Art. L.—The Earthquake of the 4th December, 1891: Notes thereon.

Transactions and Proceedings of the Royal Society of New Zealand, Volume 25, 1892, Page 362

Art. L.—The Earthquake of the 4th December, 1891: Notes thereon. By George Hogben, M.A. [Read before the Philosophical Institute of Canterbury, 2nd June, 1892.] Plate XLVI. During last year (1891), Dr. Lemon, Superintendent of Posts and Telegraphs, kindly made special efforts to obtain for me from a number of stations round Cook Strait exact records of earthquake-shocks. The first fruit of those efforts was the attempt in a previous paper to find the origin of a small earthquake on the 5th July, 1891. The facts of the earthquake now under discussion are, however, considerable enough to deserve a somewhat fuller treatment. It will be seen that the data are still insufficient to determine the depth of the origin, for the estimate of the depth given below can only be regarded as very rough indeed. I think we are justified in saying that we now know the position of one of the chief sources of the Cook Strait shocks.

Place. Time: N.Z. M.T. *Verified. Apparent Direction. Apparent Duration. Effects. Remarks. Intensity. Rossi-Forel Scale in Roman Numbers. A. M. (a)Otaki 4.39* E. to W. 30–35 secs. Crockery broken. Preceded by heavy rumbling noise “Severe.” vii. Picton 4.40* N. to S. 20 secs. Windows rattled; sash-weights swayed to and fro for 1 min. afterwards; houses shaken severely. Preceded by loud rumble “Severe.” iv. to v. Havelock 4.40* N.W. 5 secs. “Severe.” iii. or iv. Foxton 4.40* S.E. to N.W. 20 secs. Articles thrown off mantelpieces; clocks stopped; small amount of crockery broken “Sharp.” vii. Marton 4.40* N. and S. 12 secs. Articles rattled. Two shocks—first slight; second much sharper “Sharp.” iv. Wanganui 4.40.½* N.E. to S.W. 50 secs. Prolonged continuous shock; shaking of utensils “Sharp.” iv. to v. Blenheim 4.41* W. to E. 20 secs. Subsequent tremors for nearly 30 secs. “Sharp.” iii. to iv. Feilding 4.42* 30 secs. No effects, though shock most severe since 1881. Loud rumbling. Cattle alarmed for 10 min. previously “Severe.” iv. (or v.). Opunake 4.43* S.E. to N.W 10 secs. Slight rumbling. Prolonged tremor, with 4 or 5 shocks at intervals. Building creaked. “Woke me up.” [A slighter shock, 2 secs., S. to N., at 4.48.] v. (b)White's Bay 4.38* N. to S. 5 & 2 secs. Previous rumble for about 25 secs. Two shocks “Very sharp.” iii. to iv. Palmerston N. 4.38* N. to S. 20 secs. Distinct previous rumbling “Sharp.” iii. to iv. Woodville 4.40* E. to W., then N.E. to S.W. over 1 min. Crockery jingled. A noise described as “a loud explosion” woke many. Severe shake, and prolonged tremor “Severe.” v. to vi. Upper Hutt 4.41* N. to S. Clock stopped. Previous rumbling and tremors “Sharp.” v. or vi. (c)Wellington 4.40 E. to W.(?) about 10 secs. Accompanied by rumble. Basin and jug, and mirror in door quivered. “Nearly vertical”(?) “Severe.” iv. Masterton 4.42.½ W. to E. nearly 1 min. Two chimneys injured so as to require rebuilding. Observer awakened. Tremor lasted some seconds after “Severe.” v. to vi. (d)New Plymouth 4.45 N.E. to S.W. 30 secs. Rumbling and rattling of window-frames “Slight.” iv. (or iii.). Nelson 4.45(?) E. to W. a few secs. Very long and undulating “Slight.” iii. Westport about 4.40(?) 2 min. Windows rattled and buildings shook considerably. Not every one awakened “Severe.” iv. to v.

Of these times, I first rejected (c) and (d), which are not stated to have been verified by New Zealand mean time; but Wellington is probably a good record, being due to Dr. Lemon himself; and the Masterton time seems to have been carefully noted (by one who habitually keeps Wellington time), though not subsequently verified. All trials show the Upper Hutt time to be too late. Perhaps the velocity of propagation was retarded locally, or, as the previous tremors might suggest, the time was taken at a later stage of the earthquake than elsewhere. White's Bay, Palmerston North, and Woodville are all two to two and a half minutes at least too early, and possibly refer to an earlier shock.* A shock taking place at 4.28.½ at an origin below some point in the smaller circle shown on the map, propagated with a velocity of about seven miles per minute, would reach these three places at the times named, and thus explain the discrepancy completely. In several of the memoranda forwarded by the telegraph officers mention is made of previous rumbling or tremors (see especially Feilding). We must rely, therefore, principally on the times of the nine places in (a). Comparison with the result of the normal equations, indeed, shows Marton and Blenheim to be of less value than the rest, but we are not warranted, primâ facie, in excluding them from our calculations. Four methods were used in finding the origin, depending on the direction, time, and intensity. Direction.—Drawing lines in the directions given, and at right angles thereto, to include cases in which the direction given is that of the transverse vibrations (see Trans. N.Z. Institute, vol. xxiii., p. 474), we find that we can describe a circle of 15 miles radius with centre D (55 miles from Wellington, 58 miles from Wanganui), an origin within which would agree with Wanganui, Otaki, Palmerston North, Picton, Blenheim, Woodville, White's Bay, Havelock, and very nearly with one or two others. Time: Method of Circles.—Using the times for Havelock, Blenheim, Otaki, Wanganui, and Opunake, with a velocity of 11.½ miles per minute, we find an epicentrum E, 49 miles from Wellington, 60 miles from Wanganui. This, however, does not agree with several of the other times, which are presumably equally good. E', found by the same method from Havelock, Otaki, Foxton, Feilding, Wanganui, Opunake, with an assumed surface-velocity of 8 miles per minute, gives far more satisfactory results. Method of Equations.—Taking Opunake as the origin of co-ordinates, and the line Opunake—Havelock as the axis of x (Milne's “Earthquakes,” p. 206), using all the data contained in (a), and forming the normal equations, we get—

