt. With the exception of the first two (on February
23rd), none was observed outside the isoseismal 4 of the principal
earthquake (Fig. 33); and, of the rest, only the four whose dates are
given above disturbed an area of more than one-eighth of that of the
great shock. Some of them, like the shock of March 11th, were stronger
in the western part of the meizoseismal area; but the majority
affected most the eastern portion and seem to be closely associated
with the Oneglia focus.
From February 26th to April 20th, Professor Rumi made observations on
the after-shocks by means of the Foucault pendulum erected at Genoa
for demonstrating the rotation of the earth. In nearly every case, the
oscillations took place along a north-east and south-west line, or in
the same direction as the first great shock--a resemblance which
supports the inference that many of the after-shocks originated
within the Oneglia focus.
ORIGIN OF THE EARTHQUAKES.
_Recent Movements in the Riviera._--The earliest movements that
resulted in the great range of the Maritime Alps and the Ligurian
Apennines date from pre-Carboniferous times, when the central
crystalline massifs in part emerged. At the end of the Liassic epoch,
the secondary formations of the district were uplifted, and it was at
this time that the range assumed its characteristic curved form. Later
still, at the close of the Eocene period, an elevation of more than
9000 feet took place, for upper Eocene beds are found at this height
in the Maritime Alps.
Since that time, other important movements have occurred. Pliocene
deposits have been found in the Riviera at an altitude of 1,800 feet.
Recent soundings in the Gulf of Genoa have also shown that all the
valleys of the Riviera between Nice and Genoa are continued far below
the level of the sea to depths of not less than 3000 feet. Thus, at
the end of the Pliocene or beginning of the Quaternary period, there
was an elevation of nearly 5000 feet, accompanied or followed by the
erosion of the valleys which, later on, during the Quaternary period,
were submerged about 3000 feet. Even in still more recent times,
probably in the Palæolithic age, minor movements continued. Traces of
recent elevation, varying in amount from a few feet to sixty feet or
more, occur at the Balzi Rossi in the Alpes Maritimes, near Bergeggi,
and in Genoa; while evidences of submergence are to be found near
Monaco, at Beaulieu and at Diano Marina. It is important to notice
that the great movements dating from the end of the Eocene period are
almost confined to the Maritime Alps and the western portion of the
Riviera. In the parts of Piedmont lying to the north of Cuneo and in
the eastern Riviera, they produced hardly any sensible effect.
_Seismic History of the Riviera._--The movements just referred to are
those which, in course of time, have become sensible to the eye. They
represent the sum of a long-continued series of displacements that may
once have been on a large scale, but are now comparatively small. The
earthquakes that occur in the Riviera show, however, that the final
stage has not yet been reached. Their epicentres indicate the regions
in which slips are still taking place, and the magnitude of these
slips is roughly measured by the intensity of the resulting shocks.
The map in Fig. 40 is one of a series drawn by Professor Mercalli to
represent the distribution of seismic activity in Piedmont and the
Riviera. It corresponds to the period from 1801 to 1895. The whole
area is divided into a number of seismic districts, each of which is
distinguished by a particular degree of activity. In estimating this
quantity, Professor Mercalli takes intensity as well as frequency into
account. Thus, the lowest degree, represented by the lightest tint of
shading, corresponds to one or two strong earthquakes with a few
moderate or slight shocks; the eighth and highest to four or five
ruinous or disastrous earthquakes followed by trains of after-shocks.
The map shows very clearly that, during the last century, the seismic
activity was greatest in the Maritime Alps and the western
Riviera--that is, in the very districts in which the recent
mountain-making movements have been most conspicuous.[53]
[Illustration: FIG. 40.--Distribution of seismic activity in the
Riviera. (_Mercalli._)]
In all these districts, Professor Mercalli distinguishes several
well-marked seismic centres, to each of which he traces the origin of
two or more earthquakes. In the districts with which we are at present
concerned, those of the Alpes Maritimes and the western Riviera, the
most important centres are situated near Oneglia (in the sea), near
Taggia, in the valleys of the Vesubia and Tinea (near Nice), and in
the sea to the south of Nice. To the first of these centres belongs
the disastrous earthquake of February 23rd, 1887, as well as its
after-shocks on February 24th, May 20th, July 17th, and September 30th
of the same year, also the ruinous earthquakes of 1612 and 1854, and
several others of a lesser degree of intensity. All of these were
longitudinal earthquakes, the axes of their meizoseismal areas being
parallel to the neighbouring mountain-ranges. A few miles to the west
of Oneglia lies the Taggia centre, with which were connected the
disastrous earthquake of 1831, the violent earthquake of 1874, and
other strong or very strong shocks. These were for the most part
transversal earthquakes, their axes being perpendicular to those of
the Oneglia centre.
Some of the strongest earthquakes in this region originated in a
centre lying to the north of Nice in the valleys of the Vesubia and
Tinea. Among them may be mentioned the ruinous earthquakes of 1494,
1556, 1564, and 1644, and probably also the disastrous earthquake of
1227. A fourth centre, and one of considerable interest, is that which
lies at sea, a short distance to the south of Nice, and nearly along
the continuation of the valleys above-mentioned. This is the secondary
centre of the earthquake of 1887, and probably also of that of
December 29th, 1554. It is occasionally in action apart from the
Oneglia centre, as on November 27th, 1771, June 19th, 1806, and
December 21st, 1861; but such shocks, though rather strong, never
reach a high degree of intensity.
