’Quakes in New Zealand

Sun (Auckland), Volume IV, Issue 1075, 12 September 1930, Page 8

’Quakes in New Zealand

SOUTH ISLAND’S MANY SHOCKS Record for Last Year Special to THE SUN ' WELLINGTON. Today. NEW ZEALAND experienced 678 earth tremors last year The great proportion of the shocks, numbering 632. succeeded the disastrous earthquake in the Murchison area on June 17 until December 31. In the annual report of the Department of Scientific and Industrial Research, Dr. C. E. Adams, Dominion seismologist, also reveals that the North Island was far freer of seismic disturbances than the South, having only 93 shocks, compared with 624 in the South. Thirty-nine tremors were felt in both islands. The greatest period of disturbances to the South Island was June and July, in which 88 and 234 shocks were felt, respectively.

The maximum intensity of the quakes was between 9 anil 10 on the RossiForel scale, and was felt at Murchison •and Lyell on June 17. The greatest intensity of shocks in 1921 and 1922 was 8; 1923, 6; 1924, 7; 1925, 1920. 1927, and 1928, 8, on the same scale. More shocks were felt during the night than in the day, probably partly because many feeble tremors which would be noticed at night would pass unnoticed in the daytime. The series of earthquakes beginning on June 1 7 last year that caused so much damage to Murchison and adjacent parts of Western Nelson, resulted from a sudden relief of pressure in the earth’s crust, according to the director’s report on geological survey work. “THE NEXT RELIEF" "Probably the earthquakes that have shaken New Zealand during the last 18 months are to be attributed to the same cause—that is, to the stresses that had accumulated along the great earth ridge, parts of which rise above sea-level as the South Island and the eastern half of the North Island of New Zealand. The first intimation of unusual conditions along this ridge was perhaps the outflows of mud in the Gower Valley, in the northern part of Canterbury, in September to December. 1928. Next followed the earthquakes at Otira, Feilding, Murchison, Porangahau, and Opotiki. If the series is not yet complete, this distribution suggests that the next relief of pressure along the elevated ridge will take place somewhere between Otira and Stewart Island, though the numerous minor shocks still occurring in the northern part of the South Island' suggest that this region is not yet altogether stable. "The principal earthquake was due to movement along the well-known White Creek fault that crosses the Buller about seven miles west of Murchison. This fault has a general northerly course, and the country east of i it was raised and tilted, the uplift at the fault being about 15 feet, but this diminished gradually eastward till at Mangarata, 12 miles away, it is not detectable. There is, unfortunately, but one line of levels available, that

along the Buller River, and the movement north and south of this line is not known. Preliminary surveys indicate a horizontal movement of about six feet, by which amount the block east of the White Creek fault apparently overlaps the block west of it. IMPOSSIBLE TO COUNT SHOCKS “Tremors were felt at Murchison some hours before the principal shock, and innumerable after-shocks, which i for several days were so frequent that it was impossible to keep count of them. From the fact that the shocks are still continuing, nearly a year after June 17, it may be argued that the nice adjustment of very large masses of rock was disturbed, and that the focus was a considerable, depth below the surface. Observations of residents of the district, and the records of distant seismographs, prove that the aftershocks did not originate all from the same focus, and could not all be attributed to the growth of the White Creek fault. Indeed, adjustments of a considerable group of earth-blocks probably took place along all the major faults of the region. Blips from hillsides are conspicuous, and furnish some guide to the intensity of the earth movements, though the slope of the ground, the texture, structure, composition, and attitude of the rock forming the substrata, and. if the land is cleared, the time of clearing, are important in determining the size and number of slips. Though the great landslides near Murchison were caused by the main shock, there is evidence that some slips at least were due rather to shakes from local centres of eartli movement. The distribution of the slips, combined with the evidence from the seismographs at Wellington and Christchurch, makes it probable that, in addition to the White Creek fault, the following fractures or fracture zones were among the faults that were active: (1) The faults bounding the Radiant and Glasgow Ranges on the east; (2) the faults along which the valleys of the Upper Mokihinui, the Upper Karamea, and Matiri Valleys have been carved; (3) the faults about Mount Owen: and (4) the Waimea fault, south-west of the Nelson lowlands.