THE HEREFORD EARTHQUAKE OF DECEMBER 17TH, 1896, AND THE INVERNESS
EARTHQUAKE OF SEPTEMBER 18TH, 1901.
Among the earthquakes described in this volume, the Hereford and
Inverness earthquakes hold but a minor place. The damage to buildings,
though unusual for this country, was slight when compared with that
caused by the preceding shocks; there was no loss of life, not a
single person was injured by falling masonry. The interest of the
earthquakes lies entirely in the detailed study rendered possible by
numerous observations of the shock and sound,[61] and in the bearing
of this evidence on the general theory of the origin of earthquakes.
THE HEREFORD EARTHQUAKE OF DECEMBER 17TH, 1896.
The principal earthquake of this series occurred at 5.32 A.M. on
December 17th, and was preceded by at least nine minor shocks (the
first of which was felt at about 11 or 11.30 P.M. on December 16th),
and followed by two others on the same day, and by a third and last on
July 19th, 1897. The accounts of these preliminary movements will be
found on a later page, as their bearing will be more fully apparent
after the discussion of the principal shock.
[Illustration: FIG. 60.--Isoseismal and Isacoustic lines of
Hereford earthquake. (_Davison._)]
ISOSEISMAL LINES AND DISTURBED AREA.
On the map in Fig. 60, the continuous curves represent isoseismal
lines corresponding to the degrees 8, 7, 6, 5, and 4 of the
Rossi-Forel scale. The isoseismal 8, which is the most accurately
drawn of the series, is an elongated oval, 40 miles long, 23 miles
broad, and containing an area of 724 square miles. The longer axis is
directed W. 44° N. and E. 44° S. Within this curve, there are 73
places where buildings are known to have been damaged, 55 places being
in Herefordshire, 17 in Gloucestershire, and one in Worcestershire.
The most important damage occurred in the city of Hereford, which, in
1901, contained 4,565 inhabited houses. Here, no fewer than 218
chimneys had to be repaired or rebuilt. The Cathedral was slightly
injured. The finial of a pinnacle of the Lady Chapel was thrown down,
a fragment of a stone fell from one of the arches in the south
transept, and the three pinnacles of the western front were fractured.
Several churches suffered to a similar extent, while, at the Midland
Railway Station, all the seven chimney-stacks were shattered. At
Dinedor, Fownhope, Dormington, Withington, and a few other villages,
the damage was also relatively greater than elsewhere, these places
all lying within a small oval about 8-1/2 miles long, which surrounds,
not the centre, but rather the north-west focus, of the isoseismal 8.
The isoseismal 7, which includes places where the shock was strong
enough to overthrow ornaments, vases, etc., is also very nearly an
ellipse, whose axes are 80 and 56 miles in length, and whose area is
3,580 square miles. Its longer axis, running from W. 42° N. to E. 42°
S., is practically parallel to that of the inner curve. Next in
succession comes the isoseismal 6, surrounding those places where the
shock was strong enough to make chandeliers, pictures, etc., swing;
but, as most of the observers seem to have slept in darkened rooms,
the number of determining points for this curve is less than usual,
and its course is therefore laid down with a somewhat inferior degree
of accuracy. The error, however, is probably small, and we may
therefore regard the isoseismal 6 as another ellipse, 141 miles long,
116 miles broad, and containing an area of 13,000 square miles. Its
longer axis is again nearly parallel to those of the preceding
isoseismals.
The next two isoseismals are nearly circular in form. It will be
noticed that large portions of them, and especially of the isoseismal
4, traverse the sea. In these parts, the paths of the curves are to
some extent conjectural. In drawing them, the chief guides are their
trend before leaving the land and the known intensity along the
neighbouring coastlines. The isoseismal 5 bounds the area within which
the shock was perceptible as a sensible displacement and not merely a
quiver. Its dimensions are 233 miles from north-west to south-east,
and 229 miles from south-west to north-east, and its area 41,160
square miles. The isoseismal 4, which includes places where the shock
was strong enough to make doors, windows, etc., rattle, is 356 miles
from north-west to south-east, and 357 miles from south-west to
north-east, and 98,000 square miles in area; its centre coincides
nearly with that of the small oval area in the neighbourhood of
Hereford, where the damage to buildings was relatively greater than
elsewhere.
Outside the isoseismal 4, the earthquake was observed at several
places. The shock was certainly felt at Middlesbrough, 12-1/2 miles
from the curve, and probably at Killeshandra (in Ireland), 65 miles
distant. Thus, if we consider the boundary of the disturbed area to
coincide with the isoseismal 4, its area would be 98,000 square
miles, or 1-2/3 that of England and Wales; if it were a circle
concentric with the isoseismal 4, and passing through Middlesbrough,
its area would be 115,000 square miles, or nearly twice that of
England and Wales; while, if it passed through Killeshandra, its area
would be 185,000 square miles, or more than three times the area of
England and Wales.[62]
_Position of the Originating Fault._--The form, directions, and
relative positions of the isoseismal lines furnish important evidence
with regard to the originating fault. We conclude in the first place
that its mean direction is parallel to the longer axes of the three
innermost isoseismal lines--that is, north-west and south-east, or,
more accurately, W. 43° N. and E. 43° S.[63] In this case, the
elongated forms of the isoseismal lines cannot be attributed to
variations in the nature of the surface rocks. The district embraced
contains about 13,000 square miles, and it is improbable that the axes
of the three isoseismals should retain their parallelism over so large
an area, if these variations had any considerable effect. Moreover, in
the same district, an earthquake occurred in 1863, whose meizoseismal
area was elongated from north-east to south-west, or almost exactly
perpendicular to the direction in 1896.
Secondly, it will be noticed (Fig. 60) that the isoseismal lines are
not equidistant from one another. On the north-east side, they are
separated by distances of 20, 34, 55, and 51 miles; and on the
south-west side by distances of 13-1/4, 25, 60, and 77 miles. It
follows from this that the fault-surface must hade or slope towards
the north-east; for, near the epicentre, the intensity is greatest and
dies out more slowly on the side towards which the fault hades.
If we could ascertain any one place through which the fault passed,
its position would thus be completely determined. Unfortunately, there
is no decisive evidence on this point. There are, however, several
places to the south-west of Hereford where the intensity of the shock
was distinctly less than in the surrounding district, and it is
possible that this was due to their neighbourhood to the fault-line
(see p. 135). If so, the originating fault must have extended from a
point about a mile and a half west of Hereford for a distance of about
16 miles to the south-east; and a fault in this position would
certainly satisfy all the details of the seismic evidence.
NATURE OF THE SHOCK.
Throughout the disturbed area, considerable variations were observed
in the nature of the shock. These changes were due to the mere size of
the focus, to its elongated form and, as will be seen, to its
discontinuity, and also to the distance of the place of observation
from the epicentre.
At places near the epicentre, rapid changes in the direction of the
shock were observed owing to the large angle subtended by the focus;
while, at considerable distances, this angle being small, the changes
of direction were imperceptible. A further variation with the distance
was an increase in the period of the vibrations. Close to the
epicentre, the general impression was that of crossing the wake of a
steamer in a very short rowing-boat, or of riding in a carriage
without springs. At distances of a hundred miles or more, the movement
is described as being of a pleasant, gentle, undulating character,
like that felt during the rocking of a ship at anchor or in a carriage
with well-appointed springs.
