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A Study of Recent Earthquakes
by Charles Davison
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1. Coincidence of Epicentres.—In Fig. 14, which is copied from Professor Mercalli's map, are shown the areas in which buildings were seriously damaged by these four earthquakes. The curves for 1796, 1828, and 1881 are approximately concentric. In 1796, the shock was disastrous only to the west of Casamicciola; in 1828, according to Covelli, "the ground most injured was not precisely the region of Casamicciola, but that which lies between the district called Fango and that known as Casamenella, situated to the west of Casamicciola, and a short distance from it."[26] The epicentres may have varied slightly in size, but, in position, it is clear that all four were nearly or quite coincident. The meizoseismal bands in 1881 and 1883 were also similar in form and elongated in the same direction.

In the last two earthquakes there was, as we have seen, very distinct evidence of a secondary meizoseismal area surrounding Fontana, and it is remarkable that this was also noticeable in the earthquake of 1828. "Besides the centre of vibration in the district of Fango," says Covelli, "another less powerful centre showed itself in the locality of Fontana; this made itself felt more heavily than in surrounding localities; as if another centre of movement had taken place from that part, independent of the former."

2. Small Depth of the Foci.—Mallet's method, as noted above, cannot be trusted to yield accurate estimates of the focal depth, or to indicate more than its order of magnitude. But it is remarkable that the depths calculated by Dr. Johnston-Lavis for the last two earthquakes are both only a little less than a third of a mile, and it is probable that the actual depth did not differ very greatly from this amount. The nature of the shock, vertical or nearly so close to the epicentre and horizontal at a short distance from it, is merely personal testimony of the same character as fissures in masonry, and of course points to the same result.



But the most conclusive evidence on which we have to rely is the extraordinary intensity of the shock at the centre of a very small distributed area. In Great Britain, an earthquake felt over a district of equal size would hardly at the centre exceed the trembling produced in a station platform by a passing train. The curves in Fig. 18 show how the rate of decline in intensity depends on the depth of the focus. They are drawn on the supposition that the intensity at any point on the surface varies inversely as the square of its distance from the focus; the curves a, b, c corresponding to foci situated at depths of one-third of a mile, one mile, and two miles respectively, and the figures below the horizontal line denoting the distance in miles from the epicentre. Thus, the rapid decline of intensity from the epicentre outwards shows that, in each of the four great Ischian earthquakes, the depths of the focus must have been very small.

3. Suddenness of the Shocks.—In 1796, we have no record of preparatory shocks, but the evidence is scanty; in 1828 and 1881, none are mentioned; in 1883, one or two tremors and underground noises, possibly of seismic origin, gave warning to a few. Fore-shocks, for all practical purposes, were conspicuous by their absence.

Still more remarkable is the sudden advent of the great shocks. There were no preliminary tremors or rumbling sound, no animals showed signs of uneasiness and no birds fluttered screaming from trees or ground. The shock of 1828, says Covelli, "was announced by three powerful blows coming almost vertically, from below upwards;" and the same words apply equally well to the earthquakes of 1881 and 1883. The destruction of houses in every case was practically instantaneous, and coincident with the first vibration.

In all respects, tectonic earthquakes differ widely from the Ischian shocks. The epicentres of successive earthquakes are rarely coincident, but show a distinct tendency to migration along certain lines; the decline in intensity outwards from the epicentre is nearly always very gradual, and therefore indicative of a comparatively deep-seated focus; they are almost invariably preceded either by a series of slight shocks and rumbling sounds, or, in an unstable district, by a marked increase in their frequency. Distinctions, so great as these are, evidently remove the Ischian shocks from the category of tectonic earthquakes.

ORIGIN OF THE ISCHIAN EARTHQUAKES.

On the other hand, the Ischian earthquakes possess several features which connect them closely with true volcanic earthquakes.

1. They originate beneath the northern slope of Epomeo—a volcano that we have no reason to consider absolutely extinct, but rather as one subject to eruptions at long intervals of time—in a region as yet unoccupied by parasitic craters, but having the same relation to the central cone of Epomeo as those in which the recent craters of Monte Rotaro, Montagnone and Cremate are situated.

