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.
[Illustration: 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 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.
[Illustration: 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 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).
[Illustration: 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 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.
[Illustration: FIG. 28.--Epicentral isoseismal lines of
Charleston earthquake according to Mr. Sloan. (_Dutton._)]
[Illustration: 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 associated with a weaker
disturbance within a shallow focus.
[Illustration: 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 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.
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."
[Illustration: FIG. 31.--Diagram to illustrate Dutton's method of
determining depth of seismic focus.]
_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. If the 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 was found 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. The _index-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 4-1/2
miles, and the resulting depth of the Rantowles focal point would be
nearly eight miles.
VELOCITY OF THE EARTH-WAVES.
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, or because 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 at Charleston
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 plus or minus .105 miles
(or 5205 plus or minus 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 plus
or minus .147 miles (or 5192 plus or minus 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 plus or
minus .027 miles (or 4848 plus or minus 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 plus or minus .105 miles (or 4245 plus or minus .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 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
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; 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 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 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
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 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 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.
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-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 the earth-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.
[Illustration: 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. xxxv., 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. 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.
[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
y=a/(c^2+x^2)
The inclination of the curve at any point is given by
dy/dx=-2*a*x/(c^2+x^2)^2,
and this is a maximum when
d^2y/dx^2 or (3*x^2-c^2)/(c^2+x^2)^3
is zero, which is satisfied when c=x*sqrt(3)
[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.
[45] The method employed is as follows: Let t_0 be the computed time
(9h. 51m. 6s.) at the focus, _x_ seconds the error in this estimate,
_t_ the reported time at a given place, _D_ its distance from the
focus in miles, and _y_ the number of seconds required to travel one
mile; then, assuming that _y_ does not vary with the distance, we have
x+Dy=t+t_0. 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 of _x_ and _y_.
[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.