THE ANDALUSIAN EARTHQUAKE OF DECEMBER 25TH, 1884.
In most countries the principal seismic districts are of limited
extent. Thus, in central Japan, the east coast is frequently visited
by earthquakes, while the west coast is relatively undisturbed. Of the
earthquakes felt in the kingdom of Greece during the years 1893-98, 63
per cent. were observed in Zante, and were for the most part confined
to that island. In the interior of the Iberian peninsula--in Leon and
in New and Old Castile--destructive earthquakes are practically
unknown; while the littoral regions of central and southern Portugal,
Andalusia, and Catalonia are noted for their disastrous shocks.
During the eighteenth century seismic activity was chiefly
concentrated in Portugal, and culminated in the great Lisbon
earthquake of 1755. In the following century the seat of disturbance
was transferred from the west to the south of the peninsula; Portugal
remained throughout in comparative repose, while Almeria experienced
destructive shocks in 1804, 1860, and 1863, and Murcia in 1828-29 and
1864, leading up to the Andalusian earthquakes of 1884-85, described
in the present chapter.
The preparation for the principal earthquake of December 25th, 1884,
was unusually indistinct. For a day or two before, shocks were felt
here and there in Andalusia, but so weak were they that they passed
almost unperceived. During the night of December 24-25, one slight
shock was noticed at Colmeñar (Fig. 19) and another at Zafarraya. On
the 25th, a faint movement of the ground was noticed at Malaga, and a
few weak tremors at Periana; and shortly after came the great shock at
about 8.50 P.M. mean time of Malaga, or about 9.8 P.M. Greenwich mean
time.
This earthquake was investigated by no fewer than three official
committees. The first in the field was nominated by the Spanish
Government on January 7th, 1885, and consisted of four members, the
President being Señor M.F. de Castro, the director of the Geological
Survey of Spain. The report of this commission was presented to the
Minister of Agriculture, etc., on March 12th. Early in February a
French Commission, appointed by the Academy of Sciences, proceeded to
the scene of the disaster. With Professor F. Fouqué as chief, and MM.
Lévy, Bertrand, Barrois, Offret, Kilian, Bergeron, and Bréon as
members, this committee resolved itself after a time into one for
studying the geology of the central area; and, of their voluminous
report of more than 700 quarto pages (published in 1889), only 55 are
immediately concerned with the earthquake. At the beginning of April,
Professors Taramelli and Mercalli, sent by the Italian Government,
arrived in Andalusia; and their memoir, read a few months later before
the Reale Accademia dei Lincei, forms by far the most valuable
contribution to our knowledge of the earthquake.
DAMAGE CAUSED BY THE EARTHQUAKE.
The meizoseismal area (see Figs. 19 and 20) lies in a mountainous
district, almost equidistant from the cities of Malaga and Granada. In
this area, which contains nearly 900 square miles, the shock was
disastrous to all but well-built houses. Whole villages were
overthrown. In the surrounding zone many buildings escaped serious
damage, and only a few were completely destroyed. It is estimated by
the Spanish Commission that, in the province of Granada, 3,342 houses
were totally, and 2,138 partially, ruined; in the province of Malaga,
1,057 houses were totally, and 4,178 partially, ruined; while in the
two provinces together 6,463 houses were damaged; making a total of
17,178 buildings more or less seriously injured.
As usual in the South of Europe, bad construction and narrow streets
were largely responsible for the loss of property, houses that were
regularly built and made of good materials being only slightly
injured. But, in this case, the great slope of the ground, the bad
quality of the foundations, and the nature of the underlying rocks
were contributing factors. Many buildings also had been damaged by
previous shocks, and their ruin was only completed by the earthquake
of 1884.
The total loss of life is variously estimated. According to the
Spanish Commission, 690 persons were killed and 1,426 wounded in the
province of Granada, while 55 were killed and 59 wounded in that of
Malaga, making a total of 745 persons killed and 1,485 wounded. The
Italian seismologists, having additional materials at their disposal,
raise the total figures to 750 persons killed and 1,554 severely
wounded. Careful inquiries were also made on this subject by the
conductors of the newspaper _El Defensor de Granada_. In Granada
alone, they reckon that 828 persons were killed and 1,164 wounded.
From the table given in the Italian report, it appears that 330
persons were killed at Alhama, 118 at Arenas del Rey, 102 at
Albuñuelas, 77 at Ventas de Zafarraya, and 40 at Periana; the
percentage of mortality being 9 at Arenas del Rey, about the same at
Ventas de Zafarraya, and 3 or 4 at Alhama, Albuñuelas and Periana.