The most remarkable feature of the shock, however, was its division
into two distinct parts or series of vibrations, separated by an
interval, lasting two or three seconds, of absolute rest and quiet.
And this was no mere local phenomenon. With the exception of a narrow
band that will be referred to presently, records of the double shock
come from nearly all parts of the disturbed area, even from districts
so remote as the Isle of Man and the east of Ireland. The two parts
differed in intensity, in duration, and in the period of their
constituent vibrations. For instance, at Oaklands (near Chard), a
shivering motion was first felt, and then, after about three or four
seconds, a distinct rocking from side to side. At Exeter, there was a
sudden tremor lasting about two seconds, followed, after two or three
seconds, by another and more severe shaking lasting four or five
seconds. Again, at West Cross (near Swansea), an undulatory movement
for about four seconds was followed soon after by a tremulous shock.
At Liverpool, the durations of the first part, interval, and second
part were respectively estimated at about six, two, and four seconds.
As a first result of the observations, then, it appears that in the
south-east half of the disturbed area, the second part of the shock
was the stronger, of greater duration and consisted of longer-period
vibrations (as at _a_, Fig. 61); while, in the north-west half, the
same features characterised the first part of the shock (_b_, Fig.
61). A closer examination of the records shows, however, that the
boundary between the two portions of the disturbed area was not a
straight line, but slightly curved, the concavity facing the
south-east. The broken line on the map (Fig. 60), which is hyperbolic
in form, represents roughly the position of this curved boundary.[64]
[Illustration: FIG. 61.--Nature of shock of Hereford earthquake.]
Along this hyperbolic boundary-line, or rather within a narrow band of
which it is the central line, the shock lost its double character, and
was manifested as a single series of vibrations gradually increasing
in intensity and then dying away. Close to the edges of this band,
careful observers were able to distinguish two maxima of intensity
connected by a continuous series of tremors (_c_, Fig. 61). Thus,
within the band, the two series of vibrations, which elsewhere were
isolated, must have been superposed on one another; while, near the
edges of the band, the concluding tremors of the first series
overlapped the initial tremors of the second.
_Origin of the Double Series of Vibrations._--The Hereford earthquake
thus belongs to the same class as the Neapolitan, Andalusian,
Charleston, and Riviera earthquakes. As in these cases, the hypothesis
of a single focus is inadmissible. The division of the disturbed area
into two regions of opposite relative intensity, duration, etc., is
sufficient proof that a single series of vibrations was not duplicated
by reflection or refraction, or by separation into longitudinal and
transverse waves. It is equally conclusive against a repetition of the
impulse within the same focus. We must therefore infer that the focus
consisted of two nearly or quite detached portions arranged along a
north-west and south-east line, and that the impulse at the north-west
focus was the stronger of the two. The only question that remains to
be decided is whether the impulses at the two foci were simultaneous
or not.
Now, if the impulses occurred at the same instant, the waves from the
two foci would travel with the same velocity, and would therefore
coalesce along a straight band which would bisect at right angles the
line joining the two epicentres. But we have already seen that this
band is curved, and it thus follows that the two impulses were not
simultaneous. Again, since the concavity of the hyperbolic band faces
the south-east, the waves from the north-west focus must have
travelled farther than those from the south-east focus before the two
met along the hyperbolic band; in other words, the impulse at the
north-west focus must have occurred two or three seconds before the
impulse at the other.
_Position and Dimensions of the Two Foci._--There can be little doubt
that the impulse at the north-west focus was responsible for the
greater damage to buildings at Hereford, Dinedor, Fownhope, etc. The
centre of its epicentral area must therefore lie about three miles
south-east of Hereford. It is probable, also, that the corresponding
centre of the other focus is similarly placed with respect to the
south-east portion of the isoseismal 8--that is, about two or three
miles north-east of Ross. These two points are eight or nine miles
apart. Now, since, as we shall see, the mean surface-velocity of the
earth-waves was about 3000 feet per second, and the mean duration of
the quiet interval between the two series was 3-1/2 seconds, the
nearest ends of the two foci must have been separated by a distance of
not less than two miles. Moreover, since the series of vibrations from
the north-west or Hereford focus lasted a few seconds longer than that
from the south-east or Ross focus, the former must have been about two
miles longer than the latter, and we may therefore estimate their
lengths at about eight and six miles respectively. Including the
undisturbed intermediate portion, this would give a total length of
focus of about 16 miles, a result we have already inferred from the
dimensions of the isoseismal 8.
DIRECTION OF THE SHOCK.
Although no question was asked with regard to the direction of the
shock, no fewer than 469 observers made notes on this point. As a
general rule, their determinations are extremely rough, few referring
to more than the eight principal points of the compass. Moreover, in
any one place, the directions assigned to the shock are very varied.
For instance, in the city and suburbs of Birmingham, eight observers
give the direction along a north and south line, eight east and west,
eleven north-west and south-east, and five north-east and south-west,
while there are five other intermediate estimates. But, when these
directions are plotted on a map of the district, it is seen at once
that they are either nearly parallel or perpendicular to the roads in
which the observers were living; that is, the apparent direction of
the shock was at right angles to one of the principal walls of the
house. This, of course, is a result to be anticipated, for, whatever
be the direction of the earthquake-motion, a house tends to oscillate
in a plane perpendicular to one or other of its walls.
It is extraordinary to how great a distance the direction of the shock
is perceptible. Records come from Brighton (137 miles from the
epicentre), Maldon in Essex (144 miles), Harrogate (147 miles),
Douglas in the Isle of Man (167 miles), Dublin (176 miles), and
Baltinglass in Co. Wicklow (180 miles).
Nevertheless, whatever the distance may be, the sense of direction
must be most perceptible in those houses whose principal walls are at
right angles to the true direction of the earthquake-motion, and we
should therefore expect to find the observations of direction most
frequently made in such houses, or in others which approximate to this
situation. Thus, the average of all the observations within a fairly
small area should give a result not very far from the true direction
of the shock; and, the smaller the area and the farther from the
epicentre, the more reliable should be the result. Now, in Birmingham
the mean direction of the shock is E. 39° N., which differs only by 2°
from the line joining the city to the epicentre; in London it is E.
21° S., the difference being again 2°. In other cases, the
observations from different counties are grouped together, and the
mean direction is taken to correspond to the centre of the county.
Yet, even then, there is often a close agreement between the mean
direction of the shock and the direction of the county-centre from the
epicentre; the difference being not more than two or three degrees in
the counties of Buckingham, Devon, Stafford, Warwick, and York. In
other cases, where the deviation exceeds this amount, either the
number of observations is small or the county is near the epicentre
and so subtends a large angle.
Two results of some importance follow from this analysis: (1) that
while, with a few isolated observations, the "method of directions" is
almost sure to fail, with a large number of observations closely
grouped, the position of the epicentre may be determined with a fair
approach to accuracy; and (2) that, at any rate outside a radius of
forty miles, the earth-waves travelled in approximately straight lines
outwards from the epicentre.
COSEISMAL LINES AND VELOCITY OF EARTH-WAVES.
Coseismal lines were defined by Mallet as long ago as 1849, but, owing
to the difficulty of ascertaining the correct time, they have so far
been of little service in the investigation of earthquakes. In the
case of the Hereford earthquake, the distances traversed by the
earth-waves are small; but, on the other hand, the time-records are
numerous and frequently trustworthy to the nearest minute. Rejecting
all estimates earlier than 5.32 A.M., and later than 5.36, as well as
a number at 5.35, there remain fairly good observations from 381
places, and exceptionally accurate ones from 33 places. The latter
were obtained from signalmen and other careful observers who were in
possession of Greenwich time, or who compared their watches shortly
afterwards with well-regulated watches.
