574 Transactions. — Miscellaneous. 



Art. LV. — Notes on the Earthquake of the 24th June, 1891. 



By George Hogben, M.A. 



[Bead before the Philosophical Institute of Canterbury, 4th November, 



1S91.] 



Plate XXXIX. 



This earthquake is one of the most considerable recorded in 

 the Auckland District ; but there is some difficulty in arriving 

 at a definite conclusion in regard to the origin. Nevertheless, 

 the facts of the earthquake, and some attempt at an explana- 

 tion of them, should, I think, be placed on record. The data, 

 obtained through the Telegraph Department, are given in the 

 accompanying table (pp. 575, 576). 



1. For the determination of the epicentrum the usual time- 

 methods were employed. Assuming all the times to relate to 

 the same shock, by the method of circles, with velocity twelve 

 miles per minute, we obtain from the times at Auckland, Cam- 

 bridge, Thames, and Tauranga an epicentrum E, about twelve 

 miles south of Mercer ; but this does not agree with the time 

 at Mercer (probably a good observation), nor with that at 

 Helensville. By taking a point F, about twenty miles south- 

 west of E, and a slightly greater velocity, we may make our 

 solution suit Mercer better, but Thames hardly so well ; it 

 would agree with the effects observed at Eaglan. The normal 

 equations derived from the equations of observation (see Trans., 

 vol. xxiii., p. 476) give N for the epicentrum ; but, though this 

 may give the mean position of the epicentra of several shocks 

 near together in time, it does not agree closely with any of the 

 data. None of these three points — E, F, N — agree with the 

 times at Wanganui (a good observation) and New Plymouth 

 (doubtful). The investigations, however, lead us to suspect 

 two shocks follovv'ing one another at a brief interval, the first 

 proceeding from below some point within the circle whose 

 centre is A, and the second (and chief shock) from below the 

 circle whose centre is B. 



2. I next tried to arrive at some conclusion by considering 

 the intensity of the shock at various places. For this method 

 of working I am indebted to a paper on the " British Earth- 

 quakes of 1889," by Charles Davison, of Birmingham (Geolo- 

 gical Magazine, 1891). The dotted lines on the map (PI. 

 XXXIX.) show the isoseismals (lines drawn through places at 

 which the intensity was the same) : the intensity has been 

 marked on the Eossi-Forel scale. The intensity of the shock 

 was greatest within the innermost curve, the area marked VII. 

 to VIII. on the map. The shaded portion shows the area 