Comparing these latter figures with the death rates of 71 per cent. at
Montemurro, caused by the Neapolitan earthquake, and of about 45 per
cent. at Casamicciola, by the Ischian earthquake of 1883, it will be
seen that the loss of life during the Andalusian earthquake was
comparatively small--an exemption which is attributed by the Italian
commissioners to the absence of inhabited places from the immediate
neighbourhood of the epicentre, and to the fact that the destructive
vibrations occurred towards the end of the shock, thus allowing
opportunity for escape.
ISOSEISMAL LINES AND DISTURBED AREA.
Fig. 19 shows the principal isoseismal lines as drawn by the Italian
commissioners. The meizoseismal area, which included all places at
which the shock was disastrous, is bounded by an ellipse (marked 1 on
the map) 40 miles long from east to west, 28 miles wide, and about 886
square miles in area. The next isoseismal (2) includes the places in
which some buildings were ruined, but not as a rule completely, and in
which there was no loss of life. Its bounding line is also elliptical,
the longer axis being about 71 miles long and running nearly east and
west. Towards the south this zone is interrupted by the sea. It will
be noticed that these isoseismals are not concentric, the second
extending much farther to the west and south-west than in the
opposite direction. A third isoseismal (not shown in the map) encloses
the district in which the shock was "very strong," or just capable of
producing cracks in the walls of houses. It is similar in form to the
second isoseismal, reaching as far as Estepone to the south-west,
Osuna, Cordova, and Seville to the west, Jaen to the north, while
towards the east it stops short of Almeria.
[Illustration: FIG. 19.--Isoseismal lines of Andalusian
earthquake. (_Taramelli and Mercalli._)]
The French Commission have also published a map of the earthquake,
and, though the work of an experienced seismologist like Professor
Mercalli is probably more trustworthy, it is interesting to compare
his isoseismal lines with those obtained by his French colleagues,
which are reproduced in Fig. 20. The curves in this figure are drawn
so as to include the places that were, respectively, ruined, seriously
damaged, and slightly damaged, by the shock. They should therefore
correspond with the lines in Fig. 19. It will be seen that they differ
considerably in form, but at the same time they present certain points
of agreement, such as the east and west elongation of the meizoseismal
area, and the great extension of the two outer isoseismals towards the
west and south-west The greatest difference is to be found in the
eastern portion of the third isoseismal, which, according to the
Italians, extends beyond the limits included in Fig. 20, and,
according to the French, is bayed back by the great masses of the
Sierra Nevada.
Outside Andalusia the earthquake was sensibly felt to the north as far
as Madrid and Segovia, to the west at Huelva, Cárceres and Lisbon, and
to the east at Valencia and Murcia. Towards the south, the greater
part of the disturbed area was cut off by the Mediterranean, and there
are no records forthcoming from the opposite coast of Africa. The
total area disturbed by the earthquake is roughly estimated by the
French Commission at about 154,000 square miles, and by the Italian
Commission at about 174,000 square miles; but, as the shock was
strong enough to stop clocks and ring bells at Madrid, it is evident
that even the greater of these values is too small.
[Illustration: FIG. 20.--Isoseismal lines of Andalusian
earthquake. (_Fouqué, etc._)]
THE UNFELT EARTHQUAKE.
Far beyond the limits of the disturbed area, however, the long slow
waves sped over the surface, disturbing magnetographs and other
delicate instruments. More than a century before, the great Lisbon
earthquake of 1755 had caused oscillations in Scottish lakes, and on
other occasions the effects of remote earthquakes had been witnessed
at isolated places. But, in 1884, the concurrent registration of the
Andalusian earth-waves at distant observatories attracted general
attention, and in part suggested the world-wide network of
seismological stations, the foundation of which was laid before
another decade had passed.
In Italy, probable records of the earthquake were obtained at two
observatories, but, owing to the approximate times given, their
connection with it is not established. At Velletri, near Rome,
Professor Galli's seismodynamograph registered a very slight movement
at 10 P.M., and at Rome itself Professor de Rossi found a tromometer
making unusual oscillations at 10.15 P.M.[31]
The most interesting records, however, are those furnished by the
magnetographs at Lisbon, Parc Saint-Maur (near Paris), Greenwich, and
Wilhelmshaven. At Lisbon, the records are extremely clear. The curves
of the declination, horizontal force and vertical force magnets, as
seen in Fig. 21, are abruptly broken at 8.33 P.M. (Lisbon time, or 9h.
