Flexure of rails at Jedburgh.Fig.27.—Flexure of rails at Jedburgh. (Dutton.)ToList
Fig.27.—Flexure of rails at Jedburgh. (Dutton.)ToList
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 thesouth-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 1½ 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 6⅓miles, 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 8½ inches. A little beyond the 10-mile point, an embankment 15 feet high was pushed 4½ 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 8⅓feet to the west. From 12½ to 14½ 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 abroad, 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 19½ 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 19½-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 sevenmiles from Charleston, and then turns off in a south-westerly direction. For 4½ 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 16½ miles the effects of the shock became rapidly more apparent. For nearly 1½ 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 18½ 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 27¼-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 clearlyto 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 curvein 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.
Epicentral isoseismal lines of Charleston earthquake according to Mr. Sloan.Fig.28.—Epicentral isoseismal lines of Charleston earthquake according to Mr. Sloan. (Dutton.)ToList
Fig.28.—Epicentral isoseismal lines of Charleston earthquake according to Mr. Sloan. (Dutton.)ToList
Epicentral isoseismal lines of Charleston earthquake according to Major Dutton.Fig.29.—Epicentral isoseismal lines of Charleston earthquake according to Major Dutton. (Dutton.)ToList
Fig.29.—Epicentral isoseismal lines of Charleston earthquake according to Major Dutton. (Dutton.)ToList
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.
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 outmore 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.
Planes of oscillation of stopped pendulum clocks at Charleston.Fig.30.—Planes of oscillation of stopped pendulum clocks at Charleston.ToList
Fig.30.—Planes of oscillation of stopped pendulum clocks at Charleston.ToList
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 themanner 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.
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 theinitial 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 √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 allpoints where the intensity of the shock changes most rapidly. Major Dutton calls it theindex-circleand, when its radius is known, the depth of the focus is at once obtained by multiplying the radius by 1.73.
In 1858, Mallet proposed a method which bears some resemblance to the above,[43]but depending only on the intensity of the longitudinal waves. Major Dutton claims for his method that the effects of the longitudinal and transverse waves are not separated, that it takes account of the "total energy irrespective of direction or kind of vibration."
Diagram to illustrate Dutton's method of determining depth of seismic focus.Fig.31.—Diagram to illustrate Dutton's method of determining depth of seismic focus.ToList
Fig.31.—Diagram to illustrate Dutton's method of determining depth of seismic focus.ToList
Objections to Dutton's Method.—I have described this method somewhat fully, though it seems to me open to more serious objections than Mallet's first method which it is intended to replace.
We have, in the first place, no reason for supposing that the focus is either a point or a sphere, or that the initial impulse is uniform in all directions. Ifthe earthquake were caused by fault-slipping, both assumptions would be untrue, and it is for those who employ the method to prove their validity.
But of greater consequence is the fact that, if the method were correct, all earthquakes originating at the same depth must have index-circles of equal radii. If the depth of the focus were, say, ten miles, then the index-circle must have a radius of about six miles, whether the initial disturbance be of extreme violence or so weak that it is not felt at the surface at all, much less so far as six miles from the epicentre. The law of the inverse square is of course only true for a perfectly elastic and continuous medium, and the real curve of intensity is not that of the continuous line in Fig. 31, but something of the form represented by the dotted line. In this case, the rate of change of intensity is greatest at some point C', nearer than C to the epicentre, and the application of Major Dutton's rule would give a point F', nearer the surface than F, for the focus. Thus, assuming that the method can be applied in practice—and the test involved is one so delicate that it would be difficult to apply except with refined measurements—then all that we can assert is that the calculated depth is certainly less than the true depth.
Dutton's Estimate of the Depth of the Seismic Foci.—In applying the method, the chief difficulty is to obtain a series of isoseismal lines corresponding to equidistant degrees of intensity. As already pointed out, those given in Fig. 29 are merely diagrammatic; but the index-circle of the Woodstock focus, represented by the dotted line, is made to pass through the places where the rate of change of intensity wasfound to be greatest. The radius of this circle being very nearly seven miles, it follows that the resulting depth of the Woodstock focal point would be about twelve miles. Major Dutton regards this estimate as probably correct within two miles.