With evidence so abundant, a new method of drawing coseismal lines
becomes possible. According to this method, each place of observation
is indicated on the map by a mark corresponding to the particular
minute recorded. If the records were quite correct, there would be a
central area occupied by the marks corresponding to 5.32 A.M.,
surrounded by a series of zones in which the times were respectively
5.33, 5.34, and 5.35. The curves separating these zones would be
coseismal lines corresponding to the times 5.32-1/2, 5.33-1/2, and
5.34-1/2.
Owing, however, to the inevitable inaccuracy of all the time-records,
these different zones intrude on one another, and the coseismal lines
have therefore to be drawn about half-way through the overlapping
regions, special weight being attributed to the apparently more
accurate observations.
[Illustration: FIG. 62.--Coseismal lines of the Hereford
earthquake. (_Davison._)]
The coseismal lines obtained in this manner are represented by the
continuous curves in Fig. 62. The isoseismals, which are added for the
sake of comparison, are indicated by the dotted lines. It will be seen
that the coseismal lines are elongated in the same direction as the
isoseismals, but to a less extent, and this no doubt is due to the
fact that the epoch selected by the majority of observers was one not
far from, and slightly preceding, that of the maximum intensity of
the shock.
Now, the average distance between the two inner coseismals is 32-3/4
miles, between the two outer ones (so far as drawn) 35-1/6 miles, and
between the first and third 67-1/6 miles. The mean surface-velocity
between the two inner coseismals is therefore 2,882 feet per second,
and between the two outer ones 3,095 feet per second. There is thus an
apparent increase in the velocity with the distance, but the accuracy
of the coseismal lines is unequal to establishing this as a fact. The
mean surface-velocity of 2,955 feet per second between the first and
third coseismals is probably, however, the most accurate estimate of
the surface-velocity yet made in a slight earthquake.
SOUND-PHENOMENA.
_Nature of the Sound._--The sound which accompanied the shock was of
the same character as that heard during all great earthquakes. It is
often described in such terms as a deep booming noise, a dull heavy
rumble, a grating roaring noise, or a deep groan or moan; more rarely
as a rustling or a loud hissing rushing sound. As a rule, it began
faintly, increased gradually in strength, and then as gradually died
away; and this no doubt is the reason why it sometimes appeared as if
an underground train or waggon were approaching quickly, rushing
beneath the observer, and then receding in the opposite direction.
Occasionally, the sound was very loud, being compared to the noise of
many traction-engines heavily laden passing close at hand, or to a
heavy crash or peal of thunder. But its chief characteristic was its
extraordinary depth, as if it were almost too low to be heard.
According to one observer, it was a low rumbling sound, much lower
than the lowest thunder; and another compared it to the pedal notes of
a great organ, only of a deeper pitch than can be taken in by the
human ear, a noise more _felt_ than heard. It will be seen presently
how the sound, from its very depth, was inaudible to many persons.
A few observers described the sound in terms like those quoted above,
but by far the larger number compared it to some more or less
well-known type, and in many cases the resemblance was so close that
the observer at first attributed it to the object of comparison. The
descriptions, which present great varieties in detail, may be
classified as follows: (1) One or several traction-engines passing,
either alone or heavily laden, sometimes driven furiously past; a
steam-roller passing over frozen ground or at a quicker pace than
usual; heavy waggons driven over stone paving, on a hard or frosty
road, in a covered way or narrow street, or over hollow ground or a
bridge; express or heavy goods trains rushing through a tunnel or deep
cutting, crossing a wooden bridge or iron viaduct, or a heavy train
running on snow; the grating of a vessel over rocks, or the rolling of
a lawn by an extremely heavy roller; (2) a loud clap or heavy peal of
thunder, sometimes dull, muffled or subdued, but most often distant
thunder; (3) a moaning, roaring, or rough, strong wind; the rising of
the wind, a heavy wind pressing against the house; the howling of wind
in a chimney, a chimney or oil-factory on fire; (4) the tipping of a
load of coal, stones, or bricks, a wall or roof falling, or the crash
of a chimney through the roof; (5) the fall of a heavy weight or tree,
the banging of a door, only more muffled, and the blow of a wave on
the sea-shore; (6) the explosion of a boiler or cartridge of dynamite,
a distant colliery explosion, distant heavy rock-blasting and the boom
of a distant cannon; (7) sounds of a miscellaneous character, such as
the trampling of many men or animals, an immense covey of partridges
on the wing, the roar of a waterfall, the passage of a party of
skaters, and the rending and settling together of huge masses of rock.
The total number of comparisons made was 1,264. Of these, 45.4 per
cent. refer to passing waggons, etc., 15.0 per cent. to thunder, 15.5
to wind, 3.9 to loads of stones falling, 2.7 to the fall of a heavy
body, 7.2 to explosions, and 10.3 per cent. to miscellaneous sounds.
Generally, the sound adhered throughout to one of the types mentioned
above, and, if it varied at all, varied only in intensity. At some
places, however, the character of the sound was observed to change.
For instance, one person described it as like the rumbling of a train
going over a bridge, with a terrific crash, such as is heard in a
thunderstorm at the instant when the shock was strongest, the rumbling
dying away afterwards for some seconds.
_Inaudibility of the Sound to some Observers._--The total number of
observers who give a detailed account of the earthquake is 2,681, and,
of these, 59 per cent. state that they heard the sound, 23 per cent.
give no information, while 18 per cent. distinctly say that they
heard no sound; that is, roughly, out of every five observers, three
heard the sound, one made no reference to it, and one failed to hear
the sound.
In a few cases, no doubt, this failure was due to the distance of the
observer, but this is far from being a complete explanation; for, in
Herefordshire, six out of 179, and in Gloucestershire 17 out of 227,
observers heard no sound. Nor is the peculiarity a local one, for at
Clifton two out of five observers who were awake did not hear the
sound, at Birmingham four out of 23, and in London, eight out of 18.
Even in the same house, it would happen that one observer would hear a
sound as of a heavily-laden traction-engine passing, while to another
it was quite inaudible.
Again, a large number of observers who heard the sound expressly state
that they were unconscious of any while the shock lasted. The noise at
first resembled the approach of a steam-roller or traction-engine up
the street, it became gradually louder, and then ceased more or less
suddenly as the shock began; while, to others in the same places, the
sound continued to grow in loudness until the strongest vibrations
were felt.
Even when observers in the same place agreed in hearing the sound, it
presented itself to them under different aspects. Thus, at Hereford, a
crash or bomb-like explosion was noticed by some, but not by all,
observers; at Ledbury, the sound according to one began like a rushing
wind and culminated in a loud explosive report, another heard a noise
like distant thunder, which ended when the shock began, while a third
heard no sound at all. At places more distant from the epicentre, the
same diversity, both in character and intensity, is manifested. Thus,
at Birmingham, the accounts refer on the one hand to the distant
approach of a train and the rising of the wind, on the other to the
reports of large cannons and to a noise as if tons of _débris_ had
been hurled against the wall of the house; at Bangor, to muffled
thunder, wind through trees, and a loud rumbling sound.
