EARTHQUAKE TREMORS.

New Zealand Herald, Volume LXVII, Issue 20474, 28 January 1930, Page 13

EARTHQUAKE TREMORS.

LONG DISTANCES TRAVELLED.

FROM MURCHISON TO OTTAWA. MORE SEISMOGRAPHS NEEDED - Through the courtesy of the director of the Canadian Seismological Observatory at Ottawa, an interesting record of the New Zealand earthquake of last Juns, as observed in Canada, has become avail* able for inspection by local seismologists. The actual chart taken from the Ottawa seismograph is in the possession of Mr. W. F. Robinson, F.R.G.S., lecturer in encineering at Canterbury University College, and it traces with great distinctness the arrival of the earth tremors after travelling nearly half-way round the globe. The earthquake commenced at Murchison at 10.17 a.m., according to observations taken at the Christchurch Magnetic Observatory, and the first intimation of the earthquake was recorded at Ottawa 21£ minutes later. These waves, called brachistochrone waves, travelled more or less through the earth, writes Mr. Robinson in an interesting commentary, and followed a path which allowed of the shortest time of travel, being reflected from the inner or concave surface of the earth's shell. The velocity of the waves must have been about 12 miles a second.

Arrival of "Earth Rollers." A second series of energy waves, known as transverse, after being once reflected, arrived 18m. after the first, while the same waves, after being twice reflected from the earth's surface, arrived after the passage of a further sm. After another interval of 11m., or 44m. from the arrival of the first waves, the long waves or earth rollers arrived. These probably travelled through the outer shell of the earth, not far from the surface, a course which is not that of the shortest time of travel. They had a large amplitude and continued for about half-an-hour, at the end of which they began to fade until they became practically insensible nearly four hours after the first record was made. Mr. Robinson, who acknowledges his indebtedness to Mr. H. F. Skey, director of the Christchurch Magnetic Observatory, for much of the above information, revives the question of establishing more seismographs in New Zealand. "It is to be hoped that the Government department concerned will adhere to the good resolutions it made while the ground was still quivering beneath it and will bring the seismological observatories of the Dominion more up-to-date," he said. "1 doubt if the establishment of only one up-to-date and first-class instrument at Wellington will suffice. Useful as it might be for the record of a distant earthquake or for minor local shakes, the record of an important earthquake whose epicentre was near the instrument would probably be off the scale. Instruments in Country.

"Scattered stations, even if fitted with simple instruments, can, in the hands of intelligent observers of no great specialised scientific knowledge, give valuable information. Since time is a very important part of the record, it would seem that at various country post offices there should b? installed second-class (not second rate) instruments, of the selfstarter type if desired, and that a little more responsibility still should be laid on the shoulders of the postmasters, who already have enough to do. These o'fficers, by their training and general standard, are the best fitted of the civil servants to have charge of these instruments. "It should be remembered that in Japan seismology has reached a stage at which it is said to be possible to predict a coming important earthquake with reasonable certainty some days beforehand," Mr. Robinson adds. "There are authenticated cases where, the lower animals have had instinctive premonitions of earthquakes and other geophysical disturbances, therefore it should be quite possible for man to get those same warnings instrumentally."