Earthquake, 4th Dec, 1891. Paper by G. Hogben

x), 279,268x+ 93,020y+10,877u—3,675w=19,801,880 y), 93,020x+121,252y+ 6,379u—2,119w= 7,979,392 u), 10,877x+ 6,379y+ 636u— 197w= 785,488 w), 3,675x+ 2,119y+ 197u— 63w= 268,104 Hence x = 74.46 miles, y = 24.55 miles. K (see Pl. XLVI.) is the epicentrum thus found. It is 56.½ miles from Wellington, 59 miles from Wanganui. The equations do not give the absolute velocity (u is negative), or the depth of the centrum. By a method of trial we find the former to be about 7.7 miles per minute, and the latter probably a little less than 10 miles. K is 3 miles north-east of E'. The absolute velocity, with the same depth of origin, but with the epicentrum E', would be 8.4 miles per minute. These are both very slow, especially considering the marked (though not severe) character of the shock. The following table shows how these two solutions agree with the best of the recorded times:— Place. Time at Origin K. (Mean absolute velocity, 7.7 miles per minute. Depth, 10 miles.) Time at Origin E'. (Mean absolute velocity, 8.4 miles per minute. Depth, 10 miles.) Otaki gives 4h. 32.8min. and 4h. 33.2min. Picton " 4h. 32.9min. " 4h. 33.8min. Havelock " 4h. 32.2min. " 4h. 33.1min. Foxton " 4h. 32.9min. " 4h. 33.2min. Wanganui " 4h. 32.7min. " 4h. 33.1min. (Blenheim " 4h. 31.4min. " 4h. 32.5min.) Feilding " 4h. 32.7min. " 4h. 33.1min. Opunake " 4h. 32.7min. " 4h. 33.4min. (Wellington " 4h. 32.6min. " 4h. 33.3min.) (Masterton " 4h. 32.4min. " 4h. 33.1min.) Mean* Excluding Blenheim, Wellington, Masterton. " 4h. 32.7min. " 4h. 33.2min. The time at the origin should be the same from whatever place we reckon back; the close agreement, however, is worthy of note. Intensity: Method of Isoseismals.—The intensity on the Rossi-Forel scale is indicated by Roman numbers in the last column. If we trace the isoseismal corresponding to intensity iv., and smooth it out into an ellipse, we shall obtain for the focus of the latter one of the points I, S, S', S′, according to the prominence given to the estimate of the intensity at different places. S7apos; and S′ are too far to the east, for Foxton and Otaki must be both on or nearly on the isoseismal VII., an ellipse with the same origin as focus. None of the points S, S', S′ agree with the times. The focus I may be said to confirm in a general way the position already found for the origin.

The limits within which the epicentrum must almost certainly lie are shown by the inner circle, of 6 miles radius, containing all the points D, E, E', K, and I. The epicentrum of the earthquake of the 5th July, 1891, was estimated to be at A, which is 4 miles S.E. of the last-named circle. F is the position assigned approximately to the epicentrum of the earthquake of the 20th February, 1890. (Trans. Aus. Assoc. Adv. Sc., 1891, p. 46, and map.) The line HM is that marked as a probable continuation of the line of fault No. 5 in the map of New Zealand, showing faults and earthquake-rents, published in 1890 by Sir James Hector and Mr. A. McKay. The small figures on Plate XLVI. to the S.W. of E' show the soundings: it will be seen that there is a marked hollow in the sea-bed in the neighbourhood in question. It may be as well to note the last two facts referred to, though I do not propose to discuss their significance at present. The depth of the origin may have been even greater than 10 miles (though that depth agrees most closely with the observed times), for the shock was not stopped by the mountains; for the same reason it could hardly have been very shallow. Though we are not called upon to explain all the inconsistencies even of professedly good observations, yet the following hypothesis of the movements that took place removes most of the discrepancies, and is supported by the remarks in the fifth column of the table:— I. A shock, noted at White's Bay and Palmerston North, occurring 3 or 4 minutes before II. II. The chief shock, lasting nearly 1 minute, with three chief maxima, viz.:— (α.) First maximum, observed at Marton (so-called “first shock”), Blenheim, and New Plymouth (?). (β.) Greatest maximum, observed at Wellington, Masterton, Feilding, Havelock, Picton, Otaki, Foxton, Wanganui, Opunake (4.43), and Marton (“second shock”). (γ.) A lower maximum (still considerable), Upper Hutt. III. A third shock, less severe than I. and II., 5 minutes later than II.—Opunake (4.48); and possibly Nelson (4.45). The only records remaining unexplained would, if we accept this hypothesis of the history of the earthquake, be Westport, which is evidently rough. (New Plymouth and Nelson may be only approximate also, but admit of explanation as above.)

Summary.—The origin of the earthquake felt around Cook Strait on the 4th December, 1891, was beneath an area most probably including E' and K', at a depth possibly somewhat less than 10 miles; the velocity of propagation was slow—about 708ft. per second; the time, at the origin, of the chief shock was 4h. 33min. a.m., nearly; the maximum recorded intensity, vii. on the Rossi-Forel scale.* The Rossi-Forel scale was adopted as a convenient standard of intensity by the Seismological Committee of the Australasian Association at the Hobart meeting, January, 1892.