The first explanation of these apparent anomalies which presents
itself is inattention on the part of the observers; but it is one that
will not bear examination, though it may apply in some cases. The
sound is too loud, at any rate near the epicentre, to escape notice,
and it is generally heard before the shock begins to be felt.
Moreover, as described in the last chapter, three out of every four
earthquakes in Japan are unaccompanied by recorded sound, and the
Japanese as a race cannot be accused of such constant inattention. The
defect, it can hardly be doubted, is inherent to the observer, and not
dependent on the conditions in which he is placed.
That the higher limit of audibility varies with different persons has
long been known; and there can be no reason for doubting that there is
a similar variability in the lower limit. Thus, to some observers, the
sound remains inaudible throughout, however intently they may be
listening. Again, it is found that, the deeper the sound, the greater
must be the strength of the vibrations required to render them
audible. As the vibrations which reach an observer increase in period,
it may therefore happen that, sooner or later, the strength of some
does not attain or exceed that limiting value, and, at that moment,
the sound will cease to be heard. Moreover, for vibrations of a given
period, this limiting value varies for different persons. Thus, to one
observer, the sound may become inaudible, while another may continue
to hear it. Lastly, the vibrations which affect an observer at any
moment are of various strength and period. One may hear all perhaps,
while a second may be able to hear some and not others. Thus, to one
observer, the sound may be like a rising wind, to another like a heavy
traction-engine passing; one may hear the crashes which accompanied
the strongest part of the shock, while a second may be deaf to the
same vibrations; to one the sound may become continually louder and
cease abruptly, to another it may increase to a maximum and then die
away.
_Sound-Area._--While the sound was a very prominent feature of the
earthquake in and near the epicentral area, records at a great
distance are naturally difficult to obtain, and, on this account, the
number of stations for determining the boundary of the sound-area is
too small to allow of it being accurately drawn. As a rule, however,
it must lie between the isoseismals 5 and 4, but it is less nearly
circular than either of these lines. Its length, from north-west to
south-east, is 320 miles, its breadth 284 miles, and the area
contained by it about 70,000 square miles, or roughly two-thirds that
of the disturbed area.
_Isacoustic Lines._--The dotted lines in Fig. 60 represent isacoustic
lines--that is, lines which pass through all places where the
percentage of observers who recorded their perception of the sound is
the same. For instance, if we take any point in the line marked 80 and
describe a small circle with that point as centre, then 80 per cent.
of the observers within that circle would hear the earthquake-sound.
The isacoustic lines thus show how the audibility of the sound varies
throughout the sound area. To draw the curves with a close approach to
accuracy, the unit of area should be small and of constant dimensions;
but, in the present case, owing to the comparative paucity of the
observations, a smaller unit than the county would give unreliable
results.[65] At the centre of each county, the sound audibility may be
regarded as proportional to the percentage of the total number of
observers within the county who distinctly heard the sound. To draw
the curve marked 50, the centre of every county in which the average
percentage is less than 50 is joined to the centres of those adjoining
counties in which it is above 50, and these lines are then divided in
the proper ratio so as to give a point where the percentage would be
exactly 50. A number of points at which the percentage is 50 is thus
obtained, and the curve drawn through them is the required isacoustic
line. The percentage of audibility varies from 87 in Herefordshire to
23 in Essex and the east of Ireland, but the only isacoustic lines
which can be completely drawn are those that correspond to the
percentages between 80 and 50 inclusive.
The peculiar form of the isacoustic lines will be evident at a glance.
They bear little relation to the isoseismal lines. Their greatest
extensions are not along the axes of those lines, but in two
directions which are a little east of north-east and south of
south-west. They lie indeed along a hyberbolic line which, towards
the south-west, agrees closely with the curvilinear axis of the
hyperbolic band represented by the broken line in Fig. 60. Towards the
north-east, the coincidence is not so close, but this is chiefly owing
to the magnitude of the northern counties, which causes a deflection
of the isacoustic lines towards the north.
It will be remembered that the hyperbolic band is the area within
which the vibrations from the two foci were superposed. Now, the sound
accompanied each part of the shock, and ceased entirely during the
interval between them. Also, the stronger series of vibrations was
accompanied by the louder sound; but, while the difference in strength
was considerable between the two parts of the shock, it was very
slight between the two sounds. There is therefore no marked distortion
of the isoseismal lines when crossing the hyperbolic band, while the
isacoustic lines are completely diverted from their normal course.
Thus, the study of the isacoustic lines strongly confirms the
conclusions at which we have arrived above (p. 223)--namely, that
there were two distinct foci arranged in a north-west and south-east
line, and that the impulse at the former focus occurred a few seconds
earlier than that at the latter.[66]
_Variations in the Nature of the Sound throughout the Sound-area._--In
one respect, the sound exhibited a marked uniformity all over the
sound-area--namely, in its great depth; the word "heavy" being used in
one out of every four accounts of the sound, whether close to the
epicentre or near the boundary of the sound-area.
The type of comparison employed varies in different parts of the
sound-area. As we recede from the origin, the sound becomes on the
average less like thunder or explosions and more like wind. The
references to passing waggons, etc., are so numerous that it is
possible to draw curves, in the same way as isacoustic lines, which
represent equal percentages of comparison to this type out of the
total number of comparisons. The curves are somewhat incomplete, but
it is noteworthy that those corresponding to the higher percentages
cling to the extremities of the hyperbolic band, probably because the
uninterrupted duration of the sound is greater there than elsewhere.
The effect of distance from the epicentre, however, is most noticeable
in connection with changes in the character of the sound. It is only
on the immediate neighbourhood of the origin that the explosive
reports or crashes were heard in the midst of the rumbling sound. At a
moderate distance, the sound before and after the shock became
smoother, while the sound which accompanied the shock retained to a
certain extent its rougher and more rumbling or grating character.
Close to the boundary of the sound-area, the irregularities were still
further smoothed away, and the only sound heard was like the low roll
of distant thunder.
The explanation of these changes depends on the fact that, as we
recede from the epicentre, the vibrations of every period tend to
become inaudible. The limiting vibrations of the whole series will be
the first to be lost, especially those of the longest period. Thus,
near the epicentre, sound-vibrations of many different periods will be
heard, and the sound will be more complex than it is elsewhere. The
greater the distance, the narrower are the limits with regard to
period between which the audible vibrations lie, until, near the
boundary of the sound-area, the sound becomes an almost monotonous
deep growl of nearly uniform intensity.
_Time-relations of the Sound and Shock._--The principal epochs to be
compared are the beginning, the epoch of maximum intensity, and the
end. The beginning of the sound preceded that of the shock in 82 per
cent. of the observations on this epoch, coincided with it in 12, and
followed it in 6 per cent.; the epoch of maximum intensity preceded
that of the shock in 21 per cent. of the records, coincided with it in
73, and followed it in 6 per cent.; while the end of the sound
preceded that of the shock in 22-1/2 per cent., coincided with it in
27-1/2, and followed it in 50 per cent. Thus, as a general rule, the
beginning of the sound preceded that of the shock, the sound was
loudest when the shock was strongest, and the end of the sound
followed that of the shock. In other words, the duration of the sound
was in most cases greater than that of the shock.
MINOR EARTHQUAKES.
