Part 18 (1/2)
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 Herefords.h.i.+re to 23 in Ess.e.x 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 pa.s.sing 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 princ.i.p.al 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 princ.i.p.al 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 princ.i.p.al earthquake and partly coincided with them.
[Ill.u.s.tration: 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 princ.i.p.al earthquake. We may therefore infer that this shock and the princ.i.p.al 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 s.p.a.ce between the two foci of the princ.i.p.al 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 a.s.sociated with the Ross than with the Hereford focus, and then followed the princ.i.p.al 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 princ.i.p.al 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.
[Ill.u.s.tration: 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 pa.s.s 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 pa.s.s 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.