HoGBEN. — The Wanganui Earthquake. 587 



The earthquake was the severest felt in the colony since 

 the Nelson earthquake of the 12th February, 1893, and the 

 severe effects were more widely felt than in the latter. These 

 two were more severe than any others since the memorable 

 earthquake of the 23rd January, 1855. 



Besides the intensity, which was rather over viii. on the 

 Rossi-Forel scale, the prolonged character of the shocks is a 

 noteworthy feature ; there were evidently several maxima, 

 and normal and transverse vibrations appear to have been so 

 far distinct in their times of arrival as to give the impression 

 of two different shocks. 



A(a) and B(a) are first-class observations, the times being 

 verified, and other indications of consistency and the known 

 experience of the observers confirming this idea of their value. 

 All the verified times — i.e., A(a) and B(a) — together with 

 Gisborne and Nelson, were taken at first to form eleven 

 equations of observation. All were counted as of equal 

 weight, for I do not yet see my way to give greater weight 

 to the times from more distant places, as has been suggested. 

 The greater numerical value of the coefficients naturally gives 

 a steadying value, as it should, to the equations based on the 

 data from them. 



The normal equations were formed and treated as shown 

 in my previous papers. It hardly seems necessary to give 

 them here. 



An examination of the residuals clearly showed the ex- 

 istence of two sets of observations, marked A and B, and led 

 also to the rejection of the returns from Nelson and Gisborne, 

 both of which seemed a little doubtful on other grounds. 



The observations in A and B were then examined separ- 

 ately. 



Set A. — To find the origin, time at the origin, and velocity 

 of propagation {x, y, z, t, v) there are only five sets of data, 

 and we gain nothing by forming normal equations ; indeed, 

 in this case the graphic method of circles is as effective as the 

 method of co-ordinates, and much more manageable. Both 

 were used, in order to have a check on the result. They give 

 the epicentrum R ; time at the origin, 2 hours 40 minutes 11 

 seconds + 5 seconds ; velocity of propagation, 85-87 miles 

 per minute (7,440 ft.-7, 656 ft. per second, or 227,000 cm.- 

 233,500 cm. per second.) The value of z, or depth of origin, 

 is swamped by the high velocity ; and we have no means of 

 finding it from the available observations. E is fifty-one 

 miles from Wanganui and seventy miles from Wellington. 



Set B. — There being only four places of observation, with 

 the same time at each, the method of straight lines or that of 

 circles can be used. Either gives S for the epicentrum, 

 taking into consideration that the time at Wakapuaka is 