Of the twelve undoubted minor earthquakes, nine occurred before, and
three after, the principal shock, the times of the first eleven lying
between limits about seven hours apart. With three exceptions, the
records are insufficient to determine the positions of the epicentre
with any approach to exactness.
The first occurred at about 11 or 11.30 P.M. on December 16th. The
boundary of the disturbed area, which coincides nearly with that of
the fifth shock (E, Fig. 63), is 97 miles long from north-west to
south-east, 83 miles wide, and contains about 6,300 square miles. The
focus was apparently situated between the two foci of the principal
earthquake and partly coincided with them.
[Illustration: FIG. 63.--Map of minor shocks of Hereford
earthquake. (_Davison._)]
Then came three slight shocks (at about 1 A.M. on December 17th, 1.30
or 1.45 A.M., and 2 A.M.), about which little is known except that
they probably originated somewhere near the Ross focus.
The fifth shock (E, Fig. 63) occurred at about 3 A.M., and disturbed
an area 104 miles in length, 79 miles in width, and about 6,400 square
miles in area. Its boundary occupies approximately the position that
would be taken by an isoseismal of intensity between 7 and 6 of the
principal earthquake. We may therefore infer that this shock and the
principal earthquake were caused by slips along the same fault and in
about the same region of the fault. Also, as there is no evidence of
discontinuity in the vibrations of the minor shock, it is probable
that the focus was continuous, and occupied the space between the two
foci of the principal earthquake, as well as part or the whole of both
these foci.
The next four shocks occurred at about 3.30, 4, 5, and 5.20 A.M., and
were more closely associated with the Ross than with the Hereford
focus, and then followed the principal earthquake at 5.32 A.M.
A few minutes later, at 5.40 or 5.45 A.M., a very slight shock was
felt, the focus of which was possibly situated in the central region
between the two foci. The next, at about 6.15 A.M. (K, Fig. 63),
disturbed an area 41 miles long, 27 miles broad, and containing about
870 square miles. Its focus must have coincided approximately with the
Ross focus of the principal earthquake, and this was also the case
probably with the last shock of all, which occurred on July 19th,
1897, at 3.49 A.M.
ORIGIN OF THE EARTHQUAKES.
The greater part of the epicentral district is covered by a sheet of
Old Red Sandstone (Fig. 64), but, just to the north-east of the
position laid down for the originating fault (indicated by the
straight broken line), is the well-known Woolhope anticlinal, by which
Silurian beds are brought to the surface. The anticlinal axis runs
approximately north-west and south-east, and is thus roughly parallel
to the earthquake-fault. Moreover, the thinning-out and occasional
disappearance of some of the Silurian beds on the south-west side of
the anticlinal (as compared with those on the north-east side) is
suggestive of a north-west and south-east fault or rapid flexure at or
near the south-west junction of the Old Red Sandstone and the
Silurian strata. If it be a fault, it must hade to the north-east, and
would therefore satisfy two of the conditions determined by the
seismic evidence. It would lie, however, about two miles too far to
the north-east, being in fact to the north-east of the villages which
suffered most from the earthquake.
[Illustration: FIG. 64.--Geology of meizoseismal area of Hereford
earthquake. (_Davison._)]
But only a few miles to the south-east of the Woolhope anticlinal, and
almost in the same line with it, there is a second anticlinal, that of
May Hill. This is a triangular area, and is known to be bounded on all
three sides by faults. The fault on the north-east side has an average
north-west and south-east direction, and, if it were continued through
the Old Red Sandstone towards the north-west, but bending at first a
few degrees more to the west, it would pass through a point about
1-1/2 miles west of Hereford. It is worthy of notice that both this
fault and another nearly parallel to it, about half-a-mile farther
north-east, stop, according to the Geological Survey map, at the
points where they enter the Old Red Sandstone. The latter is an area
which has never been investigated with thoroughness by modern
stratigraphical methods, and in which it is difficult to trace faults.
It therefore appears not improbable that the earthquakes were due to
slips along a continuation of this fault.
Whether this be the case or not, however, it is clear that the
earthquake-fault must pass between the anticlinal areas of Woolhope
and May Hill, the former being on the north-east, and the latter on
the south-west, side of the fault. At the Hereford focus, the fault
must hade to the north-east; and, at the Ross focus, it is probable,
from the distribution of places where damage occurred to buildings,
that it hades to the south-west If this be the case, the fault must
change in hade between the two foci.
How long a time had elapsed since the last sign of growth in the
earthquake-fault took place, it is impossible to say; but it must be
many years in length. During this interval, the stresses tending to
produce movement along the fault-service had been gradually
increasing, until they were sufficient to overcome the resistance
opposed to them. It is worthy of notice that the earliest perceptible
movements were slight. Their function seems to have been to prepare
the way for the great slips by equalising the difference between
stress and resistance over a large area of the fault-surface. We
cannot trace with accuracy the transference of the seat of movement
from one part of the fault-surface to another. The first slip seems to
have taken place chiefly in the region between the two foci of the
principal earthquake; possibly it overlapped both of them partly. The
next three slips were apparently in the neighbourhood of the Ross
focus, and were followed by a fifth in the same area as the first.
Then came a series of small movements that we cannot locate further
than by saying that they were more closely connected with the Ross
focus than the other.
In consequence of the preliminary slips within and near the Ross
focus, the effective stress in that portion of the fault was
diminished; and this may be the reason why the first great slip took
place at the Hereford focus. The immediate result of such a movement
would naturally be an increase of stress in and beyond the terminal
regions, and the next slip might have been expected in an area partly
overlapping the Hereford focus, and either to the north-west or
south-east of it. Instead of this, for a distance of two miles in the
latter direction, there was not the least perceptible movement during
the principal earthquake, and the second great slip occurred in the
region beyond occupied by the Ross focus. This second slip, moreover,
occurred within two or three seconds after the other; that is, before
the earth-waves had time to travel from the Hereford to the Ross
focus. In other words, the slip at the Ross focus was not a
consequence of the slip at the Hereford focus; but both were due to a
single generative effort.
Now, a section drawn parallel to the earthquake-fault and on the
north-east side of it, would show an anticline near the Hereford focus
and a corresponding syncline near the Ross focus, with an undisplaced
portion in the intermediate region; while a parallel section on the
other side of the fault would show a syncline near the Hereford focus,
an anticline near the Ross focus, and again an undisplaced portion in
the intermediate region. If further movements tending to accentuate
such a structure were to occur (that is, if the anticlinals were to be
made more anticlinal and the synclines more synclinal), there would
therefore be two slips, one in each focus; while, along the
fault-surface between, there would be practically no displacement. At
any rate, the earlier stresses in that region may have been fully
relieved by two slight preliminary slips (those causing the first and
fifth minor earthquakes), and those resulting from the great
displacements by the first after-slip which followed in about ten
minutes.
Half-an-hour later, another slip took place at the Ross focus, and by
this the equilibrium of the rock-masses was almost completely
restored; for we have no certain evidence of any further movements
until seven months have elapsed (July 19th, 1897), when there was a
final slip in the same region of the fault.
THE INVERNESS EARTHQUAKE OF SEPTEMBER 18TH, 1901.
Between the north-east end of Loch Ness and the Moray Firth at
Inverness, there lies a tract of land not more than seven miles in
length, which is notable as one of those most frequently shaken by
earthquakes in the British Islands. In the intensity of its shocks it
is inferior to the south-east of Essex and the centre of
Herefordshire, and, in mere number, to the celebrated village of
Comrie in Perthshire. But, in the interest of its seismic phenomena,
in the light which they cast on the development of the earth's crust,
the neighbourhood of Inverness has no equal in Great Britain, and not
many superiors in any part of the world.