9m. 45s., G.M.T.). The disturbances, which are greatest on the
declination curve and least on the vertical force curve, lasted in all
three for about 12 minutes, and are quite distinct from the ordinary
magnetic perturbations. At Parc Saint-Maur, the magnetographs seem to
be ill-adapted to act as seismographs, for only a slight mark was
discovered on a re-examination of the curves, beginning at 9.24 P.M.
(Paris time, or 9h. 14m. 39s., G.M.T.) At Greenwich, Mr. W. Ellis
writes, there was "a small simultaneous disturbance of the declination
and horizontal force magnets, occurring at 9h. 15m.... Both magnets
were at this time set into slight vibration, the extent of vibration
in the case of declination being about 2' of arc, and in horizontal
force equivalent to .001 of the whole horizontal force nearly." Of the
three instruments at Wilhelmshaven, only one showed any movement at
the time of the earthquake. The declination magnet was undisturbed,
the horizontal force curve was accidentally interrupted, but the
vertical force curve indicated a very perceptible shock. Beginning at
9.52 P.M. (Wilhelmshaven mean time, or 9h. 29m. 29s., G.M.T.), the
curve was broken for four minutes, for the rapid swinging of the
needle could not be registered until the motion became fainter.
Further disturbances also occurred at 9.59, 10, 10.2, and 10.5
P.M.[32]
[Illustration: FIG. 21.--Magnetograph records of Andalusian
earthquake at Lisbon. (_Fouqué, etc._)]
POSITION OF THE EPICENTRE.
The innermost isoseismal being too large, and the time-records too
inaccurate, to give the position of the epicentre, both Commissions
resorted to observations of the direction, Professor Fouqué and his
colleagues depending chiefly on the oscillation of hanging lamps, and
Professors Taramelli and Mercalli on the fall or displacement of
statues and other objects, and all avoiding as far as possible the
evidence of fissures in buildings.
The Italian observers point out that, among the divergent directions
visible at any place, there is generally one more distinctly marked
than the others, and this, they consider, corresponds to the movement
coming almost directly from the centre of disturbance. Plotting these
directions (36 in number), they find that they converge as a rule
within the triangle formed by joining Ventas de Zafarraya, Alhama, and
Jatar, while a large number of them traverse the elliptical area,
whose boundary is represented by the dotted line in Fig. 19. This area
is about 9 miles long and 2-1/2 miles wide, its longer axis runs
nearly east and west, and its centre coincides with the western focus
of the ellipse which forms the boundary of the meizoseismal area. It
lies, moreover, close to Ventas de Zafarraya and Arenas del Rey, the
two places where the seismic death-rate was highest, while its major
axis almost coincides with the line joining them.
The evidence of hanging lamps collected by the French Commission was
more consistent than that of the fallen objects. At every place, the
plane in which the lamps oscillated was nearly constant, the
deviations being generally attributable to irregularities in the mode
of suspension. The azimuths again intersect within an elliptical area,
which, according to the Commission, differs little from the central
region of the earthquake (Fig. 20). It Is clear, however, from the map
accompanying the French report, that the majority converge towards a
narrow band extending east and west from near Arenas del Rey to near
Ventas de Zafarraya, and therefore agreeing closely with the
epicentral area as determined by Professors Taramelli and
Mercalli.[33]
DEPTH OF THE SEISMIC FOCUS.
If the depth of the seismic focus amounts to several miles, one of the
most serious objections to Mallet's method lies in the varying
refractive power of the different strata traversed by the earth-waves
(p. 28). At present we have no way of meeting this objection, and all
calculations of the depth of the focus are therefore more or less
doubtful. A difficulty in practice has also been urged, depending on
the widely differing inclinations of the fractures at any place; but
the Italian observers found that the errors from this source were
greatly reduced by avoiding all fissures in poorly-built houses, or
which start from windows or other apertures, and selecting only those
which occur in homogeneous walls directed towards the epicentre. The
best angles of emergence thus measured by them are thirteen in number,
all made at places lying within 5 and 23 miles from the centre of the
epicentral area, and, with two exceptions, inside the meizoseismal
zone (Fig. 19). The depths corresponding to the different wave-paths
vary from 5.3 to 23.0 miles, the mean depth of the focus given by all
thirteen observations being 7.6 miles.