2. In both the earthquakes of 1881 and 1883, the epicentre is an elongated band, the axis of which, if produced, would pass through the centre of the old crater of Epomeo. Along the line of this band, occur the fumaroles of Monte Cito and Ignazio Verde and the thermal springs of the Rita and Capitello. These facts, as Professor Mercalli suggests, lead us to believe that the foci of the earthquakes coincide with a radial fracture of the volcano, the course of which, as traced by him, is represented by the continuous line in Fig. 14.[27]

3. Except in their relations with actual eruptions, the Ischian earthquakes resemble closely the true volcanic earthquakes which from time to time shake the flanks of Etna. These are marked by great intensity of the shock at the centre of a comparatively small disturbed area, epicentres often elongated radially to the cone, frequent repetition with similar characters in the same districts; and as a rule they precede by a short interval, but sometimes accompany or follow, volcanic eruptions.[28]

Two other phenomena may be referred to as probably indicating some connection between Ischian earthquakes and the structure and history of Epomeo.

We have seen that, in the three earthquakes of 1828, 1881, and 1883, there is distinct evidence of a second meizoseismal area at Fontana, within which the shock was mainly subsultory. Dr. Johnston-Lavis, though recognising the possibility of the existence of two epicentres, prefers another explanation.[29] But the wide extension of the southern boundary of the area of destruction in 1883, and the limitation of several of the after-shocks to the south of the island, seem to me to favour the existence of a second focus beneath the crater of Epomeo, though, it may be, not entirely detached from the chief focus beneath Casamenella.

Again, as Professor Mercalli remarks, all historic eruptions on the flanks of Epomeo were accompanied by very violent earthquakes; while, previously to 1302, only one disastrous earthquake, so far as known, occurred in the island without being attended by an eruption. It should be noticed also that the principal shocks during the recent revival of activity (i.e., since 1762) show a continual increase in intensity, whether this be measured by the damage to buildings, the loss of life, or the extent of the area of destruction (Fig. 14).

It therefore seems legitimate to conclude that, in the recent Ischian earthquakes, we have merely so many unsuccessful attempts to force a new volcanic eruption. The passages once existing through Epomeo and its parasitic craters having become blocked, the highly heated magma beneath is compelled to find a new outlet. Its tension slowly increasing, the crust above is at last rent, or an incipient rent is enlarged, the fluid rock is injected almost instantaneously with great force into the open fissure, and its sudden arrest by the containing walls is the ultimate cause of an earthquake. With the expansion of the magma, its tension is at once correspondingly reduced, and some time must elapse before it can again reach the critical point at which a further rupture, resulting in a second shock, takes place.[30]

Thus, with each great Ischian earthquake, we are, I believe, advancing a step nearer the time, which may be close at hand or may be very remote, when the fracture will at last reach the surface, and above the site of Casamenella a new parasitic cone will rise, from which, as from Cremate in 1302, a stream of lava may flow down towards the sea.

REFERENCES.

1. BALDACCI, L.—"Alcune osservazioni sul terremoto avvenuto all' Isola d'Ischia il 28 luglio 1883." Ital. Com. Geol. Boll., vol. xiv., 1883, pp. 157-166.

2. DAUBRE, A.—"Rapport sur le tremblement de terre ressenti Ischia le 28 juillet, 1883; causes probables des tremblements de terre." Paris, Acad. Sci., Compt. Rend., vol. xcvii., 1883, pp. 768-778.

3. DU BOIS, F.—"The Earthquakes of Ischia." Japan Seism. Soc. Trans., vol. vii., pt i., 1883-84, pp. 16-42.

4. —— "Farther Notes on the Earthquakes of Ischia." Ibid., vol. viii., 1885, pp. 95-99.

5. JOHNSTON-LAVIS, H.J.—Monograph of the Earthquakes of Ischia (1885).

6. MERCALLI, G.—Vulcani e fenomeni vulcanici in Italia (vol. iii. of Geologia d'Italia, by G. Negri, A. Stoppani, and G. Mercalli), 1883, pp. 46-50, 331-332.

7. —— L'Isola d'Ischia ed il terremoto del 28 luglio 1883 (Milano, 1884).

8. PALMIERI, L., E A. OGLIALORO.—"Sul terremoto dell' Isola d'Ischia della sera del 28 luglio 1883." Napoli, R. Accad. Atti, vol. i., 1884, pp. 1-28.

