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 Zafarrayaand Cacin is represented by Professors Taramelli and Mercalli by the curvesaandbin 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 curvecin Fig. 22.
Nature of shock of Andalusian earthquake.Fig.22.—Nature of shock of Andalusian earthquake. (Taramelli and Mercalli.)ToList
Fig.22.—Nature of shock of Andalusian earthquake. (Taramelli and Mercalli.)ToList
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 differentbase-lines, the base-lineacorresponding to a place near the epicentre, andb,c,d, etc., to places at gradually increasing distances. Thus, at a place corresponding to the base-lineb, 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-linesc,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-lineg, the first part was imperceptible to all observers, and the shock consisted of a single series of horizontal undulations.
Diagram to illustrate variation in nature of shock of Andalusian earthquake.Fig.23.—Diagram to illustrate variation in nature of shock of Andalusian earthquake.ToList
Fig.23.—Diagram to illustrate variation in nature of shock of Andalusian earthquake.ToList
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 reflectedat 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 Iwill 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.
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 than39,000 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.
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 Andalusianearthquake 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 5½ 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 9.29P.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. persecond. 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.
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 thatpublished 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, someperpendicular 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.
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 2.20A.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 secondisoseismal 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.
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.
Structure of meizoseismal area of Andalusian earthquake.Fig.24.—Structure of meizoseismal area of Andalusian earthquake. (Fouqué, etc.)ToList
Fig.24.—Structure of meizoseismal area of Andalusian earthquake. (Fouqué, etc.)ToList
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 andgeological, 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.
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., andG. 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).
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., andG. 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).
[31]These times correspond to about 9.10 and 9.25P.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.
[31]These times correspond to about 9.10 and 9.25P.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 10.20P.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.
[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 10.20P.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.
[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 13¾ miles), leading to a velocity of about 3.6 kms., or 2.3 miles per second.
[34]Dr. Agamennone points out that, according to the Italian report, the difference in distance is 22 kms. (or 13¾ 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.
[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.
[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.
[37]The boundary, as drawn in this figure, differs slightly from that given in Fig. 20.
The Charleston earthquake stands alone among the great earthquakes described in this volume, and indeed among nearly all great earthquakes, in visiting a region where seismic disturbances were almost unknown. Calabria and Ischia, the Riviera and Andalusia, Assam and the provinces of Mino and Owari in Japan, are all regions where earthquake-shocks are more or less frequent and occasionally of destructive violence. But, from the foundation of Charleston in 1680 until 1886, that is, for more than two centuries, it is probably not too much to say that few counties in Great Britain were so free from earthquakes as the State of South Carolina.[38]
The practical isolation of the earthquake of 1886 left its trace on the character of the investigation. Not only were the observers untrained, but theinvestigators themselves were unprepared. For instance, the scale of intensity used in drawing the isoseismal lines was not adopted until after the first letters of inquiry were issued. On the other hand, nothing could exceed the energy and ability with which the epicentral tracts were examined by Mr. Earle Sloan and the collection of time-records made by Mr. Everett Hayden. To them, and to Major C.E. Dutton, whose valuable monograph supersedes all other accounts, we are indebted for the two chief additions to our knowledge resulting from the study of the Charleston earthquake. These are the determination of the double epicentre, and the measurement of the velocity with which the earth-waves travelled.
The land-area disturbed by the earthquake and the isoseismal lines are shown in Fig. 25, the small black oval area (which Includes Charleston) being that within which the greatest damage to buildings occurred. The chief part of the epicentre, however, lies from 12 to 15 miles to the west and north-west of Charleston, in a forest-clad district, containing only two villages and various scattered houses.
The city of Charleston, whose population between 1880 and 1891 increased from fifty to fifty-five thousand, is built on a peninsula between the Cooper River on the east and the Ashley River on the south-west. Originally, this was an irregular tract of comparatively high and dry land, intersected by numerous creeks, which, as the city grew, were filled up to the general level of the higher ground. It is on this"made land" as a rule that the more serious damage to buildings occurred.
At 9.51P.M.(standard time of the 75th meridian), the great earthquake occurred, and, one minute later, there was left hardly a building in the city that was not injured more or less seriously. "The destruction," as Major Dutton remarks, "was not of that sweeping and unmitigated order which has befallen other cities, and in which every structure built of material other than wood has been levelled completely to the earth in a chaos of broken rubble, beams, tiles, and planking, or left in a condition practically no better." The number of houses entirely demolished was not great, but several hundred lost a large part of their walls, and many were condemned as unsafe and afterwards pulled down. A board of inspectors, appointed to investigate the condition of the houses, reported that not one hundred out of fourteen thousand chimneys examined by them escaped damage, and that 95 per cent. of those injured were broken off at the roof. The total cost of the necessary repairs, it was estimated, would amount to about one million pounds.
According to the official records, 27 persons were killed in Charleston during the earthquake, but, by cold, exposure, etc., this number was brought up to not less than 83. The number of persons wounded was never ascertained.
