Abstract

The Dominion Observatory, Ottawa, Canada, has applied Byerly's method to determine the direction of faulting in 65 earthquakes, which makes a total of 75 earthquakes so analyzed. To date 10 solutions have received some confirmation by comparison with observed faulting.

The method is ambiguous in that two planes, neither of which is indicated as the fault, are defined for each solution. This does not obscure the fact that of the 75 earthquakes all but 8 resulted from strike-slip faulting. Of these 8, 5 are on the Pacific coast of North America, 2 in the Hindu Kush, and 1 was an anomalously deep-focus earthquake off the coast of Spain. Otherwise, for all circum-Pacific areas and for the Mediterranean, and for focal depths ranging from 12 to 650 km, strike-slip faulting appears to be the rule. Within any area the strike directions of the faults appear to be random. The dip direction on the other hand may have some consistent orientation. This apparent inconsistency could be explained by supposing that planes whose strikes deviate from a favored direction tend to develop very steep dips.

In any solution the intersection of the two planes defines a unique line, here called the null vector. Whichever plane represents the true solution, this vector is perpendicular to the displacement couple and so undergoes no displacement; hence its name. In the southwest Pacific the null vectors lie parallel to vertical planes striking in the direction of the associated geographical features. In other circum-Pacific areas the correlation is less certain, but it is still definite enough to suggest a considerable measure of confidence in the validity of the fault-plane techniques.

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