9. ROSSI, M.S. DE.—"Il terremoto di Casamicciola del 4 marzo 1881." Bull. del Vulc. Ital., anno viii., 1881, pp. 5-12. (In the same volume are brief notices by different writers on pp. 22, 38-42, 52-53, 67-68, 70-74.)

10. —— "Raccolta di fatti, relazioni, bibliografie sul terremoto di Casamicciola del 28 luglio 1883, con brevi osservazioni." Bull. del Vulc. Ital., anno xi., 1884, pp. 65-172.

11. —— "Intorno all' odierna fase dei terremoti in Italia e segnatamente sul terremoto in Casamicciola del 4 Marzo 1881." Ital. Soc. Geogr. Boll., 1881.

12. SERPIERI, A.—"Sul terremoto d'Ischia il 28 luglio 1883." Scritti di Sismologia, Pte. ii., pp. 207-216.

13. —— "Sul terremoto dell' Isola d'Ischia il 28 luglio 1883." Ibid., pp. 217-232.

FOOTNOTES:

[21] The shaded areas indicate the principal trachytic masses, the broken lines represent the boundaries of the craters that are still recognisable, and the dotted lines the boundaries of the areas within which buildings were damaged by the earthquakes of 1796, 1828, 1881, and 1883 (according to Mercalli). The continuous curved line shows the position of the radial fracture with which the earthquakes were probably connected. The trachytic masses and craters are denoted by the following tables:—

a. Epomeo. b. Trippiti. c. Vetta. d. Garofoli. e. Vatoliere. f. Campagnano. g. Vezza. h. Imperatore. i. C. St. Angelo. j. Lo Toppo. k. Marecocco. l. Zale. m. Rotaro. n. Montagnone. p. Bagno. q. Tabor. r. P. Castiglione. s. Cremate. t. Arso. u. Porto d'Ischia.

[22] It is possible that Monte Campagnano may form an exception to this statement.

[23] Shocks were felt in the island in 1559 and 1659, but one at least was of external origin.

[24] Prof. Mercalli, from the five estimates of the angle of emergence which he considered most reliable, found the mean depth to be about 3,280 feet.

[25] Professor de Rossi estimated the mean duration as not much exceeding ten seconds. Dr. Johnston-Lavis, on the other hand, considers the general estimate of fifteen seconds as far too low. In one case, at Casamicciola, he ranks it as high as thirty-one seconds.

[26] Quoted from the useful translation of Covelli's memoir given by Dr. Johnston-Lavis.

[27] Baldacci supposes that the thermal springs and fumaroles of Forio, Stennecchia, Montecito, Casamicciola, and Castiglione lie along a tangential fracture starting from Forio and passing by Casamicciola to near Punta di Castiglione. Mercalli, however, argues forcibly against this inference.

[28] Professor Mercalli adds, as a fourth point of contact between Ischian earthquakes and volcanic phenomena, the changes in the fumaroles and hot springs which preceded or accompanied or followed the earthquakes of 1828, 1881, and 1883.

[29] "Fontana," he says, "occupies the centre of the great crater of Epomeo..., and therefore lies immediately over the ancient chimney, which in all probability is filled by an old plug of consolidated trachyte, which must descend to the igneous reservoir. Any mass of igneous matter, that might determine the further rupture of a collateral fissure, would result in the conduction of any changes of pressure or vibrations, along the column of highly elastic trachyte; whilst the same earth-waves would be annulled or absorbed by the inelastic tufas surrounding it, so that the blow would be struck perpendicularly to the surface, and in a small area with well defined limits. The undulatory sensations, after the principal local shock, were those that arrived from the great centre of impulse beneath Casamenella."

[30] The above paragraph is a summary of the reasoning stated with admirable clearness by Dr. Johnston-Lavis. It should be mentioned that the late Professor Palmieri, relying on the extremely limited disturbed area, dissented from this view; but his difficulty is met by supposing the focus to be small as well as shallow, a supposition that is supported by the shortness of the meizoseismal band, as well as by the elongation of the isoseismal lines in the direction perpendicular to this band.



CHAPTER IV.

