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Volume 26, 1893
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Art. XXXVII.—The Nelson Earthquake of the 12th of February, 1893.

[Read before the Philosophical Institute of Canterbury, 5th July, 1893.]

Plate XLI.

The earthquake was felt more strongly at Nelson than anywhere else. Considerable damage was done in the town and neighbourhood, and it was estimated that the total loss would not fall far short of £4,000. Many chimneys were brought down, others were twisted out of position, and, according to the Colonist of the 13th February, over one hundred were injured. In several buildings ceilings and plaster were shaken down, and walls were cracked. The spire of the Cathedral was estimated by the City Surveyor to have been 3ft. out of plumb after the earthquake. Clocks were stopped, water overflowed in jugs north and south, a large amount of crockery and some statuary were broken in private houses and in shops, and in one or two instances plate-glass windows were broken. Careful observations of the direction with compass bearings seem to have been taken, and these appear to show that the chief line of movement was from south by west to north by east. In connection with this, we may note one fact recorded—viz., that the north wall of a massive stone malt-kiln was thrown away from the ends of the east and west walls, and from the floor of the upper story, to such an extent that the malt poured down upon the lower floor through the aperture caused.

The time given by the officers of the Post and Telegraph Department was 8·2 (checked by New Zealand Mean Time); another good observation was 8·1. I have therefore taken 8·1 ½ as the actual time of the beginning of the shock at Nelson.

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The report of the Colonist newspaper, to which I am indebted for most of the above details, concludes that the shock was more severe than that of the 19th October, 1868: this is almost certainly correct—indeed, it is the most considerable earthquake felt in New Zealand since the 23rd January, 1855.

The effects noted at Wellington were well marked in character, but far less in degree of intensity than at Nelson. The most important from the point of view of the present investigation were the stopping of clocks, the ringing of bells, the cracking, and in some instances the fall, of plaster, the overthrow of movable objects, the cracking of some walls, and the fall of a few chimneys, probably already out of repair.

In the Post Office buildings pendulum-clocks at right angles to the line of shock (E. and W.) were stopped, and all the western walls where the plaster of the ceiling joins the wall were cracked, and chips of plaster deposited on the dado moulding. It is interesting to remark that the seismometer at the Museum showed a large displacement, and registered movements both from east to west and from north to south—that is, it showed both the longitudinal and transverse vibrations.

As will be seen from what follows, the velocity of propagation was much greater than the average velocity of New Zealand earthquakes; and this, coupled with the undoubted fact that the shock was a compound one, made the determination of the origin more than usually difficult. A small error in time is of far more importance with a large than with a small velocity; and when two shocks follow closely on one another, and one of them only is felt at many of the places, it becomes a matter of some difficulty to determine which shock it was that was observed at any particular place. There is, in addition, the usual amount of uncertainty as to whether the same phase of a long earthquake is referred to by different observers—all being asked to give the time of the beginning of the shock. The time put down for the apparent duration is of some service in resolving this uncertainly. Using this and the other means of checking the times given, I have set down the times at the first five places in the list (a) as of greater weight than the others. The times in this list and in the second list are stated to have been all checked by New Zealand Mean Time by the Telegraph officers, who filled up the forms supplied to them. The times in the third list (c) were not so verified.

