Part 3 (1/2)

Relying, however, on knowledge obtained from the study of more recent shocks, it seems to me probable that the two foci formed parts of one fault with a general north-west and south-east direction. The slip causing the first part of the double shock apparently took place within the south-east focus, and was followed after a few seconds by one within the north-west focus, greater in amount as well as more deeply seated. In consequence of these displacements there were local increases of stress, causing numerous small slips within or near both princ.i.p.al foci; and, if we may judge from some slight shocks felt at La Sala, accompanied also by other minor slips in the intermediate region of the fault.

REFERENCE.

MALLET, R.--_The Great Neapolitan Earthquake of 1857: The First Principles of Observational Seismology_, etc. 2 vols 1862.

FOOTNOTES:

[3] _Irish Acad. Trans._, vol. xxi., 1848, pp. 51-105 (read Feb. 9, 1846).

[4] _Brit. a.s.soc. Reports_, 1850, pp. 1-87; 1851, pp. 272-330; 1852, pp. 1-176; 1853, pp. 117-212; 1854, pp. 1-326; 1858, pp. 1-136.

[5] _A Manual of Scientific Enquiry_, edited by Sir J.F.W. Herschel, 1849, pp. 196-223.

[6] _Irish Acad. Trans._, vol. xxii., 1855, pp. 397-410.

[7] The linear dimensions of the isoseismal lines are obtained by measurements from Mallet's maps. The areas are given by him in geographical square miles.

[8] Mallet, by some accident, omitted the losses at Polla and neighbouring towns from this estimate. Mercalli (_Geologia d'Italia_, pte. 3, p. 324) gives the number of killed as more than 12,300.

[9] Mallet does not make use of the term _epicentre_; he speaks of the line FE as the _seismic vertical_. The modern and accepted terms are used above

[10] _j.a.pan Seismol. Soc. Trans._, vol. xi., 1887, pp. 175-177.

[11] _Ital. Seismol. Soc. Boll._, vol. ii., 1896, pp. 180-188.

[12] Professor Omori gives the mean direction as S. 71 W., but this was obtained from observation on lamps with square, as well as with circular bases.

[13] Twelve measurements chosen at random from Professor Omori's list gave a mean direction of S. 78 W.

[14] When the accuracy of all the observations seemed equally probable, he adopted the mean of the two extremes as the true direction.

[15] If _a_ be the amplitude of a simple harmonic vibration, _T_ its complete period, _v_ its maximum velocity, and _f_ its maximum acceleration, we have v=2*pi*a/T and f=4*pi^2*a/T^2

[16] _Earthquakes and other Earth Movements_, pp. 81-82.

[17] Obtained from the formula: T=2*pi*a/v=2*pi*x*(1/3)/12

[18] If we take the maximum velocity to be 12 feet per second, and the period to be one second, the amplitude would be about 11-1/2 inches.

[19] Vol. ii., p. 299. The punctuation of the original is not followed in the above extract.

[20] _British a.s.sociation Report_, 1851, pp. 272-320.

CHAPTER III.

THE ISCHIAN EARTHQUAKES OF MARCH 4TH, 1881, AND JULY 28TH, 1883.