The only estimate made by the French Commission--and it is one that
they rightly regarded with considerable doubt--was based on a method
devised by Falb. As the sound generally precedes the shock, Falb
assumes that it travels with a greater velocity. If the velocities of
both series of waves are known, and if they start at the same instant
and from the same region, the interval that elapses between the
arrivals of the sound and shock should give the distance traversed by
them and consequently the depth of the focus. It is unnecessary to
mention more than two of the serious objections to this method. The
duration of the preliminary sound should increase rapidly with the
distance from the focus, and of this there is not the slightest
evidence. Moreover, the sound-vibrations that are first heard do not
necessarily come from the same part of the focus as those which cause
the shock, but, as will be seen in Chapter VIII., probably from its
nearer lateral margin. The French Commission, finding the average
duration of the fore-sound near the epicentre to be 5 seconds,
estimate the depth of the focus at about 7 miles--a result which
agrees remarkably with that obtained from the angles of emergence, but
which is not, on that account, entitled to credit.
NATURE OF THE SHOCK.
In the nature of the shock, there was a singular uniformity throughout
the whole disturbed area, the chief variation noticed being evidently
dependent on the observer's distance from the epicentre.
For instance, in the meizoseismal area (Fig. 19), at Ventas de
Zafarraya, a loud sound like thunder was first heard, and before it
ceased there came a violent subsultory movement preceded by a very
brief oscillation, then a pause of one or two seconds, and lastly a
more intense and longer series of undulations, the whole movement
lasting 12 seconds. At Cacin, three phases were distinguished, the
first a slight undulatory movement coincident with the sound, followed
immediately by the subsultory motion, a pause, and stronger
undulations, the total duration being 15 seconds. The variations
noticeable in this zone seem to have been apparent only, sensitive
observers perceiving a tremulous motion before the vertical
vibrations, and in the pause between them and the concluding
undulations. In both phases, the intensity increased to a maximum and
then gradually decreased. The movement at Ventas de Zafarraya and
Cacin is represented by Professors Taramelli and Mercalli by the
curves _a_ and _b_ in Fig. 22.
In the second zone (Fig. 19), the same two phases were universally
observed, but the subsultory movement was less pronounced or the
movement was partly subsultory and partly undulatory, and occasionally
both phases are described as undulatory. The motion near Malaga is
represented by the curve _c_ in Fig. 22.
[Illustration: FIG. 22.--Nature of shock of Andalusian
earthquake. (_Taramelli and Mercalli._)]
Outside the ruinous zone, the first phase rapidly lost what remained
of its subsultory form, and the pause between the two parts was
noticeably longer than near the epicentre. Thus, at Seville and
Cordova, two shocks were felt, separated by an interval of some
seconds; the second according to some observers at Seville,
terminating with vertical tremors. At Madrid, also, the two parts were
perceived, the interval between them being 3 or 4 seconds in length;
but, as a rule, outside Andalusia, only a single undulatory shock was
felt, without any preliminary sound.
That the changes observed in the shock were merely an effect of less
or greater distance, will be obvious from Fig. 23, in which the
intensity at any moment is that represented by the distance of the
corresponding point on the curve from the different base-lines, the
base-line _a_ corresponding to a place near the epicentre, and _b_,
_c_, _d_, etc., to places at gradually increasing distances. Thus, at
a place corresponding to the base-line _b_, the intensity of the
tremors during the intervening pause (represented by the short line
PN) was so slight that they frequently escaped notice, while the
preliminary tremors observed by some near the epicentre were
altogether imperceptible. At the places corresponding to the
base-lines _c_, _d_, _e_, _f_, the duration of the whole shock and of
each part gradually diminished, while the interval between the two
parts increased owing to the gradual extinction of the final
vibrations of the first part and of the initial vibrations of the
second. At the farthest of these places (_f_) the first part was so
weak that it sometimes passed unobserved. Lastly, at a place
corresponding to the base-line _g_, the first part was imperceptible
to all observers, and the shock consisted of a single series of
horizontal undulations.
[Illustration: FIG. 23.--Diagram to illustrate variation in
nature of shock of Andalusian earthquake.]
_Origin of the Double Shock._--If the double shock were observed at
only a few places, we should naturally look for some local explanation
of the peculiarity. The second shock, for instance, might be a
subterranean echo, the earth-waves being reflected at the bounding
surface of two different kinds of rock. In the case of the Andalusian
earthquake, such an explanation is precluded by the almost universal
observation of the double shock, the greater intensity of the second
part, and the longer period of its vibrations.
