EARTHQUAKES

Evening Star, Issue 21822, 11 September 1934, Page 2

EARTHQUAKES

CAUSE AND RESISTANCE ADDRESS BY MR W. L. NEWNHAM “ Although earthquakes of considerable magnitude have occurred in New Zealand at different times since the white man’s occupation of the islands, it is only since the recent earthquakes at Murchison and Napier that more than passing interest lias been evoked by them,” said Air W. .L. Newnham, chief designing engineer of the Public Works Department, Wellington, when giving an address on ‘ Earthquakes and Building Construction ’ at a meeting of the New Zealand Insurance institute, Dunedin centre, last night. Air A. Alumford presided.

‘‘ The loss of life in these two earthquakes, particularly Napier,” ho continued, “ has made New Zealanders realise that the dominion is a country of permanent seismic activity, that severe earthquakes will occur again in tiie future, and that urgent and definite action is necessary in order to mitigate the loss of life and the damage to properly when such earthquakes do occur.” Air Newnham explained that the subject of the address was such a vast one that he proposed to give only a few notes on certain aspects of the question. Compared with earthquake disasters in other countries, New Zealand, being a young and comparatively sparselysettled country, had so far been extremely fortunate, and had not suffered the loss of life and property which other countries had'experienced. Although architects and engineers were more concerned with the effects of earthquakes than with the causes, the latter aspect was very interesting. In the past all manner of theories regarding the cause of earthquakes had been advanced, but scientists now generally agreed that earthquakes were the vibrations of waves set up in the earth’s crust by the sudden movement of earth blocks, which Avere under very high stress, such movement releasing the stress and restoring equilibrium. This stress might bo caused in many ways, but it was often caused by the growth or movement of a fold in the earth’s crust, parts of Avhich plight be sinking in one place and xising in another. The earth block movement might be confined to one small area, or it might bo hundreds of square miles in extent. The movement took place along a plane of fracture called a fault, and it ivas from the study of these faults that deductions concerning the seismic activity or otherivise of a locality were made. There were also earthquakes due to volcanic activity, but they were very local in their effect, and the great majority of earthquakes had no connection with volcanic activity. Speaking broadly, tectonic earthquakes, as the movement described Avas known, were more frequent along steeper slopes of the earth’s surface. The slopes running eastward from Japan and ivestivard from the Andes were very steep, while the slopes running from most parts of Australia, Eastern America, and Western Europe were very fiat, and in these latter regions earthquakes were very rare. The effect of an earthquake wave upon a building was to generate a'force directly proportional to the weight of the building, tending to push it forward in the direction of the Avnve or accelerate it from rest to motion. It was the correct measurement of this tendency to motion or acceleration. Avhich was one of the fundamental necessities of correct earthquake-resisting design ■ and construction. Unfortunately most of the instruments or seismographs evolved by scientists ivere capable only of measuring the acceleration at a considerable distance from the main centre of the disturbance. The records obtained from them were not of much value for designing structures to resist earthquakes at or near the epicentre. During recent years efforts had been concentrated on the construction of instruments that would enable .records to be obtained adjacent to the epicentre. Some of these strong motion seismographs or accelerographs had been set up just prior to the recent Longbeach earthquakes, andsome fairly accurate records were obtained nothin twenty miles of the epicentre. Unfortunately most instruments close to the arba of major disturbances ivere put out of action at the first strong movement of the earth'." Until means AA r ero devised for accurately measuring the force of an earthquake in the area of maximum disturbance structural designing must be based on assumption. It should be borne in nxind that all work in earthquake-resisting designs Avas strictly limited to assumptions as to the forces to be provided for, and that extreme refinement in design was under these conditions not Avarrantecl. It Avas bettor to concentrate on the question of good workmanship, uniformity of construction, and standardisation of materials and methods, with resultant economy.

The speaker then dealt with gravity values of earthquakes, giving examples of various values recorded. The earthquake forces were very complex, and the means of measuring them were very limited. The scale now used almost universally for describing the intensities of was the Uossi-Forel scale devised in 1883 by the two men whoso numes.it bore. It was based for tho lessor quakes chiefly upon the sensation produced, and for tire larger earthquakes upon descriptions of ,the effects of the earthquake in wrecking structures. It was a very crude scale, but up to the present was the only possible one to uso when investigating earthquakes from an historical point of view, and when comparing earthquakes in one country with those in another.

Since the days of the white settlement, New Zealand had experienced hundreds of earthquakes of considerable magnitude, and countless minor ones. During the ten years from 1921 to 1931, for instance, 3,286 earthquakes wore recorded. Since 1855 thorp had been at least twenty earthquakes of major magnitude, culminating in tlvose at Murchison in 1929 and at Napier in 1931. Major earthquakes wore also experienced at Wairoa in 1931 and Pahiatua this year. Because the recent earthquakes and most of the major ones of the tlast hundred years had been confined to an area xtending a few hundred miles north and south of Wellington, many people concluded that major earthquakes were confined to this area, but we would be living in a fool’s paradise if this idea were allowed to grow and bo five means of preventing adequate precautionary measures against the dangers of earthquakes being taken. Practically every part of New Zealand was liable to those sudden movements of the earth's crust, and the people must educate themselves to realise that by sound design and honest workmanship in construction all buildings could bo made safe from destruction by even major earthquakes, Mr Nownhnm then made brief reference to the effects of earthquakes on buildings, and pointed out that, in order to reduce live wreckage forces, the one object to be aimed at was to keep the centre of gravity of a building as low as possible. Tin’s could partly he achieved hy making iloors and

roofs as light as possible without sacrificing their fireproofness and permanence. The use of all extraneous weights such as heavy parapets, towers, and other ornaments should be avoided as much as possible. Earthquake resistance required that buildings should be designed as one unit, not ns separate units, with columns to carry their direct loads, and that the whole should lie capable of taking lateral loads as well as vertical. Ho spoke of the construction 3 of walls and columns, and mentioned that it was essential that all parts of the. foundation should be tied together, so that they would act in unison and transmit the earthquake forces simultaneously to all parts of the structure. The shape of a building was also a consideration. The chimney problem was one not easy of solution, and so far no thoroughly satisfactory design had been evolved. One very groat source of danger in an earthquake was a composite construction, such as a wooden shop, with a brick front inadequately tied to the building. Reports of all earthquakes indicated, and investigations in New Zealand confirmed, that practically all damage was due to bad design and popr workmanship. This state of anairs must bo remedied. Codes for the of earthquake damage were now in existence, and the need for action in New Zealand was urgent. After touching upon other aspects of the question of earthquakes and building construction, Mr Newnliam said that the dangerous features‘ of existing buildings must be gradually eliminated. . A scries of lantern slides, including several diagrams and pictures of damagy

done to buildings by earthquakes, was then explained by tlic speaker, who was accorded a vote of thanks for bis address.