EARTHQUAKE INTENSITY IN AUSTRALASIA. 275 



absolute units. The diagram shows very clearly — (1) the rapid 

 rise to the first maximum in October and November. 1883; (2) a 

 second maximum in August 1884 ; (3) a very gradual decline for 

 nearly two and a half years, the shocks slowly dying away at the 

 end Jf 1886. 



Four of these shocks reached the intensity VII. on the Rossi- 

 Forel scale ; for these Mr. A. B. Biggs and Captain Shortt assigned 

 approximate origins. I have also tried to use the available data 

 to ascertain the epicentrum or epicentra ; but only in one case is 

 the conclusion at all definite — in the case of the shock-earthquake 

 of the 13th of May, 1885 : — Its epicentrum is either identical 

 with or not far from that of an earthquake already referred to, 

 January 27th, 1892, which I have discussed in another paper. For 

 the present I assume that all the important shocks proceeded from 

 the same region, east of Tasmania, about 353 miles from Launceston 

 and 365 miles from Hobart, nearly in the middle of a deep 

 depression in the bed of the Tasman sea. An examination of the 

 records, however, shows that the majority of shocks were felt at 

 one place only (or at two places near together) ;- even when slight 

 they were generally distinct in character, and the nimbling was 

 often of such a nature as to suggest that the cause of it was com 

 paratively near. Hence I conclude that there were constantly going 

 on a large number of smaller movements in the crust of the earth 

 immediately beneath Tasmania and Bass's Strait,- - which were 

 secondary to greater movements which took place about an axis, 

 or about a point in the deep sea. For three or four years a re- 

 adjustment of some kind was going on ; the larger shocks Avere 

 perhaps merely incidents caused by movements a little quicker 

 than usual, or by the sudden slipping of large glasses out of the 

 position of unstable equilibrium into which the slow movements 

 bad brought them. 



If these large or primary movements were fault-movements, one 

 would almost expect to find the axis on the edge and not in the 

 middle (or at the bottom) of a steep declivity in, the ocean bed. 

 If the primary movement was one of revolution (principally) about 

 the axis, interrupted by an occasional sliding of the mass on one 

 side of the axis upon the mass on the other side, then we must 

 look for secondary movements at some distance fi-om the axis, 

 where the displacement caused by revolution is greater. Is it 

 possible that the lesser shocks were more or less local movements 

 of this character ? 



Is there any evidence of fault-movements having taken place in 

 Tasmania within the period of these shocks ; or any evidence of 

 a change of the level of the land in Tasmania, or in S.E. Australia, 

 or in the islands between them ? If such evidence exists, I should 

 be glad to hear of it ; but, till it is forthcoming, it would be vain to 

 theorise any more.* 



• Any such information may be adcli-essed to G. Hogben, Timani, N.Z. 