In drawing the isoseismal lines (represented by the continuous curves in Fig. 25), Major Dutton made use of the well-known Rossi-Forel scale of seismicintensity, a translation of which is given below.[39]The curves range from the highest degree, 10, corresponding to disastrous effects on buildings, down to the lowest but one, 2, which would be applied to a shock felt only by a small number of persons at rest. It is evident, I think, that these lines cannot be regarded as drawn with great accuracy. The number of records (nearly 4000, from about 1,600 places), great as it is, is hardly sufficient for the purpose; and many were collected from newspapers. The circulars of inquiry also contained no distinct questions corresponding to the different degrees of the scale employed, and therefore it is not always certain that the intensity recorded was the maximum observed. But, if the curves might have varied in detail with a larger and more accurate series of observations, they must represent in their main features the distribution of seismic intensity throughout the disturbed area. Onepoint of importance is the partial earthquake-shadow in the region of the Appalachian Mountains shown by the southward incurving of the isoseismals 4, 5, and 6, and especially by the first two of these lines. Another is the close grouping of the isoseismals in the State of Mississippi, illustrating a rapid fading ofintensity as the earth-waves crossed the unconsolidated materials of the Mississippi delta.
Isoseismal lines of Charleston earthquake.Fig.25.—Isoseismal lines of Charleston earthquake. (Dutton, etc.)ToList
Fig.25.—Isoseismal lines of Charleston earthquake. (Dutton, etc.)ToList
Owing to the short distance between the epicentre and the sea-coast, it is impossible to make more than a rough estimate of the extent of the disturbed area. Even when the boundary lies on land, it traverses some districts which are thinly populated and others where the inhabitants are unobservant, and unlikely to notice the slow oscillations which were alone perceptible at great distances. The shock was, however, felt at Boston (800 miles from the epicentre), La Crosse on the upper Mississippi (950 miles to the north-west), at several places in Cuba (between 700 and 710 miles), and in Bermuda (950 miles). To the south, the limits are unknown, there being no report from Yucatan, the nearest point of which is distant about 930 miles. If we assume the disturbed area to have a mean radius of 950 miles, then it must have covered no less than 2,800,000 square miles. And, that this estimate is not excessive, will be evident from the fact that the land-area disturbed (including parts of the great lakes and inlets in the sea-coast) amounted to about 920,000 square miles.
The preparation for the earthquake seems to have begun about three months before. During June, and even earlier, slight but undoubted tremors are said to have been felt in Charleston, but no record of them was kept until about 8A.M.on August 27th, when a decided earthquake occurred at Summerville, a village twenty-two miles to the north-west. The shock and sound were simultaneous, the shock a single jolt orheavy jar, the sound loud and sudden; they were such as might have been caused by the firing of a heavy cannon or the explosion of a boiler or blast of gunpowder. At 4.45A.M.on August 28th, the shock and sound were repeated, only more strongly, the former being distinctly felt as far as Charleston. During that day and the next, there were several other shocks at Summerville, and then rest and quiet succeeded until the evening of August 31st.
At 9.51P.M.(to take one of the best descriptions), the attention of an observer in Charleston was "vaguely attracted by a sound that seemed to come from the office below, and was supposed for a moment to be caused by the rapid rolling of a heavy body, as an iron safe or a heavily-laden truck, over the floor. Accompanying the sound there was a perceptible tremor of the building, not more marked, however, than would be caused by the passage of a car or dray along the street. For perhaps two or three seconds the occurrence excited no surprise or comment. Then by swift degrees, or all at once—it is difficult to say which—the sound deepened in volume, the tremor became more decided, the ear caught the rattle of window-sashes, gas-fixtures, and other movable objects; the men in the office ... glanced hurriedly at each other and sprang to their feet.... And then all was bewilderment and confusion.
"The long roll deepened and spread into an awful roar, that seemed to pervade at once the troubled earth and the still air above and around. The tremor was now a rude, rapid quiver, that agitated the wholelofty, strong-walled building as though it were being shaken—shaken by the hand of an immeasurable power, with intent to tear its joints asunder and scatter its stones and bricks abroad....
"There was no intermission in the vibration.... From the first to the last it was a continuous jar, adding force with every moment, and, as it approached and reached the climax of its manifestation, it seemed for a few terrible seconds that no work of human hands could possibly survive the shocks. The floors were heaving under-foot, the surrounding walls and partitions visibly swayed to and fro, the crash of falling masses of stone and brick and mortar was heard overhead and without....
"For a second or two it seemed that the worst had passed, and that the violent motion was subsiding. It increased again and became as severe as before. None expected to escape. A sudden rush was simultaneously made to endeavor to attain the open-air and fly to a place of safety; but, before the door was reached all stopped short, as by a common impulse, feeling that hope was vain—that it was only a question of death within the building or without, of being buried beneath the sinking roof or crushed by the falling walls. The uproar slowly died away in seeming distance. The earth was still, and oh! the blessed relief of that stillness."
