A Study of Recent Earthquakes Part 8

3. CASTRO, M.F. de.--_Terremotos de Andalucia: 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. FOUQUe, F., etc.--"Mission d'Andalousie: etudes relatives au tremblement de terre du 25 decembre 1884, et a la const.i.tution geologique du sol ebranle par les secousses." Paris, _Acad.

Sci. Mem._, vol. x.x.x., pp. 1-772.

5. MACPHERSON, J.--"Tremblements de terre en Espagne." Paris, _Acad.

Sci., Compt. Rend._, vol. c., 1885, pp. 397-399.

6. NOGUeS, A.F.--"Phenomenes geologiques produits par les tremblements de terre de l'Andalousie, du 25 decembre 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. Fouque).

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 Wilts.h.i.+re, 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 s.h.i.+fted. 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, a.s.sam and the provinces of Mino and Owari in j.a.pan, 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, ill.u.s.trating a rapid fading of intensity as the earth-waves crossed the unconsolidated materials of the Mississippi delta.

[Ill.u.s.tration: FIG. 25.--Isoseismal lines of Charleston earthquake. (_Dutton, etc._)]

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 un.o.bservant, 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 a.s.sume 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 pa.s.sage 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 part.i.tions visibly swayed to and fro, the crash of falling ma.s.ses of stone and brick and mortar was heard overhead and without....

"For a second or two it seemed that the worst had pa.s.sed, 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.

[Ill.u.s.tration: FIG. 26.--Curve of intensity at Charleston.

(_Dutton._)]

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 a.s.sert 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 pa.s.sed 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 pa.s.sed 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.