For this importance from a seismological point of view, the district
is indebted to the great fault which traverses Scotland along the line
of the Caledonian Canal, and to the fact that this fault, although it
dates from Old Red Sandstone times, has not yet finished growing. As
results of its formation, we have the almost straight cliff along the
south-east coast of Rossshire, and the long chain of lakes, beginning
with Loch Dochfour and Loch Ness, and ending with Loch Oich, Loch
Lochy, and Loch Linnhe. As evidences of its persistent though
intermittent growth, we have the slight tremors and earth-sounds
occasionally observed at and near Fort William, and the much stronger
shocks felt in the neighbourhood of Inverness.
During the nineteenth century there were three strong earthquake
shocks in this district. The first and most severe occurred on August
13th, 1816, and was felt over the greater part of Scotland; the second
on February 2nd, 1888; and the third and weakest on November 15th,
1890. This last shock was followed by several slighter ones, the
series ending with a rather smart shock on December 14th. Between this
date and the summer of 1901 no earthquakes seem to have been felt at
or anywhere near Inverness.
PREPARATORY SHOCKS.
The date of the first shock of 1901 is not quite certain. One is said
to have been felt at Aldourie (see Fig. 66) some time in June, and a
second at Dochgarroch in July. These may have been succeeded by others
too slight to attract much notice, but the first to be generally
observed occurred on September 16th at 6.4 P.M. A weak tremor,
accompanied by a faint sound, was perceived over a nearly circular
area about 12 miles in diameter, and with its centre about 1-1/2 miles
south of Dochgarroch. On the next day, at 11 P.M., a quivering lasting
two seconds was felt at Inverness, and a weak tremor, accompanied by
sound, at Dochgarroch at 1.15 A.M. on September 18th. Nine minutes
later, at 1.24 A.M., occurred the principal earthquake, the shock of
which would be called a strong one, even in Italy and Japan.
EFFECTS OF THE SHOCK.
In Inverness, the damage to buildings, though seldom serious, was by
no means inconsiderable. One brick building used as a smithy was
destroyed, several chimneys or parts of them fell, and many
chimney-cans were displaced or overthrown. At Dochgarroch and other
places within the meizoseismal area, walls were cracked, chimneys
thrown down, and lintels loosened.
But, for this country, an unusual effect of the earthquake was a long
crack made in the north bank of the Caledonian Canal near Dochgarroch
Lochs. It occurred in the middle of the towing-path, and could be
traced at intervals for a distance of 200 yards to the east of the
Lochs, and 400 yards to the west, being often a mere thread, and in no
place more than half-an-inch wide. Soon after its formation, however,
the fissure was obliterated by heavy showers of rain.
ISOSEISMAL LINES AND DISTURBED AREA.
The map (Fig. 65) shows the area over which the earthquake was
perceptible. The isoseismal lines are drawn partly continuous and
partly dotted--continuous where some confidence can be placed in their
accuracy, and dotted where their course must be regarded as doubtful,
owing to the rarity or absence of observations.
The innermost isoseismal (shown on a larger scale in Fig. 66)
corresponds to the intensity 8 of the Rossi-Forel scale, and includes
the places where the shock was strong enough to cause slight
structural damage to buildings. It is elliptical in form, 12 miles
long, 7 miles broad, and 67 square mile in area, with its centre at a
point about 1-1/2 mile east-north-east of Dochgarroch, and its longer
axis running N. 33° E. and S. 33° W.
[Illustration: FIG. 65.--Isoseismal lines of the Inverness
earthquake. (_Davison._)]
The remaining isoseismals are less accurately drawn, owing to the
scarcity of observations made in the west of Scotland. Except towards
the west, however, the course laid down for the isoseismal 7 may be
trusted. Its length is 53-1/2 miles, width 35 miles, and area 1,500
square miles. Its longer axis is almost exactly parallel to that of
the preceding isoseismal, but the distance between the two curves is 9
miles on the north-west, and 14 miles on the south-east, side. The
isoseismal 6 is 105 miles long, 87 miles wide, and contains 7,300
square miles; and the isoseismal 5, 157 miles long, 143 miles wide,
and about 17,000 square miles in area.
The isoseismal 4 may be regarded as the boundary of the disturbed area
of the earthquake, for, so far as known, the shock was not noticed at
any point outside it. Towards the north, it was felt at Wick,
Castletown, and other intermediate places; towards the west at
Tobermory in the island of Mull; and, towards the south, at Skelmorlie
(in Ayrshire), Paisley, Belsyde (near Linlithgow), Gullane (near North
Berwick), and Dunbar. Along the east coast of Scotland, between Wick
and Dunbar, there are few places of any size where the shock was not
felt. The disturbed area of the earthquake is thus 215 miles long from
north-east to south-west, 198 miles wide, and contains about 33,000
square miles.
_Position of the Originating Fault._--The only isoseismals which are
drawn accurately enough to determine the earthquake-fault are the two
inner ones, those marked 8 and 7; but these are sufficient for the
purpose. It is clear, from the direction of their longer axes, that
the average direction of the fault must be N. 33° E. and S. 33° W.
Again, the isoseismals are farther apart towards the south-east than
towards the north-west, implying that the fault hades to the
south-east. Lastly, as the intensity of the shock is greater on the
side towards which the fault hades, it follows that the fault-line
must lie a short distance (about a mile or so) on the north-west side
of the centre of the isoseismal 8.
Now, the great fault alluded to above occupies almost exactly the
position indicated by the seismic evidence. Its mean direction from
Tarbat Ness to Loch Linnhe is N. 35° E. and S. 35° W., it hades to the
south-east, and the fault-line passes through a point about
three-quarters of a mile to the north-west of the centre of the
isoseismal 8 (Fig. 66). There can be little doubt, therefore, that the
earthquake was caused by a slip of this fault; and the evidence of the
after-shocks, as will be seen, offers additional support to this
conclusion.
The region in which the slip took place may be determined roughly from
the position and form of the innermost isoseismal. Its centre must
have been close to the point marked A in Fig. 66, which corresponds to
a point about 1-1/2 mile east-north-east of Dochgarroch. In a
horizontal direction, its length must have been at least five or six
miles; otherwise, the isoseismal 8 would have been less elongated. It
must therefore have reached from about half-a-mile north-east of Loch
Ness to about half-a-mile south-west of Inverness. Its width, measured
along the dip of the fault-surface is unknown; but the small distance
between the centre of the isoseismal and the fault-line shows that the
principal movement took place at a depth which was probably under,
rather than over, one mile.
NATURE OF THE SHOCK.
We come now to the evidence afforded by the nature of the shock, in
which there was but little variation throughout the disturbed area. At
Inverness, a gentle movement was first felt, followed by an
extraordinary quivering, which increased in force for two or three
seconds, and then decreased for two or three seconds; just as the
quivering was about to cease, there was a distinct lurch or heave,
after which the vibration was much more severe than before and lasted
several seconds longer than the first part of the shock. Dalarossie
lies about fourteen miles south-east of Inverness, and here the first
indication was a loud sound, as of an express train, coming from the
east, rushing close to, and then under, the house; this lasted for a
few seconds, and towards the end of it the house vibrated. Then
succeeded an interval of quietness for about a second, followed by a
terrific burst or crash, not unlike the crash of a loud thunder peal,
of about two seconds' duration, during which the house distinctly
heaved up once and then sank back. After another brief interval of
quietness, there was a low rumble, like the sound of a dying peal of
thunder.