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Place. Time of Beginning of Shock, N.Z.M.T. Apparent Direction. Apparent Duration. Effects. Remarks. Intensity Rossi-Forel Scale.
(a)Nelson A.M. 8·2,* or 8·1 N.E. to S.W., or S. by W. to N. by E. about 1 min. For remarks see p. 347 viii.+
Wellington 8·3* N.E. to S.W. 30 secs. For remarks see p. 348 vii.+
Kaikoura 8·3* N.E. to S.W. 15–20 secs. “Moderately severe,” Rumbling preceding, Crockery rattled iv.
Opunake 8·4* E. to W. N. to S. E. to W. 73 secs Very sharp: three shocks-sharpest for years. A few articles thrown from shelves iv. to v.
Christchurch 8·4 ½* E. to W. more than 1 min, Maximum between 8·5 and 8·5 ¼. Commenced with slight tremors; increased slowly to maximum; then decreased more rapidly. Crockery set in motion. One account says three distinct shocks iv.
(b)Wanganui 8·5,* or 8·3* N. to S., or S.W. to N.E. 90 secs., or 4 or 5 min. “Severe.” Broke battery-jar in officer's house. Water spilt out of washstand jugs. Mr. Field says began at 8·3 and lasted (gyratory motion) 4 or 5 minutes. Slackened and increased again three times. Began with slight rumble v, (?)
Timaru 8·4 ½* N. to S. 15–20 secs. No previous rumbling, Succession of small shocks, with rolling movement. Followed by a slighter shock ii. to iii.
Westport 8·3* S.E. to N.W. 1 min. “Severe.” Loud rumble preceding iii.+
Cambridge 8·2 ¾* S.E. to N.W. 20 secs, “Slight,” No damage iii.
Hawera 8·2* N.W. to S.E. 80 secs, “Severe,” Milk spilt out of pans iv. to v.
Helensville 8·4* S.E. to N.W. 15 secs. Slight, then sharp iii.
Blenheim 8·5 ½* E. to W. (n'rly) then heavy upheavals at right angles 50–60 secs, Clocks stopped, bells rung, crockery thrown down; several chimneys thrown down, others cracked and twisted. Time given for middle of shock vii. to viii.
Manaia 8·3* N.E. to S.W. 90 secs. “Severe” iii.+
Otaki 8·2 ½* N.E. to S.W. 20 secs. “Severe.” Clocks stopped, Preceded by loud rumbling vi.
Marton 8·1* N.E. to S.W. 35 secs, “Sharp.” Shook buildings; rattled crockery, Accom. panied by loud rumbling iv.+
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Place. Time of Beginning of Shock, N.Z.M.T. Apparent Direction. Apparent Duration. Effects. Remarks. Intensity Rossi-Forel Scale.
Greymouth A.M. 8·1* E. to W. S.E. to N.W. N. to S. 15 secs, Three movements, No rumbling. Heavy undulating motion iii, +
Hokitika 8·4* N.E. to S.W. 50 secs, “Slight, then severe,” One long shock, not two as reported. Newspaper states two smart shocks, with interval of 2 mins.; second one lasted 45 secs, iii.+
Ashburton 8·3* N.E to S.W. ab't 10 secs. “Sharp.” Double shock iii.
(c)Picton 8·2 N. to S. 40 secs, One chimney thrown down; houses shaken violently vii.+
Takaka 8·4 W. to E., (n'rly) ab't 1 min. Buildings swayed; crockery fell; milk spilt to eastward vii.
Collingwood 8·0(?) W. to E., or from slightly N. of W. 70 secs. “Sharp, followed immediately by severe.” Clocks stopped, A number of bricks shaken out of top of chimney at E. side. One outside chimney shifted 2in. Preceded by slight rumbling. Cattle frightened vii.
Karamea 8·0 (?) N.W. to S.E. ab't 100 secs. Clocks stopped vi.
Otira Gorge 8·3 E. to W. iii. to iv.
Waitara 8·5 S.W. to N.E. 2 shocks 30 secs, each Each shock very sharp, followed by rumbling, Water spilt, Lamp-glasses thrown off shelf; clocks stopped vi, +
Auckland 7·55 (?) S.E. to N.W. 15 secs, Slight. Two shocks in quick succession. Windows rattled iv.
Manukau Hds. 8·4 and 8·6 S.E. to N.W. Two slight shocks, faint rumblings accompanying iii,
Woodville 8·2 N. to S. 80 secs. iii,
Reefton 8·3 “Sharp.” Preceded and followed by tremors lasting ½ min, Some damage to crockery iii. to vi.
Palmerston N. 8(soonafter) 10 secs, “Smart” iii.
Oamaru
Kaipara
Waikato Dist.
Patea Shock felt. No details ii.
Hanmer Plains
Rakaia
Amberley 8·6 N.N.W. some secs. iii.
Malvern 8·10 Not severe. Preceded by long rumble
Akaroa 8·3 N. to S. iii.
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The Origin of the Shock or Shocks.

