SEISMIC ACTIVITY IN THE SNOWY MOUNTAINS REGION AND ITS RELATIONSHIP TO GEOLOGICAL STRUCTURES
Referring back to the projected section of Fig. 1, it can be seen that in fact Nos. 7, 8, 9, 10, 12, 13, 15, 26 and 31 are in reasonable proximity to this plane (shown on the section by a dashed line). The remainder, Nos. 5, 16, 19 and 20 at the south end and Nos. 17 and 28 to the north, are the ones which show least agreement with the postulated direction of faulting. It is suggested that at this distance from the source of secondary stress, the fault movement is complicated by the addition of a thrust component. Such a feature is often observed in transcurrent faults which are associated with thrust faults (De Sitter, 1956, pp. 165-169). In the case of No, 20, which actually lies between AA´ and BB´, the observed directions of motion could have been produced by a thrust along a plane almost parallel to the Berridale fault plane.
It may be remarked here that in an area so highly faulted as the Snowy Mountains, movements are very likely to occur along previously existing faults which constitute preferred planes of fracture (Bott, 1959), and consequently it is unnecessary to postulate contemporary rotation of the crustal stress system of Anderson (1951) in order to explain the high angle thrust of the Berridale fault.
V. STRAIN RELEASE ANALYSIS
In Fig. 7 the square root of energy released, which is taken to be proportional to the strain release, is plotted cumulatively on a linear time scale, Some recent shocks are included which have not yet been accurately located and which therefore do not appear in Table I. Zero time on the scale corresponds to the time immediately following the Berridale tremor. Table I includes the magnitudes from which ordinates were derived according to the formula:
Log J ½ = 4⋅5 + 0⋅9M,
which was used by Benioff (1955) in his strain release analysis of the Kern County aftershocks. In this formula, J is the energy released by a shock of magnitude M.