It will be noticed, in this account, that the two parts of the shock
were no longer consecutive. There was a short interval of rest between
them, the intermediate vibrations observed at Inverness being too weak
to be felt at Dalarossie. Still farther away, the extinction became
more marked. At Aberdeen, for instance, the shock consisted of two
parts, the first a tremble, followed, after an interval of a few
seconds, by a swinging movement of longer duration than the tremble.
In all parts of the disturbed area, the shock maintained the same
character of division into two parts, the second of which was of
greater duration and intensity than the first and consisted of
vibrations of longer period. A phenomenon of such wide occurrence was
clearly not due to local influences. It must have been caused by two
separate initial impulses, the stronger succeeding the other after an
interval of a few seconds and taking place in nearly the same region
of the fault.[67]
SOUND-PHENOMENA.
Outside the isoseismal 5, there are but few records of the
earthquake-sound; but it was heard faintly at Skelmorlie (in
Ayrshire), Belsyde (near Linlithgow), and Gullane (near North
Berwick). Towards the north, it was not observed beyond Wick and
Wathen (in Caithness). The boundary of the sound-area cannot be laid
down with any approach to accuracy, but it must have included a
district containing about 27,000 square miles.
Throughout the whole disturbed area, 84 per cent. of the observers
heard the sound. The percentage varies in different counties, from 93
in Inverness-shire to 77 in the counties of Perth and Aberdeen; but
the records in the more distant regions are too few to allow of the
construction of isacoustic lines.
In its character, the sound resembled that usually heard with strong
earthquakes, 39 per cent. of the observers having compared it to
passing waggons, traction-engines, etc., 25 per cent. to thunder, 14
to wind, 8 to loads of stones falling, 3 to the fall of heavy bodies,
4 to explosions or the firing of heavy guns, and 7 per cent. to
miscellaneous sounds. The intensity of the sound gradually diminished
outwards from the epicentre, and most rapidly near the isoseismal 7,
which abounds approximately the area in which the sound was very loud
from that in which it was distinctly fainter, and also includes nearly
all the places at which loud explosive crashes were heard with the
strongest vibrations.
In the time-relations of the sound and shock, the Inverness earthquake
resembles the Hereford earthquake of 1896. The beginning of the sound
preceded that of the shock in 72 per cent. of the records, coincided
with it in 20, and followed it in 8 per cent.; the epoch of maximum
intensity of the sound preceded that of the shock in 20 per cent. of
the records, coincided with it in 73, and followed it in 7 per cent.;
while the end of the sound preceded that of the shock in 15 per cent.
of the records, coincided with it in 34, and followed it in 52 per
cent.
Somewhat similar proportions hold over the greater part of the
disturbed area, the percentages being nearly the same in the counties
of Inverness, Ross, Nairn, Elgin, Banff, and the most distant
counties. But in Aberdeenshire an exception occurs, the three epochs
of sound and shock in most cases coinciding with one another. The
majority of the observations in this county come from the southern
part, and the line joining this district to the epicentre is nearly
perpendicular to the line of the earthquake-fault. This result has an
important bearing on the origin of the sound-vibrations. For, if the
general precedence of the sound with respect to the shock were due to
its superior velocity, the percentage of records in which the
beginning of the sound preceded that of the shock would vary only with
the distance, and not with the direction from the origin. Indeed,
with increasing distance from the origin, this percentage should
continually approach 100; while that in which the end of the sound
followed that of the shock should diminish to zero. There is, however,
no trace of either tendency, the sound being heard after the shock at
places close to the boundary of the sound-area. On the other hand, it
the sound-vibrations were to start simultaneously, or nearly so, from
all parts of the focus, but especially from its marginal regions,
then, in the greater part of the disturbed area, the sound would be
heard both before and after the shock; for the lateral margins of the
focus would be the portions nearest to, and farther from, most
observers; while, at places near the line through the epicentre at
right angles to the earthquake-fault, the three principal epochs of
the sound and shock should approximately coincide.
The inference that the sound-vibrations heard before and after the
shock come from the margins of the focus is also supported by the
observations on the relative duration of the sound and shock. If we
take only those records which are free from doubt, in 78 per cent. of
the total number, the duration of the sound was greater than that of
the shock; while, in Aberdeenshire, according to 93 per cent. of the
observers, the durations of sound and shock were equal.
We may imagine, then, that the slip within the seismic focus would be
greatest in a central region, and that it would die outwards in all
directions towards the edges. The friction arising from the slipping
in the central region would produce chiefly the comparatively large
oscillations that formed the perceptible shock; the evanescent creep
within the marginal regions would produce the small and rapid
vibrations that were sensible only as sound.
ORIGIN OF THE EARTHQUAKE.
While the seismic evidence enables us to determine the
surface-position and the horizontal dimensions of the seismic focus,
it unfortunately throws no light whatever on a point of some
importance--namely, the direction of the movement which caused the
earthquake. We cannot infer from it whether it was the rock on the
south-east or north-west side of the fault that slipped or whether
both sides slipped at once; nor, if that point had been settled, do we
know if the movement of the displaced side was upward or downward. In
the formation of the fault, however, it is clear that either the
south-east side has been depressed or the north-west side elevated;
and, as the bed of Loch Ness is below the level of the sea, that the
former movement has predominated. If the displacements which gave rise
to the earthquake were merely a continuation of the original series of
movements--and this is, to say the least, a very probable view to
take--then we may imagine that, for a distance of five or six miles,
and at a depth of about a mile or less, there was a sudden sag
downwards of the rock on the south-east side of the fault through a
distance which perhaps in no part exceeded a fraction of an inch.
Fig. 66 is an attempt to represent roughly the displacement which
caused the principal earthquake. The diagram makes no pretence to
accuracy, and the scale in the vertical direction is enormously
greater, perhaps a hundred thousand times greater, than that in the
horizontal direction. The straight line is supposed to represent a
straight line drawn before the earthquake on the surface of the rock
adjoining the fault on the south-east side and at a depth of about a
mile, and the curve the form of the same line after the earthquake.
[Illustration: FIG. 66.--Diagram to illustrate supposed
fault-displacement causing Inverness earthquake.]
The effect of this great slip would obviously be to relieve the stress
in the central region A, and to increase it suddenly in the parts
denoted by the letters B and C. It is, therefore, in these parts
especially that we should expect future slips to occur. Each slip
would of course give rise to an after-shock, and would in like manner
result in an increase of stress in its own terminal regions, though
chiefly on the side remote from the centre A.
THE AFTER-SHOCKS AND THEIR ORIGIN.
It is difficult to form any estimate of the total number of
after-shocks. The list, compiled from the records of careful observers
only, includes forty-six shocks and ten earth-sounds, the last of all
occurring on November 21st. But the list is certainly incomplete. It
contains, for instance, only one entry on September 18th between 3.56
and 9 A.M.; whereas, during the same interval, no fewer than eighteen
slight shocks were felt by one observer at Dochgarroch, while another
near Aldourie estimates the number of shocks up to October 23rd at
about seventy. The total number probably did not fall short of one
hundred.
