Travel times and amplitudes from the Salmon explosion

Salmon, an underground nuclear explosion in the Tatum salt dome in Southern Mississippi, was seismically recorded throughout North America and at some overseas stations. Data are presented from 143 seismograph stations recording signals from the Salmon event. Travel times at smaller distances generally confirm the high upper mantle velocities indicated by Gnome as recorded in the eastern United States. Travel times for the area east of the Rocky Mountain front may generally be represented by the equation t = Δ/8.3 + 8 seconds (Δ in kilometers); however, additional detail is shown indicating apparent velocities of up to 8.7 km/sec. In the 10° to 20° distance range multiple arrivals are apparent, and beginning near 16° late initial arrivals are observed which correspond to Jeffreys-Bullen travel time. Amplitudes were generally higher than expected for regional and for teleseismic distances but lower in the 13° to 22° range in the western United States. A revision to procedures for determining magnitudes is shown to be necessary.