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 Colmear (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 Seor 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. Fouqu as chief, and MM. Lvy, Bertrand, Barrois, Offret, Kilian, Bergeron, and Bron 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 Albuuelas, 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, Albuuelas 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, Crceres 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 Fouqu 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 Albuuelas; 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 velocit di propagazione del terremoto andaluso del 25 dicembre 1884." Roma, R. Accad. Lincei, Rend., vol. iii., 1894, pp. 303-310.

2. —— "Velocit 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 Andaluca: 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. FOUQU, F., etc.—"Mission d'Andalousie: tudes relatives au tremblement de terre du 25 dcembre 1884, et la constitution gologique du sol branl par les secousses." Paris, Acad. Sci. Mm., 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. NOGUS, A.F.—"Phnomnes gologiques produits par les tremblements de terre de l'Andalousie, du 25 dcembre 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. Fouqu).

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.



CHAPTER V.

THE CHARLESTON EARTHQUAKE OF AUGUST 31ST, 1886.

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 the investigators 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.

DAMAGE CAUSED BY THE EARTHQUAKE.

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 9.51 P.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.

ISOSEISMAL LINES AND DISTURBED AREA.

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 seismic intensity, 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. One point 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 of intensity as the earth-waves crossed the unconsolidated materials of the Mississippi delta.



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.

PREPARATION FOR THE EARTHQUAKE.

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 8 A.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 or heavy 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 4.45 A.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.

NATURE OF THE SHOCK.

At 9.51 P.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 whole lofty, 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 suddenly by 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.



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 the house, 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."

THE DOUBLE EPICENTRE.

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 the north-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 11-1/2 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 (27-1/2 miles) at the south end of a long, heavy trestle (Fig. 27).



There is thus a certain symmetry in the damage to this line with respect to a point about 15 or 16 miles from the Charleston terminus. The changes of intensity are most rapid at distances of about 9 and 23 miles from the terminus. Also, on the south-east side of the 16-mile point, the longitudinal displacements of the line are always to the south-east; on the other side, always to the north-west. Major Dutton therefore infers that the epicentre must be on a line drawn nearly through the 16-mile point at right angles to the railway.

Somewhat similar changes were noted along the North-Eastern Railway (B), the Charleston terminus of which is about three-quarters of a mile to the south-east of that of the South Carolina Railway. Slight flexures in the line occurred at distances of 1-1/2 and 4 miles from the terminus, and at about 6 miles the road-bed was depressed, in one part by as much as 22 inches. At about 6-1/3 miles, the joints between the rails were opened 14 inches, and there were slight sinuous flexures in the line near the 7-mile and 8-mile points. The indications of great intensity then rapidly increased, the rate of change being greatest near the 9-mile point. Here, there was a long lateral flexure with a shift of 4 inches eastward. Half-a-mile farther, the fish-plates were broken and the rails parted 8-1/2 inches. A little beyond the 10-mile point, an embankment 15 feet high was pushed 4-1/2 feet eastward along a chord of 150 feet. At the 12-mile point and beyond, fish-plates were broken, lines were bent and the joints opened; the road-bed was cut by a series of cracks, one of which was 21 inches wide, while the beginning of a long trestle was shifted 8-1/3 feet to the west. From 12-1/2 to 14-1/2 miles, several buildings were damaged or destroyed by a movement which was clearly more vertical than horizontal. Near the 16-mile point, the ground was fissured and thrown into ridges, the rails being similarly bent in a vertical plane. Soon after this, the line reaches a broad, sandy tract, and, though the thickness of the sand is probably not much more than 40 feet in any place, the disturbances diminish almost at once, and, for a distance of more than two miles, there was little damage done to the line. At Mount Holly Station (18 miles), the intensity was so slight that the houses suffered no injury more serious than the loss of chimneys. Half-a-mile farther, the ground becomes less sandy, and the effects of the shock more distinct. The lines were bent in places for about a quarter of a mile, after which they again pass into the sandy area with a decrease of damage, the last flexure being near the 21-mile point. The rate of change of intensity in this part of the line appears to have been greatest at a distance of about 19-1/2 miles from the terminus, but the exact distance is obviously somewhat uncertain.

