Chapter 7

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.

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.

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]

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.

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.

_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.

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.