QUAKE RECORDING.

Waikato Times, Volume 107, Issue 18024, 20 May 1930, Page 2

QUAKE RECORDING.

SAFETY OF BUILDINGS.

VIBRATIONS AND RESONANCE. Interesting sidelights on 'modern earthquake theory and the latest earthquake recording machines were provided in an address given by Dr. C. E. Adams before the Wellington branch of the New Zealand Institute of Architects. The subject of the address was “ The Measurement of Vibrations,” and Dr. Adams dealt generally with movements of the earth as well as the safety factor in buildings. Amply illustrated with lantern slides, his address touched on many things of intense interest to laymen. Mr A. S. Mitchell was chairman. Dr. Adams spoke firstly of early earthquake investigations which had been.done by means of much cruder devices than the ones in use to-day. The Milne seismograph was the first to be used in New Zealand, but Milne’s pattern had been improved in several ways by Shaw. Two of these Milne-Shaw machines were working at the Kelburn Observatory at present. They were really horizontal pendulums, designed more especially for the recording of distant shocks. Their main purpose was to fix accurately the time of arrival of waves from disturbances some distance away and to measure the movement of the earth. In'the case of two oversea earthquakes recorded' in Wellington last year, the earth had moved here through a distance of a millimetre or a millimetre and a half. By co-operation between stations, the speed with which the waves travelled from any particular disturbance could be found out accurately and the peculiarities of propagation investigated.

Internal Structure of Earth. From such results said Dr. Adams, knowledge of the internal structure of the earth was being gained. It was the only method of doing this, because even the deepest mine did very little more than touch-The of the interior. It was agreed .that the earth’s internal structure consisted of a core of iron and nickel, surrounded by concentric"‘sheJls of other" minerals which progressed from "substances of considerable density round the core to a range of rock Shells near the surface. Turning to local shocks, he showed a diagram of the wave mechanism near the origin or epicentre. These were becoming increasingly investigated—an accidental substerranean explosion having first set scientists on the track. The- explosions were now intentional and artificial, and fast recording machines were used near the origin. ■V' Some means of combatting resonance was very necessary , in buildings: said Dr. Adams. The natural period of a building should not for safety come within the ordinary earthquake period, which was from half a minute to a minute and a half, and some method- of ensuring this immunity should be used." It was a difficult matter, he said, and one way > of tackling it seemed to be to take observations of the period of existing buildings, and then use the results in the design of Dew ones. The oscillation of bridges and viaduots was a similar concern to the constructional engineer. But here the danger period was that of the trains or, trafflo which passed over them. DjS Adams explained an instrument' which he had designed Jointly with his son, Mr Howard Adams, for the measurement of bridge vibrations. It was the first of its kind in New Zealand, he added, and was working satisfactorily. The Railway Department has lent it to him for his address.

In New Zealand. As far as New Zealand earthquakes Were concerned, he had made efforts , to arrive at the precise positions of the epicentres of many of our shocks. He showed a map which indicated the origins of disturbances which had occurred in New Zealand last year. Five • were at the north of the South Island, four near the south of the North Island, seven were at sea in Cook Strait, and between Taranaki and Marlborough, and the remaining two were to the east of Cape Turnagain.. They seemed to be centring round Wellington, he : added. He described, an extremely simple seismometer which would be of great service in locating ..epicentres,' providing that people were interested enough to make and install them. It consisted merely of a heavy lead cylinder suspended by a piece of piano wire. Round the base of the cylinder were set eight pegs of wood in a plate attached to the earth. When a quake occurred the earth slid across, and the cylinder stayed momentarily still, knocking down pegs in the appropriate From many of . these instruments, tHe disturbances could probably be tracked down with fair accuracy.