In the neighbourhood of the Rantowles epicentre, the isoseismals in both Figs. 28 and 29 are elongated in form. Theindex-circuit, as it would be called in such a case, cannot be drawn completely, but its radius parallel to the shorter axis of the curves is about 4½ miles, and the resulting depth of the Rantowles focal point would be nearly eight miles.
The recognition of the double epicentre is, from a geological point of view, the most important fact established by the investigation of the Charleston earthquake. But of equal interest, from a physical point of view, is the estimate of the velocity of the earth-waves, which is probably more accurate than that determined for any previous shock. Owing to the existence of the standard time system in the United States, the exact time is transmitted once a day to every town and village within reach of a telegraph line; and the effect of small errors in the observations is considerably lessened by the great distance traversed by the earth-waves, sixty good reports coming from places more than 500 miles from the epicentre, and ten from places more than 800 miles distant.
The total number of time-records collected is 316, but of these 130 had to be rejected, either because they were obviously too early or too late, orbecause they were only given to the nearest five-minutes' interval. There remain 186 observations which are divided by Major Dutton into four classes according to their probable value.
In an earthquake of such great duration (about 70 seconds at Charleston), it is necessary in the first place to select some special phase of the movement as that to which the records mainly refer, and then to determine as accurately as possible the time of occurrence of this phase at the origin.
There can be little doubt as to which phase should be chosen. The shock began with a series of tremors, which passed somewhat abruptly into the oscillations that formed the first and stronger maximum. These were clearly felt all over the disturbed area, and, as the beginning of the first maximum at places near the epicentre and the beginning of the shock at distant stations were probably due to the same vibrations, this particular phase may be fairly selected as that to which the time-measurements refer.
The time of this phase at the origin can only be ascertained from the time at which it reached Charleston, and our knowledge of this depends chiefly on the evidence of stopped clocks. How unreliable this may be is well known. Clocks may indeed be stopped at almost any phase of the movement; and, whenever stopped clocks can be compared with really good personal observations, they almost invariably show a later time. At Charleston three good clocks were stopped by the vibrations from the Woodstock focus, two of them being in close agreement (p. 121); and, allowing for a few oscillations before their final arrest, Major Dutton places the time of arrival of the selected phase atCharleston at 9h. 51m. 12s.P.M.The evidence of these clocks is also supported by that of other observations, which show that 9.51 was certainly the nearest minute to the time of arrival, and favour a somewhat later instant much more strongly than one a little earlier.
Now, the distance of Charleston from the Woodstock epicentre is sixteen miles, and from the corresponding focus (with the calculated value of its depth) twenty miles. A first estimate of the velocity gives a value of a little more than three miles a second, and the time at the Woodstock focus may therefore be taken as 9h. 51m. 6s. with a probable error of a few seconds.[44]
Proceeding to the observations at a distance, we find them, even after all rejections, to be very different in value. They were therefore divided into groups consisting of observations which are as nearly as possible homogeneous.
The first group contains five records from places between 452 and 645 miles from the Woodstock epicentre. They give the time to within 15 seconds, obtained from an accurately kept clock, or from a clock or watch that was compared with such within a few hours of the earthquake. The resulting velocity is 3.236 ± .105 miles (or 5205 ± 168 meters) per second.[45]
In the second group there are eleven observations (between distances of 438 and 770 miles) which satisfy the same conditions as those in the first group, except that the time is only given to the nearest minute or half-minute. The velocity obtained from them is 3.226 ± .147 miles (or 5192 ± 236 metres) per second.
The third group included all but the above records and those obtained from stopped clocks. They are 125 in number (between distances of 80 and 924 miles), but it is uncertain whether they correspond to the selected phase of the movement, and the errors of the clocks and watches used were unknown. They give a mean velocity of 3.013 ± .027 miles (or 4848 ± 43 metres) per second.
