A Study of Recent Earthquakes Part 17

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 pa.s.sing through Middlesbrough, its area would be 115,000 square miles, or nearly twice that of England and Wales; while, if it pa.s.sed 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 pa.s.sed, 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 s.h.i.+p 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 const.i.tuent vibrations. For instance, at Oaklands (near Chard), a s.h.i.+vering 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]

[Ill.u.s.tration: 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 cla.s.s 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 repet.i.tion 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 princ.i.p.al points of the compa.s.s. Moreover, in any one place, the directions a.s.signed 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 princ.i.p.al walls of the house. This, of course, is a result to be antic.i.p.ated, 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 Ess.e.x (144 miles), Harrogate (147 miles), Douglas in the Isle of Man (167 miles), Dublin (176 miles), and Baltingla.s.s in Co. Wicklow (180 miles).

Nevertheless, whatever the distance may be, the sense of direction must be most perceptible in those houses whose princ.i.p.al 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 a.n.a.lysis: (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.

[Ill.u.s.tration: 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 establis.h.i.+ng 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 rus.h.i.+ng 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, rus.h.i.+ng 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 pa.s.sing 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 cla.s.sified as follows: (1) One or several traction-engines pa.s.sing, either alone or heavily laden, sometimes driven furiously past; a steam-roller pa.s.sing 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 rus.h.i.+ng 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, m.u.f.fled 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 m.u.f.fled, and the blow of a wave on the sea-sh.o.r.e; (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 pa.s.sage of a party of skaters, and the rending and settling together of huge ma.s.ses of rock.

The total number of comparisons made was 1,264. Of these, 45.4 per cent. refer to pa.s.sing 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 Herefords.h.i.+re, six out of 179, and in Gloucesters.h.i.+re 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 pa.s.sing, 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 rus.h.i.+ng 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 _debris_ had been hurled against the wall of the house; at Bangor, to m.u.f.fled 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 j.a.pan are unaccompanied by recorded sound, and the j.a.panese 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 pa.s.sing; 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 pa.s.s 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.