The Italian observers, who paid considerable attention to the double
shock, give a more general explanation. They regard the two parts of
the shock as corresponding in the main to longitudinal and transversal
waves starting simultaneously from the same focus (see p. 13). The
former vibrations would be vertical at the epicentre and would
gradually become horizontal in spreading outwards; the latter would be
horizontal at the epicentre and at a distance from it (_e.g._ at
Seville) nearly vertical. Also, as the longitudinal waves travel more
rapidly than others, the interval between the two parts of the shock
would increase with the distance from the origin. Owing again, to the
large size of the focus, the first part of the shock would at no place
be instantaneous, and its later vibrations might coalesce with the
earlier transverse vibrations, so that, within and near the
meizoseismal area, the second part of the shock might be stronger than
the first. A similar result might be produced in the same district if
the transverse vibrations coincided with reflected longitudinal
vibrations, and Professors Taramelli and Mercalli think that such
reflection would occur from the old crystalline rocks of the Sierra de
Almijara and possibly also from the calcareous and crystalline rocks
to the south-west of Cartama.
Satisfactory as it seems to be in some respects, this explanation is
open to serious objections, of which I will mention only two. The
first is that, though the pause between the two parts of the shock
does increase with the distance, it does not increase rapidly enough;
at Seville, it should be two or three minutes, instead of "some
seconds" in length. A more fatal objection, however, is that, if the
explanation were correct, every earthquake-shock should consist of two
parts, and this is only the case with a small minority.
On the other hand, if the velocities of the waves composing each part
were the same, the slight increase in the length of the interval is
readily accounted for, as we have seen, by the gradual extinction of
its weak terminal vibrations. But in any case, the long interval that
elapsed between the beginnings of the two parts at a place so near the
epicentre as Ventas de Zafarraya, shows that each part was due to a
distinct impulse; and, judging from the directions of the respective
movements, it would seem that the focus of the first impulse was
situated at a greater depth than the focus of the second. Whether the
epicentres corresponding to the two foci were coincident or more or
less separate is not clear from the nature of the shock; but it is
probable that they were nearly or quite detached, and that a second
epicentre was situated near the eastern focus of the ellipse bounding
the meizoseismal area.
SOUND-PHENOMENA.
In the Neapolitan earthquake, the sound was only heard in a district
of about 3,300 square miles immediately surrounding the epicentres,
while the whole area disturbed by the shock was not less than 39,000
square miles. A similar limitation was noticed in the Andalusian
earthquake. According to the Spanish Commission, the sound was heard
at only one place (Cordova) outside the provinces of Granada and
Malaga; and its audibility was a rule confined to the area within
which buildings were damaged by the shock. It was compared at
different places to the noise of a passing train or a carriage heavily
laden running on a paved road, of distant thunder, a great storm, or
the discharge of heavy guns.
At every place where the sound was heard, it distinctly preceded the
shock, frequently allowing time for escape from houses that were
afterwards ruined. Its duration within the meizoseismal area was on an
average about five or six seconds, rarely perhaps did it exceed ten
seconds. At some places in the same area, it overlapped the beginning
of the shock, but generally it was separated from the latter by a very
short interval, estimated at a second. From this precedence of the
sound, the Italian Commission conclude that the sound-waves travelled
more rapidly than those which formed the shock, an inference that
depends on the assumption that both waves started simultaneously from
within precisely the same focal limits. A different explanation, not
based on these assumptions, will be considered more fully in Chapter
VIII, dealing with the recent earthquakes of Hereford and Inverness.
VELOCITY OF THE EARTH-WAVES.
If, in a highly-civilised country, the time-records of an earthquake
vary within wide limits, it is not surprising that those given for the
Andalusian earthquake should be wholly untrustworthy. Even the clocks
in public buildings and railway stations differed by as much as 25
minutes in their indications. An interesting observation is, however,
described in the French report and is worth repeating, though it does
not lead to any accurate result. At the time of the principal shock,
two telegraph-clerks were in communication, one at Malaga and the
other at Velez-Malaga. The latter, surprised by the shock, suddenly
stopped his message; and, about six seconds later, the arrival of the
earth-waves at Malaga explained the interruption to his colleague. As,
according to the French report, Velez-Malaga is 9 kms. (or about 5-1/2
miles) nearer than Malaga to the mean epicentral point, it follows
that the velocity of the earth-waves must have been about 1.5 kms., or
nearly a mile, per second.[34]
The only observations of any real value in determining the velocity
are those given by the stopped clock at the observatory of San
Fernando (Cadiz) and by the magnetographs at Lisbon, Parc Saint-Maur,
Greenwich, and Wilhelmshaven. Taking the times at Cadiz, Lisbon,
Greenwich, and Wilhelmshaven at 9.18, 9.19, 9.25, and 9.29 P.M.