To ascertain this I employed, as usual, the methods depending on the direction, time of beginning of shock, and intensity—the time-methods being, as a rule, by far the most reliable.

1. By the Method of Directions.—Drawing lines through all the places where the apparent direction was noted, we find that a circle with centre B and a radius of 10 miles can be drawn to cut or touch the direction-lines for Nelson (N.E. to S.W.), Wellington, Picton, Blenheim, Christchurch, Grey-mouth, Hokitika, Wanganui; and a circle with centre A and radius 26 miles would agree with these, and with Takaka, Westport, Karamea, Marton, Kaikoura, and nearly with Collingwood, Hawera, Opunake, Otaki. These form most of the places. We should therefore expect the epicentrum to be within or near the circle (B), and almost certainly within the larger circle (A).

(The direction-lines must be drawn in the direction noted for each place and at right angles thereto—to include cases where the direction of only the transverse vibrations is given. One of the two direction-lines will then be the direction of the line of propagation, unless there has been reflexion, or some other cause of deviation of the waves.)

2. By Time-methods.—(æ) Straight lines, (β) circles, (γ) coordinates. (See Milne's “Earthquakes.”)

(æ) The method of straight lines is available when we have several pairs of places at which the shock was simultaneous; the epicentrum must be equally distant from each of the pair. I have used four such pairs: Wellington-Westport, Kaikoura-Wellington, Westport-Kaikoura, Opunake-Hokitika (an independent pair). All the positions given by the intersections of the equidistant lines are near together, and E3, the mean position, would thus be the epicentrum. This corresponds to a velocity (superficial) of about 58 miles per minute. E3 is near the circle (B) and within the larger circle (A).

(The limits of the velocity for E3 are 46 miles and 61 miles per minute.)

(β) The method of circles: From the times at Opunake, Wellington, Christchurch, Hokitika, with an assumed velocity of 40 miles per minute, we get the epicentrum E1. To suit this, the Nelson time should have been 8h. 1min. 8sec.; we can hardly allow it to have been quite so early, hence the velocity is probably too small (i.e., if the other times are good). Using Wellington, Opunake, Christchurch, and Nelson (origin deep), with an assumed velocity of 55 miles per minute, we obtain E2 for our epicentrum.

The point F is found from the times at Nelson, Wellington, Christchurch, Kaikoura, Opunake. The velocity of

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propagation assumed is 50 miles per minute, and this solution agrees also with Wanganui (8·4), and Westport (8·3) nearly, and is only ¼min. out for Picton (8·2).

(y) The method of co-ordinates: The times in the list (a)—all verified by New Zealand Mean Time, and apparently good times, referring to the same phase of the same shock—were employed. Christchurch was taken as the origin of co-ordinates, the line Christchurch-Hokitika as the axis of y, and the axis of x at right angles (north-easterly).

The reduced equations are—

  • Opunake), 544x + 196y + ¼u - ½w = 83,588

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    Kaikoura), 186x - 8y + 9/4u - 3/2w = 8,665

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    Wellington), 384x - 16y + 9/4u - 3/2w = 36,928

  • Nelson), 300x + 110y + 9u - 3w = 25,525

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Hence x = 145·15 miles, y = 59·6 miles, v = 493/4 miles per minute = 4,378ft. per second (velocity of propagation), and the time at the origin = 8h. 1min. 20sec. A.M. The point K near F, five miles and a half W.S.W. of Nelson, is the point thus found for the epicentrum. By trial we find that a depth of about 5 miles for the centrum best suits the data.

