A Study of Recent Earthquakes Part 16

REFERENCES.

1. CONDER, J.--"An Architect's Notes on the Great Earthquake of October 1891." _j.a.pan Seismol. Journ._, vol. ii., 1893, pp.

1-91.

2. DAVISON, C.--"On the Distribution in s.p.a.ce of the Accessory Shocks of the Great j.a.panese Earthquake of 1891." _Quart.

Journ. Geol. Soc._, vol. liii., 1897, pp. 1-15.

3. ---- "On the Effect of the Great j.a.panese Earthquake of 1891 on the Seismic Activity of the Adjoining Districts." _Geol.

Mag._, vol. iv., 1897, pp. 23-27.

4. ---- "On the Diurnal Periodicity of Earthquakes." _Phil. Mag._, vol. xiii., 1896, pp. 463-476, especially pp. 466-468.

5. ---- "On Earthquake-Sounds." _Phil. Mag._, vol. xlix., 1900, pp.

31-70--especially pp. 49-53, 60-61.

6. KOTO, B.--"The Cause of the Great Earthquake in Central j.a.pan, 1891." _Journ. Coll. Sci. Imp. Univ. j.a.pan_, vol. v., 1893, pp. 295-353.

7. MASATO, H.--"Report on Earthquake Observations in j.a.pan." _Cent.

Meteor. Obs. of j.a.pan_ (Tokio, 1892), pp. 16-18, 41, and map 30.

8. MILNE, J.--"A Note on the Great Earthquake of October 28th, 1891." _j.a.pan Seismol. Journ._, vol. i., 1893, pp. 127-151; _Brit. a.s.soc. Rep._, 1892, pp. 114-128.

9. ---- "A Catalogue of 8,331 Earthquakes recorded in j.a.pan between 1885 and 1892." _j.a.pan Seismol. Journ._, vol. iv., 1895, pp.

1-367--especially pp. 134-234, 303-353.

10. ---- "On Certain Disturbances in the Records of Magnetometers and the Occurrence of Earthquakes." _Brit. a.s.soc. Rep._, 1898, pp. 226-251--especially pp. 227, 232, 234, 241, and 245.

11. MILNE, J., and W.K. BURTON.--"The Great Earthquake in j.a.pan."

_Journ. Coll. Sci. Imp. Univ. j.a.pan_, vol. v., 1893, pp.

295-352.

12. OMORI, F.--"On the After-shocks of Earthquakes." _Journ. Coll.

Sci. Imp. Univ. j.a.pan_, vol. vii., 1894, pp. 111-200; abstract in _j.a.pan Seismol. Journ._, vol. iii., 1894, pp. 71-80.

13. ---- "A Note on the Great Mino-Owari Earthquake of October 28th, 1891." _Pub. Earthquakes Inves. Com. in Foreign Languages_, No. 4, Tokio, 1900, pp. 13-24.

14. ---- "Sulla velocita di propagazione e sulla lunghezza delle onde sismiche." _Ital. Soc. Sismol. Boll._, vol. i., 1895, pp.

52-60--especially pp. 52-57.

15. ---- "Sull' intensita e sull' ampiezza del movimento nel gran terremoto giapponese del 28 ottobre 1891." _Ital. Soc. Sismol.

Boll._, vol. ii., 1896, pp. 189-200.

16. ---- "Note on the After-shocks of the Mino-Owari Earthquake of Oct. 28th, 1891." _Pub. Earthquakes Inves. Com. in Foreign Languages_, No. 7, Tokio, 1902, pp. 27-32.

17. ---- "Note on the relation between Earthquake Frequency and the Atmospheric Pressure." _Tokyo Phys.-Math. Soc. Reports_, vol.

ii., 1904, No. 8.

18. TANAKADATE, A., and H. NAGAOKA. "The Disturbance of Isomagnetics attending the Mino-Owari Earthquake of 1891." _Journ. Coll.

Sci. Imp. Univ. j.a.pan_, vol. v., 1893, pp. 149-192.

FOOTNOTES:

[54] I have not referred to the results of this survey, for, though changes in all the magnetic elements (especially in horizontal intensity) have taken place between 1887 and 1891-92, these changes cannot be ascribed with confidence to the earthquake in the absence of a thorough knowledge of the secular variation.