respectively (Paris mean time) and the mean epicentral point as
coinciding with Alhama, the French Commission estimates roughly the
mean surface-velocity between Cadiz and Lisbon at 3.6 kms. per second,
between Cadiz and Greenwich at 4.5 kms. per second, between Cadiz and
Wilhelmshaven at 3.1 kms. per second, and between Greenwich and
Wilhelmshaven at 1.6 kms. per second. Dr. Agamennone, however,
notices that the distances from Alhama are not correctly measured, and
substitutes for the above figures 4.83, 3.43, 2.82, and 1.75 kms. per
second respectively.
These results apparently show a decrease in the velocity with the
outward spread of the earth-waves, but, as Dr. Agamennone again points
out, a comparatively small error in the time at Cadiz would neutralise
the apparent decrease. It is not to be supposed that the astronomical
clock at this observatory was wrong by more than a second or two, but
the behaviour of clocks during an earthquake is so irregular--some
stopping at once, others staggering on for some seconds before
arrest--that the Cadiz time may differ from the true time by several
seconds.
Besides this possible error, there is also considerable uncertainty in
the records from the magnetic observatories, owing to the slow rate at
which the photographic paper travels. At Parc Saint-Maur this rate is
only 10 mm. per hour, and at the other observatories about 15 mm. per
hour. Allowing, therefore, for an error of half-a-minute in the
time-record at Cadiz, of one minute in those of Lisbon, Greenwich, and
Wilhelmshaven, and of two minutes in that at Parc Saint-Maur, and
taking the mean epicentral point as determined by the Italian
observers, Dr. Agamennone, applying the method of least squares, finds
the probable value of the velocity of propagation to be 3.15 kms. (or
nearly 2 miles) per second, with a possible error of .19 kms. per
second. This result agrees closely with the value found for the long
slow undulations of more recent earthquakes.
MISCELLANEOUS PHENOMENA.
_Connection between Geological Structure and the Intensity of the
Shock._--While a great part of the injury to buildings must be
attributed to their faulty construction, the connection between the
nature of the underlying rock and the amount of damage was very
clearly marked. Other conditions being the same, houses built on
alluvial ground suffered most of all; and the destruction was also
great in those standing on soft sedimentary rocks such as clays and
friable limestones. On the other hand, when compact limestones or
ancient schists formed the foundation-rock, the amount of damage was
conspicuously less than in other cases.
The members of both the French and the Italian Commissions agree in
ascribing the peculiar form and relative positions of the isoseismal
lines to geological conditions. To the east of the epicentre, the
schists and crystalline limestones form a deep, uniform, and compact
mass; while, to the west, the old crystalline rocks are covered by
jurassic, cretaceous, and eocene formations, constituting a less
homogeneous and less elastic mass, in which the intensity of the shock
would fade off much more rapidly, with the result that the epicentre
occupies the western focus of the elliptical boundary of the
meizoseismal area (Fig. 19).[35]
That mountain-ranges have an important influence on the form of
isoseismal lines is evident from both maps (Figs. 19 and 20), but
especially from that published by the French Commission (Fig. 20).
The resistance offered by the Sierra Nevada to the propagation of the
earth-waves is shown in the former map by the approximation of the
first and second isoseismals at the east end, and in the latter by the
great bay in the third isoseismal line. Whichever interpretation of
the evidence is the more accurate, the action of the mountainous mass
is clearly to lessen rapidly the intensity of the shock--an effect
which is probably due to the abrupt changes in the direction and
nature of the strata encountered normally by the earth-waves. On the
opposite side of the epicentre, the waves meet the Sierra de Ronda
obliquely. In traversing this range, the shock lost a great part of
its strength, while it continued to be felt severely along its eastern
foot, thus giving rise to the south-westerly extension of the third
isoseismal in Fig. 20, and, though to a less extent, that of the
second in Fig. 19.
_Fissures, Landslips, etc._--The earthquake resulted in many
superficial changes, such as fissures, landslips, and derangement of
the underground water-system--all changes of the same order as the
destruction of buildings--but, so far as known, in no fault-scarps or
other external evidence of deep-seated movements.
