Hogben. — Theory of Neto Zealand Earthquakes. 507 



about 353 miles east of Launceston ; it lies on the western 

 slope of the Thompson Basin, the great trough in the Tasman 

 Sea. The same region contains the origins of at least three 

 well-marked Tasmanian earthquakes — namely, those of 13th 

 July and 19th September, 1884, and 13th May, 1885. 



From April, 1883, to December, 1886, 2,540 shocks were 

 recorded by Captain Shortt, R.N., of Hobart, and his assistants, 

 nearly all being very slight. Probably the earthquakes just 

 named above were due to the principal movements, and the 

 numerous smaller after-shocks or tremors indicated the slight 

 adjustments of the Tasmanian land-mass. We have yet to dis- 

 cover what connection, if any, there is between these and other 

 movements in or near Australia and Oceania, and those of New 

 Zealand. Most of the evidence available seems to point to the 

 hypothesis of a general elevation of the floor of the Western 

 Pacific ; but the evidence so far is very meagre and discon- 

 nected. 



In the case of the New Zealand earthquakes, on the other 

 hand, I think that a careful study of the map and of the facts 

 mentioned in the text will immediately suggest the theory 

 that our earthquakes are incidents in the history of folding 

 and similar movements that have been going on for ages, the 

 axes of the folds being parallel to the general axis of the country. 



The origins of the New Zealand seismic region will be seen 

 to arrange themselves in groups as follows : — 



Group I. — Earthquakes felt most strongly on south-east coast of 

 North Island ; epicentra form a strip 180 miles from the coast, parallel 

 to the axis of New Zealand, and to axis of folding of older Cainozoic rocks 

 in Hawke's Bay. Chief shocks: 17th August, 1868; 7th March, 1890; 

 23rd and 29th July, 1904; 9th August, 1904 (intensity IX on R.-F. scale); 

 8th September, 1904; prob. 23rd February, 1863 (IX, R.-F.) ; &c. 



According to Captain F. W. Hutton, F.R.S., the geological 

 evidence shows that New Zealand rose considerably in the 

 older Pliocene period, and was then probably joined to the 

 Chatham Islands. At a later period subsidence occurred, 

 followed again by elevation in the Pleistocene period, with 

 oscillations of level since. The seismic origins of this group 

 are at the foot of a sloping submarine plateau, about two hun- 

 dred miles wide (marked B on the map), which culminates 

 to the east-south-east in the Chatham Islands. This elevation 

 is separated from the New Zealand coast by a trough from 

 1,000 to 2,000 fathoms in depth, which is widest and deepest 

 at A A — that is, between these origins and the mainland. 



Group II. — (a.) South-east of Otago Peninsula. Shocks: 20th No- 

 vember, 1872, &c. 



(b.) A striD south-east of Oamaru. Shocks: February, 1876; April, 

 1876; &c. 



