SEISMIC ACTIVITY IN THE SNOWY MOUNTAINS REGION AND ITS RELATIONSHIP TO GEOLOGICAL STRUCTURES
Fig. 2. Isoseismal map for Berridale earthquake.
Fig. 3. Isoseismal map for Rock Flat earthquake
Another conclusion reached by Davison from his investigations was that the fault must reach the surface near the centre of the innermost isoseismal. It can thus be inferred from the positions of the Berridale and Rock Flat epicentres in relation to their isoseismals that for the case of Berridale the fault dips to the north-west, while for Rock Flat the fault dips to the west.
Independent estimates of the focal depths of these two shocks were obtained from the macroseismic data, using the empirical formula of Blake (1941):
J − j = −5⋅35 log10 cos θ, θ = tan−1 (Rj/h),
where J is the maximum intensity, Rj, is the radius of the area enclosed within isoseismal j, and h is the depth of focus. Where the isoseismals did not close, Rj, was estimated on one side of a line drawn through the epicentre. The results are given in Table II. The scatter is similar to that obtained for the Robertson isoseismals (Cleary and Doyle, 1962).
If no information was available apart from that shown in Table II, it would be concluded that (i) both foci were certainly within the crust, probably at depths of between 10 and 20 km., and (ii) the Rock Flat tremor was probably shallower than the Berridale tremor. Thus the depths of foci obtained from instrumental data are to some extent corroborated.
