FAULTS IN THE EARTH

Taranaki Daily News, 7 February 1931, Page 1 (Supplement)

FAULTS IN THE EARTH

LINES OF DANGER ZONES

(By

“Scriblerus.”)

Although geological research and survey in New Zealand commenced at a very early date in the colonization of the Dominion and was carried to a high point of efficiency as early as the 60’s, co-ordination of geological information with intensive study of earthquake activity is really only a development of the past decade.

Briefly, New Zealand forms a portion of -the more or less continuous belt of mountains which geologists state was raised around the borders of the Pacific Ocean in comparatively recent ages. This belt which is sometimes known as the “fire girdle of the Pacific” is characterised by “chronic and acute seismicity.” Dr. Adams writes that at times earthquakes occur within the belt itself, though a large proportion have their epicentres on its submerged frontal slope.” The epicentre of an earthquake is the point on the surface of the ground vertically over the point below ■the surface Where the vibratory impulses of the ’quake originate. Earthquake activities, as well as volcanic eruptions, are “manifestations of the adjustments constantly occuring in the earth’s crust.” New Zealand, Professor James Park tells us, is traversed by a number of powerful faults “that group themselves into three main systems. The chief of these run parallel to the contour of -the mountain chains and traverse both North and South Islands.

It was on this main system that the disastrous Buller district earthquake of June 17, 1929, occurred which resulted in the loss of 17 lives and serious damage to property. Also it was in this system that the recent Hawke’s Bay, disaster originated. The fault on which the quake had its' epicentre has not yet been decided. > . Of the remaining two systems it m enough to say that one group of faults follows a north-west —south-east course, running at right angles to the majoi system. One fault crosses the Otago province, and another Cook Strait. The third system traverses the Kakanuian and Maniototian schists of Central and Western Otago with a general trend from north to south. In the major system, the Whakatane fault commences at the northern extremity of the Alps, and runs north along the edge of the Spence and. St. Arnaud mountains. The 'Buller district, lying north west of this fault, was devastated in 1929. Northwards across .the Cook Strait the dislocation can be traced until it emerges in thyi North Island near Wanganui, and at continues on past Ruapehu and Lake Taupo to Whakatane. The more noticeable features of this line of weakness are described (by Professor' Park as being evident at Whakatane, where the cliffs behind the township contain strata which is “broken, sharply folded, faulted, sheared and up tilted.” At Nelson, where tile fault leaves the South Island “the Miocene brown coal-measures are tucked by it against the Triassic rocks of the Richmond and Hope Hills.” Apparently the recent- calamity in Hawke’s Bay has occurred in close proximity to the Wairarapa or Clarence fault, which, is the most important line of weakness in New Zealand. Earthquakes occurring on this fault shake large areas, and in effect might be described as “world shakers. Quakes on this line are described as tectonic and are caused by the slipping of earth blocks against their neighbours along fractures. The Clarence fault is a, dislocation in the crest of a gigantic ridge, or crustal fold, which forms the 'South Island and the eastern part of the North. This ridge, which is a portion of the eastern border of the Pacific, traverses a line running through New Zealand on a course north-east for 1600 miles nearly to Samoa. This great boundary line projects eastward from New Zealand far into the Pacific. In the past the most severe earthquakes have had their- origin along the great shears that probably cut the submerged flank of the main fold about 200 miles east of the North Island. Here it is probable there is a third parallel fault. The chief manifestations of the Clarence fault follow the Raukamara Range southward past Lake Waikaremoana across the coastal plains of Hawke’s Bay and past Napier and Hastings. From then on it continues down the valley between the Ruahine, Tararua and Rimutaka Ranges and the Puketoi Hills. During the great earthquakes of 'the fifties earth-rents traceable for many miles opened along this latter portion of its course on the west side of Wairarapa Lake. In the South Island it follows the Clarence Valley, its course being traceable by movements of recent date, and by a line of depression along the surface of the ground for a distance of fifty miles. Earthquakes in the great Clarence or Wairarapa fault have been frequent even since the district has been colonized. The Wellington quakes of. January, 1855, probably originated in the portion, which forms the eastern boundary of the Rimutaka Range and the western boundary of 'the Wairarapa Valley. Great earthquakes occurring in October, I'B4B, probably came from the same region. The >chief shocks of both series did extensive damage to property, and caused the formation of large rifts in. the earth’s surface. The Cheviot quakes of November I’6, 1901, have also been' ascribed to tectonic action at the southern continuation of the fault.

In the past there - have been many earthquakes of a more or less violent character -which have affected the towns and district of Hawke’s Bay. The Government seismologist stated recently that those felt most strongly on the south-east coast of the North Islandoriginated from a strip 180 miles from ■the coast, parallel to the axis of New Zealand, and to the axis of folding of older rocks in Hawke’s Bay. He enumerates the chief shocks as occurring on August 17, 1868; March 7, 1'800; July 23 and 29, 1904; August 9, 1904; and September 8, 1904, and probably on February 23, 1863. ' The three _ latter were very strong shakes and registered an intensity of 9 on the Rossi-Forel scale as compared with 10 registered during the Buller quake in June, 1929. On the other hand Hastings and Napier were both situated on or near the Clarence or Wairarapa fault which definitely was the seat of the Wellington shakes of the fifties. It is inopportune for a layman to venture an opinion until the experts have given their decision, but it does seem significant that as a result of the recent earthquake there should be a rising of the sea floor to a height of 7 feet above the old level, m such close proximity to the line of dislocation formed by tha Clarence or Wairarapa, fault.