There is again a rough symmetry in the damage to the line, the central point being about 14 miles from the Charleston terminus. A line drawn through this point at right angles to the North-Eastern Railway (or rather to that part of it between the 9-mile and 19-1/2-mile points) should pass through the epicentre. It meets the corresponding line for the South Carolina Railway in a point which is indicated in Figs. 27 and 28 by a small circle (W). Houses and other buildings are rare in the surrounding district; but, as the intensity of the shock diminished outwards in all directions, this point must mark approximately the position of the epicentre. As it is close to the Woodstock Station on the South Carolina Railway, it is called by Major Dutton the Woodstock epicentre.

The Charleston and Savannah Railway (C) uses the same lines as the North-Eastern for the first seven miles from Charleston, and then turns off in a south-westerly direction. For 4-1/2 miles from the junction the signs of disturbance were few and unimportant. The railway then crosses the Ashley River, the banks of which slid towards one another and jammed the drawbridge; but for four miles farther there was no serious damage done to the lines. At about 16-1/2 miles the effects of the shock became rapidly more apparent. For nearly 1-1/2 mile the entire railroad was deflected into an irregular curve, the displacement being greatest at the bridge, where it crosses the Stono River. Here, it was as much as 37 inches to the south. After Rantowles Station (18 miles), there were many displacements, both lateral and vertical. At 18-1/2 miles, a long southward deflection began, the amount of which reached 25 inches at the 19-mile point, 50 inches half-a-mile farther on, and was still greater at 20-2/3 miles. For two miles more, sinuous flexures were continuous, but, at the 22-2/3-mile point, they rapidly disappeared, the railroad passing on to higher and firmer ground. Between 25 and 27 miles, there were occasional slight flexures in the line or depressions of the railroad; but, after the 27-1/4-mile point, they seldom occur, and, when they do, are of little consequence.

Some of the effects described in the last paragraph may, as Major Dutton suggests, be due to the varying nature of the surface-rocks. It is important to notice, however, that disturbances of the lines were exceedingly rare in the section that lies nearest to the Woodstock epicentre, and that they increase in violence for some distance from that region, the maximum intensity being reached a mile or two to the west of Rantowles Station. This points clearly to the existence of a second focus. Unfortunately, there are very few houses or other objects in the neighbourhood, and the position of the corresponding epicentre cannot be determined accurately. Major Dutton places it in the position indicated by a small circle (R), and calls it the Rantowles epicentre from its vicinity to the station of that name.

If the meizoseismal area had been a thickly populated one, the evidence of ruined and damaged houses would have provided materials for the construction of isoseismal lines surrounding the two epicentres. It is difficult, as it is, to gauge the equality of the effects on objects so different as railway-lines and buildings; and the isoseismals shown in Figs. 28 and 29 can therefore lay no claim to accuracy.

Fig. 28 shows the epicentral isoseismals as they are drawn by Mr. Earle Sloan. They do not correspond to the degrees of any definite scale of seismic intensity; but they may be taken as representing the impressions of a very careful observer, who traversed the district immediately after the occurrence of the earthquake, and who, when drawing these lines, was biassed by no preconceived theory.

Major Dutton, relying chiefly on Mr. Sloan's written notes, interprets the evidence differently, and obtains the series of curves shown in Fig. 29. In this case, also, the isoseismals correspond to no expressed standard of intensity. They are intended merely to represent the forms of the curves, and, by their less or greater distance apart, the more or less rapid rate at which the intensity varied.

The chief difference between the two maps concerns the form of the Woodstock isoseismals. Major Dutton draws them approximately circular, leaving the map blank towards the north, where hardly any evidence was forthcoming. Mr. Sloan attributes the scantiness of effects here to a diminution of intensity, and makes the lines curve in towards the epicentre. They certainly must do so in crossing the North-Eastern Railway; and the somewhat southerly trend of Mr. Sloan's curves to the east of this railway seems to me to furnish the better representation of the distinctly greater intensity in that region.





More important, however, than this divergence of opinion is the agreement in one respect between the two sets of curves. Both show a marked expansion around the points known as the Woodstock and Rantowles epicentres, especially about the former, and a contraction in the intermediate region. The evidence of these isoseismals therefore confirms that of the damaged railway lines, and establishes Major Dutton's inference that there were two distinct foci, the epicentres of which were about thirteen miles apart.

ORIGIN OF THE DOUBLE SHOCK.

