A major simplification of teleseismic epicenter location procedures can be achieved by regression in the space domain. If distance residuals are used instead of travel-time residuals the normal equations become linear. Observations must be weighted by ∂t/∂▵; however, linear weighted regression is extremely fast as compared to unweighted nonlinear regression. The new program features several other innovations; separate estimation of focal depth, self-initialization using the nearest station as trial epicenter, and second-degree approximations to the Herrin tables instead of travel-time tables in core. On the basis of a relocation of event LONGSHOT, the new routine is found to be about ten times faster, absolutely convergent, and more compact than any of the standard programs in current use.

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