If somewhat sensational in form, this report gives an extremely vivid and generally accurate account of the great shock. Other observers in Charleston concur in dividing the movement into five phases. The preliminary tremors and murmuring sound lasted about twelve seconds, and, although they increased in strength, they were succeeded somewhat suddenlyby the violent oscillations of the second phase, followed by a third phase of much less intensity and a fourth of stronger oscillations, these three phases lasting altogether about fifty seconds. The fifth phase, in which the tremors died out rather rapidly, continued about eight seconds; so that the total duration of the earthquake was not less than seventy seconds. The variation of the intensity with the time is represented roughly by the curve in Fig. 26.
Curve of intensity at Charleston.Fig.26.—Curve of intensity at Charleston. (Dutton.)ToList
Fig.26.—Curve of intensity at Charleston. (Dutton.)ToList
At Charleston, there were thus two decided maxima of intensity, nearly equal in strength, though the first seems to have been slightly more powerful than the second. As in the Andalusian earthquake, the intervening tremors were imperceptible at a distance from the epicentre, and the earthquake appeared in the form of two distinct shocks, separated by an interval the average duration of which was estimated at slightly less than half a minute. At most places, the first shock is described as the stronger, but the difference in intensity of the two parts could not have been great, for both were noticed at several places more than 600 miles from the epicentre.
Visible Earth-Waves.—Many persons in the meizoseismal area assert that they saw waves moving along the surface of the ground. At Charleston, according to an observer who was facing a street-lamp at the time, "the progress of the waves as they passed thehouse, going towards the south-east, was plainly observed, although they travelled with incomparable swiftness. The shadow of each moving ridge cast from the gas-light was distinctly seen. The waves were not in long rollers, but had rather the appearance of 'ground-swells' in deep water," the height of which from crest to trough he estimated at not less than two feet. In the words of another observer, "The vibrations increased rapidly and the ground began to undulate like the sea. The street was well lighted, having three gas-lamps within a distance of 200 feet, and I could see the earth waves as they passed as distinctly as I have a thousand times seen the waves roll along Sullivan's Island beach. The first wave came from the south-west, and as I attempted to make my way ... I was borne irresistibly across from the south side to the north side of the street. The waves seemed then to come from both the south-west and north-west, and crossed the street diagonally, intersecting each other, and lifting me up and letting me down as if I were standing on a chop sea. I could see perfectly, and made careful observations, and I estimate that the waves were at least two feet in height."
For seismological purposes, it is unfortunate that the epicentral district should be one containing so few buildings and other objects that could preserve the effects of the shock. It is for the most part a barren, forest-clad region, in places swampy, with occasional scattered houses. But it is crossed by three lines of railway diverging from Charleston,and the damage which they suffered supplements to some extent the defects arising from the scarcity of buildings. These railway lines are the South Carolina, the North-Eastern, and the Charleston and Savannah, denoted by the letters A, B, and C, respectively, in Figs. 28 and 29.[40]It will be convenient to follow Major Dutton, and trace the variation of intensity exhibited along each line.
For six miles along the South Carolina Railway (A) the damage to the line, though indicative of a strong shock, was of little consequence. In the first half of this distance no repairs were required, but at 3-2/3 miles the rails were bent and the joints between them opened; at 5 miles, the fish-plates were torn from their fastenings and the joints between the rails opened seven inches; and at nearly 6 miles the joints were again opened, and the road-bed depressed six inches. After this point, deflections of the line and elevations and depressions of the road-bed were no longer rare. Near the 9-mile point, the intensity of the shock seemed to increase most rapidly; lateral displacements of the line became more frequent as well as greater in amount. The distortions of the lines were probably greatest between 10 and 11 miles; here they were often displaced laterally, sometimes depressed or elevated, and occasionally twisted into S-shaped curves, while many hundred yards of the track were shoved bodily towards the south-east. "The buckling always took place when this lateral shoving encountered a rigid obstacle, usually a long rigid trestle. At thenorth-western end of the trestle the accumulation of rails resulted in a sharp kink. Corresponding extensions of the track by the opening of the joints and shearing of the fish-plate bolts occurred some distance to the north-westward." At 11½ miles, the lines were again stretched and the joints opened by about seven inches; but, from this point for more than four miles, the sharp kinks revealing a sliding of the track were entirely absent, though there were still long slight flexures in the lines and changes of level in the road-bed. The railway in this section traverses a district which is partly a swamp and partly a rice-field; and thus it may be, as Major Dutton suggests, that the ground was less fitted to preserve the effects of the shock.[41]At about 18 miles, the line reaches higher and firmer ground; and, from here to Summerville (21-2/3 miles), there were many sinuous flexures. For six miles farther, violent distortions of the rails ceased to occur, the rate of decrease in intensity being most marked near the 23-mile point. The last flexure occurred at Jedburgh (27½ miles) at the south end of a long, heavy trestle (Fig. 27).