Some of the fissures were of great length. One of the most remarkable
occurred at Guevejar, a village built on the south-west slope of the
Sierra de Cogollos. It was in the form of a horse-shoe, and was about
two miles long, from ten to fifty feet wide, and of great depth. In
its neighbourhood, innumerable small cracks appeared, some
perpendicular and others parallel to the great fissure. The ground
within, a bed of clay resting on limestone, also slid down towards the
river. Houses near the centre of the fissured tract were shifted as
much as thirty yards within the first month, and others near its
extremity about ten feet; while the accumulation of the material at
the south end of the fissure resulted in the formation of a small
lake, of about 250 to 350 square yards in area and about 30 feet deep.
All streams within the fissured zone disappeared, and the spring,
which provided the drinking-water of the village, ceased to flow.
The underground water-system was generally affected throughout the
central area. In some places, mineral springs disappeared; in others,
new springs broke out or old ones flowed more abundantly. At Alhama,
the increased flow was accompanied by a permanent rise in temperature
from 47° to 50° C., and by a marked change in character.
AFTER-SHOCKS.
Frequent after-shocks are a characteristic of the earthquakes of
Southern Spain. After the Cordova earthquake of 1170, they continued
for at least three years. The Murcian earthquake of 1828 was followed
by 300 minor shocks during the next twenty-four hours, and for more
than a year slight tremors were often felt. For some time after the
great earthquake of 1884, the movements of the ground were extremely
numerous in the immediate neighbourhood of the epicentre, farther away
they were rarer and of less intensity, and outside the area of damaged
buildings they were nearly absent.
Thus, during the night of December 25-26, 110 after-shocks were
counted at Jatar, from 14 to 17 at Alcaucin, Ventas de Huelma, Motril,
Cacin, Durcal, Malaga, etc.; about 11 at La Mala and Albuñuelas; 9 at
Velez-Malaga and Lenteje; and from 5 to 7 at Frigiliana, Riogordo, and
Cartama. The strongest of these shocks occurred at 2.20 A.M., and,
though none was violent, several helped to complete the ruin of many
houses that had been damaged by the principal shock.
From this time, after-shocks occurred almost daily until the end of
May, after which they became much less frequent. According to the list
given in the Italian report, which closes at the end of January 1886,
237 shocks were felt, 23 up to the end of December, 30 in January
1885, 25 in February, 27 in March, 46 in April, and 43 in May. In June
1885, only three are recorded, and the average number during each of
the following seven months lies between five and six. This list,
however, does not include the very weak shocks,[36] for nearly all
those contained in it were felt as far as Malaga or its neighbourhood.
The shocks varied considerably in intensity as well as in frequency,
five of them being much more violent than the rest. One that occurred
on December 30th was felt strongly in all the damaged area, two others
on January 3rd and 5th caused fresh injury to buildings, a fourth, on
February 27th, disturbed an area bounded roughly by the second
isoseismal of the principal earthquake (Fig. 19), while the fifth and
strongest, that of April 11th, was felt over a large part of the zone
beyond.
At places within and near the meizoseismal area, earth-sounds were
sometimes heard without any sensible shock; occasionally, also,
tremors were felt with no attendant sound; but, as a rule, the shocks
were accompanied by sound, and in every such case, as in the principal
earthquake, the sound preceded the shock, or at most was partly
contemporaneous with it.
Several of the after-shocks resembled the principal earthquake in
their division into two parts separated by an interval of rest or
weaker movement from half a second to a second in length, though the
whole duration of the shock itself in no case exceeded five or six
seconds. Occasionally, the likeness was still closer, in the
succession of sound, subsultory motion and concluding horizontal
undulations.
GEOLOGY OF THE MEIZOSEISMAL AREA AND ORIGIN OF THE EARTHQUAKES.
The meizoseismal area and surrounding zones lie in the midst of the
mountainous region that separates the basin of the Guadalquiver from
that of the Mediterranean, the essential structure of which, according
to the geologists of the French Commission, is outlined in Fig. 24. In
this sketch-map, the lightly-shaded bands correspond to an upper
series of crystalline schists, and the cross-shaded bands to the lower
series of mica-schists and dolomites that form the anticlinal folds of
the Sierra de Ronda, the Sierra de Mijas, and the Sierra Tejeda.
In addition to the faulting and intense folding in the direction of
their strikes, these rocks are also intersected by three nearly
parallel transverse faults of post-Triassic age, which, aided by
subsequent denudation, have cut up the whole range into a number of
distinct sierras. They are represented by the broken lines in Fig. 24.