In the fourth group, we have the evidence of 45 stopped clocks (at places between 20 and 855 miles), which apparently give a velocity of 2.638 ± .105 miles (or 4245 ± .168 metres) per second. At six places, however, the times indicated by stopped clocks can be compared with good personal observations; and these show that the time of traverse from the origin obtained from the former is on an average 1.28 times the time of traverse obtained from the latter. If a similar correction be made for all the stopped clocks, the corrected velocity of the earth-waves would be from 3.17 to 3.23 miles (or 5100 to 5200 metres) per second.
In obtaining the mean value of the velocity from all the observations, those of the fourth group are omitted, and the weights of the first three groups aretaken inversely as the squares of the probable errors—that is, as 2: 1: 4. The resulting mean velocity is 3.221 ± .050 miles (or 5184 ± 80 metres) per second; and, though it does not follow that all other estimates are erroneous (for the velocity may vary with the strength of the earthquake and with other conditions), it is probable that this result is more nearly accurate than any other previously obtained.
Fissures and Sand-Craters.—In point of size, there was nothing remarkable about the fissures in the ground produced by the Charleston earthquake. The largest were only a few hundred yards long, and, except near the river-banks, they rarely exceeded an inch in width. They seem, however, to have been unusually abundant; for, within an area of nearly 600 square miles surrounding the two epicentres, they were almost universal, and over a much wider area they still occurred in great numbers, though with somewhat less continuity.
From many of these fissures water was ejected, carrying with it large quantities of sand and silt, and so abundantly that every stream-bed, even though generally dry in summer, was flooded. By the passage of the water, some part of the fissures was often enlarged into a round hole of considerable size, ending in a craterlet at the surface. Certain belts within the fissured area contained large numbers of these craterlets, of all sizes up to twenty feet or more in diameter. One near Ten-Mile Hill was twenty-one feet across. In this district, they were apparently larger and more numerous than elsewhere; manyacres of ground being covered with sand, which, close to the orifices, was two feet or more in depth.
Here and there, the water was ejected with considerable violence, as was manifest from the heights to which it spurted. The testimony of witnesses on this point is of course doubtful, for the earthquake occurred after nightfall, but in a few places the branches and leaves of trees overhanging the orifices were smirched with sand and mud up to a height of fifteen or twenty feet.
Effects on Human Beings.—It is interesting to notice the behaviour of different races under the influence of a violent earthquake, and perhaps no greater contrast could be observed than between the calmness exhibited by the Japanese in the presence of disaster and the wild fear merging into helpless panic that characterised the residents, and especially the negroes, of Charleston. "As we dashed down the stairway," says a writer already quoted (p. 108), "and out into the street, from every quarter arose the shrieks, the cries of pain and fear, the prayers and wailings of terrified women and children, commingled with the hoarse shouts of excited men.... On every side were hurrying forms of men and women, bareheaded, partially dressed, some almost nude, and all nearly crazed with fear and excitement.... A few steps away, under the gas-lamp, a woman lies prone and motionless on the pavement, with upturned face and outstretched limbs, and the crowd which has now gathered in the street passes her by, none pausing to see whether she is alive or dead ... no one knows which way to turn, or where to offer aid; many voices are speaking at once, but few heed what is said."
Between the selfish rush for safety here described and the calm interest of the most distant observers, Major Dutton records nearly every possible variety of mental effects, the actions resulting from which may be roughly classified as follows:
A. No persons leave their rooms.
B. Some persons leave their houses.
C. Most persons run into the streets, which are full of excited people.
D. People rush wildly for open spaces and remain all night out-of-doors.
In the map of the isoseismal lines (Fig. 25), the dotted curves bound the areas in which the effects corresponding to the three highest degrees of the above scale were observed. The curve for the first degree (A) coincides of course with the isoseismal line of intensity 2.
It will be seen that there is a certain rough correspondence between these curves and the isoseismal lines. The curve D and the isoseismal 8 are close together; in other words, people thought it wiser to camp out-of-doors for the night if the shock was strong enough to damage buildings slightly. The curve C and the isoseismal 6 are similarly connected; that is, if the movement made pictures swing, etc., people rushed into the streets. On the whole, the curve B and the isoseismal 3 roughly coincide, or, if the shock was not strong enough to make doors and windows rattle, some persons left their houses and public meetings were dispersed.
