HoGBEN. — Nelson Earthquake of 12th February, 1893. 351 



The Origin of the Shock or Shocks. 



To ascertain this I employed, as usual, the methods depend- 

 ing on the direction, time of beginning of shock, and intensity 

 — the thne-methods being, as a rule, by far the most reliable. 



1. By the Method of Directions.— Drsiwing lines through 

 all the places where tlae 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 epicen- 

 trum 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. — (a) Straight lines, (/3) circles, (y) co- 

 ordinates. (See Milne's "Earthquakes.") 



(a) The method of straight lines is available when w^e 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 in- 

 dependent pair). All the positions given by the intersections 

 of the equidistant lines are near together, and Eg, 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 Eg are 46 miles and 61 

 miles per minute.) 



{(i) 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 Ei. To suit 

 this, the Nelson time should have been 8h. Imin. Ssec. ; 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 Eg for our epicentrum. 



The point F is found from the times at Nelson, Welling- 

 ton, Christchurch, Kaikoura, Opunake. The velocity of 



