WHEN THE GROUND MOVES

Taranaki Daily News, 23 July 1931, Page 8

WHEN THE GROUND MOVES

EARTHQUAKE THEORIES.

LECTURES ON CAUSES.

Popular interest in the "earthquake” series of lectures arranged by the Wellington Philosophical Society, has in no way abated, says the Post, and the Dominion Museum lecture room was again full to overflowing when Professor C. A. Cotton gave the third lecture of the’series, one on “Earth Movements and the Causes of Earthquakes." Reversing the order of the title, Professor Cotton first touched upon the cause of earthquakes. The old hypothesis, he said, connected earthquakes with volcanic activity. There was sometimes something, in this view, but the connection between the two was now generally considered to be only indirect. The theory that earthquakes were caused by the collapse of subterranean caverns could be put aside. There was another theory that earthquakes were caused by the slipping of soft sediment on the steep slopes of the ocean floor, but this was a confusion of thought as regards caustj, and effect. West of Cook Strait was an earthquake centre which gave Wellington its little shakes; but in this region there was no steep slope and no sliding. Most big earthquakes were due to the slipping of a fault, although this did not exclude the possibility of the arching or folding of strata as well. The Murchison earthquake was due to a vertical fault movement, and possibly that of Napier also. Most big earthquakes could be traced to vertical rather than horizontal fault movement, but the San Francisco ~earthquake of 1906 was caused by a horizontal movement along a fault. The effect had been traced for 200 miles along a Ime parallel to the coast of California. Professor Cotton here showed lantern slides showing the effects of this horizontal move ment. One slide showed the front drive I of a house which before the earthquake finished opposite the front door, as. mos drives do. But after the earthquake it was many feet to one side. "This is the only joke of the lecture,” remarked Professor Cotton, "but it could have been.no joke for the people concerned.” Other shdes showed similar effects upon fences in ew Zealand, resulting from the Napier earthquake and from previous earthquakes. Professor Cotton briefly disccussed what is known as the elastic-rebound theory of earthquakes, and somewhat tickled the imagination of his audience by his description of migrating mountain masses. Earthquakes, he said, could be prophesied on the elastic-rebound theory, in fact it was the only way in which they could possibly be forecasted, but the prophets would not be ab’e to be more accurate than within ten to twenty years. The best that they could say would be that an earthquake would take place some day in a particular locality. Professor Cotton went on to describe the

earth movements that have taken place in New Zealand: in remote, as well as ip more recent times. Many excellent photographs and diagrams showed what was meant by fault scarps, and thein significance, and an interesting account was given of the formation of Wellington’s harbour. A subject ! difficult to those not well versed in geo logy was introduced when the lecturer concluded with a description of the theories accounting for the crumpling of fold mountains.' He mentioned, amongst others, the theory of Hobbs, the American geologist, which postulates a sinking of the Tasman Sea to account for the crumpling of the Southern Alps. The majority of the audience were probably somewhat staggered by the stupendous movements that must have taken place at some time in the world e history to produce the present mountain ranges, and even if they did not altogether grasi. the full geological details involved, they came away from the lecture profound lv grateful to Professor Cotton for a illuminating lecture on an admittedly dim cult subject.