SEISMIC ACTIVITY IN THE SNOWY MOUNTAINS REGION AND ITS RELATIONSHIP TO GEOLOGICAL STRUCTURES
In 1909, 21 well-equipped seismograph station was set up at Riverview, and this has been in operation continuously since that date. Unfortunately for the present purpose, the instruments were of long period and relatively insensitive to the effects of local tremors, being incapable of detecting tremors of maximum intensity less than Mercalli IV (magnitude > 4) originating in the Snowy Mountains (Burke-Gaffney, personal communication). Nevertheless, the seismologists at the Observatory had made a practice of systematically investigating all which were recorded and of assigning approximate epicentres to them (Burke-Gaffney, 1951). In the 40-year period to 1950 no epicentres were found in the Snowy Mountains region although a considerable number was recorded from the Dalton-Gunning area. During the same period many small tremors felt at Gunning-Dalton were not recorded at Riverview (Joklik, 1950; Joklik and Casey, 1952). Also in the same period there were occasional reports of small shocks being felt in the Snowy Mountains area which were not recorded at Riverview (for instance, Sydney Daily Telegraph, 24th March, 1917). Occurrence of such local tremors has since been confirmed by information supplied by several persons in answer to the questionnaires which were sent out in connection with the tremors of 1st September, 1958, and 18th May, 1959.
No information about the seismicity of the region was available from other observatories in Sydney, Melbourne and Adelaide.
L. A. Cotton (1915) operated sensitive horizontal pendulums in tunnels near Burrinjuck dam in 1914–1915 to measure any deflection of the earth’s crust under the load created by the filling of the reservoir. His records showed a number of small tremors attributable to local earthquakes (personal communication) but from the nature of the installations it was not possible to determine epicentres.
Preliminary tests by the Snowy Mountains Authority in co-operation with the Australian National University commenced in 1956, using a Willmore and later Wood-Anderson seismographs. After several local tremors had been recorded by these instruments, the Authority decided to install a seismograph network capable of locating the origins of such movements. Dr. Dean S. Carder, of the U.S. Coast and Geodetic Survey, gave valuable advice at this stage on instrumentation and network operation. Four stations were set up, at Wambrook, Jindabyne, Geehi and Cabramurra, equipped with short-period Benioff seismographs. These stations came into operation in 1958.
