A Study of Recent Earthquakes

by Charles Davison

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.