SEISMIC ACTIVITY IN THE SNOWY MOUNTAINS REGION AND ITS RELATIONSHIP TO GEOLOGICAL STRUCTURES
For the Berridale aftershocks, on the other hand, the corresponding curve shows a rapid decrease in activity after about 400 days. In Fig. 8 this curve has been replotted on a logarithmic time scale, and is compared with the strain release curve obtained by Benioff (1955) for the Kern County aftershocks which occurred to the north of the White Wolf fault. Bath and Richter (1958) interpreted these aftershocks as being due to a secondary release of strain produced by motion south of the fault. The similarity between the two curves is obvious, and supports the conclusion that similar mechanisms were responsible for their generation. The outstanding differences are the much smaller release of strain energy, and the much longer delay before onset, for the Berridale after shocks. It seems probable that these two features are related.
The possibility has been considered that the Berridale sequence was caused by the filling of the nearby Lake Eucumbene. It has been observed, however that the maximum rate of filling of the Lake occurred about the same time that activity in the area tailed off. It may be remarked also that the Berridale earthquake occurred before filling had proceeded to any significant extent. Carder (1945) has published the results of investigations into the effect of the filling of Lake Mead upon local seismicity. This study was not based on magnitudes, but from the data provided on the number of felt shocks, it appears that activity did not taper off appreciably, but showed a fluctuation corresponding to the seasonal loading of the dam.
VI. GEOLOGICAL SETTING
Fig. 9 is a geological map of the Snowy Mountains area, with epicentres superimposed. A circle has been drawn round each epicentre, with radius proportional to the magnitude of the shock.
The Snowy Mountains area is a complex, block-faulted region formed by the differential uplift during the Tertiary era of thoroughly compacted Palaeozoic sediments and granites. Considerably more than half the region is now composed of granite and it is certain that the proportion of granite would be much higher at relatively shallow depths.
The Kosciusko plateau rising to more than 7,000 feet forms the central block. This is tilted gently downwards towards the north without any marked transverse dislocation, but on the south-east it terminates abruptly against the gorge of the Crackenback River.