Feeling of Nausea.—A feeling of nausea was experienced by many persons at the time of the earthquake, somewhat rarely it appears in the neighbourhood of the epicentre and even outside the isoseismal7, but more frequently beyond these limits, and perceptible as far as the broken line in Fig. 25. The most distant places at which it was noticed are Blue Mountain Creek (New York) and Dubuque (Iowa), which are respectively 823 and 886 miles from Charleston.
As Summerville lies six miles to the north-west of the Woodstock epicentre and Charleston 16 miles to the south-east, it is probable that many of the after-shocks were unfelt and a still greater number unrecorded. In Charleston, seven shocks, all much slighter than the principal shock, were felt during the night of August 31—September 1, and thirty before the end of September. Of these, the shock of September 3rd, at 11P.M., was the strongest, but those which occurred on September 1st, 2nd, 21st, and 27th were also described as severe, and the remainder as moderate or slight. For weeks after the great shock, curious sensations were distinctly perceptible during the still hours of the night "as though the crust of the earth were resting on a gelatinous mass in constant motion." The last shock felt in Charleston seems to have been one recorded on March 18th, 1887.
At Summerville, many shocks occurred that were scarcely perceptible in Charleston, and those noticed in both places were usually stronger, and the motion more nearly vertical, at Summerville. "The peculiar characteristic of all of them was the deep, solemn, powerful boom, like the report of a heavy cannon, usually accompanied by a quick, short jar. Sometimes it was prolonged into a heavy roar or rumble,as if many reports were delivered in a volley. The number of them was never recorded." On September 3rd, Mr. W.J. McGee, of the United States Geological Survey, arrived at Summerville. During the evening of that day, detonations were heard at intervals, averaging perhaps half-an-hour, accompanied occasionally by very slight spasmodic tremors of an instant's duration. They were much like peals of thunder at a distance of half-a-mile or more, though rather more muffled. "It was my impression," Mr. McGee remarks, "that the sound was sometimes about as grave as the ear can perceive, resembling somewhat the tremulous roar sometimes accompanying combustion in locomotives." These sounds continued, but with diminishing frequency, throughout the remainder of the year and as late as July 1st, 1887.
Major Dutton's valuable monograph is a record of the earthquake-phenomena. He offers no theory as to the cause of the shock, and is therefore in no way responsible for the account given in the remaining part of this chapter.
That there were two seismic foci he has shown, I think, conclusively; and my object is now to trace out briefly the probable nature of the movements that produced the double shock.
Referring to Figs. 28 and 29, it will be seen that, according to both Mr. Sloan and Major Dutton, the isoseismals surrounding the Rantowles epicentre are distorted along a line which runs from a few degrees east of north to a few degrees west of south. Their oval form is in all probability connected with afocus elongated in about the same direction. Near the Woodstock epicentre, the isoseismals are drawn differently in the two maps, and in neither case do they offer any sure guide as to the form of the seismic focus. If, however, we follow Mr. Sloan's interpretation of the evidence, and suppose the earthquake to have been fault-formed, then it is probable that in this region the fault bends round slightly towards the east.
The only other evidence on this point is that afforded by the regions of defective intensity, real or apparent, along the three railway-lines diverging from Charleston. One of these occurred near Mount Holly Station on the North-Eastern Railway (B, Figs. 28 and 29), another for four miles starting from the 11½-mile point on the South Carolina Railway (A), and a third along the Charleston and Savannah Railway (C) over a distance of four miles from the Ashley River. In the first two cases, Major Dutton suggests that the less amount of damage was due to the nature of the soil traversed by the railway; but it is on the softer ground that the effects of an earthquake-shock are generally the more disastrous. On the whole, it seems to me probable that the three tracts referred to are really regions of less intensity, and it is worthy of notice that they lie along a nearly straight line.
To show the bearing of these remarks, let CD (Fig. 32) represent the section of a fault and EF that of the surface of the earth, and suppose the rock-mass A to slip slightly and suddenly downwards. Then the particles of A at the surface of the fault will, by impulsive friction, be drawn sharply upwards, and those of B correspondingly downwards; so that theearth-waves in the two rock-masses will start in opposite phases of vibration. Along the line of fault, every particle of rock, being urged upwards and downwards almost equally, will remain practically at rest. Thus, regions of defective intensity may arise from partial interference by the spreading of either earth-wave in the adjoining rock-mass.