In the last chapter, it was shown that the double shock of the Andalusian earthquake could be due only to two distinct impulses taking place either within the same focus or, more probably, in two detached foci. Similar reasoning applies to the Charleston earthquake. The double maximum or double shock was observed in no less than fourteen States. Moreover, the interval between the two maxima at Charleston appears from Fig. 26 to have been about 34 seconds in length. Thus, the duplication of the shock cannot have been a merely local phenomenon, nor can it have resulted from the separation into two parts of the earth-waves proceeding from a single disturbance. Each maximum must therefore be connected with a distinct impulse.

Combining this inference with Major Dutton's discovery of the double focus, no doubt can remain as to the origin of the repeated shock. It is clear, also, that the impulse at the Woodstock focus was the stronger of the two; for the isoseismals spread out more widely round the corresponding epicentre, and there was no rapid decline of intensity from that point, such as might be associated with a weaker disturbance within a shallow focus.



Again, since the earlier part of the shock is almost uniformly described as the stronger, it follows that the Woodstock focus was the first in action. A curious fact recorded by Major Dutton supports this inference. In Charleston, four clocks were stopped by the shock, the errors of which at the time were certainly less than eight or nine seconds. The planes in which their pendulums oscillated are shown by the lines lettered A, B, C, and D in Fig. 30, the broken lines W and R representing respectively the directions from Charleston of the Woodstock and Rantowles epicentres. Clock A stopped at 9h. 51m. 0s., B at 9h. 51m. 15s., C at 9h. 51m. 16s., and D (which had been reset to the second earlier in the day) at 9h. 51m. 48s. Now, if the plane of oscillation coincided nearly with the direction of the shock, the only effect would be a temporary change in the period of oscillation; but if it was at right angles to the direction of the shock, the clock might be stopped by the point of the pendulum catching behind the graduated arc in front of which it oscillated. The planes of the first three clocks, it will be seen, were approximately at right angles to the direction of the Woodstock epicentre, and B and C were indeed stopped in the manner just described; while the plane of shock D was nearly perpendicular to the direction of the Rantowles epicentre. As the pendulums of B and C might make a few staggering oscillations before their final arrest, Major Dutton assigns 9h. 51m. 12s. as the epoch of the first maximum at Charleston; and, as the interval between the two maxima was about 34 seconds, this would give about 9h. 51m. 46s. for the epoch of the second maximum—a time which agrees very closely with that given by clock D. Thus, clocks A, B, and C must have been stopped by the Woodstock vibrations, and clock D about half-a-minute later by those coming from the Rantowles focus.

DEPTH OF THE SEISMIC FOCI.

Two methods of estimating the depth of the seismic focus have been described in the preceding pages—namely, Mallet's, depending on the angle of emergence, and Falb's, based on the interval between the initial epochs of the sound and shock. To these, Major Dutton adds a third method, in which he relies on the rate at which the intensity of the shock varies with the distance from the epicentre.

Dutton's Method of determining the Depth of the Focus.—If the seismic focus is either a point or a sphere, and the initial impulse equal in all directions, and if the intensity of the shock diminishes inversely as the square of the distance from the focus, then the continuous curve in Fig. 31 will represent the variation of intensity along a line passing through the epicentre E. The form of the curve on these assumptions does not depend in any way on the initial intensity of the impulse; it is governed solely by the depth of the focus. The deeper the focus, the flatter becomes the curve, as we have seen in discussing the Ischian earthquakes (p. 68). In all directions from the epicentre, the intensity at first diminishes slowly; but the rate of change of intensity with the distance soon becomes more rapid, until it is a maximum at the points C, C; after which it again diminishes and dies out very slowly when the distance becomes great. It will be evident from Fig. 18 that the deeper the focus the greater also is the distance EC of the points where the intensity of the shock changes most rapidly. It may be easily shown, indeed, that this distance always bears to the depth of the focus the constant ratio of 1 to sqrt(3), or about 1 to 1.73.[42]

Now, if a series of isoseismals could be drawn corresponding to intensities which differ by constant amounts, we should have a series of circles like those surrounding the Woodstock epicentre in Fig. 29, the distance between successive lines at first decreasing gradually until it is a minimum at the dotted circle and afterwards gradually increasing. This dotted circle is obviously that which passes through all points where the intensity of the shock changes most rapidly. Major Dutton calls it the index-circle and, when its radius is known, the depth of the focus is at once obtained by multiplying the radius by 1.73.

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