Part 9 (1/2)
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 acc.u.mulation 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).
[Ill.u.s.tration: FIG. 27.--Flexure of rails at Jedburgh.
(_Dutton._)]
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 s.h.i.+ft 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 s.h.i.+fted 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 pa.s.s 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 pa.s.s 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 pa.s.sing 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 bia.s.sed 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.
[Ill.u.s.tration: FIG. 28.--Epicentral isoseismal lines of Charleston earthquake according to Mr. Sloan. (_Dutton._)]
[Ill.u.s.tration: FIG. 29.--Epicentral isoseismal lines of Charleston earthquake according to Major Dutton. (_Dutton._)]
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 a.s.sociated with a weaker disturbance within a shallow focus.
[Ill.u.s.tration: FIG. 30.--Planes of oscillation of stopped pendulum clocks at Charleston.]
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 a.s.signs 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.