5. “Remarks on Earthquakes in the Amuri District, South Island,” by Alexander McKay, F.G.S.
The author commenced by stating that, for twelve months previous to the end of August last, booming noises, proceeding from the ground, had been heard in the district surrounding the Hanmer Plains, and that towards the end of that month earthquakes began to be experienced; these premonitions were followed by the great shock of the 1st September, which did nearly all the damage that happened to buildings, and opened most of the fissures that are yet to be seen. This was followed by the shock of the 28th September, and, after a like period, by that of the 23rd October, and those of the 26th and 28th of the same month; there being just about a lunar month between the first and second and the second and last series of shocks. Mr. McKay then described the effects the earthquakes had produced, and gave a detailed account descriptive of the fissures opened at many places along the Waiau-ua and Hope Valleys, more especially those seen near the mouth of Gorge Creek near Hopefield, at Hopefield, and at and near Glynn Wye. The present ruined condition of the buildings at Glynn Wye was described, and the manner in which the fences
had been broken and shifted 8 feet 6 inches to the east of the line in which they originally stood. Next it was shown that the evidences consisting of slips and earth-rents are confined to a narrow belt of country extending S.S.W. from the east corner of the Hanmer Plain to the Upper Hope Valley, abreast of and about six miles to the north of Lake Sumner, in the Hurunui Valley. Beyond this point to the westward the line of dislocation was not examined. All these rents and fractures lie along a line of previous earthquake disturbance, the old fractures indicating this being traceable on the surface where the line does not run along river-beds, liable to be flooded and leave the surface shingles rearranged. The eastern continuation of this line of old fractures caused by earthquakes was described as extending to the eastern base of Mount Fyffe, near Kaikoura. Glynn Wye was described as being the point on this line at which the most violent disturbance of the surface took place; while Westport, 60 miles to the north, Christchurch, 65 miles to the south, and Kaikoura on the east coast and Hokitika on the west coast, were the limits to which the earthquake extended as a shock violent enough to do damage to buildings, &c. Mr. McKay said that, while not touching the question what the primal cause of earthquakes may be, he felt sure that the Amuri earthquakes, in as far as they were expressed at the surface and had been studied by him, were due to crushing movements along the old earthquake-line; and he went on to show that in the northern part of the South Island, and, indeed, throughout the islands of New Zealand, there are many old faults, showing a great vertical displacement, running coincident with earthquake-rents opened but recently, though not for the first time. The whole of the northern part of the South Island, it was stated, was being elevated, and a series of parallel fractures gave relief to the resulting strain, which relief, at the moment of its happening, produced the earthquake.
Sir James Hector considered that this paper, as a simply-told narrative of the observed facts, would become classical in the literature of earthquakes, and he complimented the author on its excellence. He did not quite agree with all the author's deductions, however. The mere linear extension of fault-lines did not determine a liability to earthquakes. There must be a lateral stress or condition of strain in some part of the fault-line. As he had pointed out last year, a violent concussion might originate from a slickenside surface in a fault. He quite agreed that in this particular case there was nothing to connect this shock with volcanic action, present, past, or future. It seemed to be a localised fault-movement, no doubt produced by the jar of a wide-spreading earthquake-shock of the ordinary mild character.
Mr. McKay, in reply, stated that we could only deal with what was open to observation. The facts went to prove that from some point not far from Glynn Wye the force of the shock diminished in all directions; and practically there or thereabouts the centre of the disturbance must be placed. As to the influence of the great faults, it mattered little whether the earthquake produced the faults or the faultings were the cause of the earthquake—both were effects of a greater movement behind either or both; but, as the faults and earthquake-rents were in this district on the same lines, the earthquakes were always most severe in the near vicinity of those lines. Mr. McKay said he spoke not of the numerous lesser faults that are to be found all over the country, but of the five or six greater faults whose movements have stamped with peculiarity the physical features of the whole district.