Epicenters and origin times pertaining to surface focus events, mainly underground nuclear explosions, have been determined using the method described in this paper with estimates comparable to those of United States Coast and Geodetic Survey (USCGS) determinations. Data used are crude estimates of apparent phase velocity and backbearing, supplied by a medium aperture seismic-array station (at Gauribidanur in South India) using time lags across the array arms, and the preliminary seismogram readings available from just four Indian World-Wide Network of Standard Seismographs (WWNSS) stations and less than five foreign stations. The method essentially solves a system of time-term equations until an ideal “minimum variance” is achieved to best fit the observed onset times. Provision has also been made in the method to give an allowance for onset-reading errors, if any to the extent of ±0.53 second for epicentral distances up to 10,000 km. Twenty-five Russian tests, LONGSHOT in the Rat Islands and five Nevada tests were tried with the present technique and results compared with corresponding USCGS and United Kingdom Atomic Energy Authority (UKAEA) determinations. For the epicentral regions under study, station time-residuals have also been obtained.

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