280 Transactions. 



Art. XXIX.— A Note on East Coast Earthquakes (N.Z.), 1914-17. 

 By George Hogben, C.M.G., M.A., F.G.S. 



[Bead before the Wellington Philosophical Society, 12th December, 1917 ; received by 

 Editors, 31st December, 1917 ; issued separately, 24th June, 1918.] 



Between February, 1914, and November, 1917, about thirty earthquakes 

 (not counting shocks of intensity I or II, Rossi-Forel) were recorded at 

 places on or near the east coast of New Zealand. The chief of these were 

 the earthquakes of 6th-7th October, 1914; 28th October, 1914; 22nd 

 November, 1914 ; and 5th-6th August, 1917. All the shocks came from 

 a region to which in a former paper* I have referred the earthquakes of 

 9th August, 1904; 9th March, 1890; and 17th February, 1863. 



The most marked effects of the first three earthquakes (October and 

 November, 1914) appear to have been noted at Tokomaru and other places 

 a little to the south and south-west of East Cape ; but they were sharply 

 felt from Timaru and Greymouth to Auckland. Their intensity was suffi- 

 cient to throw down chimneys in the Tokomaru district, and to stop some 

 clocks in Wellington and other places. 



The movements originating the vibrations appear to have taken place 

 in each of these cases beneath the sea-bed, probably both at the north- 

 east and south-west ends of the three lines Aa, Bb, Cc, beginning at the 

 north-east point, A, B, or C, and ending at the south-west point, a, b, or c ; 

 suggesting the existence of three fault-planes whose position is shown by 

 the lines on the map. These conclusions rest on the instrumental records 

 from Wellington and Christchurch (Milne seismographs), and from Apia, 

 Riverview (Sydney), and Batavia (Wiechert instruments), and a large 

 number of memoranda from telegraph officers in New Zealand, forwarded 

 to me by the courtesy of the New Zealand Post and Telegraph Depart- 

 ment — which it would take too long to discuss here. (It is quite possible 

 that better records would have made the three lines Aa, Bb, and Cc coincide.) 



A fact worth noting is that the Milne seismograms of the 6th October 

 at Christchurch and Wellington both showed a considerable tilt of the 

 ground down on the eastern side (7 mm. at Wellington, 5-4 mm. at Christ- 

 church, or 2-28 and T76 seconds of arc respectively). This might have 

 been attributed to instrumental causes were it not that a Milne-Ewing 

 duplex pendulum at Wellington, set up on an independent column about 

 9 ft. from the column on which the Milne seismograph was placed, showed 

 a tilt of corresponding amount nearly towards the south-east. (The 

 needle of the duplex pendulum is, of course, free to move in any direction 

 horizontally, whereas the Milne seismograph records only the E.-W. com- 

 ponent of the motion.) 



The earthquake of the 5th-6th August, 1917, was most severely felt 

 in the district between Masterton and Castle Point, especially at the 

 former placef — probably because of the alluvial character of the ground 

 on which the town is built. The epicentral area is near F, in lat. 42° 41' S., 

 long. 178° 12' E., which is not far from the origin of the earthquake of 



* G. Hogben, Notes on the East Coast Earthquake of 9th August, 1904, Trans. 

 N.Z. Inst, vol. 37, pp. 421-24, 1905. 



f Probably the intensity at Masterton may be described as between VII and VIII 

 on the Rossi-Forel scale. 



