A spherical hyperbola program initially used for the determination of T-phase origins has been modified to allow the determination of earthquake epicenters by use of time differences in P-wave travel. Theoretical P-wave travel-time differences for pairs of seismic stations have been calculated and plotted on charts by a digital computer utilizing data from the Jeffreys-Bullen tables. The epicenter determination is accomplished in the following manner: If three stations observe an event, then the three different P-wave arrival times give two independent travel-time difference values (Δt). Each Δt value determines a time difference curve. The intersection of the two time-difference curves determines the epicenter location.

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