Part 23 (2/2)

9. ---- ”On Tidal Periodicity in the Earthquakes of a.s.sam.” _Journ.

Asiat. Soc._, vol. lxxi., 1902, pp. 139-153.

FOOTNOTES:

[69] According to some reports, the earthquake was felt in Italy. At Livorno, the first movements were registered by seismographs at 11.17 A.M. (G.M.T.), and tremors were noticed by some persons at rest at about 11.15 A.M. At Spinea, a sensible undulatory shock from south-east to north-west, and lasting about four seconds, was felt at the moment when all the seismographs were set in motion by the Indian earthquake. In spite of the great distance, the perception of the earthquake in Italy is not impossible, but the records seem to me to refer to local tremors rather than to the very slow evanescent oscillations of a very distant earthquake.

[70] All the times in this section are referred to Madras mean time, which is 5h. 20m. 59.2s. in advance of Greenwich mean time. In the next section it will be found convenient to use the latter standard.

[71] It may be useful to give references to works in English in which the princ.i.p.al instruments for registering distant earthquakes are described. For Cancani's vertical pendulum, see _Brit. a.s.soc. Rep._, 1896, pp. 46-47; Darwin's bifilar pendulum, _Brit. a.s.soc. Rep._, 1893, pp. 291-303, and _Nature_, vol. 1., 1894, pp. 246-249; Milne's horizontal pendulum, _Seismology_, pp. 58-61; Rebeur-Paschwitz's horizontal pendulum, _Brit. a.s.soc. Rep._, 1893, pp. 303-308.

[72] The beginnings of the second and third phases are shown more clearly in the record of the vertical pendulum at Catania, a record, however, that will not bear the reduction necessary for these pages.

[73] _Geol. Mag._, vol. x., 1893, pp. 356-360.

[74] _Irish Acad. Trans._, vol. xxi, 1848, p. 52.

[75] _Irish Acad. Trans._, vol. xxi., 1848, pp. 55-57.

[76] _Neapolitan Earthquake of 1857_, vol. i., 1862, pp. 376-378.

[77] _j.a.pan Seismol. Soc. Trans._, vol. i., pt. II., 1880, pp. 33-35.

[78] _Geol. Mag._, vol. ix., 1882, pp. 257-265.

CHAPTER X.

CONCLUSION.

In this concluding chapter, I propose to give a summary of the results at which we have arrived from the study of recent earthquakes, and this can, I think, be done best by describing what may be regarded as an average or typical earthquake, though it may be convenient occasionally to depart slightly from such a course. Few shocks have contributed more to our knowledge than the majority of those described in this volume; but, on certain points, we gain additional information from the investigation of other earthquakes, and these are referred to when necessary for the purpose in view.

FORE-SHOCKS.

At the outset, we are met by a question of some interest and great practical importance--namely, whether there are any constant signs of the coming of great earthquakes by means of which their occurrence might be predicted and their disastrous effects mitigated.

Excluding the Ischian earthquakes, which belong to a special cla.s.s, it is evident that there is generally some slight preparation for a great earthquake. For a few hours or days beforehand, weak shocks and tremors are felt or rumbling noises heard within the future meizoseismal area. But, unfortunately, it has not yet been found possible to distinguish these disturbances from others of apparently the same character which occur alone, so that for the present they fail to serve as warnings.

In j.a.pan, where the organisation of earthquake-studies is more complete than elsewhere, it is possible that a vague forecast might be made, if the distribution of the fore-shocks of the earthquake of 1891 should prove to be a general feature of all great earthquakes. It was at first supposed that this earthquake occurred without preparation of any kind; but a closer a.n.a.lysis of the records shows that during the previous two years there was a very decided increase in the seismic activity of the district, and also that the distribution of the epicentres marked out the future fault-scarp, and at the same time exhibited a tendency to comparative uniformity over the whole fault-region.

For the present, then, the only warning available is that given by the preliminary sound, which may precede the strongest vibrations by as much as five or ten or even more seconds. Though two or three seconds may elapse before its character is recognised, the fore-sound thus allows time for many persons to escape from their falling houses. Some races, however, are less capable of hearing the sound than others, and this may be one reason why j.a.panese earthquakes are so destructive of human life.

DISTURBED AREA.

It is usual with some investigators to measure the intensity of an earthquake roughly by the extent of its disturbed area. The depth of the seismic focus must of course have some influence on the size of this area, and this condition is only neglected because we have no precise knowledge of the depth in any case. Thus, Mr. Oldham regards the Indian earthquake of 1897 as rivalling the Lisbon earthquake of 1755, which is generally considered to hold the first place, because its disturbed area was not certainly exceeded by that of the latter.

That disturbed area is, however, an untrustworthy measure of intensity will be evident from the following table, in which the earthquakes described in this volume (omitting those of Ischia) are arranged as nearly as may be in order of intensity, beginning with the strongest:--

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