SEISMIC ACTIVITY IN THE SNOWY MOUNTAINS REGION AND ITS RELATIONSHIP TO GEOLOGICAL STRUCTURES
In the inset to the figure the Berridale and Rock Flat earthquakes are included using a reduced scale. Also included is a tremor of magnitude 3½, which was recorded by a Willmore seismograph at Wambrook on 11th March, 1957, before the installation of the network. The considerable reduction in the rate of strain release since the Berridale earthquake is evident.
Because the Berridale aftershocks are believed to represent release of secondary strain produced by the Berridale earthquake, this sequence will be considered separately. The remainder then represent the effect of continuance of the tectonic forces which had been responsible for that earthquake. The figure shows a gradual increase in the rate of strain release for about 600 days, after which it becomes approximately constant. It can be seen also that with the passage of time the frequency of occurrence of shocks decreases, while the magnitudes of the shocks increase. These facts may be interpreted as follows: The Berridale earthquake, was accompanied by significant relaxation of stress over a considerable area of the Snowy Mountains. With the gradual rebuilding of the stress field, shocks first occurred in places where resistance to movement was not great, later the stress field increased to a value where more tightly bound dislocations were forced to move. It is possible that this process will continue until a large movement produces sufficient stress relaxation to allow the system to return to its initial condition.
Fig. 8. Comparison of strain release curves of Berridale and Kern County (Calif.) after-shock sequences, using a logarithmic time scale.
