EARTHQUAKES

Nelson Evening Mail, Volume LXIV, 17 January 1931, Page 2

EARTHQUAKES

STATION REPORTS SOME BUSY DAYS As previously stated the continued earth tremors on the West Coast of the South Island are to form the subject of a special inquiry by the Government Seismologist (Dr. C’. E. Adams), who leaves Wellington shortly for Takaka. He will take with him a seismograph of Japanese make, recording the three components on smoked paper, instead of photographically. Special packing precautions will be necessary, and the instrument will be placed in charge of a capable person, and run in conjunction with a special clock, in view of the lack of timing facilities on the West Coast, equal to those of Kelburn Observatory. From the records thus made and those received at Kelburn it is hoped to determine the places of origin of the disturbances with some accuracy. It is customary to install two instruments, taking the impressions of shocks at right-angles to each other. With two such stations at work it is possible to work out fairly accurately the centre of the disturbance, but with three separated stations it is possible to make the verification of the central spot and also the distance virtually accurate. Reading seismographs, despite the formal rules laid down, is not the simplest of matters, but with the direction indicated from three sources, the results are remarkably accurate, the intersection of the lines of direction giving the actual spot. With fewer stations, it is sometimes only possible to tell that the centre of the disturbance is somewhere within the circumference of an ascertained circle. By having two components at right-angles, the direction at one point may be ascertained. Records of vertical disturbances show as a rule that the seismograph recording them is more or less directly over the disturbance itself. Records of earth tremors are sent in to the Observatory From many stations, most of them by officers in the postal service, and these are of great value, both as recording the impressions received and the strength of the tremors on the Rossi-Forel scale. The 17tli December was a particularly busy day in the receipt of such messages at the Observatory. On the previous day there had been recorded shocks ranging from sto 6 on the Rossi-Forel scale at Takaka. 5 at Blenheim, 4£ at Westport, and from 3 to 5 at Kaliauranga Point and elsewhere, but on 17th December shocks were felt at such widely separated spots at Opotiki, Akaroa, Eketahuna, Christchurch, Westport, Waipawa, Stratford, Cambridge, Blenheim, Wanganui, Farewell Spit, Motu, and Poro-o-Tarao. At Takaka on 21st December shocks of from 6 to 7 on. the Rossi-Forel scale were felt, accompanied by a loud booming. A SIMPLE RECORDER Like the honorary recorders who assist the Government by sending in meteorological records such as rainfalls, these earthquake enthusiasts endeavour to make their reports as illuminating as possible, while priding themselves on their accuracy. Reports from such a wide range as those of 17th December would have been invaluable in determining the centre of the disturbance had the recorders been furnished with some simple form of inexpensive instrument giving the direction of the tremors at the various stations. There is one such at Stratford, where the recorder, Mr Burrell, has made his own. It com sists of a heavy lead cylinder, suspended on a long wire, the cylinder moving almost in touch with a plate of glass. Round it, standing on the glass; are light blocks of wood, just touching the round cylinder at eight points of the compass. • The lightness of the blocks on tile smooth glass does not aneot the movement of the pendulum, but after it has become still again, the blocks show its greatest extent of movement, and the direction of the tremor. Dr. Adams has at the Observatory an instrument some adaptation of which would be admirable for the recording of tremors at such stations. It is on the principle of the instrument invented by Professor T. A. Jagger, consisting of a lead cylinder standing vertically and , attached to its support by two springs, and furnished with a long arm carrying , a pen which records spirally ofl a cardboard disc rotated by a clock, thus 1 making a record something like that of a gramophone. This instrument will , record shocks only at right angles to the i arm, and therefore it needs two to give the direction of the tremor.