EARTHQUAKE SCIENCE

Hokitika Guardian, 4 August 1931, Page 3

EARTHQUAKE SCIENCE

FAULT SYSTEM

LECTURE BY GEOLOGICAL director. WELLINGTON, July 29. “The recent earthquake was not an isolated phenomenon, but merely a twinge in the growing pains of New Zealand,” declared Dr J. Henderson, Director of the Geological Survey at the fourth of a .series of “earthquake” lectures arranged by the Wellington Philosophical Society. The three previous lectures of this popular series hav.e dealt with earthquakes from a world point of view, but the latest talk brought the subject near home, so to speak. Dr Henderson dealt with the fault systems of New Zealand, and with their relation to those bordering the Pacific, intimating that, geographically speaking, New Zealand cannot bo regarded as an isolated unit. It has a very definite relationship to the vast Pacific Ocean and, as part of the border of that big basin, its geological, and therefore seismological, problems are closely connected with those elsewhere.

EARTH’S RIGID CRUST. The fire girdle of the Pacific, or line of active volcanoes on its margin, said I)r 'Henderson,- marked belts of mountains that persisted around it and crossed from continent to continent without losing identity. Close in front of the mountains, the ocean floor, elsewhere generally featureless, drops down in steep narrow trenches, or “deeps." For example, a section of the earth at the fortieth degree of south latitude, not far from Napier, showed the Andes towering over the Atacama deep, aim Neiv Zealand overlooking the Tonga trench. The surface of the' earth, the. lecturer pointed out, was not wrinkled evenly like the skin of an apple. The crust was rigid and adjusted itself to the shrinking core in Hakes, some sinking to form the ocean floor, and some pinching up to form the continents, with crinkling pud ; folding along . the margins into mountain chains. • It , was therefore fairly probable that the earth's features were formed by compression forcing up the continents and mountains above the oceans and lowlands in earth-blocks with wedge-shaped or curved bottoms.

Dr .Henderson discussed the formation ol New Zealand and its piobable effect on the Napier earthquake. He suggested that the South Island was more tightly compressed than the North Island, being folded up in higher earth-folds. The maps he showed made it clear that the mountain ranges were marked off by faults arc-shaped in plan,'many high along the western side and tilted gently to the east, and some apparently pinched up from both s d s. Sections across Hawkes Bay il'ustrat 1 d the structure and indicated how the earth-blocks moved when Napier was shaken by the recent earthquake. This, .he said, was not an isolated phenomenon, but was merely a twinge in the growing pains of New Zealand. Although no definite conclusion could be reached, it would appear that the Kaikoura-Spenser mountains and the TaupoMJotorua district were among th“ most unstable parts of New Zealand.

NEW ZEALAND FORTUNATE. The Pacific, said Dr Henderson, had been formed by heavy earth blocks sin A tig towards the earth's centre and fracturing the margins of coiiC-mids. Tt consisted of two basins, the larger :n the north, with its floor pressing most strongly at present against Asia and ■North Australia, the most active sr.s mic region in the world. New Zealand, however, was structurally co inected with the nothern boundary ot Australasia through the nprth-westwa' >1 trending Auckland . peninsula and the many island ridges on the Justvalas an nlatform. These; lines of islands were all parts of. a system of foldjl, probably formed when the main basin of . the Pacific sank in late Tertiary times. \ll were' lines of hut fortunately for New Zealand the movements now were concentrated along the outer arcs, where the earthquakes were numerous and strong. The main axis of New Zealand, however, was forced up by the sinking o» the South Pacific, which fortunately for us, was pressing more- heavily on South America than on New Zca and, so much -so, that four “world shakers occurred annually there to one e\eiy few years in New Zealand.