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 22½ per cent., coincided with it in 27½, 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.