[Illustration: FIG. 24.--Structure of meizoseismal area of
Andalusian earthquake. (_Fouqué, etc._)]
One of these faults, that which passes near Motril, traverses the
meizoseismal area, whose boundary, as laid down by the French
Commission, is indicated by the dotted line on the sketch-map.[37] In
the neighbourhood of Zafarraya, the fault intersects the broken
anticlinal fold of the Sierra Tejeda, and the epicentre is thus
situated in one of the most disturbed tracts of the whole region. The
evidence, both seismic and geological, is insufficient to support any
precise view as to the origin of the earthquake, but there can be
little doubt that it was closely connected with movements along one or
more of the system of faults that intersect not far from Zafarraya.
REFERENCES.
1. AGAMENNONE, G.--"Alcune considerazioni sui different metodi fino
ad oggi adoperati nel calcolare la velocità di propagazione
del terremoto andaluso del 25 dicembre 1884." Roma, _R.
Accad. Lincei, Rend._, vol. iii., 1894, pp. 303-310.
2. ---- "Velocità superficiale di propagazione delle onde sismiche
in occasione della grande scossa di terremoto dell' Andalusia
del 25 dicembre 1884." _Ibid._, vol. iii., 1894, pp. 317-325.
3. CASTRO, M.F. de.--_Terremotos de Andalucía: Informe de la
comision nombrada para su estudio dando cuenta del estado de
los trabajos en 7 de marzo de 1885._ (Madrid, 1885; 107 pp.)
4. FOUQUÉ, F., etc.--"Mission d'Andalousie: Études relatives au
tremblement de terre du 25 décembre 1884, et à la constitution
géologique du sol ébranlé par les secousses." Paris, _Acad.
Sci. Mém._, vol. xxx., pp. 1-772.
5. MACPHERSON, J.--"Tremblements de terre en Espagne." Paris, _Acad.
Sci., Compt. Rend._, vol. c., 1885, pp. 397-399.
6. NOGUÉS, A.F.--"Phénomènes géologiques produits par les
tremblements de terre de l'Andalousie, du 25 décembre 1884 au
16 janvier 1885." _Ibid._, pp. 253-256.
7. ROSSI, M.S. de.--"Gli odierni terremoti di Spagna ed il loro eco
in Italia." _Bull. Vulc. Ital._, anno xii., 1885, pp. 17-31.
8. TARAMELLI, T., and G. MERCALLI.--"I terremoti Andalusi cominciati
il 25 dicembre 1884." Roma, _R. Accad. Lincei, Mem._, vol:
iii., 1885, pp. 116-222.
9. Paris, _Acad. Sci., Compt. Rend._, vol. c., 1885, pp. 24-27,
136-138, 196-197, 256-257, 598-601, 1113-1120, 1436 (the last
three by F. Fouqué).
FOOTNOTES:
[31] These times correspond to about 9.10 and 9.25 P.M., Greenwich
mean time. The earthquake stopped a clock at the Royal Observatory of
San Fernando (Cadiz), at 8h. 43m. 54.5s. mean local time,
corresponding to 9h. 8m. 44s., G.M.T.
[32] The earthquake is also said to have been registered at the
observatory of Moncalieri, near Turin, but I have not been able to
ascertain the time of occurrence. A movement felt at about 10.20 P.M.
at Ramsbury, in Wiltshire, was attributed to the earthquake, though
the time is about an hour too late. On December 26th, an astronomical
clock was stopped at Brussels and its pillar displaced; and, on the
evening of the same day, the large telescope at the observatory was
also found to have been shifted. These effects, it is suggested, were
caused by the Andalusian earthquake, but the connection between them
seems to me very doubtful.
[33] The French observers have also applied a method depending on the
time of occurrence of the shock. Joining places where the recorded
times were the same, they notice that the perpendicular bisectors of
these lines intersect within an area which agrees practically with
that determined by the azimuths. The inaccuracy of the time-records
must, however, lessen the significance of this result.
[34] Dr. Agamennone points out that, according to the Italian report,
the difference in distance is 22 kms. (or 13-3/4 miles), leading to a
velocity of about 3.6 kms., or 2.3 miles per second.
[35] It should be remembered that it is not improbable that there were
two detached epicentres, coinciding roughly with the two foci of this
curve.
[36] Only eight are recorded during the night of December 25-26. On
several occasions during April and May 1885, groups of slight shocks
were felt; but as their individual times are not given, they are
regarded as equivalent to one shock each in the above totals.
[37] The boundary, as drawn in this figure, differs slightly from that
given in Fig. 20.