[55] From the formula a=x*g/y, where _a_ is the maximum horizontal acceleration, _g_ the acceleration due to gravity, _y_ the height of the centre of gravity, and _x_ its horizontal distance from the edge about which the body was overturned.

[56] These estimates are made, on the supposition of simple harmonic motion, from the formula 2*a=alpha*t^2/(2*pi^2), where 2_a_ is the total range or double amplitude, _a_ the maximum acceleration, and _t_ the period of the vibration.

[57] Professor Omori finds that the mean daily number of earthquakes _y_ during the month _x_ (reckoned from November 1891) may be approximately represented by the equation--

y = 16.9 / (x + 0.397);

or, taking the semi-daily earthquake numbers during the five days between October 29th and November 2nd, 1891, by the equation--

y = 440.7 / (x + 2.314),

where _y_ denotes the number of earthquakes observed during the twelve hours denoted by _x_, the time being measured from the first half of October 29th. It is interesting to notice that, taking account of the mean annual frequency of earthquakes in ordinary years, the number of shocks observed at Gifu during the two years 1898-99 should, according to the latter formula, be 163; the actual number recorded was 160.

[58] The last violent shock before the end of 1893 occurred on September 7th, 1892, and its effects on the frequency of after-shocks is shown by the daily numbers recorded at Gifu during the first fortnight in September. These are--2, 2, 2, 3, 5, 5, 28 (on September 7th), 8, 8, 5, 4, 3, 2, 4, 3.

[59] The periodicity of after-shocks is discussed in the papers numbered 4, 12, 16, and 17 at the end of this chapter. In these, the existence of diurnal and other periods is clearly established.

Professor Omori also shows that the mean daily barometric pressure is subject to fluctuations with maxima occurring on an average every 5-1/2 days, and that earthquakes are least frequent on the days of the barometric maxima and minima, and more frequent in the days immediately preceding and following them.

[60] Of the j.a.panese earthquakes of 1885-92 originating beneath the land, twenty-six per cent. were accompanied by a recorded sound; but less than one per cent. of those originating beneath the sea and not more than ten miles from the coast.

CHAPTER VIII.

THE HEREFORD EARTHQUAKE OF DECEMBER 17TH, 1896, AND THE INVERNESS EARTHQUAKE OF SEPTEMBER 18TH, 1901.

Among the earthquakes described in this volume, the Hereford and Inverness earthquakes hold but a minor place. The damage to buildings, though unusual for this country, was slight when compared with that caused by the preceding shocks; there was no loss of life, not a single person was injured by falling masonry. The interest of the earthquakes lies entirely in the detailed study rendered possible by numerous observations of the shock and sound,[61] and in the bearing of this evidence on the general theory of the origin of earthquakes.

THE HEREFORD EARTHQUAKE OF DECEMBER 17TH, 1896.

The princ.i.p.al earthquake of this series occurred at 5.32 A.M. on December 17th, and was preceded by at least nine minor shocks (the first of which was felt at about 11 or 11.30 P.M. on December 16th), and followed by two others on the same day, and by a third and last on July 19th, 1897. The accounts of these preliminary movements will be found on a later page, as their bearing will be more fully apparent after the discussion of the princ.i.p.al shock.

[Ill.u.s.tration: FIG. 60.--Isoseismal and Isacoustic lines of Hereford earthquake. (_Davison._)]

ISOSEISMAL LINES AND DISTURBED AREA.

On the map in Fig. 60, the continuous curves represent isoseismal lines corresponding to the degrees 8, 7, 6, 5, and 4 of the Rossi-Forel scale. The isoseismal 8, which is the most accurately drawn of the series, is an elongated oval, 40 miles long, 23 miles broad, and containing an area of 724 square miles. The longer axis is directed W. 44 N. and E. 44 S. Within this curve, there are 73 places where buildings are known to have been damaged, 55 places being in Herefords.h.i.+re, 17 in Gloucesters.h.i.+re, and one in Worcesters.h.i.+re.

The most important damage occurred in the city of Hereford, which, in 1901, contained 4,565 inhabited houses. Here, no fewer than 218 chimneys had to be repaired or rebuilt. The Cathedral was slightly injured. The finial of a pinnacle of the Lady Chapel was thrown down, a fragment of a stone fell from one of the arches in the south transept, and the three pinnacles of the western front were fractured.