TERRA INFIRMA

Wanganui Chronicle, Volume 76, Issue 188, 11 August 1933, Page 8

TERRA INFIRMA

HAWKE’S BAY ’QUAKE A SCIENTIFIC REVIEW THE CAUSES DISCUSSED. Although it is two years and six months since the disastrous Hawke’s Bay earthquake, it has taken most of that time for scientists to investigate thoroughly all tho phenomena connected with the upheaval. The results of their labours have been correlated in an exceedingly interesting and very comprehensive special Hawke’s Bay earthquake number of the New Zealand Journal of Science and Technology. A general description of the whole occurrence is given by Mr. F. R. Cal laghan, who writes a very vivid summary. The geological aspects of the earthquake are ably presented by Dr. J. Henderson, Dr. P. Marshall, and Mr. S. W. S. Strong, while Dr. C. E. Adams, Dr. M. A. F. Barnett, and Mr. R. C. Hayes contriute the seismological section. Observations on the damage done to buildings are made by Mr. A. Brodie and Mr. A. G. Harris. The whole, together with tho photographs, maps, and plans, makes as comprehensive a survey as could be wished for. Without any apparent preliminary warning the earthquake occurred at 10.47 a.ni/on February 3, 1931. The swiftness of the catastrophe was one of tho features, and those articles raise the question whether there had been preliminary indication. They appear to have boon slender, and certainly not portentous. “It is asserted,” states Mr. Callaghan, “that in one or two instances where building operations re-

quiring precise levelling were in pro gross, structures in which care had been taken to ensure correct levels were found to bo out of alignment, and gave evidence that tilting had preceded the actual earthquake. It is difficult, however to substantiate completely these claims. ’ ’ It has since been noted as a significant fact that two days prior to the shock an unusually heavy swell was breaking along tho foreshore at Napier and scattering spray across the Marine Parade, though there was an entire absence of wind during the day to account for such a marine disturbance. Hundreds of After-Shocks. It has been found from comparison of seismograph records that the epi-centro of the great shock was near the coastline of Hawke’s Bay, from five to 15 miles north of Napier. Records of the after-shocks show that there were no fewer than 596 in the month of the disaster, and that they continued

throughout the year, the aggregate being 938. The monthly records show a constantly-diminishing number, until in December there wbro only 12. The writers of the seismological aspect of the earthquake remark that tho region of tho shore line of Hawke’s Bay and the area of the bay itself had not been a region of earthquake origins during recent years. Th e actual region which was so disturbed on February 3, 1931, and for some months after, had been inactive during the past decade at least. Perhaps the stresses in the crust had been gradually accumulating over a considerable period until a state of unstable equilibrium had been reached, and a violent dislocation resulted. Discussing the after-shocks, the same writers stated that they originated beneath the bay itself, and the majority in the southern portion. A distinct lino of weakness, they state, can be traced in a northerly direction from Cape Kidnappers, and another, extending from the cape in an east-north east direction for .10 or 15 miles, and then striking north-east right across the mouth of the bay. ’Che main shock gave rise to an unstable condition of the sloping ocean bed to the eastward, resulting in gradual adjustment and frequent earthquake shocks. Earth Block Uplifted. Movements in the bed of the Pacific arc discussed as being the cause of earthquakes in New Zealand, and it is demonstrated how a great thrust from the Pacific is still operating. The actual operation of these forces is traced with great care over the whole arer by Dr. Henderson, who writes on tho geological aspects of the earthquake. He points out that this immense thrust movement can be distributed by innumerable crustal adjustments of tho earth. Pressure from the sea floor and counter-pressure from the highlands act as a “couple,” the deeplying rocks opposite the forces of the Pacific are “contained,” whereas those underlying Hawke’s Bay arc free to move upward, provided the upward component of the thrust is able to overcome gravity and friction. He reaches the conclusion that an earth block 60 miles long in a north east direction and at least 10 miles wide at one pointy ha* been uplifted. Many details of well-as-certained changes in level are quoted by the writer, who draws the conclusion that a large portion of this earth block was tilted gently and uniformly to the north-west, and that the inference is that the. whole block was so tilted.