’QUAKE ZONES

Sun (Auckland), Volume III, Issue 880, 25 January 1930, Page 28

’QUAKE ZONES

AREAS IN AUSTRALIA NEW ZEALAND ON EDGE Three zones subject to earthquakes exist in Australia, according to Sir Edgworth David, who, however, points out that the Commonwealth and Tasmania are outside of the main ’quake belt, which encircles the Pacific Ocean. The main boundary of | this belt, he says, is along the east seaboard of New Zealand, tending thence to New Guinea. In an interview with the Sydney “Morning Herald,” he says that the recent very severe quake having its centre near Murchison, in the South Island of Now Zealand, has proved that the Dominion was on the edge of the main volcanic belt. Similarly New Guinea was on the continuation of the belt, together with New Britain and New Ireland. On the north-east side of New Guinea there was the very active volcanic belt from which from time to time shocks preceded. The mainland of that vast island was also subject to earthquakes. Along the main cordillera of New Guinea movement in the way of elevation was still taking place, he said, the tendency being for New Guinea, under the Influence of stupendous forces, to come, from subsidence in the floor of the ocean, nearer to Australia. In the process, not only were the main ranges being elevated; but along the southern coast recent formations such as coral reefs were being slowly but irresistibly raised above sea-level. Such elevation, in recent geological time, has amounted to 1.000 feet, and there could be little doubt that in the near geological future a range of mountains could be raised near the present south coast of New Guinea. In this process of elevation and subsidence, the earth’s crust became bent, and, being composed of rocks of a brittle nature, the crust tended to crack open when its elastic limits were exceeded. “It Is these cracks, formed along areas undergoing elevation or subsidence,” said Sir Edgworth, “which are the chief sources of earthquakes. In the case of the recent Tasmanian quake, it seems clear that it had its . origin in some important crack in the earth’s crust in Bass Strait. , "Speaking generally, one may say there are three main centres liable to shocks of sufficient violence to bring down chimneys and cause loss and

damage. The first is the Great Rift Valley of Australia. St. Vincent and Spencer Gulf (South Australia) owe their origin to large slices of the earth's crust having subsided. This region can be traced along the western ranges, past Lake Torrens, and in the direction of Lake Eyre, both of which owe their origin to subsidences—on the western side of the geological faults which hound the Finders Range, the Great Rift Valley of Australia.” The second area was Bass Strait, said Sir Edgworth. It was obvious that Tasmania was once joined to the mainland. It has been severed from I it chiefly by the formation of what may- be termed the “Rift Valley of Bass Strait,” having its main trend in an east-west direction. There is evidence on both sides of the strait of geological faults, probably connected with the Sorrento crack. The third area in South-east Australia was that on the Main Dividing Range along a line from Kosciusko to the Blue Mountains, particularly' in the region of Kurrajong Heignts. There had been in comparatively late geological time considerable movement of the earth’s crust along the eastern escarpment of the Blue Mountains, from Mulgoa to the eastern foothills, at the beginning of the long ascent to the heights. The great' pressure apparently coming from the Pacific Ocean and the Tasman Sea had forced the earth’s crust westward, thus producing a “fold,” which had necessitated the Zig-zag railway and the long deviation up the mountains. King Tides and the Solstice “As regards the most recent quake,” added Sir Edgworth, “it may be ..hat stresses which had long been accumulating were liberated by the extraattractive pull of the sun, due to its still being near the vertical. Near the summer solstice a stronger pull was exercised on the ocean. Thus it produces the phenomenon of the king tide, examples of w-hich have been seen at Sydney-. It is quite possible that the small extra attraction may have been the trigger action which touched off the earthquake crack in Bass Strait. “While Australia and Tasmania are fortunate in being fairly stable portions of the earth’s crust, and should not be in danger of damage, it would be imprudent,” said Sir Edgworth, “to build structures of very unstable equilibrium, such as tall chimneystacks, in areas where experience has taught us that the intensity of an earthquake shock can be eight or nine. The only spots where such quakes have so far been recorded are Yorke’s Peninsula aud Spencer and St. Vincent Gulfs, and the extreme southern-east corner of South Australia.”