Art. LVI.—Notes on the Earthquake of the 5th July, 1891, in Cook Strait: an Attempt to define the Epicentrum.
[Read before the Philosophical Institute of Canterbury, 1st October, 1891.]
This was a slight earthquake, without any marked effects, and I do not propose to deal with it at any length. Its sole interest is derived from the fact that the observations of it, though not many in number, were sufficiently exact to enable us to ascertain the epicentrum very nearly. The data—all obtained through the Telegraph Department—are these:—
|Blenheim||10.53 p.m.||W. to E. (but uncertain)||10 secs.|
|Upper Hutt||10.55 p.m.||N.E. to S.W.||3 secs.|
|Masterton||10.53 p.m.||N.E. to S.W.||about ¾ min.|
|Woodville||10.54 p.m.||W. to E.||5 secs.|
|Marton||10.53 p.m.||N.E. to S.W.||8 to 10 secs.|
|Foxton||10.55 p.m.||N. to S.||5 secs.|
|Wellington||10.55 p.m.||S.E. to N.W.||5 secs.|
|Picton||10.52 p.m.||N. to S.||10 to 15 secs.|
|Featherston||10.52 p.m.||N.W.||2 secs.|
The times are those of the beginning of the shock: the first six are stated to have been verified by New Zealand mean time, the last three being given to the best of the officers' belief to N.Z.M.T. No qualification of the times at the Upper
Hutt, Foxton, and Wellington appears to be required on account of the time being a multiple of five minutes, as the observers at those stations have been in the habit of giving the time to the nearest minute—in the case of Wellington, to the nearest half-minute. At three places the shock is said to have been preceded by rumbling; at Picton “the house shook and the windows rattled;” at Foxton “crockery rattled about considerably;” and at Masterton the Postmaster was awakened by the shock. An important detail is mentioned by the Foxton observer—namely, that the shock he records was “preceded by a slight premonitory shake,” how long before he does not state. This seems to show that there were two shocks, and I think we are justified in assuming that the times at the Upper Hutt and Wellington, as well as that of Foxton, belong to the second shock.
The simultaneous arrival of the vibrations at Blenheim, Masterton, and Marton gives us for the epicentrum a point in Cook Strait sixty-five miles from each of these places (forty-four miles N. 24° W. from Wellington). The inclusion of Picton and Woodville with them gives us almost exactly the same spot. If we take in Featherston also, we must suppose an epicentrum five miles further to the south-east.*
For the second shock the time—10.55—at the Upper Hutt, Foxton, and Wellington implies an epicentrum two miles north-west of the first epicentrum. A circle of three miles radius would include all the epicentra found, and if we reject Featherston a circle of one mile radius would satisfy all the data. The velocity of propagation is eighteen miles per minute.
The agreement of the observations is remarkable, for with the utmost allowance for probable errors the epicentric area found is very small. The velocity must lie between fifteen miles and a quarter per minute and eighteen miles per minute, and the time at the origin between 10.48 ½ p.m. and 10.49 ½ p.m. Compared with time-observations those of direction are usually of small value; they may, however, be used cautiously for testing our results. In the present case it will be found that (lines being drawn in the given directions, and at right-angles thereto, so as to include cases of normal and transverse vibrations) the direction of the epicentrum found agrees almost exactly with Upper Hutt and Marton, and is in accordance to the nearest half-quadrant (or within 22 ½°) with all the rest except Blenheim, Masterton, and Picton, the errors of these directions being 25°, 31°, and 27° respectively.
We have thus as good a confirmation as we can expect of
[Footnote] * If the clock at the last place was forty seconds slow the same epicentrum would be given as for the other places.
the conclusion already arrived at by means of the times. The observations are not such as to enable us to ascertain the depth of the origin; they are opposed to one at great depth—indeed, the origin was probably very shallow.
Dr. Lemon has kindly allowed me to supply earthquake memorandum forms to a large number of telegraph-stations near Cook Strait, so that I trust this attempt is only the first instalment towards the exact determination of the epicentric areas in that district.
Postscript.—The earthquake of the 4th December, 1891, for which the number of data is very large, gives nearly the same origin. It will form the subject of a separate paper.