SEISMIC ACTIVITY IN THE SNOWY MOUNTAINS REGION AND ITS RELATIONSHIP TO GEOLOGICAL STRUCTURES
III. ISOSEISMALS
Following both the Berridale and Rock Flat earthquakes, questionnaires based on the U.S.C.G.S. questionnaire cards were circulated to many people over a wide area, and from the elicited information isoseismal maps were constructed (Figs. 2 and 3).
Anderson (1951, p. 124), following Davison, believes that elongation of isoseismal curves in a particular direction indicates the direction of the fault-line, and states that this has been ``abundantly verified'' for some well-known faults in Britain. In the present cases, the Berridale isoseismals are fairly symmetrical, but those for Rock Flat have a marked elongation in a meridional direction. Both sets of isoseismals appear to be distorted near the coast; it is well known that increase of local intensity occurs when waves emerge into alluvium or unconsolidated sediments.
Fig. 1. Computed epicentres, with standard errors. The inset shows focal depths of tremors in the vicinity of the Berridale earthquakes.
