EARTHQUAKES

Sun (Auckland), Volume 1, Issue 12, 5 April 1927, Page 8

EARTHQUAKES

ONR OWN FAULT (Written for THU SU2>.? THE visit of an American scientist. and a conference of New Zealand scientists in Wellington last week, have combined to give to earthquakes some of the publicity usually secured by saints, sinners or statesmen. Which emphasises the fact, unrecognised by most of us. that New Zealand is one of the noted earthquake countries. Though we have but one • worldshaking” quake to our record, that of ’55, which permanently raised several feet part of Wellington province as large as Yorkshire, it is well to remember that our innings has not been closed. In 1922 an area of about 100 square miles on the northern shores of Lake Taupo was depressed by amounts ranging from a few inches to ten and twelve feet. Earthquakes may be divided into volccinic quakes and crust movement quakes. Volcanic eruptions often cause violent earthquakes, due to the explosive action at the vent. These are never destructive over any great area, the centre being too close to the surface. Four-fifths of the destructive quakes, which devastate large areas and whose vibrations shake the globe, are due to crust movements and occur independently of volcanic action. And in volcanic countries the most severe shakes have their centres in parts away from the volcanic vents. The recent Morrinsville disturbance, was but one New Zealand illustration of this. ’Quakes of this type are due to the growth of faults, the movement of one huge rock mass against another along fracture lines in the crust. Segments of the crust bounded by such faults tend to move periodically, the most favoured explanation being at present that of isostacy. By this is meant that condition of form to which gravitation tends to restore bodies which, like the earth, are in a condition of rotation. All such bodies must possess a form of equilibrium. When this is reached no further change will occur provided the distribution of matter remains the same. A THREE-MILE CRACK This is not so with the earth, the land being constantly denuded by the weather and the material deposited in the oceans, thus disturbing the form of equilibrium. Other possible contributing causes are contraction of the earth due to cooling, effect of gravitation on crust segments, and the flow of surface layers of the crust. The amount of movement necessary to cause a severe quake may be only a fraction of an inch. But a movement of several feet may occur and even extend to the surface. Thus in the Taupo quakes of 1922 several fault scarps were formed. One of these extended about three miles, a great crack running roughly north and south, the western side depressed an average amount of six feet. Similar fault scarps were formed during the Cheviot quakes in the South Island in 1901. New Zealand, situated on a line of crustal instability,- practically encircling the Pacific, has, in addition, a good collection of its own faults. These fall into three systems. Those of the major system run parallel with the main lines of elevation of the islands, north-east and south-west. One of these is the well-known Moanataiari fault, which traverses the Thames goldfield, a feature of economic importance from its influence on the distribution of gold. This fault, while not having an Act of Parliament to itself as did Pelorus Jack, was sufficiently known to the authorities to be constituted by Act the eastern boundary of the Thames Drainage Board’s area. Running almost parallel with this is the Cook Strait fault, marking a dislocation between the two islands and indicating that the North Island has been thrust eastward some distance relatively to the South Island. The second system of faults follows a course at right angles to the first. Belonging to this is the Whakatane fault, crossing the centre of the North Island from Whakatane through Lake Taupo and Ruapehu to Cook Strait, and in the South Island skirting the Waimea Plain. THE WISDOM OF PRECAUTION Along this fault lies our thermal and volcanic district. Parallel with it is the Wairarapa fault, following the east side of the Rimutaka Range from the sea north to Masterton. In the quake period of ’55 earth rents traceable for miles occurred along this fault line west of Lake Wairarapa. The faults of the third system run roughly north and south across Central and West Otago. The faults of all these systems invariably follow the direction of the main valleys and inland depressions, which most probably owe their origin to the presence of the faults. Reference has already been made to the great Wellington quake of *55. One of similar intensity to-day would mean a disaster rivalling that of ’Frisco in 1906, allowing of course for the comparative sizes of the two cities. One earthquake, equal in intensity to any of several experienced in the Taupo country in 1922, occurring say along the Moanataiari fault from a possible centre in the Hauraki Gulf, would

WHAT THE QUEEN SAW

mean loss of life and thousands of pounds’ worth of damage. This fault is apparently of very recent origin geologically, and it cannot be said that further movements along it are impossible. In 1897 part of the land bordering Disenchantment Bay in Alaska was raised 47ft, an indication of what earthquakes can do. Such a summary shows the wisdom of taking precautionary measures in the construction of buildings, particularly the larger ones in our growing cities. In architectural and engineering practice in this direction we can learn much from Japanese, American and Italian investigators. The scientist who gives us a word of warning is neither an alarmist nor a prophet. He is merely an expert knowing whereof he speaks. And he speaks of probabilities due attention to which may save us much in lives and wealth and cannot, in any case, involve us in any great expense. RALPH H. WARD.