Diagram to explain origin of regions of defective intensity.Fig.32.—Diagram to explain origin of regions of defective intensity.ToList
Fig.32.—Diagram to explain origin of regions of defective intensity.ToList
If this be the correct explanation, the path of the originating fault may be taken as that indicated by the broken line in Fig. 28, a line which is nearly parallel to the chief branches of the isoseismal curves.[46]As both epicentres lie on the west side of this line, the fault must hade or slope in this direction. The distortion of the Woodstock isoseismals towards the north-west confirms the latter inference, for the intensity of the shock is greater on the side towards which the fault hades.
From the comparative absence of earthquakes in South Carolina, we may infer that the fault is one subject to displacements at wide intervals of time. The gradually increasing stress along its surface was relieved at one or two points in or near the Woodstock focus on August 27th and 28th, and perhaps during the preceding months. But the first great sliptook place suddenly in that focus, and spread gradually southwards—for there was no interruption in the movement—until about half-a-minute later it reached the Rantowles focus, where the second great slip occurred. Eight or ten minutes afterwards there was another slip—in what part of the fault is uncertain—and this was followed at irregular intervals by many small movements gradually diminishing in frequency and in focal area. Within a year from the first disturbance, the fault-system attained once more its usual condition of rest.
1.Dutton, C.E.—"The Charleston Earthquake of August 31st, 1886."Amer. Geol. Survey, Ninth Annual Report, pp. 209-528.2.Nature, vol. xxxv., 1887, pp. 31-33; vol. lxiii., 1901, pp. 165-166.
1.Dutton, C.E.—"The Charleston Earthquake of August 31st, 1886."Amer. Geol. Survey, Ninth Annual Report, pp. 209-528.
2.Nature, vol. xxxv., 1887, pp. 31-33; vol. lxiii., 1901, pp. 165-166.
[38]The authorities for this statement are Mallet's Catalogue of Recorded Earthquakes (Brit. Assoc. Rep., 1852, pp. 1-176; 1853, pp. 117-212; 1854, pp. 1-326), which closes with the year 1842, and Fuchs'Statistik der Erdbeben von 1865-1885. According to Mallet, there was an earthquake in S. Carolina in November 1776, and the New Madrid earthquake of December 16th, 1811, was felt at Charleston. Fuchs records two earthquakes at Charleston on May 12th, 1870, and December 12th, 1876; and two in S. Carolina on December 12th and 13th, 1879.
[38]The authorities for this statement are Mallet's Catalogue of Recorded Earthquakes (Brit. Assoc. Rep., 1852, pp. 1-176; 1853, pp. 117-212; 1854, pp. 1-326), which closes with the year 1842, and Fuchs'Statistik der Erdbeben von 1865-1885. According to Mallet, there was an earthquake in S. Carolina in November 1776, and the New Madrid earthquake of December 16th, 1811, was felt at Charleston. Fuchs records two earthquakes at Charleston on May 12th, 1870, and December 12th, 1876; and two in S. Carolina on December 12th and 13th, 1879.
[39]1. Recorded by a single seismograph, or by some seismographs of the same pattern, but not by several seismographs of different kinds, the shock felt by an experienced observer.2. Recorded by seismographs of different kinds; felt by a small number of persons at rest.3. Felt by several persons at rest; strong enough for the duration or direction to be appreciable.4. Felt by several persons in motion; disturbance of movable objects, doors, windows; creaking of floors.5. Felt generally by every one; disturbance of furniture and beds; ringing of some bells.6. General awaking of those asleep; general ringing of bells; oscillation of chandeliers, stopping of clocks; visible disturbance of trees and shrubs; some startled persons leave their dwellings.7. Overthrow of movable objects, fall of plaster, ringing of church bells, general panic, without damage to buildings.8. Fall of chimneys, cracks in the walls of buildings.9. Partial or total destruction of some buildings.10. Great disasters, ruins, disturbance of strata, fissures in the earth's crust, rock-falls from mountains.
