THE SHRINKING EARTH

Taranaki Daily News, 18 February 1931, Page 4

THE SHRINKING EARTH

RECORDING THE MOVEMENTS FIVE SEISMOGRAPH STATIONS. ONE CENTRE AT NEW PLYMOUTH. At the direction of Dr. C. E. Adams, Government Seismologist, a WoOd-An-derson seismological machine has been insUlJed in the private garage of a i’rankieigh Park .resident at New Plymouth for the purpose of estimating the distance and position of earthquake shocks which occur in New Zealand frolu time to time. This, it is stated, is one of four Widely spread stations whose records are being used with those made -by instruments at the Dominion Observatory in Wellington.. With the results of five stations to go by, Dominion seismologists are in a better position to study earthquakes, than they have ever been before. The machine at New Plymouth, which has been installed for more than a week, is of a Californian type, and radically different from the various forms of horizontal pendulum seismographs. The moving part ,is a small vertical metallic cylinder with a mirror attached. A beam of light strikes the mirror in the usual way and is reflected back to the surface of a revolving drum on w-hich is fixed a strip of photographic paper. REGULAR RECORDS.

Records come 'in regularly from those who are in charge of instruments at distant stations , and the seismologists at Kelburn have a great number of interesting ones made during the last few .weeks. At Hastings there is an instrument of the Jagger type, developed by Dr. Jagghr, of the Volcano Observatory, Hawaii. It is being looked after by Mr. Henry de Denhe, of Nelson Street, a’ solicitor, who has-been playing a prominent part in relief work since the disaster. This instrument was set, up on Friday, February 6, three days after the first big shock. It has been, working practically cofitiriubiisly ever since. In ■'principle, the Jagger seismograph is similar to tKb‘old Milne type; It-has a vibrating weight, with a' short period of oscillation, to- which is attached a long, light pdipter which, serves to magnify the .motion of the pivoted weight. At the end of this pointer is a sharp stylus which .-traces. a line on the smoked surface of. a slowly revoly-: ing cardboard disc. . • At .Takaka, an Imamura strong-mo-tion three-component seismograph has been installed since Dr. Adams visited the region for the purpose of setting it up about January 22. The Imamura is a Japanese type of seismograph which gives extremely good records of two horizontal compopehts at right angles as ■Well as the vertical component.. It has been working well, and traced ilrstejasa records of til© big shocks very clearly on the smoked paper used for recording. Smoked paper records, as given by the Imamura and. Jagger instruments, are “fixed” by immersion in shellac. MACHINE AT ARAPUNI. A seismograph of the Milne type is jn operation at Arapuni under the car© of officers at the hydro-electric works. It is admirably situated for its records to be used with those of other stations in fixing the origins - of earthquakes in the recently-affected area. The Milne is an instrument with a long beam supported horizontally which swings under the influence of earth tremors. . ;■ At the Dominion Observatory itself a number.of instruments is.working. M°St important, perhaps, are the two MilneShaw machines set up witp their beams at fight angles, one - pointing east and west and. the other north and south. These instruments are improvements of the Milne instrument. The beam is a horizontal one, swinging laterally. Light from a small lamp shines on to a mirror and is reflected back to the surface of photographic paper fixed on a drum revolved by clockwork. Tri addition to these two Milne-ShaW machines, there is a Wood-Anderson seismograph, of the type already mentioned, an improvised Jagger machine, a Galitzin seismograph Which records vertical motion only, and a clinograph, which, although useful, is not primarily designed for seismological work. As far as recording goes, instrunients like the Milne, Milne-Shaw, Wood-An-derson and Galitzin, which all record on photographic paper; have much in common, CONVERSION OF MOTION. The mechanism converts the motion of the earth into the motion of a tiny beam of light. This is reflected on to ’surface of a slowly revolving drum round which A strip of sensitised paper is fixed. The record stays on the machine for a day, and when removed and developed appears like a long photograph of a series of parallel straight, and sometimes wavy, lines. The paper lias' been on the drum with the two ends together, sb that what seems to be a number of parallel lines is really a continuous record. The drum in addition to turning round has been moving steadily along its axis. On; all the machines except the Wood-Anderson, the drum makes one revolution .in an hour, so that each long Ijne on the chart is an hour’s record. On the Wood-Anderson, however, the drum revolves completely every 15 minutes. This gives a more widely spread record.

All iiistrurr.mis arb connected with a clock which gives an electrical impulse each minute, resulting in an eclipse or deflection of the light-beam for a few seconds. In this way, by slight breaks which occur at minute intervals along the line of record, seismologists can determine at exactly what minute and second any given wive begins. The most interesting thing about the records is the manner in which by d study of the waves corresponding to a certain disturbance scientists can determine the distance from the instrument of the epicentre, which is the spot on the surface of the earth directly above the origin of a. Sfeismiie disturbance. The first wave to reach a distant instrument comes through the earth by the shortest route. The next wave may be a reflection of the preliminary wave, which menus that the wave from the origin has come up to the surface at an intermediate point, been reflected into the earth again, and has come out at the instrument. There may be one op more of these reflections before the type of wave arriveb.’ This second main wave also takes the shortest route and i$ caused by transverse stresses in the edrth. It, too, uiay have 1 reflections. Then finally th > surface wave comes bustling in. It has come all the way along t|ie surface. Prom the difference in the times at which the waves arrive an estimate of the distance can be made. SERIES OF WAVES. From the position of each station circles cgn be d ,-a -"'I l "ith ra.cj.ii in each case to the distance of the disturbance given from the record at the place. These circles should intersect

theoretically at the same point, but for various reasons they rarely dp. It is a difficult matter in many cases to say from a seismograph record just where a quake begun. If it is a severe ope, close by, the separate waves will be jumbled together, as often happened with the Murchison earthquakes. The latter seem from, the. records to have been more sevete than the ones recently experienced. A point that often puzzles the layman was explained by onp of the seismological workers at Kelburn, Wellingt.n. The waves in the lines on the seismograph charts are not all of the same size or so close together. He explained that tremors came in with all periods of vibration from a fraction of a second to 20 secoflds or more,