Chapter XI: 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.