_Origin of the Earthquakes of 1887._--The most important feature in
the principal earthquake of 1887 is its origination in two distinct
foci, which are sometimes in action almost simultaneously, but more
often separately. The earthquakes belonging to the two foci differ
greatly in intensity and number, and the stronger part of the shock in
1887 originated in the focus associated with the more disastrous and
more frequent earthquakes.
The existence of two foci would of course give rise to a meizoseismal
area elongated in the direction of the line joining them. It is clear,
however, that the Oneglia focus was also extended in the same
direction; for, in the after-shock of February 24th, the isoseismals
drawn by Professor Mercalli are parallel to this line; and this was
also the case in the shock of March 11th. As both foci were under the
sea, it is difficult to locate them with precision; but it seems very
probable that they occupy portions of a submarine fault that runs
parallel or nearly so to the Apennine axis between the meridians of
Oneglia and Nice.
A brief period of preparation is a characteristic of the Riviera
earthquakes. In 1887, two at least of the preliminary shocks on
February 23rd (those of about 2 and 5 A.M.) originated in the Oneglia
focus. At 6.20 A.M. the first and weaker movement took place in the
western focus; and, a few seconds after the resulting vibrations
reached the eastern focus, the second and greater slip took place
there. The occurrence of seismic sea-waves is probably evidence of the
formation of a small, though sensible, fault-scarp in the same region.
To relieve the additional stresses thus brought into action along the
fault-surface, numerous small slips took place in different parts,
some as far to the west as the Nice focus, but the greater number
probably within or close to the focus in the neighbourhood of Oneglia.
REFERENCES.
1. BERTELLI, T.--"Osservazioni fatte in occasione di una escursione
sulle Riviera Ligure di ponente dopo i terremoti ivi seguiti
nell' anno 1887." _Boll. Mens. dell' Oss. di Moncalieri_,
vol. viii., 1888, Nos. 6, 7, 8.
2. CHARLON, E.--"Note sur le tremblement de terre du 23 février
1887." _Bull. del Vulc. Ital._, anno xiv., 1887, pp. 18-23.
3. DENZA, F.--_Alcune notizie sul terremoto del 23 febbraio 1887_
(Turin).
4. ISSEL, A.--"Il terremoto del 1887 in Liguria." _Boll. del R. Com.
Geol. d'Italia_, anno 1887, supplemento, pp. 1-207.
5. MERCALLI, G.--_I terremoti della Liguria e del Piemonte_.
(Naples, 1897, 146 pp.)
6. ODDONE, E.--"I dati sismici della Liguria in rapporto alla
frequenza ed alla periodicità." _Boll. della Soc. Sismol.
Ital._, vol. ii., 1896, pp. 140-151.
7. OFFRET, A.--"Sur le tremblement de terre du 23 février 1887.
Discussion des heures observés dans la zone épicentrale."
Paris, _Acad. Sci., Compt. Rend._, vol. civ., 1887, pp.
1150-1153.
8. ----. "Tremblements de terre du 23 février 1887. Heures de
l'arrivée des secousses en dehors de l'épicentre." _Ibid._,
pp. 1238-1242.
9. ROSSI, M.S. DE.--"Relazione sui terremoti del febbraio 1887."
_Bull. del Vulc. Ital._, anno xiv., 1887, pp. 5-17.
10. ----. "Bibliografia: Sul terremoto ligure del 23 febbraio 1887."
_Ibid._, pp. 60-62, 107-112, 115-128.
11. TARAMELLI, T., and G. MERCALLI.--"Il terremoto ligure del 23
febbraio 1887." _Annali dell' Uff. Centr. di Meteor. e di
Geodin._, vol. viii., parte iv., 1888. (Roma, 298 pp.)
12. UZIELLI, G.--_Le commozioni telluriche e il terremoto del 23
febbraio 1887_ (Turin).
13. _Nature_, vol. xxxv., 1887, pp. 438, 462, 534-535; vol. xxxvi.,
1887, pp. 4, 151-152.
14. Paris, _Acad. Sci. Compt. Rend._, vol. civ., 1887, pp. 556-557,
606-612, 634-635, 659-667, 744-745, 757-758, 759-760, 764-766,
822-823, 830-835, 884-890, 950-951, 1088-1089, 1243-1245,
1350-1352, 1416-1419; vol. cv., 1887, pp. 202-203; vol.
cviii., 1889, p. 1189; vol. cix.; 1889, pp. 164-166, 272-274,
660.
FOOTNOTES:
[47] The above times and all others in this chapter are given in Rome
mean time, which is 50m. earlier than Greenwich mean time.
[48] Professor Uzielli has also published a map of the isoseismal
lines for the Italian part of the disturbed area.
[49] It seems doubtful whether this movement was connected with the
earthquake. M. Offret does not include Nice in his list of
observatories at which magnetographs were disturbed.
[50] This is the time given by M. Offret. According to M. Mascart, it
should be 6h. 25m. 40s.
[51] In order to test the truth of this explanation, M. Moureaux
suspended a bar of copper at the Parc Saint-Maur observatory by two
threads in the same way as the horizontal force-magnet. The direction
of this bar was also registered photographically, and it remained
unmoved during the Verny earthquake of July 12th, 1889, and the
Dardanelles earthquake of October 25th, 1889, while one or more of the
magnets were disturbed. The experiment, however, was ineffective; for,
in order that the magnet may rest in a horizontal position, its centre
of gravity must be at unequal distances from the two points of
support.
[52] The hour-marks in Fig. 38 refer to Paris mean time, and those in
Fig. 39 to Genoa mean time.
[53] In the seventeenth century, the maximum seismic activity was
manifested in the neighbourhood of Nice, and in the eighteenth century
in Piedmont.