This agrees within the limits of errors of observation with Westport, and also with Wanganui, if we take the mean of the two observations (both by good observers).

The degree of agreement is shown by the time at the origin as calculated back from each place; it should be the same, of course, from whatever place we reckon.

Time at Place of Observation. Time at Origin below K, in Minutes and Decimals.
Christchurch 8·41/2 1·35min.past 8.
Kaikoura 8·3 1·32"
Wellington 8·3 1·32"
Nelson 8·11/2 1·33"
Opunake 8·4 1·33"
Westport 8·3 1·21min.past 8.
Wanganui 8·4 1·20"
Picton 8·2 (not checked) 0·88"

The other places do not give a time at the origin agreeing with this; but the errors are all (or very nearly all) of one sign, and vary from-1min. to - 3·96min., occurring in groups. Examination of the several groups leads us to suppose that there were several shocks, all nearly below K, the first deep, about 25 miles down, the second higher up, and the third about 5 miles below the surface. At some of the more distant

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Nelson earthquake of 12th Feb 1893

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places only the deep-seated shocks were felt. At Wanganui Mr. Field noted vibrations for 4 or 5 minutes with several distinct shocks (but possibly several maxima of the same shock). At Timaru the first shock, and a later one (about 11/2 or 2 minutes later), were observed. Of all these positions found for the epicentrum, E2 best corresponds with the Nelson observations of direction; but it is possible that, if these observations were those of the transverse vibrations, K, or a place a little to the north of it, would agree equally with them.

It is, of course, most likely that the epicentrum would be an area large enough to include all the places, K, F, E2, E2 (epicentric area on map, PI. XLI.). The amount of damage done at Nelson was greater—far greater—than that reported from any other place. It is probable, therefore, that the angle of emergence there was nearly that of the maximum intensity—i.e., between 56° and 45°. This would agree with either K or E2, with a depth of 5 miles for the origin.

The origin might be guessed at with a tolerable degree of probability by the use of isoseismals. Looking at the last column in the table given above, we see that the isoseismal of intensity, vii. on the Rossi-Forel scale, would be drawn outside Picton, Takaka, Collingwood, Wellington, Blenheim; but would have all the other places outside it. An ellipse might be so drawn with a focus not far from the epicentric area (K, F, E3, E2).

3. Intensity.—The maximum intensity of this earthquake was as far above the average of our ordinary mild New Zealand shocks as its velocity of propagation was. The intensity at Nelson was evidently viii. (Rossi-Forel scale), or a little above it.

If a = amplitude of the largest vibration in the motion of any earth-particle, and T = the period of the largest wave, then 4π2a/T2 = intensity of shock defined mechanically = destructive effect = maximum acceleration due to the impulse.

Now, Dr. Holden, Director of the Lick Observatory, has given equivalents of the degrees of earthquake-shocks on the Rossi-Forel scale in terms of the acceleration due to the velocity of the shock itself (American Journ. Sci., 1888, No. 210).

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Thus a shock of intensity viii. corresponds to 500mm. per second. We should not probably be far wrong if we gave 600mm.-800mm. per second as the measure of the intensity of our present earthquake—or, in other words, from 1/16 to 1/12 of the acceleration due to gravity.

Summary.—The earthquake of the 12th February; 1893, originated below an area within 5 or 6 miles of Nelson, 23

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to the south and west. The principal shock took place at 1min. 20sec. past 8 a.m., or thereabouts, at a depth of 5 miles approximately. The velocity of propagation was 4,378ft. per second; the intensity of the shock, measured by the velocity of the earth-particle, about 2ft. per second, or rather more than viii. on the Rossi-Forel scale.

Theory suggested.—The principal shock was preceded by others at a much greater depth, and we may, if we please, imagine a succession of rock-falls (or slidings or crushings) to have taken place in the interior of that portion of the earth's crust underneath the epicentric area K, F, E2, E2.