A Study of Recent Earthquakes Part 10

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 are taken inversely as the squares of the probable errors--that is, as 2: 1: 4. The resulting mean velocity is 3.221 plus or minus .050 miles (or 5184 plus or minus 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.

MISCELLANEOUS PHENOMENA.

_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 quant.i.ties of sand and silt, and so abundantly that every stream-bed, even though generally dry in summer, was flooded. By the pa.s.sage 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; many acres 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 j.a.panese 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 hoa.r.s.e 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 p.r.o.ne and motionless on the pavement, with upturned face and outstretched limbs, and the crowd which has now gathered in the street pa.s.ses 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 cla.s.sified 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 s.p.a.ces 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 isoseismal 7, 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.

AFTER-SHOCKS.

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 princ.i.p.al 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 11 P.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 ma.s.s 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 m.u.f.fled. "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.

ORIGIN OF THE EARTHQUAKE.

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 a focus 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 11-1/2-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-ma.s.s 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 the earth-waves in the two rock-ma.s.ses 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-ma.s.s.

[Ill.u.s.tration: FIG. 32.--Diagram to explain origin of regions of defective intensity.]

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 slip took 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.

REFERENCES.

1. DUTTON, C.E.--"The Charleston Earthquake of August 31st, 1886."

_Amer. Geol. Survey, Ninth Annual Report_, pp. 209-528.

2. _Nature_, vol. x.x.xv., 1887, pp. 31-33; vol. lxiii., 1901, pp.

165-166.

FOOTNOTES:

[38] The authorities for this statement are Mallet's Catalogue of Recorded Earthquakes (_Brit. a.s.soc. 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.

[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).

[42] If _c_ be the depth of the focus, _a_ the intensity at unit distance from the focus, and _y_ the intensity on the surface at distance _x_ from the epicentre, then