Studies of some of the geologic effects of underground nuclear explosions that have been carried on during the last 3 years by the U. S. Geological Survey on behalf of the U. S. Atomic Energy Commission are summarized. Movement has been triggered on some faults within a few thousand feet of explosions in Nevada and in Amchitka Island, Alaska. The permanent relative vertical displacement on the faults always has been in the same direction as the last recognizable natural tectonic displacement and has been as much as 1 m (meter); horizontal displacement of as much as 15 cm (centimeters) has occurred on a few faults although no previous horizontal displacement had been apparent. The strike length of triggered faulting ranges from about 1 to 8 km (kilometers) and seems to correlate linearly with yield and magnitude of the explosions.

High-speed motion picture photography of two faults from both the air and the ground provides the following data on the motion of the faults.

    1. Fault motion started at the first arrival of seismic energy.

    2. Rupture velocity along triggered faults, measured over a distance of 0.210 km, was 2.1 km/sec (kilometers per second) in alluvium of Hot Creek Valley, Nevada, which has a compressional velocity that ranges from about 2 km/sec to 3.3 km/sec and which has a density of 2.3 g/cc.

    3. The velocity of displacement of alluvium at the Yucca Fault plane in Yucca Flat was a little more than 1 m/sec (meters per second). Velocity of displacement of alluvium at the fault plane in Hot Creek Valley was between 6 and 30 m/sec.

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