The majority were certainly very slight, and, at another time, would
hardly have attracted any notice. There were, however, three of much
greater importance than the rest. These occurred on September 18th at
3.56 and 9 A.M., and on September 30th at 3.39 A.M. The isoseismal
lines of all three are elongated ovals, their longer axes are parallel
to the fault, and their centres lie on the south-east side of the
fault-line. The shocks were therefore evidently due to slips several
miles in length along the fault. At present, we are concerned more
with the position of their epicentres. These are indicated by the dots
lettered B, C, D in Fig. 67; the dot marked A denoting the centre of
the principal earthquake, and the continuous line the path of the
fault.
Thus, within two and a half hours, the great slip was followed by one
with its centre at B, near the south-west margin of the principal
focus. About five hours later, the scene of action was suddenly
transferred to a region with its centre at C on the north-east margin.
Both slips affected a portion of the fault-surface several miles in
length, and must therefore have increased the area of displacement,
slightly towards the north-east and considerably towards the
south-west. Only small movements occurred during the next twelve days
until 3.39 A.M. on September 30th, when another long slip took place,
with its centre at D, still farther to the south-west, and therefore
again extending the area and amount of displacement in this direction.
[Illustration: FIG. 67.--Map of epicentres of after-shocks of
Inverness earthquakes. (_Davison._)]
Turning now to the weaker after-shocks and earth-sounds, we find them
affecting chiefly three regions of the fault. One of these is close to
Dochgarroch, another near Inverness, and the third between Aldourie
and Drumnadrochit; the effects of the slips in the last two districts
being, as before, to extend the area of displacement a short distance
(perhaps half a mile) to the north-east and not less than six miles to
the south-west underneath Loch Ness.
The unequal division of the after-shocks between the two sides of the
principal centre (A, Fig. 67) is very marked. The positions of the
epicentres of forty-four shocks and earth-sounds can be determined
with more or less accuracy, and, of these, only ten lie to the
north-east of the principal centre, while thirty-four lie to the
south-west, six or seven of the latter being beneath Loch Ness.
One other point may be referred to before leaving these minor shocks.
So far as regards the stronger shocks, there was a continual decrease
in the depths of the seismic foci. This is shown by the progressive
approach of their epicentres towards the fault-line; the distances in
the three chief after-shocks being 1.7, 1.0, and 0.5 miles
respectively; and in one of the latest shocks (that of October 13th at
4.24 P.M., E, Fig. 67) the distance is no more than one-tenth of a
mile. The focus of this shock must, indeed, have been quite close to
the surface near Dochgarroch. This constant diminution in the depth of
the foci shows that the great slip was followed by a sudden increase
of stress upwards as well as laterally, and explains why that slip did
not leave any perceptible trace, either as fault-scarp or fissure, at
the surface.
SYMPATHETIC EARTHQUAKES.
It is remarkable that, of the 56 recorded after-shocks, at least six
were felt or heard only at Dalarossie and other places in the valley
of the Findhorn, a valley which lies about 13 or 14 miles to the
south-east of the great fault. That they had no connection with that
fault is certain, for two of them were so strong that, if they were so
connected, they could not have escaped the notice of one or more of
the watchful observers between Drumnadrochit and Inverness. The
probable explanation of these after-shocks is that they were due to
slips of a fault running along the Findhorn valley;[68] and that the
great displacement near Inverness on September 18th led to a sudden
increase of stress within the rocks for many miles around, which, at
and near Dalarossie, was sufficient to precipitate the slips referred
to.
CONCLUSION.
At first sight, two earthquakes could hardly be more unlike than the
Japanese earthquake of 1891 and the Inverness earthquake of 1901. In
the rice-fields of central Japan, as we have seen, the roads for many
leagues were edged with ruins, the fault-slip was prolonged up to the
surface and visible as a scarp forty, if not seventy, miles in length,
plots of ground were compressed and their boundaries altered, the
hillsides were scored by landslips, places can now be seen from one
another that formerly were hidden by a mountain ridge, and the total
number of after-shocks within little more than two years amounted to
above three thousand. On the other hand, when we examine the
distribution of the after-shocks in space, we find that, though no
part of the fault was exempt from slips, they favoured three regions
in particular--one, the most important, a central region, yet not
coincident with that in which the principal shock was most intense;
and the other two surrounding the extremities of the fault. With the
lapse of time, the after-shocks on the whole became weaker and
occurred less frequently, and the average depth of the foci gradually
diminished. Moreover, in two districts distant forty-five and
fifty-five miles from the fault, the frequency of the shocks during
the month succeeding the earthquake was suddenly increased to ten and
sixteen times the normal rate.
It is interesting to notice so close a similarity in character,
subsisting with so vast a difference in the scale of intensity. The
identity of the powers at work in shaping the structure of both
islands Is manifest. In Japan, we see the mountain-making forces
acting with violence and producing effects that are only too apparent
to the eye. In Scotland, whatever may have happened in former
geological epochs, the changes in surface-structure are now taking
place with almost infinite slowness, and hundreds or thousands of
years must elapse before Loch Ness makes any visible progress in its
march towards the sea.
REFERENCES.
1. DAVISON, C.--_The Hereford Earthquake of December 17, 1896._
(Birmingham, 1899.)
2. ---- "The Inverness Earthquake of Sept. 18, 1901, and its
accessory shocks." _Quart. Journ. Geol. Soc._, vol. lviii.,
1902, pp. 377-397.
FOOTNOTES:
[61] The study of the Hereford earthquake is based on 2,902 records,
coming from 1,943 places; that of the Inverness earthquake on 710
records from 381 places.
[62] The disturbed area of the Hereford earthquake of 1896 was
probably greater than that of any other British earthquake of the
nineteenth century; that of the Pembroke earthquake of 1892 being more
than 56,000 square miles, of the Pembroke earthquake of 1893 about
63,600 square miles, while that of the Essex earthquake of 1884 (a far
stronger shock in the meizoseismal area) is estimated at about 50,000
square miles.
[63] The approximate circularity of the two outer isoseismals is due
to the fact that the vibrations propagated to such great distances are
those which start from the comparatively small central region of the
focus.
[64] The above statement summarises the evidence of the majority of
the observers in each portion of the disturbed area. In this, as in
other similar cases, discrepancies in the observations are
unavoidable; but it is important to notice that they are least
frequent in the observations evidently made with the greatest care.
[65] Except in the case of Yorkshire, where the three Ridings are
regarded as separate counties.
[66] The Derby earthquake of March 24th, 1903, was also a twin
earthquake. The centres of the two foci were situated near Ashbourne
and Wirksworth, above eight or nine miles apart, along a line running
N. 33° E. and S. 33° W. The two parts of the shock coalesced along a
rectilineal band about five miles wide running centrally across the
lower isoseismals in a direction at right angles to their longer axes.
The isacoustic lines are also elongated in the direction of this band.
In this case, the impulses at the two foci must have taken place at
the same instant. (_Quart. Journ. Geo. Soc._, vol. lx., 1904, pp.
215-232.)
[67] If the foci of the two impulses had been detached, there would,
with so small an interval between the two parts, have been a variation
in the nature of the shock like that observed during the Hereford
earthquake.
[68] This part of Inverness-shire has not yet been mapped by the
Geological Survey, but a fault is known to exist in the Findhorn
valley near Drysachan Lodge, which lies about eleven miles down the
valley from Dalarossie.