272 



PROCEEDINGS OF SECTION' A. 



where a = amplitude of largest wave, T = its period, V = velo- 

 city of impulse given by tlie shock, and I = intensity of shock, or 

 destructive effect, defined mechanically as =i maximum acceleration 

 due to the impulse, a and T are taken from the records of 

 twenty-one carefully selected earthquakes in Japan, for which these 

 elements Avere observed by Ewing, Milne, and Sekiya. The 

 determination is possibly the best possible at present, and forms 

 the basis of the remarks in this paper. 



I. Nein Zealand. — For New Zealand we have, for the years 

 184S-92, the records of 926 earthquake shocks; but in the earlier 

 years only the severest shocks were recorded, and until December 

 1889, when the present system of observation through the officers 

 of the Telegraph Department was begun, most of the shocks of 

 intensity I. -1 1 1, were probably neglected. Now I am convinced, 

 however, that comparatively few shocks pass unnoticed ; those 

 that do would be all, or nearly all, of the degree I. or II., on the 

 Rossi-Forel scale. As there is such a difference in the quality of 

 the records, we shall get the best estimate of the average intensity 

 of shock in New Zealand by taking only recent years into account. 

 I have therefore based my calculations on the records of the three 

 years 1890-92. 



The following table shows the number of shocks in each year, 

 classified according to intensit}", and the total intensity in absolute 

 units : — 



Average maximum intensity of shock 



78. 



