502 Transactions. 



Art. LVI. — Brief Notes on the Theory of New Zealand Earth- 

 quakes. 



By G. Hogben, M.A. 



[Re%cl before the Wellington Philosophical Society, Mh October, 1905.] 



Plates LIII and LIV. 



From time to time papers have appeared in the Transactions 

 of the New Zealand Institute and of the Australasian Associa- 

 tion for the Advancement of Science dealing with individual 

 earthquakes. Captain Hutton's monograph on the Amuri 

 earthquake of the 1st September, 1888, marked a new epoch in 

 seismological science in New Zealand, as it treated the facts 

 of that disturbance in accordance with the ideas of modern 

 seismology. I have published during the last sixteen or seven- 

 teen years several other papers in which I have endeavoured 

 to follow the same general lines in regard to all the important 

 shocks that have occurred. Although, perhaps, we cannot 

 yet with any certainty elaborate a complete theory of the earth- 

 movements in the New Zealand region, nevertheless it appears 

 to me that the time has arrived when we may attempt to co- 

 ordinate the facts in our possession by some general explana- 

 tion of them so far as the earthquakes enable us to do so. 



The most accurate observations made in the colony have 

 been those afforded by the two Milne horizontal pendulums 

 installed at Christchurch and Wellington respectively. Some 

 of the inferences to be obtained from their records, or seismo- 

 grams, may be most readily understood by means of a few 

 general remarks ; as illustrations I take the copies of four 

 seismograms on the New Zealand instruments (Plate LIII, 

 figs. 1-4). 



Fig. 1 on Plate LIII is a copy of the Christchurch record 

 of the great Guatemala earthquake of the 19th April, 1902. 

 The time is Greenwich mean civil time. 



Fig. 2 shows a severe earthquake, on the 20th November, 

 1902, from an origin probably near 21° S. lat., 172° W. long — 

 that is, north-east of Tonga (Wellington record, G.M.C.T.). 



Fig. 3 is the Wellington record of the earthquake at Cheviot, 

 New Zealand, on the 16th November, 1901, beginning at 7.47 a.m., 

 New Zealand mean time. 



Fig. 4 is a copy of the seismogram taken at Wellington of 

 the East Coast earthquake of the 9th August, 1904 (G.M.C.T.). 



The small letters a, b-h, above fig. 1 mark the beginning 

 of the eight phases into which Professor Omori divides the 



