SEISMIC ACTIVITY IN THE SNOWY MOUNTAINS REGION AND ITS RELATIONSHIP TO GEOLOGICAL STRUCTURES
Origin times obtained in this manner from each station are compared; gross discrepancies may require re-interpretation of the records. Epicentral distances are calculated using the adopted origin time and an assumed depth, and corresponding arcs are drawn from each station on a map of the area. The epicentre will be located at the intersection of the arcs: the coincidence of arcs thus provides a further check on the validity of the interpretation.
In practice, it is found that shocks in the area which are large enough to be recorded at all stations can generally be interpreted without difficulty. Occasionally a station in close proximity to the epicentre may record a “false S” (Richter, 1958, p. 293), which can be readily identified by the above technique.
When first approximations to origin time, latitude, longitude and focal depth have been calculated as above, these are fed into the computer together with weighted arrival times, station co-ordinates, and wave velocities. Using a least-squares iterative process, optimum values of the parameters are derived, with standard errors.
The computed epicentres, focal depths and origin times, with standard errors, are listed in Table 1. The epicentres have been mapped in Fig. 1; standard errors in latitude and longitude are indicated by rectangles about the epicentres. The high accuracy of the determinations was made possible by the close proximity of the network to the shocks. In a few cases the programme did not give a convergent solution for depth of focus, although the other parameters were well determined; in the figure these epicentres are represented by crosses. The projected section shows the depths of foci, with standard errors, for the relatively active region between Wambrook and Jindabyne. It may be noted that the majority of these foci are at depths close to 10 km.
In the eight months prior to the Berridale earthquake only three movements in the Snowy Mountains were recorded, none of which were in the Berridale region. In the fifteen months following the event, at least 25 minor shocks occurred, which were confined almost exclusively to an area within 20 km. of the Berridale epicentre. Activity subsequently diminished in that area, but at the same time movements became more numerous in other parts of the Snowy Mountains. Although the Berridale tremors probably represent an aftershock sequence, their behaviour is atypical in that no activity was observed in the few days following the major shock. This is in contrast to the Robertson-Bowral sequence, in which about 80 aftershocks were recorded within three days of the earthquake (Cleary and Doyle, 1962).