[39]1. Recorded by a single seismograph, or by some seismographs of the same pattern, but not by several seismographs of different kinds, the shock felt by an experienced observer.
2. Recorded by seismographs of different kinds; felt by a small number of persons at rest.
3. Felt by several persons at rest; strong enough for the duration or direction to be appreciable.
4. Felt by several persons in motion; disturbance of movable objects, doors, windows; creaking of floors.
5. Felt generally by every one; disturbance of furniture and beds; ringing of some bells.
6. General awaking of those asleep; general ringing of bells; oscillation of chandeliers, stopping of clocks; visible disturbance of trees and shrubs; some startled persons leave their dwellings.
7. Overthrow of movable objects, fall of plaster, ringing of church bells, general panic, without damage to buildings.
8. Fall of chimneys, cracks in the walls of buildings.
9. Partial or total destruction of some buildings.
10. Great disasters, ruins, disturbance of strata, fissures in the earth's crust, rock-falls from mountains.
[40]In order to simplify these figures, the rivers, most of the inlets, and other details are omitted. Small figures are added along the railway lines to denote the distance in miles from the stations in Charleston.
[40]In order to simplify these figures, the rivers, most of the inlets, and other details are omitted. Small figures are added along the railway lines to denote the distance in miles from the stations in Charleston.
[41]If this were so, the decrease in intensity would be only apparent; but it may have been real, and a possible explanation on this supposition is given later on (p. 135).
[41]If this were so, the decrease in intensity would be only apparent; but it may have been real, and a possible explanation on this supposition is given later on (p. 135).
[42]Ifcbe the depth of the focus,athe intensity at unit distance from the focus, andythe intensity on the surface at distancexfrom the epicentre, theny=a/ (c² +x²)The inclination of the curve at any point is given bydy/dx= -2ax/ (c² +x²)²,and this is a maximum whend²y /dx² or (3x² -c²) / (c² +x²)³is zero, which is satisfied whenc=x√3
[42]Ifcbe the depth of the focus,athe intensity at unit distance from the focus, andythe intensity on the surface at distancexfrom the epicentre, then
y=a/ (c² +x²)
The inclination of the curve at any point is given by
dy/dx= -2ax/ (c² +x²)²,
and this is a maximum when
d²y /dx² or (3x² -c²) / (c² +x²)³
is zero, which is satisfied whenc=x√3
[43]British Association Report, 1858, pp. 101-103.
[43]British Association Report, 1858, pp. 101-103.
[44]The above time would have to be increased by one second if the depth of the focus were very small, and diminished by one second if it were as great as 23 miles; the difference in either case being less than the probable error.
[44]The above time would have to be increased by one second if the depth of the focus were very small, and diminished by one second if it were as great as 23 miles; the difference in either case being less than the probable error.
[45]The method employed is as follows: Let t0be the computed time (9h. 51m. 6s.) at the focus,xseconds the error in this estimate,tthe reported time at a given place,Dits distance from the focus in miles, andythe number of seconds required to travel one mile; then, assuming thatydoes not vary with the distance, we havex+Dy=t+ t0. An equation of this form is obtained from each observation, and the method of least squares is then applied to determine the most probable values ofxandy.
[45]The method employed is as follows: Let t0be the computed time (9h. 51m. 6s.) at the focus,xseconds the error in this estimate,tthe reported time at a given place,Dits distance from the focus in miles, andythe number of seconds required to travel one mile; then, assuming thatydoes not vary with the distance, we havex+Dy=t+ t0. An equation of this form is obtained from each observation, and the method of least squares is then applied to determine the most probable values ofxandy.
[46]This seems to me the more probable course. It is possible, however, that the fault-line may pass from Mount Holly Station to the east of the Woodstock epicentre as shown in Fig. 28, and then to the west of the Rantowles epicentre, the fault changing its direction of hade in the intermediate district.
[46]This seems to me the more probable course. It is possible, however, that the fault-line may pass from Mount Holly Station to the east of the Woodstock epicentre as shown in Fig. 28, and then to the west of the Rantowles epicentre, the fault changing its direction of hade in the intermediate district.