On the Origin Time of the First Nuclear Test Explosion (TRINITY, 1945)

Modern analysis applied to analog seismograms from 1945 gives a new estimate for the origin time of TRINITY, which took place on July 16 that year in Southern New Mexico, with yield reported as 21 kilotons. Radio timing services failed for this important event, but were available to determine the origin time of an earlier 108 ton TNT explosion, which was conducted very near the TRINITY site, on May 7, 1945. We have scanned and digitized the vertical-component analog seismograms recorded at Tucson Observatory (TUO, at a distance of 437 km) for both events. The regional signals at Tucson include Pn and Pg, and presumably Sn and Lg though these are not obvious. The ground motion from infrasound signals is stronger than from seismic signals, for both explosions, but is not suited to detailed analysis (their time-dependent velocity profiles lead to different shapes in infrasound waveforms at TUO recorded ten weeks apart). We applied modern methods of analysis to the regional seismic window, and find that the faint signals of May 7 provide a satisfactory cross-correlation peak when compared with the signals from TRINITY, enabling us to provide a best estimate of its origin time as 11:29:24.5 (GMT) to the nearest tenth of a s. This result to be good to within a few tenths of a s, in view of various uncertainties, and is significantly different from official reports. It is good to know that modern cross-correlation methods can be effective and simple to use, in application to analog recordings of complicated weak regional seismic signals.