EARTHQUAKES IN NEW ZEALAND

Dominion, Volume 35, Issue 262, 4 August 1942, Page 4

EARTHQUAKES IN NEW ZEALAND

Seismologist’s Comments PRESENT SERIES THOUGHT DUE TO ADJUSTMENTS “On the fuller data now available it is possible that the original estimate of the strength of Sunday’s earthquake as 7 plus at Wellington was a little under-estimating it,” the acting Dominion Seismologist, Mr. Hayes, said yesterday afternoon. “The figure of 8 on the scale may ultimately be found more correct. Earthquakes of the latter strength are fairly common in New Zealand, and they are not always followed by after-tremors; “While the earthquake of June 24, centred in the Walrarapa, for which the strength estimate has been placed at 9 plus at the epicentre, can certainly be classed as of major magnitude, there is uo evidence as yet that would suggest Sunday’s disturbance was of the same order. The June 24 quake was followed by a large number of after-tremors, running into several hundreds in the first 24 hours. Up to 9 o’clock this morning the seismographs at Kelburn have only recorded about a dozen after-tremors to Sunday’s shake, and only one of these was of sufficient strength to be felt locally or noticed by such things as rattling windows or creaking walls.”

Discussing the damage caused by the latest disturbance, Mr. Hayes said it was possible that Its widespread nature and seriousness was due to the fact that its direction differed slightly from that of June 24, and the strain placed on the already damaged buildings therefore fell in a different way. Mr. Hayes regards the slight earthquake recorded at 4.48 p.m. on Saturday as a preliminary movement associated with the later disturbance. The strength of this one he placed at approximately 4 locally. Sequence of Quakes.

Reviewing the sequence of earthquakes in New Zealand, Mr. Hayes said that records over the past century showed that iu the years-from 1848 to 1855 there was a series of disturbances of strength 10 on the scale, with quite a number of strength 9. After that, till 1929, there was a quieter period, with occasional ehocks of strength 9. In 1929 began a fiveyear period of heavier disturbances, which included the Buller (Murchison), Hawke’s Bay and Pahiatua quakes. “Following those earthquakes,” Mr. Hayes said, “we felt that it was probable‘that a further period of comparative quiet might be expected. The present series is possibly due to adjustments taking place in the strata between the centres of the Buller, 1929, and Hawke’s Bay, 1931, disturbances. “The preliminary investigations bf the earthquake which occurred at 7.44 p.m. on Sunday, July 26, show that it was of deep-seated origin,” said Mr. Hayes, “possibly as deep as 50 miles or more into the earth’s crust, and its epicentre was at sea near the entrance to Cook Strait, a little to the northeast of Stephen Island. This is, of course, entirely different to the epicentres provisionally assigned to the June 24 and last Sunday’s quakes. The June 24 disturbance was centred in the Masterton area, and Sunday’s shock seems likely to have come from somewhere near the same place. The June 24 quake had its origin close to the earth’s surface, possibly no deeper 1 than about 15 miles down.”