Byerly has developed a method by which the direction of faulting in a normal-focus earthquake may be determined from the distribution of initial motion compressions and dilatations. Using a method recently devised by Stechschulte it is possible to obtain travel-time curves for an earth stripped to any focal depth and so to obtain in each case a virtual surface focus. Byerly's original theory may then be applied directly.

This technique has been applied to obtain values of extended distance for all depths of focus given in the Jeffreys-Bullen tables. The values are listed in tabular form.

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